Note: Descriptions are shown in the official language in which they were submitted.
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5INJECTION MOLDING PROCESS FOR THE PRODUCTION OF
ORIENTED THERMOPLASTIC AND PARTICULATE MATTER
COMPOSITE ARTICLES
FIELD OF THE INVENTION
This invention relates to articles formed of fiber or flake reinforced
thermoplastic composite materials and their manufacture by injection molding
processes. In particular, it relates to articles formed of cellulose fiber reinforced
thermoplastic compositions and, more particularly, to foam compositions which
have strength, modulus and density values comparable to typical hardwoods or
softwoods; and injection molding processes for their manufacture.
BACKGROUND TO THE INVENTION
United States Patent No. 5,234,652 to Woodhams et al, issued
August 10, 1993, describes processes for the melt phase extrusion of high
molecular weight polyethylenes to produce extrudates having substantially both
an increased modulus and increased strength in the flow direction. In the process
described therein, the high molecular weight plastic material, at or near its melt
temperature, is forced through a die having a converging passage so as to produce
a highly oriented extrudate. The plastic material at the die interface is in
substantially extensional i.e. plug flow through the converging die passage.
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The invention described in United States Patent No. 5,234,652 is
based upon the proposition that polymer chains, when fully extended and orientedin parallel fashion, confer greatly increased strength and modulus to the resulting
oriented extrudate. Although this concept has been extensively applied to fibersand films, allel~lpls to apply this concept to thicker sections have been limited by
the natural tendency of polymer chains to quickly recover their unstretched
equilibrium conformations at elevated temperatures. This strain recovery often
manifests itself in a phenomenon called die swell, in which the molten extrudateelastically retracts and expands as it exits the heated die. In United States Patent
No. 5,234,652, a process is described wherein a polymer is extruded in the semi-solid state, viz. melt phase extrusion under conditions that forcibly extrude and
extend the flexible polymer chains in the flow direction and retain such imparted
orientation in the extrudate. Under such melt state extrusion conditions at low
extrusion temperatures, sufficiently high molecular weight and plug flow, the
molecular relaxation times are sufficiently long that product orientation is largely
retained during and after cooling to ambient temperatures. High molecular weightpolyethylenes were identified as particularly suitable for this process.
Various extrusion processes are known for the continuous
production of integral structural foam products. Of particular relevance is United
States patent Nos. 3,764,642 - P.E. Boutillier, issued October 9, 1973. These
processes use the so-called "Celuka die" and provide a high-density, rigid skin
extruded product of desired size having an inner foamed core.
Whereas hydrostatic extrusion of polymers has been known for
some time (N. Inoue, M. Nishihara, HYDROSTATIC EXTRUSION, Theory and
Applications, Section 4, Polymers, Elsevier Applied Science Publishers, pp. 333-362, 1985) the process is normally restricted to ram extrusion which entails
deformation of a billet under conditions similar to the hydrostatic extrusion ofmetals. The prior art with respect to extrusion dies is extensive, as will be
understood from the published text by W. Michaeli (EXTRUSION DIES, Design
and Engineering Computations, Hanser Publishers, 1984). However, the precise
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conditions for achieving steady smooth extrusion of highly oriented polymers
without melt fracture or die swell are not generally known by those in the
extrusion industry.
W 094/11176- Woodhams R.T. published May 26, 1994 describes
a process for the continuous production of a high modulus article comprising a
composite of an oriented plastic m~teri~l and an oriented particulate material, said
process including the steps of: (a) continuously forcing an orientable plastics
m~teri~l, while it is close to or at its softening lelllpe-~lule and in admixture with
an orientable particulate material, through a converging passage of which the
cross-sectional area ~imini~hes in the forward direction of plastic flow, thereby
to produce an extrudate; (b) deforming the extrudate, while it is maintained at or
close to its melt temperature, to produce an oriented, deformed extrudate; and (c)
cooling the deformed extrudate to preserve the orientation and provide said
composite. The foamed and unfoamed composite articles have strength, modulus
and density values comparable to typical hardwoods or softwoods.
However, such an extrusion process does not readily lend itself to
the manufacture of non-uniformally shaped articles such as baseball bats, tennisrackets and the like which cannot be made by continuous extrusion processes
wherein the thermoplastics composite material is maintained under tension. Thereis a need therefore of a batch process for the manufacture of such articles formed
of an oriented composite material.
Injection molding processes are known for the manufacture of
thermoplastic articles wherein a melted thermoplastic material is injected underpressure into a suitably-shaped mold, the material cooled and the cooled
thermoplastic article removed. In contrast to extrusion molding which is carriedout under continuous, steady state conditions, injection molding is intermittent and
discontinuous. Further, the physical size dimensions of an extrudate section aregenerally constant and uniform whereas injection molding allows of significant
dimensional variations in the article.
Due to the shortage of high quality hardwood lumber and esc~l~ting
prices of certain types of hardwood species, there have been attempts to
manufacture baseball bats having the performance characteristics, physical
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properties, and aesthetic quality of traditional hardwood bats derived from ash,maple or hickory. Today it is possible to purchase hollow aluminum bats but
these have not yet met with favour in professional sports. Hence there is a desire
to find a substitute m~teri~l for hardwood that would duplicate the desirable
features of a hardwood bat. These desired material characteristics include a
density of hardwood near 0.64 g/cm3, a modulus of elasticity near 12 GPa, a
flexural strength of 100 MPa and possess a substantial impact resistance in order
to resist fracture in repeated use over a prolonged period of time under adverseconditions of weather and abuse. The obvious solution is to simply mold a
hollow bat from any one of several reinforced engineering plastic m~teri~
However an ex~min~tion of the technical literature reveals that there are few
compounds having modulus values greater than 10 GPa even when reinforced
with expensive graphite fibers. Moreover these molded composites tend to be
brittle and easily fracture on impact. Therefore there has been no cost effective
solution to the problem of clesigning a bat having hardwood-like properties.
The solution to this problem requires a substantial increase in
strength, modulus and impact resistance of a thermoplastic resin while at the same
time reducing the effective density to that wood. I have discovered that under
certain conditions of temperature and pressure, ordinary thermoplastics may be
elongated and deformed to produce highly oriented structures having substantially
improved mechanical properties. Furthermore I have devised a means for
injection molding wood fiber composites under conditions that simultaneously
orient the wood fibers and also the polymeric chains comprising the binder matrix
to effect the necessary increase in strength and stiffness. Furthermore, the
oriented structure resists impact and is exceptionally tough due to the longitudin~l
orientation of the entire composite structure. To meet the density requirement,
the composite is made hollow or internally foamed while ret~ining the orientation
in the outer surface. By these methods, the color, texture, weight, density,
appearance and feel of the molded composite closely resembles that of hardwood.
Using an automated injection molding process as herein described, wood
substitutes may be employed for the economic manufacture of hardwood baseball
bats and other like wood articles such as furniture components (e.g. table legs).
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In the example of a baseball bat the die is designed to yield a composite havinga solid section in the grip region where the greatest stress occurs. In the thicker
outer section of the bat, the core is either hollow or foamed (according to
pre~elt;nce) to maintain the overall density limitation. The solid outer skin issufficiently tough to resist normal abrasive wear and impact, so that the bat
should possess extreme durability unlike wood, which has a tendency to split, the
surface is easily damaged, and wood is affected by moisture and humidity. If
desired the surface of the bat may be texturized or decorated according to
methods known int the industry.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for the
manufacture of an oriented thermoplastic article by injection molding.
It is a further object of the present invention to provide a process
for the manufacture of an oriented thermoplastic composite article by injection
molding.
It is yet a further object to provide aforesaid oriented composite
articles made by injection molding.
Accordingly, in its broadest aspect the invention provides an
improved injection molding apparatus for the manufacture of an article formed ofa thermoplastics material having a melt index selected from the range 50C -
350C, comprising an article-shaping mold in communication with a melted feed
means for intermittently feeding said thermoplastics material under pressure andat a te-~-pe,;~ture above said melt index in a first mold-filling and article-shaping
amount to said mold; said mold having (i) cooling means within said mold for
cooling said melted thermoplastic m~teri~l to form a cooled, said article; (ii)
means for opening said mold to allow removal of said cooled article; (iii) meansfor sealably closing said mold for said mold to operably receive, intermittently,
a second and subsequent mold filling amount of said melted thermoplastics
material; (iv) an extrudate receiving aperture; (v) an inner surface defining anarticle-shaped inner cavity; the improvement wherein said melted feed means
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comprises (a) a die having a converging passage of which the cross-sectional area
~1imini~h~s in the forward direction of plastic flow and produces an extrudate for
said mold, and (b) means to control the prero-lll telll~ ature of said
thermoplastics material prior to passage into said converging die.
S I have found that certain thermoplastic materials containing
reinforcing fillers, the latter being substantially in flake or fibrous form, when
injection molded under conditions that promote molecular orientation as well as
particulate orientation, produce articles having greatly increased strength and
modulus, rivalling the structural pel~ollllance of softwoods and hardwoods. These
injection molding processes include the step of orienting both the thermoplasticpolymeric chains comprising the matrix and the dispersed particulate material, in
the longitu~in~l flow direction, and which solidifies the molded article in thatprefelled orientation, while substantially preventing relaxation of the polymeric
chains during the subsequent rapid cooling period in the mold to ambient
lelllpel~lu-es.
Flow orientation of the thermoplastic elastic melt deforms the
equilibrium conformations of the chain molecules in the converging die and
imparts a prefelelltial orientation of the chain segments in the flow direction.Such chain orientation increases the strength and modulus of the solid oriented
structure in the direction of orientation. The term "orientable particulate" applies
to anisotropic particles in which one dimension is much larger than another and
which may be substantially oriented in one direction - parallel or planar
arrangement. Such overlapping particles, typified by flakes and fibers, and
embedded in a polymeric matrix are, thus, normally considered "reinforcing
fillers".
The observed mechanical properties are a result of additive, or
synergistic effects of these two mech~ni~ms of reinforcement. The theory of suchreinforcement is well understood in practice (short glass fibers, asbestos, mica,
talc, wollastonite, wood fibers).
By the term "orientable particulate material" is meant m~teri~l in
the form of fibres, flakes and the like, which can substantially orient in parallel
or planar arrangement as hereinbefore described. Preferably, the orientable
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particulate material is derived from a cellulosic m~teri~l.
By the term "softening tc~ )e-~lule" is meant that temperature near
the melting point of a crystalline type polymer, or in the case of an amorphous
polymer, the tem~ ~ture near the glass transition temperature where there is an
abrupt change in viscosity.
Thus, in a preferred aspect the invention provides a process of
making by injection molding an article formed of a high strength and high
modulus cellulosic-thermoplastic composite, which process comprises:
intimately admixing shredded cellulosic fibers or cellulosic particles with
a thermoplastic polymeric material which has a softening point below about
220"C;
passing the mixture through a die having a converging passage and of
which the cross-sectional area limini~hes in the forward direction of plastic flow;
forcing the molten mass through the converging die passage under conditions thatimpart longitudinal orientation to both the cellulosic particles and the
thermoplastic polymer molecules in the direction of flow to produce an elongatedsegment;
feeding a said article-forming amount of said elongated segment to a mold
having cooling means, while rapidly cooling said oriented segment to preserve
said orientation and provide a cooled solidified article.
The invention preferably provides a process as hereinabove defined
in which the converging passage is provided in a streamline die which convergingpassage has a geometry which provides a decreasing strain rate of the elastic melt
in the flow direction.
The invention more preferably provides a process as hereinabove
defined in which the converging passage in the die has a geometry which providesa constant elongation rate of the elastic melt in the flow direction within the
converging zone.
In a more preferred aspect, the invention provides a process for the
batchwise production by injection molding of an integral structural foam
composite article of a plastic composite material comprising a solid oriented skin
said process including the steps of
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intimately admixing suitably orientable particulate m~tPri~l with a
thermoplastic material which thermoplastic has a softening point below about
220C;
forcing the ad~ lure through a converging die by injection pr~ssule at a
te.-lp~l~lure near the softening point of the thermoplastic material, so as to impart
predominantly longitudinal orientation to both the particulate m~t~ri~l and the
thermoplastic polymer chains throughout the extruded segment;
feeding a said article-forming amount of the oriented melted extrudate to
a mold having cooling means and under conditions which permit foaming to take
place in the core of the melt extrudate while rapidly cooling the article and
~ai~ ing a highly oriented, essentially solid outer skin on its surface.
The mean density of the product is most easily controlled by the
use of a foaming agent in which the preferred orientation of the solid outer skin
is maintained. Preferably the admixture is passed through a lubricated die whichhas been lubricated adjacent thereto so as to provide substantially plug flow
(slippage). The lubricating agent may be also or either provided in admixture
with the thermoplastic-particulate feed materials.
In a preferred embodiment, the particulate material is a cellulosic
material obtained by the grinding, coating and fiberizing of suitable cellulosicfillers derived from wood, paper or agricultural residues.
The combined influence of fiber orientation and polymer matrix
orientation greatly increases the resultant strength and modulus in the direction
of flow so that the integral foam having a highly oriented skin compares
favourably with the mechanical properties of wood. The average density of the
injection molded composite may be readily adjusted by controlled foam
expansion. In the case of "damp" cellulosic material, the moisture content of the
wood filler can act as the "blowing agent". The endothermic cooling action of
the water during vaporization and expansion helps to reduce the interior
temperature of the molded article in thicker sections, thereby reducing the
external cooling requirements. The cooled article is characterized by a solid skin
and a finely textured foam core wherein the overall density is determined by thedensity of the core and the thickness of the outer skin. Thus, the final product
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density may be controlled within certain limits to that of various wood species.The die head pressures reached are generally 145 MPa (21,000 psig) and permit
cycle times of 30 seconds using a conventional reciprocating single screw
injection molding machine.
In conventional extrusion processes it is common practise to
minimi7e elastic deformations which cause die swell. One purpose of the present
invention is to maximize elastic deformations during passage through the
converging die and permanently retain such deformations in the mold by rapid
quenching. This is achieved by deforming the melt near its softening point wherethe relaxation times of the polymer chains are sufficiently long to permit mold
cooling before substantial elastic recovery is able to take place. Using
conventional reciprocating screw injection molding methods, for example, the
injection pressure could be very large, perhaps excessive, at such low
te---pel~tures near the softening point of the mixture where the viscosity is also
very large. Accordingly it has been found necessary to employ die lubrication tofacilitate the injection process, and allow the use of reduced injection pressures
and temperature, at least within the limit of conventional operating pressures.
The injection of a lubricant adjacent to the converging die reduces surface friction
at the metal interface so that an increased flow rate is favoured, with the result
that the process can be operated at much lower temperatures and pressures.
Injection molding at reduced tempertures is essential for preserving the orientation
after passage through the converging die. After startup, the melt temperature isgradually reduced until the preferred temperature profile is attained. "Solid state"
injection conditions are approached gradually to avoid excessive pressures during
startup.
The cellulosic component may be derived from any number of
available sources such as ground wood, sawdust, wood flour, ground newsprint,
m~g~7ines, books, cardboard, wood pulps (mechanical, stone ground, chemical,
mechanical-chemical, bleached or unbleached, sludge, waste fines), l~min~ted
foils and various agricultural wastes (rice hulls, wheat, oat, barley and oat chaff,
coconut shells, peanut shells, walnut shells, straw, corn husks, corn stalks, jute,
hemp, bagasse, bamboo). The resin component may comprise virgin or recycled
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(waste) thermoplastics derived from the polyolefin family (polyethylenes,
polypropylenes and copolymers thereof), vinyls (chiefly copolymers of vinyl
chloride), and styrenics (including ABS and maleic anhydride copolymers thereof)and in some cases, mixtures of such polymers. Since wood, or cellulosic, fibres
S tend to decompose at l~",pel~tures above 220C, resins which must be processed
above this limiting temperature are generally excluded. Thus, a majority of the
so-called engineering resins may not be employed in the process of the inventionwhen cellulosic fibres are present, since their softening temperatures are too high
and would require processing temperatures greater than the cellulose
decomposition te-npeldlure of 220C. However, when inorganic fillers are
present, there need be no temperature limitation. The process of the invention is
of less value with thermosetting resins such as phenolics, urea-formaldehyde
resins, polyesters and epoxy resins, since these liquid resins are normally
processed in a different manner.
Preferably, the plastic material is one of the four classes comprising
the polyethylenes, polypropylenes, vinyls and polystyrenes. Typically, the
viscosities of these resins are characterized by the melt flow index, or simply the
melt index (MI). Injection grade resins typically posses melt index values
between 1 and 20 dg/min or g/10 min. After mixing with a cellulosic filler the
melt index may be substantially below 1 dg/min which would be difficult to
process by conventional injection molding means. Accordingly the use of die
lubrication to facilitate flow and maintain the peak injection pressure within
practical limits becomes necessary. Moreover, the application of extreme
pressures can induce unfavourable changes in the plastic flow properties of the
thermoplastic resin leading to embrittlement and increased resistance to plasticdeformation during injection.
In order to utilize waste paper, newsprint, cardboard materials and
the like, it is necessary to first shred the paper and then pass it through a hammer
mill to open and at least partially fiberize the shredded paper according to enduse. Wood waste may be passed through a hog mill before subjecting it to fine
grinding with a hammer mill, Wiley Mill or Szego Mill. Minor cont~min~nts
such as sizing, paper additives, inorganic fillers, adhesives, paper glazes, wax
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coatings, pigments, food residues, and inks are normally tolerated without
appreciable detriment to the extruded composite. However, the tolerance of such
impurities or conta-,linallts is largely determined by the intended application. Polar
waxes, such as m~ ted polyolefins or fatty acids, may be advantageously
employed during grinding to aid the grinding process and provide a pretreated
fibrous material which is rendered hydrophobic, den~ified and made free-flowing
for use in gravity fed machinery. The surface treatment of the cellulosic fillers
during grinding eliminates fine dust which can cause explosive mixtures with airor health hazards due to inhalation. The wax pretreatment not only renders the
wood fibres hydrophobic, but also facilitates subsequent dispersion in
thermoplastic resins whenever conventional single or twin screw compounding
machines are employed. The wax coating on the fibers becomes liquid at elevated
temperatures and accelerates the release and dispersion of fibers into the molten
thermoplastic matrix. Thus less mixing energy is required to achieve a
satisfactory dispersion.
A weighed quantity of the cellulosic filler, thus fiberized and
surface treated, is admixed with an appropflate resin and subjected to intensivemixing in a thermokinetic mixer such as a Gelimat (Werner and Pfleiderer Inc.)
or K-Mixer (Synergistics Group Inc.). The intensive mixing not only separates
the loosely bonded wood fibres from each other, but further disintegrates the
individual wood fibres into much smaller wood fragments, with some wood fibres
- being reduced to a tenth of their original size. Despite this aggressive action, the
resulting cellulosic fragments impart high strength and stiffness to the composite.
Tests have shown that these composites may be reground and remolded numerous
times (about 20) without significant loss of their mechanical properties. This
remarkable durability is attributed to the extraordinary toughness of the cellulosic
fragments which resist further breakage during reprocessing. It is usually
necessary to employ dispersing agents or coupling agents in order to effectivelymix wood or paper fibres with the thermoplastic resin, particularly with non-polar
polyolefin resins, such as polyethylene and polypropylene which do not
spontaneously wet cellulosic surfaces. In such cases, it has been found
advantageous to add certain polar polymers or waxes such as carboxylic
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polyolefins, m~le~ted polyolefins, or fatty acids. These additives can have a
profound effect on the mechanical properties of the resulting composites in somecases. They may be added during the grinding and fiberization stage as describedabove or simply added to the resin mixture prior to intensive mixing according
S to pre~el~nce. Prelleal~ent of the cellulosic fibres during the grinding stage is
particularly desirable in order to increase the bulk density of the fibrous filler and
provide a reinforcing filler which is acceptable for use in other types of
compounders such as twin screw mixers, single screw mixers and Banburys.
Various other additives may be incorporated during the compounding stage such
as antioxidants, ultraviolet stabilizers, lubricants, fire retardants, pigments,blowing agents, or crosslinking agents.
The wood fibre concentration may be varied but the mixture
becomes more difficult to injection mold at concentrations of wood fiber greaterthan about 80% by weight. In order to maximiæ the orientation of the wood
fibres, the die preferably employs a converging flow with surface lubrication.
The lubrication helps to promote extensional flow and slippage of the molten
extrudate (as opposed to conventional shear flow) so that all the wood fibres
throughout the cross section are preferentially oriented in the direction of
extrusion, i.e. the flow orientation imparts a parallel wood-like grain throughout.
This uniform parallel orientation maximizes the strength and stiffness in the
longitu-lin~l direction.
Preferential orientation of the wood fibres partially contributes to
the strength and stiffness of the composite. Added stiffness and strength is
imparted by passage through the converging die of the composite under conditionsthat extend and orient the polymer chains (or crystalline fibrils) such that themolecular orientation is, subsequently, "frozen in" by the rapid cooling in the
mold before the melted composite has an opportunity to relax. The orientation
is conducted near the softening point of the resin (whether crystalline or
amorphous) such that the relaxation times of the polymer chains are very long
ie. it requires a long time to recover equilibrium chain conformations after elastic
deformation in the converging die region, relative to the total time of cooling in
the mold. Consequently, the lowest practical extrusion temperature is, preferably,
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elected in order to prolong polymer relaxation times, thereby preserving most ofthe orientation imparted by the converging die in the finished product. To
minimi7P the applied pressure and promote extensional flow in the die zone,
surface lubrication is employed. Suitable lubricants include silicone oils (Dow
Corning Inc.), liquid paraffins, ACuflow (Allied-Signal Inc.), glycerine, castoroil, fatty amides and tit~n~tes (Kenrich Chemical Co.). The die assembly may
be fitted with a porous metal lubrication insert ring to promote even distribution
of the lubricant before it enters the die. The lubricants are metered at controlled
rates using high pressure pumps. Vapours or gases may be introduced under
pressure by such means to essentially reduce the interfacial friction to zero
(Brzoskowski et al, Rubber Chemistry and Technology 60(5), 945-956, 1987).
Water may be injected at such ports in order to provide simultaneous lubricationand cooling of the interface. In certain cases it may be desirable to admix liquid
and gaseous lubricants for minimi7ing friction in the converging zone. It will be
evident that any friction encountered by the semi solid compound as it passes
through the die assembly will limit the mold-filling rate. Although both internal
and external lubricants as such are commonly employed as additives in plastics
processing, their effectiveness is often limited by their dispersion throughout the
resin so that only a fraction of the additive is available to reduce the surfacefriction, and therefore is not quite as effective as the method herein describedwhich deposits the lubricant selectively at the die interface where it is required.
The conditions for maximum lubricity are complex although well known in the
art (i.e. tribology) particularly with respect to frictional behaviour in mechanical
devices e.g. automotive engines.
In a further aspect, the invention provides a high modulus article
comprising a composite of an oriented plastics material and an oriented particulate
material made by a process according to the invention as hereinbefore defined.
Thus, I have surprisingly discovered that suitably induced
orientation characteristics may be retained in (a) thermoplastic materials and (b)
thermoplastic composite materials whether integral structural foams or otherwisesolid or hollow articles, by the process of the present invention as conducted
under non-steady state conditions. Thus, the intermittent and discontinuous
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process steps inherent in injection molding operations have been found conduciveto the production of oriented thermoplastic articles.
I have surprisingly found that the injection pressures necessary to
pass the oriented thermoplastic out of the die into the mold at reduced melt
temperatures fall within practical values for commercial use. Further, I have
found that the desired lelllpel~ture can be achieved and suitably maintained in the
reservoir and die to enable orientation to be obtained and retained in the finalmolded and solidified article.
Yet further, notwithstanding the intermittent variable pres~ule
changes to which to the melt plastic material is subjected in the converging diepassage, as the thermoplastic material is forced through the converging passage
into the mold, orientation is produced and maintained. It is absolutely necessary
that laminar, plug flow be induced in order to effect sufficient orientation to
provide a high strength and high modulus product. Temperature uniformity is a
requirement in the reservoir in order to avoid channeling of the melt rather than
a constant velocity throughout the cross section of flow. Such tempe~dture
uniformity is induced by properly positioned StdtiC mixers equipped with a cooling
feature that partially overcomes the low thermal conductivities of such viscous
melts. Further, I have found that sufficient foam expansion can be obtained to
result in an integral structural foam. I have also found that no significant warping
or distortion of elongate article products has been observed.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be better understood, plefe -ed
embodiments will now be described by way of example only with reference to the
accompanying drawings, in which:
Fig. 1 is a schematic representation of the major components of an
apparatus for carrying out the relatively low temperature plastic and melt phasefeeding and injection molding process of the invention, and
Fig. 2 is a schematic representation of an alternative injection
molding assembly of use in the practice of the invention.
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DETAILED DESCRIPTION OF THE DRAWINGS
With reference to Fig. 1, the apparatus for carrying out the
injection molding within the process of the present invention is shown generallyas 10 and comprises four main components consisting of an injection molding
drive and feed screw assembly unit 12, a reservoir 14, a converging die 20 and
a split mold 22 arranged in co-axial lengthwise serial communication. Mold 22
is suitably shaped in the embodiment shown to form a baseball bat 24.
Injection molding unit 12 has a housing 24 containing drive motor,
gears, hydraulics and electrical controls (not shown). Unit 12 has a longitudinal
aligned heated barrel 26 which supports a feed hopper 28 by which thermoplastic
resin and, optionally combined with cellulosic fibre m~teri~ls, are introduced into
barrel 26 containing a rotating and reciprocating screw 28. The resin composite
may be, for example, a mixture of polyethylene and wood fibres which is gravity
fed into the heated barrel 26 where it is advanced by screw 28 and simultaneously
heated and plasticated. Barrel 26 connects with a tubular cooling reservoir 30
having a cylindrical body 32 having a cooling jacket 34 and an inner
circumferential annular lubrication channel 36 operably fed with a lubricant (not
shown) via an annular porous metal ring 38 connected with a lubricant port 40.
Reservoir 30 contains a static mixer 42 to promote thermal cooling and
homogenization of the melt.
Screw 28 injects the molten plastic composite mixture into reservoir
30 where it is cooled to a desired lower temperature. Static mixer 42 uniformly
cools and equilibrates the desired melt tel-lpt;l~tule as the mixed composite
material advances. Lubricant is introduced through port 40 and is chosen so as
to minimi7e interfacial friction and promote plug flow (wall slippage) of the
composite. Lubricant port 40 may optionally comprise a single aperture or a
plurality of apertures defined by portions of body 32 of reservoir 30.
A non-return valve 44 between barrel 26 and reservoir 30 allows
molten thermoplastic material to enter reservoir 30 and prevents return to barrel
26.
Reservoir 30 connects with a die 46 having a converging passage
with respect to the longitudinal axis and of which the cross-sectional area
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tlimini~hes in the forward direction of plastic flow to produce an oriented
extrudate as it enters the cavity mold 22.
Mold 22 in the embodiment shown is a split mold formed of two
mating members 50 and 52 in the closed position. Mold 22 in its closed position
has an inner surface 54 defining an article-shaped cavity 56 which receives the
injected and oriented plastic composite. In the embodiment shown, the article isa typically-shaped baseball bat 24 of a substantially elongate form having a central
longitudinal axis co-axial with the longitudinal axis of cavity 56, and also thelongitudinal axis of the converging passage of die 46.
Mold 22 is supported by stand 58 and has opposing side hydraulic
clamps 60 which actuate side mold members 50, 52 to intermittently open and
close mold 22. Mold 22 further has cooling lines 62 which receive cooling fluid,generally, cold water to effect rapid cooling of the plastic composite to well below
the solidification l~lllpeldture of the plastic to prevent spontaneous relaxation from
its oriented form.
In the case of polyethylene mixtures the exit temperature from
injection molding unit 12 may be as high as 200C before it enters reservoir 30.The melt is then cooled to a lelllp~ dture near the softening point (i.e. near 130C)
where the plastic exhibits its greatest elasticity. Under these conditions the elastic
melt is forcibly injected into split mold 22 where it is rapidly cooled to preserve
the orientation imparted to the melt after it passes through converging die 46.
The lubricant facilitates the sliding of the melt so that shear flow is minimi7ed and
maximum elongation of the melt is obtained throughout its cross section. The
lubricant also reduces the pressure needed to inject the melt into the split mold
despite the high viscosity of the melt near its softening point. Injection pressures
will depend upon the resistance to flow but commonly are less than 30,000 psi
and may be less than 10,000 psi depending upon the area reduction ratio and rateof filling. The elongational flow aids longitudinal orientation of fibrous fillers
such as wood fibres, thereby maximi7ing the mechanical strength of the molded
part. After a short residence time in the water cooled mold, the molded part is
ejected into a water bath to ensure complete cooling of the part, particularly the
interior region which may remain molten for an extended period due to the low
2l3~2~
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thermal conductivities of most polymeric resins. The injection molding machine
is adjusted to inject just the correct amount of plastic needed to fill cavity 56.
Split mold 22 is designed to facilitate smooth laminar flow of the
resin as it enters cavity 56. An automated cut-off device 64 located at the exit of
die 46 is positioned to sever molded article 24 from die 46 during ejection frommold 22. After ejection, hydraulic actuators 60 are reactivated whereby mold
22 is closed and the cycle repeated.
Under the action of reciprocating screw 28 the oriented composite
melt is forced into elongated mold cavity 56. Cavity 56 is, in the case where anintegral structural foam composite is required, maintained at atmospheric pressure
whereby moisture contained in the wood fibre filler vaporizes and causes the
molten interior of the composite material to foam and expand as it proceeds along
elongate cavity 56. In the example of a baseball bat, a blowing agent may be
employed, causing the semi-solid resin mass to increase in diameter as it expands
into the mold, thereby forming an integral foam with a solid skin. The expansionof the resin as it enters mold cavity 56 occurs spontaneously due to the pressure
exerted by the blowing agent. The highly oriented solid skin contributes most ofthe strength and stiffness to the composite, whereas the foam core reduces the
average density of the article. Thus, baseball bat 24 is formed inside the mold
cavity and has a foam core 66 surrounded by a solid skin 68 with a variable
thickness of 1-3 mm. The length of bat 24 is of normal size, being about 33
inches in length and 2 inches in diameter at its outer end.
Mold cavity 56 may be so shaped as to suitably define the outline
of other articles such as cricket bats and furniture legs, and, indeed, many other
articles generally formed of wood. Other suitable resins for use with wood fibres
include polypropylenes, vinyl resins and polystyrenes.
As depicted in Fig. 2, the injection process may alternatively use
an assembly, shown generally as 100, which comprises a reciprocating extruder
unit 102 coupled with a heavy duty hydraulic injection unit 108. The injection
assembly 100 comprises extruder unit 102 with reciprocating screw 104 which
feeds reservoir 106 of hydraulic injection unit 108. The thermoplastics m~t~
is fed into extruder 102 through hopper 122 where the resin is heated and
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plasticated before ejection from extruder into injection chamber 106. Units 102
and 108 are interconnected with barrel head passageway 110 containing a three
way shuttle valve 112. After filling the temperature controlled chamber of
injection unit 106, injection plunger 114 forces the thermoplastic resin throughconve~ging die 116 into mold cavity 118. Reservoir 116 is lubricated as in Fig.
1 in order to minimi7~ friction and promote extensional flow in the converging
passageway. Lubrication port 124is inserted in injection reservoir 116 to provide
said lubrication. The mold cavity is contained in split mold 120 which is
designed to cool the injected rein and eject the molded article therein after
solidification. Three-way shuttle valve 112 automatically directs the cooled resin
from hydraulic injection unit 108 into mold cavity 118. This design enables the
temperature gradient in the cooled injector unit 108 to be programmed separatelyfor achieving maximum orientation during injection without exceeding the peak
pressure limit. Moreover the diameter of the melt reservoir is independent of the
dimensions of the extruder and allows for greater flexibility in design and
operation.
Figs. 1 and 2 thus illustrate two injection molding assemblies and
methods of modifying conventional injection molding machines to permit low
temperature lubricated injection molding in the manufacture of highly oriented
thermoplastics materials and composites thereof. It will be apparent that there are
many variations of this process and apparatus that can achieve the desired result.
CONVERGING DIE
The temperature conditioned melt in the reservoir is forcibly
extruded through a converging die which is characterized by an area reduction
ratio (cross section area of inlet region divided by the cross section area of the
exit). The area ratio determines the degree or extent of orientation of the polymer
as it passes through the converging region. The area ratio may be any value
between 5 and 30 depending upon the degree of orientation desired for a
particular application. The exact contour of the die and the maximum incident
angle are critical to the smooth passage of the semi-solid ductile thermoplasticwithout causing melt fracture or turbulence. The angle of incidence and the area
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ratio will determine the necessary length of the die section.
Extensional flow inside the converging die can be facilitated by
appr~,iately lubricating the skin of the polymer stream so that surface frictioniS minimi7ed. This having been achieved, the velocity distribution or strain rate
in the die zone is determined by the particular die geometry and has an important
influence on the extent of deformation and the overall rate of injection.
T ~min~r extensional flow is, thus, maximized by suitable
lubrication of the die surfaces such that friction is minimi7ed and plug flow ismaximized. In general the maximum angle of incidence of the die surface to the
flow direction shall be less than 17 degrees and preferably about 10 degrees. The
streamline surface of the die passage shall be highly polished and free from edges
and corners which might disrupt smooth laminar flow. The convergence profile
of the die is designed such that the elastic limit of the polymer is not exceeded at
any stage during its passage and smooth laminar flow is maintained. Preferred
contours are those having a constant or decreasing strain Mte along the
converging passage. Such prerelled contours are related to the viscoelastic
properties of the polymer or polymer compound. Modern finite element
modelling (FEM) methods help to define such die contours for maximum
efficiency and even predict the resultant flow patterns.
In general, the rheology of the compound, the interfacial friction
in the die zone, and the operating conditions will determine the optimum die
configuration for a preferred degree of orientation. The die draw ratio is defined
as the effective area of the die inlet to the effective area of the die outlet and is
designed to impart permanent elastic deformation and preferred orientation to the
fibrous composite. In order to promote maximum orientation of the wood fibres
and at the same time orient the polymer matrix in the flow direction it is desirable
to force the extrudate through a converging die having a streamline horn shape
in the example of a simple rod profile such that the draw ratio or area reduction
ratio in passing from the large entrance region to the smaller exit region imparts
a permanent deformation to the viscoelastic melt. The contour of the die may be
mathematically correlated to the viscoelastic deformation of the melt in terms of
the elongational strain rate or elongational stress in the converging zone.
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Experience has shown that a constant or ~liminishing elongational strain rate ismost effective.
The draw ratio inside the die must not exceed the natural draw ratio
of the melt for the conditions of temperature and velocity flow gradient appliedto the extrudate. Since shape retention and strain hardening become much more
pronounced as the telllpel~ture is decreased, it is desirable to extrude the molten
composite near its softening point, where the viscosity is quite large and the
relaxation times are long for the reversible elastic deformation. Obviously, there
is a lower limit to the extrusion temperature since the extrudate may become tooviscous to extrude and the die pressure could become excessive - which can
rupture the die. The greatly increased viscosity near the softening point also
implies longer relaxation times so that the elastic deformation is retained for a
longer period of time, typically, at least several minutes. This is sufficient time
for the melt to solidify in the mold and freeze the molecules so the polymer
orientation imparted by the converging die will be permanently retained, at least
near the surface where the orientation is most effective. Molecular orientation can
sometimes be observed by the use of polarized light since the extrudate will
exhibit birefringence as long as molecular orientation persists. Long relaxationtimes and pure elastic deformation, as opposed to viscous deformation, are
favoured by the use of high molecular weight resins. For best results it is
desireable to employ resins having a pronounced melt elasticity region, a
characteristic of polymers having high molecular weights. For practical reasons
the molecular weight cannot be too large (say above 500,000 daltons) since
mixing with fillers becomes quite difficult. Furthermore the viscosities and
attendant pressures become excessive. At the other extreme, molecular weights
less than about 30,000 daltons have limited elasticity in the melt region and
possess very short relaxation times. Thus in practise it is necessary to select
polymers in at the limit of their processability with respect to molecular weight,
tell-pellllre and pressure for each composition. This will vary from one type ofpolymer and another but in general favours polymers having melt index values
below 5 and preferably below 1 dg/min. However, the choice of molecular weight
will be limited by the ability of the polymer to be mixed with wood fibres or
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other particulate filler so that a co"-plo",ise is usually necessary between ease of
mixing and retention time of orientation in the extrudate. Therefore, the highest
molecular weight consistent with ease of mixing is normally prerelled.
MELT RESERVOIR
Plasticating injection molding machines with reciprocating screws
normally require high temperatures to effect proper melting and homogenization
of the solid feed. This temperature is normally well above the softening point of
the resin. Therefore to reduce the exit le~peldlllre of the molten material it is
ejected into a cylindrical reservoir which is maintained at a lower te~-pe dture.
The volume of the intermediate reservoir determines the residence time during
which the plastic mass is cooled before entering the converging die. In order tominimi7e channelling of the melt in the reservoir, a static mixer,~ such as the
commercial Kenics or Koch types, preferably, is placed inside the reservoir to
help maintain tel~peldture uniformity. Alternatively, a water cooled spider may
be introduced inside the cylinder to accelerate cooling of the molten material in
the central region. Various other devices may be employed to remove excess heat
from the molten mass before it enters the converging zone.
LUBRICATION
To achieve stiffness and density values comparable to wood it is not
sufficient to extrude resin composite compositions under melt phase conditions as
normally practiced in industry. Under conditions normally employed in industry
there is not sufficient fiber orientation or molecular orientation retained in the
extrudate to approach typical mechanical prope.lies of common woods, ie. 10
GPa flexural modulus and 100 MPa flexural strength. By reducing the
temperature of the melt so that it becomes highly viscous and elastic, the
extrudate may be oriented in the converging die, thereby significantly increasing
the mechanical prope ~ies. Without lubrication, pressures in the die can become
excessive and cause mechanical failure of the mandrel or die. It should be
cautioned that the common interpretation of "lubricant" as applied to plastics
processing may not be approp.iate in this context. Lubrication in this context
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.
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applies strictly to the reduction of friction between the plastic melt, the die and
the mold as in the case of metal extrusions. The rate of strain throughout the die
must be maintained below a critical value to prevent polymer melt fracture.
Thus, the die convergence is gradual and preferably less than about 20 degrees
and more, preferably, less than 15 degrees (half angle), at its maximum slope.
All internal surfaces must be highly polished to minimi7e friction. Since certain
resin additives including moisture may be corrosive or abrasive under these
conditions, the choice of alloy for the die requires proper consideration. The
ability to melt phase extrude using lubricated flow at relatively low temperatures
offers several im~,~t advantages including:
(a) increased production rates since less cooling time is required, especially
for thick sections;
(b) precise control of extrudate dimensions;
(c) reduced energy consumption;
(d) greatly increased mechanical properties in the extrudate;
(e) ease of extruding highly viscous materials not normally processable;
(f) ability to extrude with large filler concentrations; and
(g) reduced heat history for heat sensitive m~tPri~
The lubricant is thus, designed to minimi7e friction during passage
into the mold cavity and maintain a low viscosity fluid interface between the metal
and the polymer. Typical lubricating fluids include silicone oil, glycerine, fatty
acids, fatty amides, glycerides, esters, and castor oil. A small pressure pump
continuously or intelllliUently introduces the lubricant into the melt reservoirwhere it spontaneously wets the interface to create an externally lubricated plug
of material. Lubrication greatly reduces the injection pressure, lowers the critical
molding temperature, contributes to a smooth molded surface, speeds injection
rates, and minimi7.~s melt fracture. Under some conditions, the lubricant may befavourably incorporated into the resin feed, provided it does not adversely affect
the operation of the injection molding machine.
FOAMING AGENTS
In many applications a solid article is preferred since a high density
2134424
.
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may not be undesirable. However in pMctise there are many other applications
where a low density is important for economy as well as weight efficiency. In
order to retain the desirable mechanical properties of a solid molded structure and
at the same time reduce the density it is convenient to produce either a hollow
profile or a structural foam such that the bending moment is maximi7ed. The
foaming action is promoted by the proper choice of commercial blowing agent of
either endothermic or exothermic type, recommended for such thermoplastics. In
these examples, the natural moisture content of the cellulosic filler is normally
sufficient to effect foam expansion without added chemical blowing agents. For
reproducible results the moisture content of the cellulosic compound must be
carefully controlled.
The choice of other foaming agents - endothermic, exothermic,
physical or chemical, will depend upon several factors, including extrusion
temperature, type of resin and cost. Residual moisture in the wood fibres is
usually sufficient to impart the desired degree of expansion so that blowing agents
need not be employed. Wood fibres are hygroscopic and will absorb moisture
from the air if not adequately protected. Under normal circumstances, wood
fibres in equilibrium with air may contain 3-7 percent moisture, the precise
amount depending upon the humidity. Some of this adsorbed moisture will be
vaporized during mixing, but a sufficient proportion can be retained to permit
foam expansion of the wood fibre composites during injection molding. The
release of adsorbed water vapour occurs immediately after the molten extrudate
enters atmospheric pressure so that the molten extrudate may expand to several
times its compressed volume, thereby lowering the temperature, via endothermic
vaporization of the adsorbed moisture. If a hollow extrudate is led into a watercooled vacuum sizer (or sizing die) the molten interior can be caused to foam
inwardly while the cooled surface remains solid and unexpanded. Such
continuous foam profile extrusion mèthods are well known in the plastics industry
but without such oriented skin as described in this solid phase extrusion process.
It is the imparted orientation that confers the extra strength and stiffness needed
to simulate the properties of wood.
The blowing agent may be any of the commercial types including
2134~24
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volatile liquids, gases, or endothermic or exothermic blowing agents. One of themore common types of chemical blowing agents in commercial use is
azodicarbonamide (Uniroyal Celogen AZNP 130) or Activex 235 (Uniroyal
Chemical Division). Both have been used successfully with wood fibre
composites. However in the case of cellulosic fillers, the natural moisture content
is usually sufficient to effect the desired expansion in those applications where
water is not deleterious (as with polyamides, polyesters and polycarbonates).
Water is prefelled since the endothermic expansion helps to cool the interior ofthick molded parts and shorten the molding cycle. However special care is
required to control the cell size of the foam structure. Since natural woods mayhave densities ranging from 0.3 to 1.1 g/cm3, the amount of blowing agent and
design of the mold can be adapted to produce structural forms within this range.Gas injection molding is a feasible alternative to foam injection where hollow
objects are desired. Nitrogen or carbon dioxide may be introduced into the central
region of the plastic melt stream through a nozzle 70 located inside the
converging die near the exit. The presure, telllp~ re and gas solubility will
determine the extent of foaming versus the formation of a central cavity inside the
molded part. Thus a hollow configuration or foam core may be obtained through
choice of gas. The choice of pressurized gas may also influence the pore size ofthe reslllting foam structure. In this manner hollow structures or structures with
foam cores may be produced with solid oriented external surfaces.
DISPERSING AGENTS AND COUPLING AGENTS
To promote complete and uniform dispersion of the wood fibres it is desirable toemploy surface active agents which prerelentially wet the surfaces of the wood
fibres and thereby increase the rate of dispersion of wood fibres in the molten
polymer matrix. These surface active agents may also provide increased adhesion
(coupling) between the surface of the wood fibre and the matrix polymer if
propelly selected. Thus it has been found useful to employ carboxylated
polyolefins as dispersing agents and/or coupling agents in polyolefin composites,
as for example in U.S. Patent No. 4,442,243, - R.T. Woodhams, issued April 10,
1984. For example, maleated polyethylenes are effective dispersing agents for
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polyethylene composites, whereas maleated polypropylenes are more effective
with polypropylene composites. The polar acidic functional groups of the
polyolefin waxes preferentially wet or react with the cellulosic fibres to increase
their compatibility with the resin matrix. The quantity of dispersing agent
required generally depends upon the total surface area of the wood fibre
component and the percentage of surface coated, and is usually 1-5 parts by
weight of the wood fibre constituent. The optimum amount is readily determined
by experiment.
RESINS
The three most important resin classes are the polyolefins
(polyethylene and polypropylene), vinyls (vinyl chloride homopolymers and
copolymers), and styrenics (homopolymers and copolymers of styrene including
ABS) with respect to cellulosic fillers because of the thermal instability of
cellulose above 220C. All of these polymers may be converted into foamed
products. The process is equally applicable to other thermoplastics having
softening temperatures less than about 220 C (ie. below the decomposition
temperature of cellulose), for example, the acrylic resins (homopolymers and
copolymers of acrylic monomers). Of course, there is no temperature restriction
when inorganic fillers are employed. The process applies equally to recycled or
waste resins, and in special cases to commingled resins. The quality of the
product will depend upon the composition and compatibility of the individual
commingled resins.
FILLER AND CELLULOSIC COMPONENT
Although inorganic fillers are commonly employed to modify the
propellies of thermoplastics, organic cellulosic fibres derived from wood, paperor agricultural residues offer an abundant source of an inexpensive reinforcing
filler. To facilitate their dispersion in molten thermoplastics it is recommended
to use certain polar waxes as wetting and dispersing agents. For maximum
reinforcement it is important that the fiber aspect ratio (ratio of length to
diameter) of the fibrous filler be greater than a certain critical value. It is also
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known that the reinforcing properties of fibrous fillers are greatest at high
concentrations, preferably above 70 percent by weight. The combined
requirement of high aspect ratio and high filler concentration leads to excessive
viscosities that cannot be processed by conventional methods. It has been found
that intensive mixing in a thermokinetic turbine mixer (commercially available
under the Gelimat tr~den~me) permits mixing such highly viscous compounds
prior to injection molding. Furthermore, the injection molding of such extremelyviscous compounds at low le-~p~ldLures cannot be accomplished without reducing
injection pressures by the methods outlined herein. The combined influence of
polymer orientation and fibre orientation is necessary to achieve satisfactory
pt;lroln~allce when compared with hardwoods.
It has been found practical to employ unbleached mechanical pulps
as reinforcing fillers since the unbleached forms impart a natural wood color and
texture to the composite. Pulps used for the manufacture of paper also possess
large fiber aspect ratios (about 100) and are therefore superior to ground wood
which has a much smaller mean aspect ratio (commonly referred to as wood flour
with fibre aspect ratios about 10). Other sources of cellulosic fillers may be
derived from agricultural residues including nut shells, bagasse, coconuts, ricehulls and other grain hulls, flax, hemp, cotton linters and husks, corn stalks,
bamboo, straw, etc. In general, the value of the filler after fiberizing is primarily
dependent upon the cellulosic fibre content and the fibre aspect ratio.
The use of high intensity thermokinetic mixing is an important
aspect of this discovery, since the intense mixing action not only disperses theindividual wood fibres, but further disintegrates them into tiny fragments. As aresult, the quality of the raw wood fibre component is not particularly criticalsince even very short fibres may be usefully employed that otherwise would be
considered of no commercial value for paper manufacture eg. waste sludge from
paper recycling and fines from pulp mills. Most forms of lumber (hardwoods and
softwoods), plywood, sawdust, tree cutting waste, agricultural waste etc.
containing a major proportion of cellulose may be usefully employed in this
process. All forms of paper, cardboard, m~g~ines, books, newspapel~,
telephone books, wrapping paper etc. may be converted into a finely shredded
21344~24
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form. Minor quantities of binding adhesives, plastic coatings, inorganic fillers,
starch sizes, etc. do not significantly affect the final prop~l~ies of the extruded
composite.
The process may also employ l~min~tecl cartons eg. Tetra Pak
beverage containers, which comprise a l~min~t~ of aluminum foil, cardboard and
plastic. Wax treated cardboard, plastic coated cardboard milk containers, glossym~g~7ine papers, and other forms of waste paper which are not particularly
suitable for reconversion into paper or cardboard products may be usefully
employed in this process. Thus the process is ideally suited for both waste
plastics and waste paper utilization due to its ability to accommodate a wide
variety of otherwise unusable waste materials. For economic reasons, the
granulated (chipped) resins recovered from plastic bottles or film (prior to
pelletizing) are prerelled since pelletizing can substantially increase the resin cost.
The injection molding processes according to the invention apply
to virtually all types of thermoplastics and compounds thereof within the limitsof processability. In the example of the manufacture of a baseball bat, a
preferred compound may be polypropylene containing wood fibres as a
reinforcing filler. Various blowing agents may be employed to produce a
structural foam as illustrated. The blowing agent (which may also be water)
forcibly expands the diameter of the extrudate so that it fills the cavity. In other
applications a solid object or hollow object may be manufactured by suitable
choice of mold design and injection molding features. Such mold designs are
well known in the art. The in-line flow of the viscoelastic compound into the
laterally sectioned mold greatly reduces the resultant back pressure so that mold
filling becomes possible at this low temperature while mold -filling is aided by the
foaming action. Complex shapes or configurations of molded objects may be
more difficult to manufacture by this process due to the extreme viscosities of the
material at these low molding temperatures. Therefore the orientation process isnot ideal for all types of injection molding applications.
MECHANICAL PROPERTIES
The mechanical properties of molded parts are highly dependent
. 213142~
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upon the degree of orientation of the resultant polymeric matrix. This degree oforientation is partly related to the area reduction ratio of the resin compound as
it passes through the converging zone prior to entry into the mold. The theory
and practise of such polymer orientation is described in previous patents relating
to extrusion (Woodhams and Tate - U.S. Patent 5,234,652, Aug. 10, 1993;
Woodhams - Oriented Thermoplastics and Particulate Matter Composite Material).
This means of orientation is now extended to injection molding by the method
outlined. Whereas normal injection molding methods do not normally permit the
attainment of sufficient strength, stiffness, density, and toughness to meet theextreme requirements of a baseball bat, the partial orientation of polymer
molecules by low temperature injection molding greatly increases the mechanical
strength and stiffness of the resulting composite bat. Under such conditions thecomposite structure now equals or exceeds the performance of common hardwood
bats manufactured from hardwoods such as ash, hickory or maple. The solid skin
helps to preserve the strength, stiffness, durability and impact toughness of the bat
in use. Note that the interior of the grip section of the bat is not foamed and
therefore retains maximum strength and impact toughness in this critical stress
region.
MOLD DESIGN
With the exception of the lateral opening feature, the mold
char~ctPrictics are common to those in current practise. Care must be taken thatall surfaces leading into the mold are streamlined, smooth and highly polished to
minimi7P. friction. A small vent port at the extreme end of the mold allows the
mold to remain at atmospheric pressure during the filling cycle.
MECHANICAL PROPERTIES OF WOOD FIBER COMPOSITES
Flexural properties were measured using the ASTM D-790
procedure . For circular extruded rods the supporting jig was modified as in
ASTM D-4476 to accommodate the curvature of the test specimens. The Izod
fracture toughness measurements followed the ASTM D-25 procedure.
The mechanical properties of extruded polyethylene wood fiber composites
2131g29
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(WFC), polypropylene WFC and polystyrene WFC are summarized in Tables 1,
2, 3, and 4, respectively. At 50 percent wood fibre content, the flexural modulus
values for these three polymeric composites were between 3 and 5 GPa.
The following examples will illustrate the method of manufacture.
In a typical experiment, 184.5 grams polypropylene (Himont
6301S) having a melt flow index of 10 g/10 min is mixed with 62.5 grams of
thermomechanical pulp (Abitibi-Price Spruce), 3.1 g of a dispersing agent
(F~tm~n Chemical Products Epolene F43 wax) and variable amounts of blowing
agent (Uniroyal Celogen AZNP 130). Mixing was carried out in a laboratory
scale one liter Gelimat mixer (Werner-Pfleiderer Inc., New Jersey) set at 3300
rpm and dump te---pel~ture 170C. The mixing times were about 30 seconds.
The molten discharge was cooled and granulated in a Brabender granulator prior
to injection molding.
The above composition was injection molded using an Engel ES
80/20 machine equipped with an accumulator. The results are s~lmm~ri7ed in
Tables 1 and 2.
Table 1. Injection Molding Conditions (Non-lubricated)
Injection pressure 4.84 MPa
Clamp pressure 14.5 MPa
Cycle times - injection 9.1 s
- cooling period 50 s
mold open time 2.0 s
Temperature profile in barrel - zone 1 177C
- zone 2 199C
- zone 3 221C
- nozzle 210C
Mold temperature 50C
Shot size 28 g
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SL439
Table 2. Mechanical Properties of Polypropylene Composites
Property Unfilled Foam 25% Wood Fibres
Tensile strength (MPa) 21.4 37.0
Flexural strength (MPa) 42.9 67.2
Notched Izod (J/m) 18.9 18.0
Reverse Notched Izod (J/m)369 128
Unnotched Izod (J/m) 486 212
Heat deflection lempe ature49.0 84.5
at 264 psi (C)
Melt flow index (g/10 min)17.3 3.1
Density (g/cm3) 0.78 0.95
Tables 1 and 2 illustrate the difficulty of injection molding wood
fibre composites using conventional injection molding machinery. Due to small
gates and sprues the Melt Flow Index, which is a measure of the melt viscosity,
must be maintained above 1 g/10 min or the mold cavity will not fill even with
the maximum injection pressure. The ASTM test specimens were 3 mm thick
which produced molded parts having a skin thickness of one millimeter on each
surface with a one millimeter foam core. Flexural modulus values were less than
3 GPa depending upon the extent of foaming, which is not an adequate substitute
for hardwoods (which generally possess flexural modulus values greater than 10
GPa). The low strength (37 MPa) and low unnotched Izod values (212 J/m)
indicate the possibility of brittle fracture at low impact energies. hence
conventional injection molding does not satisfy the requirements for a hardwood
replacement for which flexural strength values greater than 100 Pa would be
required as well as an increased stiffness at the ~lesign~tP~d density of wood. Thus
molecular orientation by the methods described above have been employed to
increase the mechanical properties of the composite.
DECORATIVE FINISHES
Decorative finishes may be applied in the mold or after forming
using standard techniques such as painting, application of decals, or text~lri7ing.
~ 213~ 12~
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For professional use, baseball bats may require certification. Furniture
components may be made fire lelaldal1t if required. Antioxidants and ultravioletradiation stabilizers are available for inlpalLing outdoor weather resistance.
The examples herein provided illustrate a new method of forming
oriented objects by injection molding using flow convergence at low le",pelatures
to forcibly align and solidify polymer molecules in their extended configurations,
thereby producing articles with enhanced physical and mechanical properties. Theprocess is applicable to thermoplastics posses~ing long stress relaxation times
under the temperatures and conditions of molding. The process is also applicableto thermoplastic composites containing organic or inorganic fillers. In one
example, a means for producing an oriented composite with a foam core is
illustrated, eg. a baseball bat. Similarly, oriented hollow objects may be formed
by gas injection molding. The novel features of the injection molding machine
include a cooled reservoir with means for uniform cooling of the melt, a
converging lubricated passage, and a laterally opening mold. The process is
conducted under conditions that maximize the extensional elongation of the elastic
melt and retain such forced elongation in the solidified product. A means is
described for forming a baseball bat using foam expansion and having mechanical
properties and densities comparable to hardwood bats.
While the invention has been described in detail and with reference
to specific embodiments thereof, it will be apparent to one skilled in the art that
various changes and modifications can be made therein without departing from thespirit and scope of the invention as described and claimed.
