Note: Descriptions are shown in the official language in which they were submitted.
~S49~3
Field of the Invention
The present invention relates to injec-tion molded articles of
thermoplastic resin compositions containing reinforcing materials and/or
fillers, and which exhibit improved appearance, especially in surface
gloss. The invention also relates to a method of production of such
injection molded articles.
Description of -the Prior Art
Heretofore, injection molding of thermoplastic resin has, in general,
been based on a technique, in which a resin mixture is molded in a metal
mold by utilizing the plasticity of the thermoplastic resin, that is to say,
by rendering the thermoplastic resin followable by heating it in, such as,
screw, etc., and is then solidifed in the mold by cooling it to obtain the
molded article. mus, it is necessary to cool the molded resin mLxture to
a temperature below the heat distortion temperature of the resin employed,
in order to attain solidification of the resin mixture, so as to release and
remove the molded article from the mold in a satisfacto~y manner. For this
reason, the temperature of the metal mold is held usually below the heat
distor'_ion temperature of the resin employed. Alsc it has c~rrently been
practiced to cool the metal mold to a temperature closely above the dew
point by using a refrigerant, in order to increase the productivity. Even
when, in cooling the metal mold, the sensible heat of the molten resin is
used for, such as, heating and regenerating, the temperature of the metal
mold in accordance wi-th the principle of the technique, should be kept
below the hea-t distortion ten~erature of the thermoplastic resin. The
molten thermoplastic resin mixture, upon contact with the cold surface of
the metal ~ld, is cooled abruptly and loses its flowability sharply near
the surface of the mold, whereby the fitness onto the mold surface is
greatly in~aired to result in
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a considerable irregularity on the surface o~ the molded
article.-
When reinforcing materials and/or fillers are
employed, with compatibility of the reinforcing substancesand/or of fillers with the thermoplastic resin being, in
general, low, micronous interspace over the inter~ace
between the particle surface of the reinforcing materials
and the thermoplastic resin may be formed, causing so-called
silver streak to appear on injection molded articles. Thus,
only molded articles of poor appearance with silver streaks,
surface irregularity due to exposure of the reinforcing
materials and/or fillers on the outer face, can be obtained.
As explained above, in producing molded articles
from thermoplastic resin compositions, especially those
containing reinforcing materials and/or fillers by injection
molding, it is important to prevent solidification of the
molten composition by cooling while it is flowing within
the mold cavity~
As a measure for preventing such peripheral solidi-
fication of the resin mixture, it has been proposed to elevate
the temperature of the metal mold. However, an increase of
the temperature o~ metal mold will naturally re~uire a longer
cooling time, and this will result in molded article being
taken out of the mold, while still incompletely solidified
and thus exhibiting poor dimensional stability. Therefore,
in actual practice, the temperature of the metal mold is
adjusted at a temperature compromising the adverse effects
of these contradictory conditions.
Explaining one typical example of the conventional
art, there is proposed in Japanese published examined patent
application No. 22,020/1970, a technical measure, in which
the inner surfaces of the metal mold are pre-heated super-
ficially by introducing a high temperature fluid into
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the mold cavity prior to the injection of molten resin mixture
thereinto. This method however, brings forth difficulties
in that, since the fluid is heated a~ter it is in the mold,
the residual 1uid may cause various streaks and spoil over
the surface of the molded articles. And, since in some
cases, heated fluid is introduced into the mold, inhomo-
generous heating of the mold inner surface may occur at
portions where the mold cavity exhibits a projection or a
recess, such as, cause ~or a rib or a stub resulting sink
marks, irregular gloss and so on of the molded articles.
In extreme cases, the resin may even adhere to the mold
surface making it difficult to remove therefrom. This can
even result in the molded article to be broken at time of
removal and thus molded articles of satisfactory appearance
cannot be expected.
The surface gloss is a very important feature that
determines the marketability of the injection molded article.
The surface gloss corresponds to a reproductivity of the
smooth surface of the metal mold, which in identical molding
conditions depends on the degree of finish of the mold inner
surface. Thus, surface gloss is best, when a mold having
perfect mirror surface i~ used. However, this resulting
best surface gloss depends, on the other hand, upon the
composition of the resin mixture. In general, this surface
gloss decreases with increase of the content of additives
such as reinforcing materials and/or fillers. In particular,
an injection molded article of a resin composition containing
- additives in an amount adequate to impart suf~icient re-
inforcement or filling effect will exhibit under ordinary
conditions of injection molding poor gloss which is
considerably inferior than that of the molded article of
resins having no additives.
Summary of the Invention
One object of the present invention is therefore
to provide injection molded articles of thermoplastic resin
compositions containing reinforcing materials and/or fillers,
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said molded articles exllibitiny surface yloss comparable to
those of injection molded resin articles without additives
and which exhibit no surface defect such as silver streak
etc.
Another object of the present invention is to
provide an improved method of injection molding which permits
the manufacture of injection molded articles exhibiting
superior surface characteristics mentioned above from thermo-
plastic resin compositions containing reinforcing materials
and/or fillers, in a shorter molding cycle.
A further object of the invention is to provide an
injection molding apparatus suitable for realizing the above
identified improved method of injection molding.
According to the present invention, there are
provided injection molded articles of thermoplastic resin
compositions containing reinforcing materials and/or fillers
in an amount at least 4 % by weight, comprising smooth skin
layer constituted substantially of only the resin component
over the outer surface of the molded article, said layer
bestowing on the article a mirror surface having surface
yloss (ASTM D 523; ~0) welL maintained within a decrement
of reflectivity of 10 % or less, preferably below 5 %, and
more preferably 3 % or less, based on the perfect mirror
reflection of the individual resin.
The injection molded articles according to the
present i.nvention can be produced by a method of injection
molding thermoplastic resin compositions containing at least
4 % by weight of reinforcing materials and/or fillers,
comprising, selectively pre-heating by a high-fre~uency
induction heating, the inner surface of the metal mold
superficially to a temperature above the heat distortion
temperature of the resin employed, before the injection of
the molten resi.n mixture into the mold, so as to permit
sufficient flow of the molten resin mixture in contact with
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the inner surEace of the metal mold to form a smoo-th skin
layer wi-th a thickness of prefexably 1 - 100~ consisting
substanti.ally o:E only the resin component.
The characteris-tic feature of the present invention
resides in that the innex surface of the metal mold is
selectively heated only superficially to a temperature above
the heat distortion temperal:ure of the resin using a high-
frequency induc-tion hea-ti.n~, whereby a smooth mirror surface
with increased gloss is b~ought about on the molded resin
article. The heat distortion temperature herein usea
corresponds to tha-t prescrihed by ASTM D 64~ (18.6 Kg/cm2
Fiber Stress). By -the words "selectively heat the surface
superficially", it is meant, that the inner surEace of the
metal mold is heated instantaneously only to a depth of a
skin layer using hiyh-frequency induction heatinyO
,
Such instantaneous heating can only be achieved
by a special~heating method of high-frequency induction
heating. It is essential for attain.ing the objects of the
present invention, that the temperature in the skin layer of
~j
the mold inner surface is elevated at a fast rate. The
actual rate of heat elevation is determined by taking into
account of the actual heat distortion temperature of the
resin employed, size of the molded product, the mold releas-
ing temperature which is de-termined suitably in accordance
with the foregoing factors, and t,o on for each resin.
It is recommended, however, to heat to a predetermined
temperature at a heat elevation rate of 80 C per minute or
more, preferably 4~0 C/min. or higher and most preferably
at least 1~00 C/min. By employing such instantaneous
heating, only a thin layer over the inner surface of the
metal mold can be heated above the heat distortion tempera-
ture of the resin without the heat being conducted into the
interior of the mold metal and without causing the whole
metal mold to be heated, so as to accomodate to the prompt
heat removal at time of cooling. Thus, it is possible to
shorten the molding cycle with simultaneous attainment of
higher surface quality of the molded articles. Furthermore,
by employing the high-frequency induction heating, such things
that may cause to contaminate the metal mold as the heating
fluid mentioned previously is completely excluded, thus
eliminating the possible deterioration of the molded product.
Other advantages of the use of high-frequency induction heat-
ing may be recited as follows:
~5 a) facilitated temperature control,
b) enabling either homogeneous heating over the
whole surface of the mold or selective heating
including local heating of speciic area beside
the above mentioned superficial heating, thus
enabling voluntarily -to heat either of the
whole mold or of local part of the mold,
c) eliminating adverse effects of heat on the
operators
d) offers a push-button automatic operation.
The invention is further explained below with
reference to the drawings appended, so as to facilitate
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the understanding o~ the principle of the present inven-tion.
Fig. 1 shows an emb~dimen-t of the apparatus to be
employed for performing the method of
injection molding according to the present
invention in schematic illustration.
Fig, 2 is another embodiment comparable to Fig. 1.
Fig. 3 shows only the portion of metal mold
employing a high-frequency inductor pending-
ly inserted within the mold cavity, in
vertical section.
Fig. 4 shows another embodiment of inductor of
built-in type also in vertical section.
Fig. 5 is a graph showing a typical temperature
distribution within the mold.
Fig. 6 illustrates the correlation between the
gloss (20) in % and the gloss (60) in
~ for molded articles according to the
present invention.
Fig. 7 illustrates the size and shape of the
specimen used in Examples.
Fig. 8 is an explanatory illustration of weld line
on the molded product in lateral section.
Fig. 9 is a microphotograph (X7000) of a cros~
section of a shaped article employing
high impact polystyrene according to the
present process.
Fig. 10 is a microphotograph of the similar article
to Fig. 9, according to conventional proces3.
Fig. 11 is a microphotograph (X440) of a cross~section
of a ~haped article employing gla~s fiber
reinforcedætyrene-acrylonitrile re3in (herein
referred to "SAN-GF") according to the present
process.
Fig. 12 is a microphotograph of the similar article to
Fig, 11, according to conventional proces~.
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Detailed Description of the Invention
. . ~
As shown in Figs. 1 and 2, the apparatus according
to the presen-t invention consists of an injection molding
machine and a high-frequency induction heating device. The
high frequency induction heating device is composed of a
high-frequency oscillator 1 and an inductance coil (inductor)
2 installed near the inner surface of the metal mold and
connected to the oscillator 1. The injection molding machine
is composed of an injection cylinder segment 3 for realizing
the melting and injecting of the resin mixture and a mold
segment consisting of a stationary split mold half 4 and a
movable split mold half 5. In the embodiment shown in Figs.
1 and 3, the inductor is inserted in the mold cavity by being
pinched between the two mold halves of the split metal mold.
In the embodiment shown in Figs. 2 and 4, the inductor 2 is
built in internally of the mold.
In Fig. 3, the mold segment and the inductor of
Fig. 1 are shown in an enlarged view. The inductor 2 for
the high-frequency induction heating is placed between the
stationary mold half 4 and the moving mold half 5. When
it is energized by a high frequency oscillation, it can be
recognized that only the temperature in the surface layer
of the metal mold (at points A and B) is increased steeply
and the temperature in the bulk of the mold (at points C
and D) is kept almost unchanged, as shown in Fig. 5. The
temperature-time diagram shown in Fig. 5 illustrates
exemplarily the course of temperature changes at various
portions of the metal mold after high-frequency induction
heating without employing water-cooling of the mold.
The split metal mold is once opened, when the temperature
of the mold surface has reached a predetermined temperature.
The inductor 2 is withdrawn from the space between the fixed
mold half 4 and the movable mold half 5. Subsequently, the
split mold is closed again to carry out the injection mold-
ing of thermoplastic resin mixture in a conventional manner.
.,
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g
Fiy. 4 shows another performance form of the
apparatus according to the present invention in which the
inductor is arranged in the metal mold as a built-in
installation. The mold shown is a two-sectioned center-
direct gate type split mold for manufacturing dish-like
article having a diameter of ~bout 10 cm. The portions A
and A' constitute the mold cavity (that determines the
shape and appearance of the molded article) and are made
of the usual mold metal, SC metal such as S-45C, S~55C etc ,
plate metal thereof, ultra hard mold metal (alloyed tool
steel) or a mold steel such as NAK, SKD 11 or the like.
The symbols B and B' denote the inductor for high-frequency
induction heating. The inductor is prepared in such a
manner, that a copper tube is wound into spiral and is
consolidated by embedding in a hardened resin such as epoxy
etc.
C and C' indicate insulation layers for high-
frequency wave made from a non-magnetic metal, as will be
explained afterwards in detail. D and Dl represent the
matrix, in which other functional mechanisms requisite for
injection molding are embedded. This matrix is furnished
with, such as, guide pin, flange and holes for fixing the
metal mold, studs and so on. For the material of the
matrix, any mold metal can be employed. An ordinary steel,
for example, an SC steel, such as S-45C or S-55C may be
recommended for its durability~ The cooling water may be
circulated through perforations arranged either in the
matrix or, in order to upgrade efficiency, in the portions
A and A'.
It may be possible in accordance with the
configuration of the molded article, to unite or intergrate
the portions C, C' with D, D' so as to use only one and the
same material, for example, Be-Cu or so on.
In case the inductor is merely embedded in the mold
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metal r portions of mold metal close to the inductor will be
heated, and thus, causing the interior of the mold metal to
be heated~ Due to such useless heating inside the mold
metal, the oscillator tends to be subject to overload, which
may cause an actuation of the overload breaker, resulting in
a cessation of also the necessary superficial heating.
The inventors having taken no-tice of the fact that
there is a selectivity among materials in their susceptibility
to high-frequency induction heating, studied various materials
and found it best to use non-magnetic metals. In consequence
of the study, it was found that successful results were
obtainable, when the portion of the metal mold exposed to the
molten resin mixture and to be heated is constituted of a
material subject to the high-frequency induction heating,
such as a ferrous metal containing predominant amount of
iron such as steel, for example, S-~5C, S-55C, NAK or so on,
and the portion of the metal mold requiring no heating is
constituted of a non-magnetic metal.
~ ere, in general, non-magnetic metals other than
Be-Cu alloy are soft and are not suited well for the matrix
metal with regard to their durability.
The inventors had therefore made their efforts to
direct to overcome such circumstances and have found, that
the portion of the metal mold where no heating is required
can be isolated from the high-frequency wave, when a thin
layer of a non-magnetic metal is interposed between the
inductor and said metal mold portion.
A thickness of this non-magnetic metal layer of
0.5 mm or more will offer a sufficient insulation for the
purpose of the present invention, whereas a thickness below
0.5 mm may bring forth faulty result. For instance, an
aluminum foil of 0.1 mm thickness will fuse down upon the
high-frequency induction heating and will not build up
an insulating layer.
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Thus t an injection molding process and apparatlls
thereof comprising a metal mold equipped with embedded
inductor for heating selectivity the superficial layer of
the mold inner surEace contacting the injected resin
mixture according to the present invention, in which an
insulating layer against the high-frequency wave is inter
posed between the inductor an~ the portion of the metal mold
requiring no hea~:ng, and which enables steep heating and
cooling utilizing high frequency induction heating, is most
effective.
The non-magnetic metal to be used according to
the present invention includes Cu, Al, Be and alloys
consisting predominantly o* these metals inclusive of bronze,
beryllium copper and so on. Ceramics, glass, wood and the
like are also non-magnetic substances, but these are not
suitable for use as material for molds for their inferior
heat conductance, durability and so on.
The injection molded articles according to the
present invention acquires from the superficially heated
inner surface of the metal mold a corresponding reproductive
surface with excellent surface gloss. The gloss is well
kept within a range in which the decrement of reflectivity
from the ideal perfect mirror reflection is at the most 10 ~,
based on the perfect mirror reflection of the employed resin
itself.
Here, by the perfect mirror reflection of the resin
is meant an intrinsic measure of gloss determined in relation
to the index of refraction of each individual resin employed,
which corresponds to a percentage indication of the reflec-
tivity measured at a prescribed standard angle of incidence
t60 in the specification of the present invention) relative
to the reflectivity of smooth surface of a glass having an
index of refraction of 1.567, measured at the same standard
angle of incidence, as is instructed ky JIS Z 8741.
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The perfect mirror reflection for each index of refraction
of resin can be seen from the ~ollowing table:
Index of Refraction Gloss at Angle oE Incidence
of Resin of 60 for Perfect Mirror
Reflection of Resin in %
(Gs 60)%)
1.500 89.1
1.520 92.4
1.540 95.7
1.560 98.9
1.580 102.1
1.600 105.1
A perfect mirror reflection of an~ voluntary resin
can be estimated from the above table by interpolating from
the yiven values. For instance, in a so-called high impact
polystyrene (HIPS) reinforced by 5 - 20 % by weight of a
rubber (e.g. poly~utadiene), -the "individual resin employed"
here is polystyrene. Index of refraction n of a polystyrene
lies in general at about 1.592 and the perfect mirror
reflection is calculated to be 104.6. ~ high impact poly-
styrene molded article obtained according to the present
invention exhibits therefore a surface gloss value (Gs 60)%)
of from about 94.6 % (decrement of 10 %) to about 104.6 %.
In another example of resin of an ABS copolymer
with monomer ratio acrylonitrile/styrene of 30/70 and
containing about 5 to 35 % by weight of butadiene, the
individual resin employed is the acrylonitrile/styrene
copolymer with monomer ratio 30/70. Here, the index of
refraction n equals to 1.577 and the perfect mirror reflec-
tion calculates to 101.6 %. For the case where the acrylo-
nitrile/styrene ratio corresponds to 25/75, the refractive
index n equals to 1.579, corresponding to a perfect mirror
reflection value of 101.9 %. Thus, if ABS resins exhibiting
A/S ratios of 30/70 and 25/75 are employed, the surface
gloss values (Gs (60)~) of the injection molded articles
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obtained fall within the range of from 91.6 to 101.6 ~ and
from 91.9 to 101.9 % respectively. Likewise, injection
molded articles of polyphenyleneether (PPE) resins according
to the present invention show surface gloss values (Gs 60)
in a range from about 80 to 108 %, especially from 90 to 108
%, depending also on the individual PPE used.
In this specification, the evaluation of the sur-
face gloss of molded article is based on the value of
Gs(60)% of ASTM D 523, in accordance with th~ current
practice for evaluating molded articles of plastic materials
for their appearance and gla~e, wherein a % indication of
gloss at insident angle of 60 is employed for the evaluation.
Following strictly the instruction of ASTM D 523, it is
prescribed to employ a Gs(20)% value measured at an incident
angle of 20 for estimating the gloss, when the Gs(60)% value
exceeds 70 %. In this regard, measurements were performed for
estimating Gs(60)% and Gs(20)% values on molded articles
according to the present invention, in order to clear the
correlation between them. The results are shown in Fig. 6.
If the instruction of ASTM D 523 is followed exactly, the
gloss of a molded article showing Gs(60)% value greater than
70 % must be expressed by the corresponding Gs~20)% value
obtainable from Fig~ 6. In this specification however, the
evaluation is performed exclusively by Gs(60)% value,
following the conventional practice in the art since the
di~ference in relation to the Gs(20)% is apparent.
/
Injection molded articles according to the present
invention exhibit the surface gloss intrinsic of the employed
thermoplastic resin itself and show no noticeably faulty
appearance, such as, so-called flow mark, jetting, weld line
and silver streak, caused from the irregular flow of the
thermoplastic resin mixture containing reinforcing material
and/or filler. In the injection molded articles according
to the present invention, it is preferable even for articles
of complicated configurations, such as lattice and the like,
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not to men-tion articles of simpler shapes, to have the said
thermoplastic resin skin~layer of thickness 1 - 100 ~. The
excellent surface gloss may be at~ributed to the fact that
the molten resin mixture injected into the mold cavity is
permitted to keep its flow even on the surface of the metal
mold, due to the preliminarily conducted superficial heating
of the metal mold, so that a smooth skin layer constituted
substantially of only the resin component without reinforcing
material or filler may be formed over the inner surface of
the metal mold under the filling up of dents and pits due to
the reinforcing material or filler.
In a usual injection molding, weld line appears at
the junction of flow of the molten resin mixture inside the
mold as a conflux line in a form of thin groove having a
depth of 3 ~ 5 ~ or more and a width of over 10 ~, as shown
in Fig. 8, in a lateral section~ In injection molded
articles according to the present invention, substantially
no noticeable weld line indention having depth less than
1 ~ and width less than 5 ~l occurs.
Flow mark can be formed by, for example, a dis
turbance of flow of the molten resin mixture and an irregu-
larity in pressure transmission at a portion of varying wall
thickness of the molded article, due to cooling and solidifi-
cation of the molten resin mixture on the metal mold.
Injection molded articles according to the present invention
shows no such flow mark.
Silver streak appears on the surface of molded
articles upon the solidification of the resin mixture in a
form of streak of silver color while volatile materials etc.
in the resin mixture are being volatilized. In injection
molded articles according to the present invention, no
silver streak can be found.
Jetting occurs often at the ~ate of the mold as
~S~3
a trace of partially projec-ting line in the mold due to the
acceleration of the flow of resin mixture through a narrow
throat. This is also excluded in molded articles according
to the invention.
A11 the deteriorations in the appearance mentioned
above are based on the irregularity in the flow of molten
resin mixture inside the metal mold, and hence, are avoided
in the molded articles according to the presen-t invention by
the improvement of the flow of resin mixture on the mold
surface.
According to the present invention, there occurs no
solidification of the resin mixture by cooling upon entrance
thereof into the metal mold and a uniform flow of the molten
resin mixture over the whole inner surface of the metal mold
can be warranted, since the inner surface of the metal mold
has been heated preliminarily above the heat distortion
temperature of the resin. This results in a uniform surface
gloss with substantially the same gloss regardless of the
portion of the mold, say the gate end or the dead end thereof.
Expressing the difference in the gloss of the molded
article per unit length along the line from the gate end to
the dead end of the mold as "gloss gradient", this gradient
is very low and amounts to 0 - 0.5, preferably to 0 - 0.2 and
most preferably to 0 - 0.1 %/cm for injection molded articles
according to the present invention. In contrast thereto,
the gloss gradient lay in most cases in the range from 1 to
5 %/cm, or the conventional injection molded articles hav-
ing a ratio of resin flow length L to the thickness t of the
molded article L/t = 20 - 30 or more. This shows the
remarkable excellency of the molded articles according to
the present invention in their gloss and gloss irregularity.
While, by the method according to the present
invention, it is possible to obtain injection molded articles
115~9;~;~
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exhibiting superior surface gloss from thermoplastic resin
mixtures containing 4 % by weight or more of reinforcing
materials and/or fillers, the method according to the
present invention brings about injection molded artlcles
showing no such surface defects, as silver streak, jetting,
weld line etc., the same is true when resin mixture contain-
ing less than 4 % by weight of relnforcing materials and/or
fillers or individual resin is used.
As for the thermoplastic resin capable of being
employed in the present invention, those based on styrene
and on polyphenyleneether are to be recited as typical
exmaples therefor.
By the resins based on styrene are meant all the
resins containing styrene as predominant monomer component
together with other subsidiary comonomer components and/or
reinforcing component. Concretely, the followings may be
enumerated:
Polystyrene, acrylonitrile/styrene resin (AS resin),
rubber reinforced styrene base resins such as HIPS and MIPS,
butyl acrylate rubber~acrylonitrile/styrene copolymer (AAS),
ethylene-propylene rubber/acrylonitrile/styrene copolymer
(AES), ABS resins including acrylonitrile/butadiene/styrene
copolymer, acrylonitrile/butadiene/styrene/~-methyl styrene
copolymer and acrylonitrile/methyl methacrylate/butadiene/
styrene copolymer, and so on.
Among these resins based on styrene, those which
show marked effect according to the present invention are
the afore-mentioned rubber reinforced styrene base resins.
These resins tend to offer rough surface upon injection
molding, due to the large rubber particles contained, which
are subjected to deforrnation upon movement inside the metal
mold. Therefore, according to the conventional practice of
injection molding, only gloss values around 60 ~ are reached
~L5~9~3
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by rubber reinforced styrene base resins con-taining 4 % by
weight or more of the rubber componen-t. In contrast thereto,
it is possible according to the method of the present inven-
tion, to attain gloss values of about 90 - 100 % by the same
resins. sy employing ABS resins, it is possible, according
to the present invention/ to obtain higher gloss values
which are usually above 94 %.
The resins based on polyphenyleneether to be used
according to the present invention are those containing as
the principal component (more than ~0 ~) a polyphenyleneether
expressed by the general formula
' ~1 -
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R2 n
wherein Rl and R2 denote each an alkyl group having 1 - 4
carbon atoms and n represents the polymer-sation degree,
a polyphenyleneether graft-copolymerised with styrenic
compound or a resin mixture consisting of 20 - 80 % by weight
o~ one of these polyphenyleneethers and 80 - 20 % by weight
of a polymer based on styrene.
The -thermoplastic ~esin composition to be used
according to the present invention can contain other addi-
tives which are commonly employed in the art, such as, flame
resisting agent, lubricating agent and so on.
Examples of the polyphenyleneethers expressed by
the above general formula are: poly(2,6-dimethylphenylene-
1,4-ether), poly(2,6-diethylphenylene-1,4-ether), poly(2-
methyl-6-ethylphenylene-1,4-ether), poly(2-methyl-6-
propylphenylene-1,4-ether), poly(2-ethyl-6-propylphenylene-
1,4-ether), poly(2-methyl-6-butylphenylene-1,4-ether),
poly(2-ethyl-6-butylphenylene-1,4-ether) and so on.
,
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The styrenic compound in the polyphenyleneether
having graft-copolymerised a styrenic compound as mentioned
above includes styrene and its derivatives such as alkylated
styrene, halogenated styrene and so on. Example therefor
are: styrene, ~-methyl-styrene, 2,4-dimethyl-styrene,
monochlorostyrene, dichlorostyrene, p-methyl-styrene, ethyl-
styrene and so on.
It is possible to use concurrently upon the
polymerisation another copolymerisable vinyl compound such
as, methyl methacrylate, acrylonitrile, methacrylonitrile,
butyl acrylate and so on. It is also possible to graft two
or more styrenic compounds concurrently.
The constituent components of the polymer based on
styrene as previously stated may be the same with the above
compounds to be used concurrently upon the graft copolymeri-
sation.
The polymer based on styrene includes, according to
the present invention, also the so-called rubber reinforced
resins, such as rubber reinforc~d polystyrene, acrylonitrile/
butadiene/styrene copolymer resin and polystyrene resin
containing EPDM rubber.
When polyphenyleneether resins as identified above
are used, the injection molded articles obtained according to
the present invention will exhibit, in general, surface gloss
value of more than 80 %.
It is of course possible to use other injec-tion
molding resins such as polyethylene, polypropylene, poly-
carbonate, polyoxymethylene, nylon and so on.
The filler to be incorporated in the thermoplastic
resin composition according to the present invention includes
those of inorganic nature, for example glass fiber, glass
,~
.
., .
, . .
~L~S~9Z3
--19--
beads, calcium carbonate, mica, asbestos, and so on and
powder and hollow material oE metals such as iron, copper,
zinc and aluminum, as well as oxides and hydroxides of these
metals, each having a predominant particle size of 5-mesh or
below.
The total amount of filler in the resin composition
according to the present invention falls in general within a
range of from 5 to 70 % by weight, based on the total compo-
sition.
The high-frequency oscillator which can be employed
according to the present invention may be of electromotive
generator, electron tube or thyristor inverter types. A
frequency in the range from 50 Hz to 10 MHz can be used,
while a frequency of from 1 to 1000 KHz may be recommended
in practice. The power output of the high-frequency oscil-
lator may be in the range from 1 to 5000 KW, to be determined
suitably in accordance with the size of metal mold to be
heated, the temperature contemplated and the rate of tempera-
ture elevation intended.
Here, the heating power P through a high-frequency
induction heating is calculated by the equation.
8~ a f ~s n I _4
in which P is the heating power, a is the radius of the
induction coil, f is the frequency, ~s denotes the specific
magnetic permeability, n indicates the number of coil wind-
ings per meter, I represents the electric current in the
coil and ~ is the specific resistance of mold metal.
For instance, a pertinent power output of an
oscillator, in which the inductor is made from copper tube
of 5 mm diameter by winding it at an interval of 5 mm into
a swirl and the distance between the inner surface of
;
9;~3
-20-
the metal mold and the incluctor set a-t 1 cm, lies within
a range Erom 0.1 to 10 KW per 1 cm of the surface area of
molded article, on condition that the frequency used is
400 KHz and the temperature of the metal mold of S 45 C
is elevated by 40 - 50 C from the starting temperature
of about 40 - 90 ~C within a heating time of 10 - 15 seconds,
as in the ordinary injection mo]ding. At power outputs less
than 0.1 KW/cm2, the rate of temperature elevation of the
metal mold is too low to be prac-tical and eventually may
suffer from overload, causing the overload breaker to actuate
and the heating to cease. When the power output exceeds over
10 RW/cm2, the rate oE temperature elevation becomes too steep
to control the mold temperature and, in case of large metal
mold with greater heating surface, a uniform heating becomes
no longer possible. If there is a temperature inequality of
more than 50 C over the inner surface of the metal mold,
gloss irregularities, sink marks and so on over the surface
of the injection molded article will tend to occur.
Hereinbelow, the invention is further explained in
detail by reciting Examples, which however should of~er no
restriction on the scope o~ the present invention.
Example 1
. . _
An AS resin composition containing 20 % by weight
of glass fiber having a diameter of 13 ~ was injection molded
by an injection molding machine of ordinary in-line type
shown in Fig. 1. The split mold made of an ordinary steel
of S 45C was used and provided for molding a dish-like article
having a diameter of 10 cm, a depth of 2 cm and average wall
thickness of 3.5 mm. The mold is equipped with a center-
direct gate.
Inductor was prepared by, having a copper tube with
diameter of 3 mm was wound at an interval of 5 mm into a whirl
and shaped to fit the proEile of the mold cavity and then was
consolidated by embedding it in an epoxy resin into a flat
- plate.
~3L5~23
-21-
The temperature of ~he injection cylinder was
adjusted so as to obtain a resin mi~ture temperature of 240
C. Before injecting the resin mixture into the mold, the
inductor prepared as above was put between the two mold
halves. After actuating the oscilla-tion at 400 KHz, 6 KW,
for 15 seconds, the split mold was once opened to draw out
the-inductorbefore it was closed again. During this procedure,
the cooling water was made not to circulate in the mold metal.
Then, the molten AS resin composition containing glass fiber
was injected into the mold at an injection pressure of 60
Kg/cm2 for 10 seconds, as in ordinary injec-tion molding.
Thereafter, cooling water was circula-ted in the mold metal
for 20 seconds to cool the molded article. Then, the molded
article was taken out of the mold. The molding cycle amounted
to 60 seconds in total.
The appearance of the molded article was excellent
and was comparable to that of molded article of AS resin only
and showed no fault such as silver streak and exposure of
glass fiber on the outer face. Gs(60~% of the molded article
was excellently high as much as 102 %.
Example 2
An ABS resin composition containing 20 % by weight
of glass fiber of a diameter of 15 ~ was molded by an ordinary
type injection machine at a resin temperature of 240 C. The
split mold employed was made of S55C steel and was so
constructed that a dumnbell specimen having a shape prescribed
in JIS K 6871 and a rectangular plate specimen can be molded.
Inductor was prepared by winding a copper pipe with diameter
of 3 mm into whirl at an interval of 5 mm and embedding it in
an epoxy resin plate of thickness of 2 cm. The procedure of
injection moldiny was the same as in Example 1, but with ~00
KHz, 6 KW, 10 seconds of high-frequency oscillation, 10
seconds of injection molding duration, 15 seconds of water
cooling, 50 seconds of total cycle time and 50 Kg/cm2 of
injection pressure.
~22-
The surface of the so molded article was covered
by a skln layer of the Ass resin employed. A molded article
exhibiting beautlful appearance and superior gloss was
obtained.
The molded article was evaluated for its properties
according to the instruction of JIS K 6871. The results were
as given in Table 1.
As is seen from Table 1, the molded article obtained
revealed an excellent appearance~ superior gloss and other
favorable properties.
Example 3
A PS resin composition containing 50 % by weight of
200-mesh iron powder was molded by an ordinary in-line type
injection machine at a resin temperature of 220 C. The split
mold having an edge gate was so constructed, that a pair of
flat rectangular dishes of 5 cm x 8 cm x 0.5 cmr which can be
coupled together to form a case having hinge, are molded
simultaneously.
Inductor was prepared by winding a copper tube having
a diameter of 5 mm into whirl at an interval of 5 mm and
embedding it in an epoxy resin plate of 2 cm thickness.
This inductor was placed be-tween the two mold halves.
The inductor was actuated by 400 XHz, 6 KW for 15 seconds.
After the inductor was withdrawn from the mold, the injection
molding was carried out as in Example 1.
The surface of the thus molded article was the same
as that of ordinary molded product of PS resin without addi-
tives, showing no exposure of iron powder on the outer face.
The specific gravity of the molded article was found to be
1.3, exhibiting a solid touch heretofore unseen in conventional
PS molded product. GS (60)% of the molded ar-ticle was 99 %.
~,
, ,, ~
-23-
Comparison Example 1
Using the same injection molding machine, same mold
and same resin composition as in Example 2, injection molding
was carried out under the condition of resin temperature of
240 C, mold temperature of 60 C, in~ection duration of 10
seconds, cooling time of 15 seconds, total injection cycle
of 40 seconds and injection pressure oE 50 Kg/cm2 without
preheating by high-frequency induction. The properties of
the so obtained molded article were as given in Table 1.
Table 1
Comparison of Injection Molding with
and without High-Frequency Induction Preheating
~ _ Method of _ Compa-
Property Examina-Unit 2 Example
. . . ..... ~ . .. _
Tensile strength JIS K 6871Kg/cm 1000 1000
Elongation JIS K 6871 ~ 2 2
Flexural modulus ASTM D 790Kg/cm255000 55000
Flexural strength ASTM D 790Kg/cm 1300 1300
Izod impact JIS K 6871 Kg-cm/cm 30 25
strength
(3.2 mm thick,
without notch)
Heat distortion JIS K 6871 C 104 100
Gloss ASTM D 523Gs (60)~o 98 45
Example 4
It was intended here to make comparison of surface
gloss values of molded articles at various mold temperatures
for various resins employed.
A sprit mold made of ultra hard mold metal (NAK
metal) with mirror finished mold inner surface for molding
.
:
~5~3
-2~-
a flat square piece with a perforation of 1.5 cm ~ and having
a thickness ~f 3 mm, as shown in Fig. 7, was used, the gate
of which was positioned at E in Fig. 7 and was a res-trict gate
of 4W x 8L x 2 mm. Inductor was prepared by winding a
copper tube having a diameter o:E 5 mm into a flat whirl at
an interval of 10 mm and embedd:ing it in an epoxy resin plate
of a thickness of 3 cm. The high-frequency oscillator with
continuous variable output power was used at 7 KHz, 10 KW.
For the injection molding machine, Toshiba IS 80 (5 oz
injection moldiny machine) was employed. The injection
molding was carried out by the temporarily adopted inductor
in accordance with the present invention under ordinary
condition using various resin composition. The results were
as given in Table 2.
It is seen in Table 2, that injection molded
articles having high gloss an~ showing almost no gloss gra-
dient, namely "gloss unequality", can be obtained according
to the present invention. In other words, it is seen, that
there is considerable difference in the surface gloss between
the portions E and F for conventional product, whereas there
is no such difference for the product according to the
present invention in addition to the high gloss thereof.
.
23
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In the molded articles according to the present
invention, the faulty surface defects of flow marks, jettings
and silver streaks are complete]y avoided and also the weld
line is no-t noticeable for all resins examined.
In the molded products by the conventional practice,
a high gloss product is never obtained even by selecting the
resin employed, as can be seen ~rom Table 2. Flow marks and
jettings were found, silver streaks occurred in molded product
containing filler and weld line was noticeable. Improvements
in the appearance and gloss obtained by alteration of the
molding condition, especially of the mold temperature is
limited and does not reach the level according to the present
invention.
Comparison Example 2
It is contemplated to exemplify the preheating of
the metal mold by introducing a heated fluid into the mold.
The metal mold of Example 4 was used. As the heat-
ing fluid, steam of 10 Kg/cm2 was employed. The metal mold
was provided with inlet and outlet slits where molten resin
cannot flow into but steam at parting planes in such a manner
that an inner surface of the mold could be heated. In the
inlet, a check valve was arranged and a trap was installed
at the outlet. In order to allow to maintain the steam
pressure within the metal mold, O rings were employed. It
was so difficult to seal up the mold against the steam of
10 Kg/cm2, that practical industrial application of such
technique would be impossible, since the metal mold has a
kick out pin and O-ring is not much effective for sealing.
The preheating of the metal mold was performed with the
steam leaking. The steam pressure within the mold was kept
at 3 - 4 Kg/cm for 30 seconds, before it was discharged.
The injection molding was carried out subsequently. The
mold temperature at this moment was 120 C. The so obtained
molded product was not always fine. Occasionally, speckles
. ' ~ .
.
11~4~3
-28-
on the molded product were recognized, though the reason
thereEor was not identified whe-ther it had been caused from
the water rest or the corrosion inhibitor contained in steam.
There was also rest around an ejector pin on the molded
product, said rest being likely due to mold sweat.
Moreover, a labor safety problem, maintenance of
metal mold for, such as, corrosion prevention etc., problem
of durability of O-ring and so on add thereto, wherefrom it
may be concluded that this procedure is far less applicable
in industry as compared with the present inventio~.
Example 5
The procedures of Example 1 were followed with the
exception that instead of the 20 % GF-containing SAN resin,
a PP resin containing 30 ~ by weight of glass fiber was
employed. Results comparable to the SAN resin were obtained.
Gs(60)% of the molded article was 88 %.
Example 6
Using a split metal mold, for molding a pair of
casing halves for audio cassettes (called cassette halves),
an HIPS resin composition was injection molded.
The inductor was prepared by winding a copper pipe
with diameter of 5 mm into flat whirl at an interval of 5 mm
and embedding it in an epoxy resin plate of a thickness of
3 cm. This inductor was placed between the mold halves and
energized by a high frequency output of 7 KHz, 20 KW for 15
seconds. After it was withdrawn from the mold, the injection
molding was carried out as in Example 1.
The molded article exhibited a complicated configu-
ration having ribs, bosses, perforations and embossed pattern
which might have exhibited flow marks, weld lines and the like
if molded by the conventional injection molding technique.
The molded article showed however, a superior appearance with
. ~, .
.
,
,
~L~S4~3
~29-
no flow mark and no visible weld line but improved feeling of
the embossed pattern. The dimensional accuracy was the same
as that of injection molded article of prior axt. No distor-
tion was recognized.
By examining the gloss at a flat portion of the
molded article, the Gs(60)~ values for the article according
to the invention and for the article of conventional method
were found to be 98 % and 45 % respectively.
Example 7
The split metal mold employed was the one shown in
Fig. 4, in which the parts A and A' were made of a NAK metal
and the surfaces thereof contacting the injected resin compo-
sition were mirror finished. This mold has a center directgate and produces a molded article of dish-like configuration
having a diameter of 10 cm, a depth of 2 cm and an average
thickness of 3.5 mm~ The inductors at -the portions B and B'
were prepared each by winding a copper pipe of a diameter of
5 mm into flat whirl of an interval of 15 mm and embedding
it in an epoxy resin plate of a thickness of 15 mm. The mold
parts C and C' are made of bronze plate having a thickness of
3 mm. The mold parts D and D' are made of mold steel S-45C.
Using this split mold, a commercially available SAN
resin composition containing 20 % by weight of glass fiber was
injection molded. The injection cylinder temperature was
adjusted so that the temperature of the resin composition was
held at 240 C. The inductors were energized by a high-
frequency oscillator of 4 K~Iz, 8 KW, for 10 seconds.
Subsequently, the injection molding was carried out at an
injection pressure of 60 Kg/cm2 in an injection period of
10 seconds as in a conventional manner. Then the mold was
cooled ~or 20 seconds before the molded article was withdrawn.
The total molding cycle was 45 seconds.
The so obtained molded article showed the same
~L~549~3
-30-
appearance as that of the molded article made of exclusively
SAN resin. No silver streak nor the projection of glass
fiber outside the surface of the molded article was able to
recognize. The gloss Gs(60)% was 102 %.
Comparison Example 3
_ . .
For the sake oE comparison, an injection molding
was conducted in the same condition but without employing
said hiyh-frequency induction heating prior to -the injection/
which resulted in a product with discolored appearance showing
many silver streaks and having a Gs(60~ value of 45 %.
Example 8
The apparatus and the procedures were the same as
in Example 7, except that the resin employed was changed to
HIPS, ABS and PPE and the molding temperature was varied so
as to match each resin. The molded article of each resin had
excellent appearance overturning the generally accepted
evaluation of the injection molded articles. Thus~ all the
molded articles obtained exhibited gloss values Gs(60)%
exceeding above 100 % regardless of the position on the
article and showed no gloss irregularities, faulty jettings or
so on.
In Table 3, the conditions of injection moldiny
appearance and gloss of the molded article for each indi-
vidual resin composition are recited together with those of
comparison Example 4 in which the procedures of Example 7
were followed except that the preliminary heating of metal
mold by high-frequency induction was discarded.
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