Note: Descriptions are shown in the official language in which they were submitted.
~2 ~5 ~6 ,
METHOD AND APPARATUS FOR PROI)UCTION
OF FOAMED THERMOPLASTIC MATERIAL
BACKGROUND OF T~E INVENTION
Field of the Invention
This invention relates to a method and apparatus for
producing foamed thermoplastic materials by homogeneously
mixing thermoplastics with a foaming agent and other desired
additives and continuously extruding the mixture.
Description of the Prior Art
There are various methods for producing foamed
thermoplastic materials and extruders are now in wide use for
this purpose. The producing of foamed thermoplastic materials
using an extruder is practically advantageous in that, aEter a
thermoplastic is homogeneously mixed with a foaming agent or
other additive under pressure, the mixture is extruded under
low pressure to continuously manufacture foamed thermoplastic
materials of desired shapes, e.g., sheets or plates.
In the above method of manufacture, however,
importance has been attached to the fact that a foaming agent
or other additive should homogeneously be mixed with the molten
therm~plastic and the mixture extruded after the molten plastic
composition containing the foaming agent is uniformly cooled
sufficiently to be suitable for foaming.
As a result, there have been proposed various methods
and apparatus for homogeneously mixing a ther~oplastic with a
foaming agent or the like and uniformly coollng the plastic
composit~on containing the foaming agent. U.S. Patent Mo.
3,751,377 discloses a method and apparatus whe~ein a static
~25S~3~6
1 mixer is installed following an extruder having a screw in
order to mix the molten plastic with a foaming agent and
uniformly cool the mixture to a temperature fit for foaming.
Although such an arrangement has succeeded in producing thick,
lar~e, low-density foamed materials to a certain extent, it has
drawba~ks in that the extrusion rate is reduced because the
flow resistance of the plastics to the static mixer is strong,
the s~:atic mixer is partially deformed or damaged particularly
when :it is cooled from the outside and the dispersion
properties of the mixed foaming agent are insufficient.
Attempts have been made to remedy such shortcomi.ngs.
As disclosed in U.S. Patent No. 4,454,087, a rotary mixing
cooler and a ~îgzag mixer as a kind of static mixer are
simult:aneously installed following the extruder. Although the
~ig2ag mixer is prevented from deforming in.this method because
it is not cooled, there is still a desire to manufacture
further low-density foamed materials and those in which the
improvea homogeneous dispersion of an additive is ensured.
On the other hand, U.S. Patent No. 4,419,014
discloses a method for homogeneously mixing molten plastics,
rubber and the like with other additives, wherein an
extruder-mixer is equipped with a cavity transfer mixer
directly coupled to the front end of the screw contained in the
extruder.
The present inventors have conceived of the
application of the cavity transfer mixer used in the above
extruder-.mixer to the manufacture of foamed thermoplastic
~Z ~8 ~
1 materials and found that further improved foamed thermoplastic
materials can be manufactured by introducing the cavity
transfer mixer into the conventional extrusion process.
SUMMARY OF THE I~3VENTION
The present invention is directed to solving the
problems heretofore posed in the manufacture of foamed
thermoplastic materials based on the above-described knowledge
of the prior art. According to the present invention, in a
method and apparatus for the production of foamed thermoplastic
materials, the apparatus comprising an extruder for melting and
e~truding the thermoplastic material and a cooling unit for
cooling the molten thermoplastic containing a foaming agent to
a temperatu-re suitable for foaming, an improvement is provided
wherein a rotor is supported in a stator between
the extruder and the cooling unit, the gap between the
statcr and the rotor being used as a passageway for
the molten thermoplastic and wherein a number o~ isolated
cavities are respectively ormed on the inner face of the stator
and the outer face of the rotor arranged
opposite thereto, the cavities on both the stator and
the 3:0tor overlapping one another during their
rotations and forming a mixer by causing the molten plastic to
be transferred between the cavities. The molten plastic is
mixed with the foaming agent or any other additive in the
upstream side of the mixer.
An object of the present invention is to provide a
method a~d apparatus capable of homogeneously mixing a molten
~2 55~ 6~
1 ~hermoplastic wit~ a large amount of foaming agent so that
highly foamed, low-density, thic~ thermoplastic materials are
obtained.
Another object of the present invention is to pro~ide
a method and apparatus capable oE ensuring the improved
homogeneous dispersion of a foaming agent or other additive in
a mo]ten plastic, so that uniformly foamed thermoplastic
materials having excellent physical properties and quality are
obtained. Moreover, ~oamed materials having a homogeneous
disp~rsion of a nucleat ~ agent such as f~ powdery talc
cells and excellent post processability are also
obtainable.
Still another object of the present invention is to
provide a method and apparatus capable of homogeneously mixing
a plurality of incompatible thermoplastics, so that many kinds
o~ ~lerm~ plastics beoome muxable- M~veover, foa~ materials having
desired properties can be manufactured.
A further object of the present invention is to
provide a method and apparatus capable of homogeneously mixing
2Q thermoplastics with a foaming agent which is considered not
easily mixable therewith. This is advantageous to the
- manufacture of foamed materials since a foaming agent can be
selected from many kinds of materials.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a vertical sectional view of an embodiment
of the apparatus of the present invention.
Figs. 2a-h a~e schematic views illustrating the
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1 principles of mixing and agitation using a cavity transfer
mlxer.
Fig. 3 is a vertical sectional view of another
embodiment of the apparatus of the present invention.
Fig. 4 is a vertical sectional view of a modified
cooling unit of tlle apparatus of Figs. 1 and 3.
Fig. 5 is a view taken on line V-V of Fig. 4.
Fig. 6 is a vertical sectional view of still another
embodiment of the apparatus of the present invention~
DETAILED DESCRIPTION OF THE INVF,NTI3N
Thermoplastics which can be subjected to
extrusion-~oaming according to the present invention are not
parti~ularly _lmited. Representative thermoplastics include
polystyrene, styrene-acrylonitrile copolymer,
styrene-acrylonitrile-butadiene copolymer, styrene~maleic
anhydride copoly~er, styrene-ethylene copolymer,
poly-~-methylstyrene, polyethylene, polypropylene,
ethylene-propylene copolymer, ethylene-vinyl acetate copolymer,
polyvinyl chloride, polymethyl methacrylate, polyamide, etc.
These copolymers may be used independently or in combination.
Since relatively incompatible poly~ers can may be uniformly mixed
according to the present invention, a wide range;-o~ ~ermo plastics
can be selected. Accordingly, thermoplastics having desired
physical properties are readily producible.
Foaming agents which can be used in the present
invention are also not limited. Volatile or decomposable
foamlng agents are normally used.
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~25S~366
1 As volatile foaming agent tllere may be ~entioned
aliphatic hydrocarbons such as propane, butane, isobutane,
pentane, neopentane, isopentane~ and the like; alicyclic
hydrocarbons such as cyclobutane, cyclopentane, cyclohexene,
and the like; methylchloride; methylene chloride;
dichlorofluoromethane; chlorotrifluoromethane;
dichlorodifluoromethane; chlorodifluoromethane;
trichlorofluorome-thane; trichlorotrifluoroethane; and
dichlorotetrafluoroethane. As decomposable foaming agents
there may be mentioned dinitrosopentamethylenetetramine;
trinilrosotrimethylenetriamine; p,p'-oxybis(benzene sulfonyl
hydra~ide); azodicarbonamide, and the like. These foaming
agents may be ~lsed independently or in combination.
~ A fnaming agent is added on the upstream side of a
mixer according to the present invention. The normal mixing
methocl comprises heating and melt~ng thermoplastics i~ an
extrucler and supplying a foaming agent to the extruder under
press~Lre. Another method is supplying thermoplastics
containing a foaming agent to an extruder.
When a large amount of foaming agent is mixed in
order to obtain a low-density foamed material, e.g., 100 parts
by weight of thermoplastic mixed with 5-50 parts by weight of a
volatile foaming agent, it is preferred to pre-knead the molten
thermoplastic and a foaming agent in an extruder or by any
other mixing means and then supply the mixture to a mixer.
~ccording to the present invention, an additive
normally used is added when the foamed material is made.
~552'3~i6
1 As aclditives, there are nucleatin~ agents
retardants, stabilizers, lubricants, plasticizers, coloring
agents, fillers, etc.
Extruders for use in melting and extruding
thermoplastics accordi.ng to the present invention are single or
twin screw extruders, whose screws are preferably equipped
with pins or other mixing means after the foaming agent is
supplied under pressure.
Cooling units usable according to the present
invention are those developed and conventionally used for the
manufacture of foamed thermoplastic materials J such cooling
units being provided with heat exchanging means and being
capable of regulatin~ the temperature of various plastics. It
is preferred to employ a cooling unit having a rotary shaft
lS equipped with vanes inside the outer cooling cylinder as
disclosed in U.S. Patent Nos. 4,~54,087 and 2,669,~51 and
Japanese Patent Nos. 544/73 and 42026/79. It is also preferred
to use a cooling extruder which is larger in dia~eter than the
eY~I~er for melting the thermo plastics so as to uniformly cool the thermo
plastics by slowly turning it with the screw with a
smaller pumping action.
Mixers usable according to the present invention are
cavity transfer mixers for mel~ing thermo plastics having rotor
supported in stator the gap between the stator
and the rotor being used as a passageway for
the ~olten plastics. A number of isolated cavities are formed
on the inner face of the stator and the outer face of
~ ~5~ ~6
1 the rotor , respectively. The cavities formed on the
stator and the rotor are positioned so that they
overlap one another during rotation. The cavities may be
semi-spherical, cylindrical and rhombical but should preferably
be a semi-spherical shape which causes little congestion of
molten plastics. The cavities should be cross-stitched (refer
to the drawings) on the inner face of the stator and
the outer face of the rotor in the respective axial and
circumferential directions. The total area of the opening of
the ~avities should be increased, in terms of the transfer face
thereof, to over 60% of the area of the inner face of the ~-
sta-tor or the outer face of the rotor The mixer
shou:!d be positioned in a location where the temperature and
pressure are highest during the extrusion-foaming process.
Accordingly, the mixer may be coupled to the front end of the
screw of the extruder so that the mixer may rotate
synchronously with the screw or installed so that the m~xer can
rotat:e independently. In case of the latter, heating and
mixing are conveniently regulated because the number of
rotations is freely controllable according to the kind of
thermoplastics and kind of foaming agents and other additives.
In other words, the mixer may be rotated at a low speed when a
fire retardant which is subject to heat decomposition and a
fibrous material readily cut upon the application of high shear
force are used and may be rotated at a high speed when a
volatile foaming agent having a viscosity different from that
of the plastic is employed.
As dies usable according to t~e present invention,
~s~
l there are ordinarily T-shaped dies, coat
hanger dies, Eish -tal~dies, circular dies, etc. with sizing
dies as occasion demands.
The construction and efEects of the present invention
will now be described in concrete terms according to selected
em~odiments of the invention.
Referring to Fig. 1, an embodiment o~ the apparatus
of the present invention will be described. In Fig. 1, there
is shown an arrangement of a main extruder 1, a cooling
extruder 2 installed parallel to the main extruder 1 and having
its a~is shifted from that of the latter and a mixer 3
installed between the extruders 1, 2.
The main extruder 1 comprises first barrel 4; a first
cantilever screw 5 rotatably inserted in the first barrel 4; a
first kneader means 6 installed at the front end o the first
screw 5; a column 7 installed at the front end of the screw -
shaft; a number of pins 8 which protrude from the periphery of
the column 7; an inlet 9 for introducing a foaming agent under
pressllre, the inlet 9 being positioned opposite to the boundary
between the first screw 5 of the first barrel 4 and the first
kneader means 6; an outlet 11 formed at the end where the screw
of the first barrel 4`floats; an inlet 12 for materials, the
inlet 12 being formed at the end where the screw of the first
barrel 4 is supported; a material hopper 13 installed at the
inlet 12 and heaters 14 installed on Lhe periphery of the first
barrel 4.
~le coolinO e~truder 2 co~prisas a se^ond bar-ei 16
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~ 5~866
l having a spiral reErigerant passage~ay 17; an inlet 18 and an
outlet 19 of the refrigerant passageway 17; a second cantilever
screw 20 rotatably inserted into the second barrel 16; die 21
fixed to the end where the screw 20 of the second barrel 16
floats, the die 21 being equipped with an outlet forthe~ plas-tics;
a bearing 23 for rotatably supporting the base of the second
screw 20; packing 24; a packing pusher 25; a passageway 26 of
a refrigerant for cooli.ng the packing; an inlet 27 for the
injection of khermo-plastics,th~inlet 27 being formed at the end
where the screw 20 o:E the second barrel 16 is supported; and a
pipe 28 for supplying a refrigerant to the inside of the second
screw 20.
. The mixer 3 comprises a fixed cylinder 30; a
cant~lever rotor 31 rotatably inserted in the fixed
cyli:~der 30j' the axls thereof being perpendicular to those of
the ~irst and second screws 5, 20; an inlet 32 formed on one
side where the rotor 31 in the fixed cylinder 30 is
supported and which communicates with the outlet 11 of the main
extnlder l; an outlet 33 formed on the other side where the
2Q roto:- 31 in the fixed cylinder 30 floats and which
communicates with the inlet 27 for the injection of thermo-plastics
to th2 coolingextruder 2; a stator 34 installed on the inner face
of the fixed cylinder 30; a number of isolated semi-spherical
cavities 35, 36 for~ed on the inner face of the stator 34 and
on the outer face of the rotor 31, the cavities 3S on
the inner face of the stator 34 partially overlapping the
cavities 3~ on the periphery of the rotor 31 during
l rotation; a bearing 37 for ro~atably supporting the rotor
31; packing 38; a packing pusher a passageway 40 of
a reErigerant for cooling the packing; a heater 41 arranged on
the periphery of the fixed cylinder 30, and a pipe 42 for
supplying a refrigerant to the inside of the rotor 3l.
lhe operation of the apparatus shown in Fig. l will
now be described. By rotating the first and second screws 5, 20
in the directions of arrows A, B, and by rotating the rotor
31 separately from the first and second screws 5, 20 in
the direction of arrow C, a thermoplastic.material is supplied
from the material hopper l3 to the first barrel 4. The
material, i.e., plastic is transported by the first screw 5 in
the direction of arrow D and heated and caused to melt by the
heater l4. A oaming agent is added through the inlet 9 to the
molten plastic ~lnder pressure and the oaming agent added is
pre-mixed with the thermo plastic by pins ~ ofthe first kne~der m.eans
6. The thermoplastic containing the foaming agent is
subsequently passed through the outlet l1 and inlet 32 before
being supplied into the fixed cylinder 30 of the mixer 3. In
2~. the mix~r 3, the ther o plasti~ is agitated and muxed by the cavities
36 o the rotor 31 and the cavities 35 of the stator
34 ar.d the foaming agent is homogeneously dispersed in the
thermo plastic.
Referring to Figs. 2a-h, the typical mixing and
agitating principle will be described using linear material.
The striation extruded from the bottom of the cavi~y 35
on the left of Fig. 2a is extended along the inner periphery
.
1 thereof and, as shown in F'ig. 2b, the front end of the ~- ~-
striation is pulled by the edge i of the cavity 36 oE the
rotor 31 rotating in the direction of arrow C and caused to
change its direction so as to conform to the state shown in
Fig. 2c. As shown in Fig. 2d, the front end of the
striation is folded by the edge ii and, as shown in Fig. 2e, the
fron~ end thereof is cut by the edge ii and the stator 34 and,
as sho~l in Fig. 2f, the front end thereof is bent by the edge
iii and, as shown in Fig. 2g, the front end thereof is cut by
the edge iii and the stator 34 and, as shown in Fig. 2h, the
front end thereof is bent by the edge iv. The same operation
is repeated thereafter and the front end of the linear material
is successively cut and the cut portion is accumulated within
the cavities.
According to this principle, the plastic material is
extended to form a;thin plate and cut to pieces wherein a
foa~ing agent or any other addicive is ho~ogeneously dispersed
in the thermo plastics. ~hel~ plastic wi~ the foa~ng agent
homog~neously dispersed therein is supplied to the second
barrel 16 of ~he cooling extruder 2 through the outlet 33 and
the inlet 270 The thermo plastic containing ~e foc~ming age~t is
transported by the rotation of the second screw 20 in the
direction of arrow E and is cooled by refrigerant passing
through the refrigerant passage 17 to a temperature suitable
for foaming. It is conveyed and extruded out of the outlet 22
for foaming purposes.
Re.erring to Fig. 3, another e~bod~ment o' the
present invention will be described. In Fig. 3, like reference
12
*2 5 ~ 6~
numbers designate like parts as shown in Fig. 1 and 2
description of these components will be omitted. In the
apparatus of Fig. 3 a cooling unit 44 comprising a couplerl45
is coupled to the outlet of the stator 34 of the mi.xer 3; an
intermediate body 46 is coupled to the coupler 45; an inner
core 47 is provided in the center of the side of the
intermediate body 46; an external cylinder 48 is concentrically
fitted to the inner core 47 with one end coupled to the
intermediate body 46; and a die 49 is fixed to the other end of
the external cylinder 48. A passageway 50 for the therm~ plastic
material formed between the inner core 47 and the external
cylinder 48 communicates with the mixer 3 through a
communicating passageway 51 formed in the coupler 45 and the
intermediate body 46. A cooling space 52 is formed in the
~5 inner core 47 and an inlet 53 and an outlet 54 for refrigerant.
are passed from the periphery of the intermediate body 46 up to
the cooling space 52 therethrough. A spiral refrigerant
passageway 55 is formed in the external cylinder 48 and an
orificè. 56 is formed in the ~ie 49. An L-shaped pipe 58 is used
to communicate the outlet 11 of the main extruder 1 with the
inlet 32 of the mixer 3. A static mixer 59 is arranged on the
mixer end side of the communicating pipe 58 and a throttle
nozzle 60 is arranged on the extruder side of the communicating
pipe 58. Numeral 61 designates a heater arranged on the
periphery of the com~unicating pipe 58~
The operation of the apparatus shown in Fig. 3 will
now be described. The velocity of the molten plastic e~.truded
13
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l from the o~tlet ll of the extruder l is accelerated by the
throttle nozzle 60 and the foaming agent supplied from the
inlet 9 is added to the accelerated the~ plastic under pressure.
The thermo plastic containin~ ~e fo~ng agent is ~ansported to the
static mixer 59 wherein the the~mo plastic and ~e ~oaming agen-t are
preliminarlly mixed. Subsequently, -~e therm~ plastic containing the
foam:ing agent is discharged from the static mixer 59 and is
coveyed to mix~r 3 ~-here the therno plastic and the foaming agent are
thoroughly kneaded under the same action as in the case o~ the
firsl: embodiment. The plastic with the foaming agent
homoc,eneously dispersed in the mixer 3 is then supplied to the
thenm~ plastic passage 50 of the cooling unit 44. The therm~ plastic
conta:~ing the:Fcaming agent is cooled in the ~erm~ plastic passage
50 b~ the refrigerant passing through the passageway 55 while
movi~g in the direction of the arrow E and is then discharged
from the outlet 56 for foaming.
In carrying out the method of the present invention,
a cocler 63 as shown in Fig. 4 or the cooling unit 44 as shown
in Fig. 3 may be used in place of the cooling extruder 2 shown
in Fio. 1. The cooler 63 shown in Figs. 4 and 5 and installed
on the downstream side of the mixer 3 according to the first
and second embodiments of the invention is described in detail
below. The cooler 63 comprises an outer cylinder 64 having a
spiral refrigerant passageway 65; a cantilever main shaft 66
rotatably inserted in the outer cylinder 6~; a ~erm~ plastic inlet 67
formed through the neighborhood o~ the end where the main shaft
of the outer cylinder 54 is supported; a die 68 wl.h an
l4
~>~ 6~
orifice 69; and a pipe 70 for supplying a refrigerant
to a cooling space provided in the main shaft 66. The main
shaft 66 comprises a large diameter end portion 66A rotatably
supported on the outer cylinder 64 through a bearing 7.1, a
central small diameter portion 66B and a large diameter ront
end portion 66C, an annular protrusion 66D being installed in a
pOSi!iOn slightly closer to the downstream side of the central
s~al:l diameter portion 66~ facing the plastic inlet. A nymber
of hllrdle type kneading bars 72 are protruded from a portion
farther downstream than the annular protrusion 66D of the
central small diameter portion 66B.
In the cooler 63 shown in Figo 4 an.d thus
const:ructed, t~e plastic containing the foaming agent and
introduced in the outer cylinder 64 from the mixer 3 through
the F~lastic inlet 67 is forced over the annular protrusion 66D
in t~.e direction of arrow E and kneaded by the knea~.ing bars 72
of t~.e main shaft 66 rotating in the direction of arrow F and
then extrudec from the orifice 69 through the gap between the
larve diameter front end portion 66C and the outer cylinder 64.
Exa~ples 1, 2 and 3 and Comparative Example 1
. _ _ _ .
The apparatus of Fig. 1 was used in Examples 1, 2 and
3 and had the following specifications: an internal diameter of 50 mm of the
first bar.rel 4 of extruder 1; an internal diameter o~ 65 mm of the
second barrel 16 of cooling extruder 2; an internal diameter of
50 mm of the stator 34 of mixer 3; a gap of 0.4 mm be.tween the
stator 34 and the ro.or 31; six cavities respectively in
the circumferential directions of the s~ator 34 and the rotor
~ , .
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.
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l 31 and seven rows of cavities 35, 36 in the axial
directions thereof; respective diameters of 23 and 24.5 mm of
the semi-spherical cavities 35, 36; depths of 8 and 9.5 mm of
the cavities 35, 36; distances oE 22 mm between the centers of
the cavities in the axial direction; l06 rpm of the
rotor of mixer 3; a temperature of 123C o~ the molten material
at the exit of the cooling extruder 2; a width of lO0 mm and a
~ height of 1 mm of the orlfice 22 o~ the die 21.
i~ lO0 parts by weight of polystyrene [Styron 679 of
Asah:~ Kasei ] as a base resin was uniformly mixed with 0.3
part by weight of fine powdery talc as a nucleating agent and 2.0
part~ by wei~ht of hexabromocyclododecane as a fire retardant
and supplied to the main extruder l, which was operated so as
to e~:trude the material at a rate of 55 kg per hour. In
addition, l2.5 parts by weight of dichlorodifluoromethane were
added as 2 Eoaming agent from the inlet 9 for the foaming agent
to lO0 parts by weight of the base therm~ plastic under pressure. As
a result, foamed pla~es about 250 mm wide, 25 mm thiclc and 40
kg/m3 in density were obtained through a sizer attached to the
2a. d~e 21 (see Table 1)O
The results shown for Examples 2 and 3 were obtained
through similar tests carried out by changing the quantity of
the foaming agent and the rpm of the rotor 310 The
comparative example of Table 1 shows a. case where the mixer 3
is removed from the apparatus of Fi~. 1.
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Example 4 and Comparative Exam~ 2
The apparatus of Fig. 1 was used in this exaTnple and
had the following specifications: an internal diameter of 50
mM of the firs t barrel 4 of extruder 1; an internal diameter of
5 65 mm of the second barrel of cooling extruder 2; an internal
dia~eter o 90 mm of the stator 34 of mixer 3; a gap of 0 . 2 rnm
between the stator 34 and the rotor 31; ten cavities
respectively in the circumferential directions of the stator 34
and t:he rotor 31 and seven rows of cavities 35, 36 in
10 the axial directions thereof; respective diameters o~ 27 and 28
mm of the semi-spherical cavities 35, 36; depths of 8 and 9 . 5
mm of the cavities 35, 36; distances of 25 mm between the
cent~rs of the cavities in the axial direction; 100 rpm of the
rotor of mixer 3, .a temperature of 159 C of the molten
15 material passing through the cooling extruder 2, a diameter of
60 mm and a gap of 0. 6 mm of orifice 22 of the die 21 in the
form of a circular slit.
100 parts by weight of polystyrene ~Styron 691 of
Asahi. Kasei3 as a base thermo plastic was unifor~y m~ed with 2.0
20 parts by weight of fine powdery talc as a nucleat.ing agen-t ar.d
supplied to the main extruder 1, which was operated so as to
extrude the material at a rate of 28 kg per hour. In addition,
3 . 5 parts by weight of butane were added as a foaming agent
from the inlet 9 for the foaming agen~ to 100 parts by weight
~5 of the base thermo plastic under pressure. As a result, uniformly minu~ely
foam.ed sheets about 633 ~n wide, 25 mn thick and 179 Kg/m2 in the unit weight were
cooled and sized through a cooling I[~drel attached to the die at the dis-
tance and slit at one place in the extrusion direction.
:
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1 Mbreover, the foamed materials thus obta~ed were aged for
seven days in ~e rcom te~era-ture and then the cell diameter was measured
according to ASTM D 2842-69, wherein the materials were heated
at 120C for 12 seconds for examining secondary foaming
thickness as assessing theformability The results obtained
are shown in Table 2.
A similar test with the mixer 3 removed from ~he
apparatus used in Example 4, was conducted t:o obtain foamed
sheets ab~lt 2 mm thick, 633 mm wide and 176 y/m2 in the unit weight
adjusting the temperature of thermoplastic to 158C. However,
a number of concentrated ~owdery talc were seen in the sheets
with rough cells. Secondary foaming was also inferior.
Table 2
Diameter o cell (mm): Secondary foam
Machine Traverse ~ertical
direction direction direction thlc~ness (mm)
Exa~ple 4 0.22 0.21 0.21 3.28
Comparative
Exa~ple 2 0.28 0.31 0~27 3.04
Example 5
The apparatus used in Exa~ple 4 was used in this
example with the exception that the die 21 and the cooling mandrell
according to the Example 1 were use~.
In the apparatus thus arranged, polyethylene
(Yukaron HE~30 made by Mitsubishi Yuka K.K.) was
~J used as a base th ~ o plastic. lO0 parts by weight of the base thermo
plastic was mixed with lO parts by weight of a mutual impreg-
nation polymer (Piocelan (trade mark) of Sekisui Kaseihin
Kogyo K.K.) obtained by impregnating styrene monomer in a
polyethylene plastic while polymerizing the monomer, which
~ ~`rc~ 4rk
-, 19
.
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~'2 S''~
1 is co~,~sed of 30/O by ~eicJht of ethylene and 70% by~eight o~ styrene and
has 18.6~ by weight of cross-l~ing ratio, ~hecross~ ng r~t:io;c~
for examble,be obtained by mP,asuring an amount of the insoluble co~ponent
of the unit sample in boilin~ xylene, and Wl~ 0.5 part by ~-eight of talc
as a nucleating agent. A m~ure ~us obtained was supplied to the ~x~der
at a ra~e of 30 kg ~er hour.
On the other hand, l4 parts by weight of a mixture of
70% by weight of dichlorodi~luoromethane and 30% by weight of
butane were supplied às a ~oaming agent under pressure. The
extrusion was then carried out at 'l00 rpm for the mixer 3 and
coo:ed a temr~ra-ture of llOaC for the molten thermo plastics passing
through the cooling extruder for foaming purposes. The foamed
materials obtained were externally fine, uniform foamed plates
about 20 mm thlck, 230 mm wide and 33 kg/m3 in dénsity.
Comparative Example 3
This example was carried out according to the method
of Example 5 except that the mixer 3 was removed from the
apparatus. High density foamed spots due to inferlor mixing
and dispersion of PIOC~N Resin and a number of foa~ing agent gas
ac~mulations were occured. Only foamed plates having uneven
surfaces were obtained.
Referring to Fig. 6, another emdobiment of the
present invention will now be describ,ed. In Fig. 6, like
reference characters designate like and corresponding parts of
Figs. l and 4, wherein an extruder 1 and a cooler 63 are
arranged in parallel but their axes are shifted from each
other. A pipe 32a for supplying the thermoplastic material
connects the outlet 11 of the extruder l and the injection port
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67 of a cooler 63. Numeral 31 designat.es a rotor
concentrically connected to a column 7 installed at the front
end of a screw 5, and with a number of recessed isolated
semi-spherical cavities 35, 36 respectively formed on the outer
face of the rotor 31 and the inner face o:E a barrel 4
arranged opposite thereto. ~e cavities 35 of ~le ban eï 4 and ~he
cavities of 36 of the rotor 31 respectively overlap one
another therebetween to form a mixer where the plastic is
transferred.
The length of the rotor 31 should be two to
eight times (preferably four to eight times) as large as the
diameter of the screw S . If the length is less than twice as
large as the diameter, kneading will be insufficient, whereas
heating will become too great if it is more than eight times
the l~tter.
The length of the column 7 is normally one to seven
times (preferably two to five times) as large as the diameter
of the screw S. If the length is less than the diameter of
the screw, pre-kneading will be lnsufficient, whereas the
2Q kneading effect will not be further improved if it is greater
than :;even times.
The cross-sectional area where therm.o plastic is al10~7ed to
pass ~hrough the column 7 is made greater than (preferably 1.5
to 3 times as large as) that at the front end of the screw 5.
Otherwise, the quantity of the ther~ plastic supplied to the col~r~ 7
will be excessive and make sufficient kneading impossible. As
the protrusions 8 provided on the column 7, there are columnar
-. 21
l pins, cutout screw ~ hts and Dulmage screws.
The screw 5 and the main shaft 66 are resyectively
rotated in the directions of arrows A and F and the ~aterial,
i.e., therm~ plastic is supplied to the barrel 4 throuyh the ;nlet 12.The
th~ plastic is sent in ~he direction of arrow D by the screw 5
and heated and neltingby the heater l4 during that time. The
foaming agent is added from the inlet 9 t.o the n~lten thermo plastic
under pressure, whereby the foaming agen-t and the therm~ plastic are
preliminarily mixed together. Subsequently, the thermo plas-tic
containing the foaming agent is forced to enter the gap between
the rotor 3l and the barrel 4 and is kneaded by the
cavities 35, 36 so that the foaming agent may be uniformly
dispersed in ~he thermo plastic.The kneadin~ principle is the sa~e
as that described in conjunction with Fig. 2. The ~herm~ plastic
wher~in the foaming agent has been disp'ersed uniformly is
conveyed t~ the outer cylinder 64 of the cooler 63 through the
supply pipe 32a and then in the direction of arrow E over the
annu'lar prot.usions 66D. The thermo plastic is kneaded by
hurd'le-shaped kneading bars rotating in the direction of arrow
F and, after being appropriately cooled, is passed through the
uap l)etween the large diameter front end portion 66C and the
oute~- cylinder 64 and extruded from the outlet 69 for foaming.
Example 6 and Comparative Example _
The apparatus shown in Fig. 6 was used and had the
following specifications: an internal diameter of 50 mm of the
first barrel 4 of extruder 1; a diameter of S0 mm of the screw
5; a length of 250 mm of the colu;nn 7; a length or 250 mm oL
22
l the ro-tor 31; a gap of 0.4 mm between the ro-tor
31 and the ba~rel 4; six cavities respectively in the
circumferential direction of the ro-to.r 31 and the barrel.
4 and seven rows of cavities 35, 36 in the axial directions
thereof; a respective diar~ter of 23.0 and 24.5 mm of the se.~i-spherical
caviti.es 35, 36; a ~epth of 3 and 9.S mm of-~e cavities 35, 35; a distance
of 22 mm bet~,een the cavities in the axial directions; 106 rpm oE ~e scr~w 5,
a te~-erature of 123C oE the ~olten material passing through the
cool.er 63, a width of 100 mm and a height of 1 mm of the ori.fice
69 of the die 6~.
100 parts by weight of polystyrene [Styron 679 of
Asahi.~asei] as a base ~lerm~ plastic was uniformly mixed with C.3
part by weight of talc as a nucleating age~ and , 2.0 parts by
weight of hexabromocyclododecane as a fire retardant and
supplied to the main extruder 1, which was operated so as to
extru~e the material at a rate of 55 kg per hour. In addition,
12.5 parts by weight of dichlorodifluoromethane were added as a
foami.~g agent from theinlet 9 for the foaming agent to 100 parts
by we:ight of the base thermo plastic under pressure. As a result,
foamed plates about 250 mm wide, 25 mm thick and 40 kg/m3 in
densi y were obtained through a sizer attached to the die 21
(see Table 33.
In Comparative Example 4, the rotor 31 was
removed to dispense with the cavities 35 of the barrel 4.
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1~s is obvious from Table 3, uniformly foamed
materials can be obtained according to the method and apparatus
of the present inven~ion.
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