Filling apparatus for filling foamed particles of a thermoplastic resin into a mold

A filling apparatus for filling foamed particles of a thermoplastic resin into a mold. The apparatus includes a weigh hopper equipped with a weighing means, a foamed particle feeder adapted to control the amount of the foamed particles of the thermoplastic resin to be fed to the weigh hopper according to the weight of the foamed particles contained in the weigh hopper, a particle feed hopper adapted to temporarily keep a fixed weight of the foamed particles, which have been weighed in the weigh hopper, a holding hopper which acts as a pressurized tank, and a filling machine for filling the foamed particles into a mold. The filling apparatus allows for the successive weighing and pressurized containment of the foamed particles prior to being fed into the mold cavities.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
The present invention relates to a production method of an expansion-molded 
article and a filling apparatus of foamed particles of a thermoplastic 
resin for use in such a method. 
2) Description of the Related Art 
Foams of thermoplastic resins have characteristics and properties such that 
they are light-weight and excellent in thermal insulating property and 
cushioning property. Therefore, the foams are widely used in various 
shapes or forms in thermal insulating materials, cushioning materials, 
packaging materials, etc. As a method of producing such foams, there has 
been widely known the foamed-in-place molding in which a thermoplastic 
resin is first of all impregnated with a blowing agent, the thermoplastic 
resin is expanded to produce foamed particles, and the foamed particles 
are then filled into a mold and heated with steam or the like, thereby 
expansion-molding them. 
At the early time the foamed-in-place molding started to be used in 
industry, the foamed particles were fed under pressure by compressed air 
or the like to fill them in an amount as full as possible into the mold, 
thereby molding them. According to this method, the constant volume of 
molded articles can be obtained. However, such a method involved a 
drawback that the constant weight of molded articles can not be obtained 
because scattering of amounts of the foamed particles to be filled into 
the mold becomes extremely great due to scattering of pressures upon 
filling under pressure, densities and particle sizes of the foamed 
particles, etc. As described in Japanese Patent Application Laid-Open No. 
27117/1987, it has accordingly been used to measure the volume of foamed 
particles by means of a metering device so as to always fill a fixed 
volume of the foamed particles into a mold. 
According to the method wherein the volume of the foamed particles is 
measured to fill them, the constant volume of the foamed particles is 
always filled into the mold. It is therefore possible to provide the 
constant weight of molded articles so long as the foamed particles always 
have a fixed expansion ratio (density). Even if expansion of the 
thermoplastic resin particles are conducted under as much control as 
possible in the production process of the foamed particles, it is however 
difficult to entirely do away with the scattering of expansion ratios of 
the resultant foamed particles. In the method wherein the fixed volume of 
the foamed particles are always filled into the mold to mold them, the 
resultant expansion-molded articles therefore vary in density and weight 
due to scattering of the expansion ratios of the foamed particles. In 
conclusion, the method wherein the volume of the foamed particles to be 
filled is kept constant could not furnish a solution for the provision of 
always the constant weight of molded articles. With the manufacture of a 
variety of precise industrial products, there has been a demand for making 
the weight of every products uniform in recent years in particular (for 
example, core materials for automobile bumpers). The conventional method 
could not meet such a demand. 
SUMMARY OF THE INVENTION 
With the foregoing in view, the present invention has as an object the 
provision of a production method of an expansion-molded article, by which 
the demanded constant weight and volume of molded articles can be provided 
even if foamed particles somewhat scatter in expansion ratio, and a 
filling apparatus of foamed particles of a thermoplastic resin for use in 
such a method. 
In one aspect of this invention, there is thus provided a method of 
producing an expansion-molded article by filling foamed particles of a 
thermoplastic resin into a mold and then causing the foamed particles to 
expand and fusion-bond under heat, which comprises measuring the weight of 
the foamed particles to be filled to fill a fixed weight of the foamed 
particles into the mold, thereby expansion-molding them. 
In another aspect of this invention, there is provided a filling apparatus 
of foamed particles of a thermoplastic resin, comprising a weigh hopper 
equipped with a weighing means; a foamed particle feeder adapted to 
control the amount of the foamed particles of the thermoplastic resin to 
be fed to the weigh hopper according to the weight of the foamed particles 
contained in the weigh hopper; a particle feed hopper adapted to 
temporarily keep a fixed weight of the foamed particles, which have been 
weighed in the weigh hopper, and then to transfer them; and a filling 
machine for filling the foamed particles into a mold.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
As exemplary base resins for the foamed particles of the thermoplastic 
resin useful in the practice of this invention, may be mentioned 
polyolefin resins such as low-density polyethylene, linear low-density 
polyethylene, high-density polyethylene, polypropylene, ethylene-propylene 
random copolymers, ethylene-propylene block copolymers, and mixtures of 
two or more these resins; styrene resins such as polystyrene, 
poly-p-methylstyrene, styrene-maleic anhydride copolymers, 
styrene-acrylonitrile copolymers, styrene-butadiene-acrylonitrile 
copolymers; polyvinyl chloride; vinyl chloride-vinyl acetate copolymers; 
polyvinylidene chloride; and the like. 
The foamed particles used in this invention can be obtained in accordance 
with a method in which resin particles containing a blowing agent therein 
are expanded under pressure in a pressure vessel, a method wherein a resin 
and a blowing agent are melted and kneaded in an extruder, the resultant 
melt is extruded and expanded in the form of a strand and the strand is 
then chopped into foamed particles, a method wherein resin particles and a 
blowing agent are dispersed in a dispersion medium in a pressure vessel, 
the resultant dispersion is heated under pressure to impregnate the resin 
particles with the blowing agent and the dispersion is then released under 
atmospheric pressure to expand the particles, or the like. 
Although the foamed particles obtained in the above-described manner are 
left to stand under atmospheric pressure after the expansion to age them, 
they may be treated under pressure with, in general, an inorganic gas, 
preferably, an inorganic gas containing nitrogen in a greater amount such 
as nitrogen gas or air to incorporate the inorganic gas in the particles, 
thereby applying an internal pressure (in general, a pressure higher than 
0.2 kg/cm.sup.2 .multidot.G in terms of gauge pressure) to the foamed 
particles prior to their molding in a pressurizing tank. 
The foamed particles optionally applied with the internal pressure are 
weighed so as to fill them in a fixed weight into a mold. The weight of 
the foamed particles to be filled into the mold is preset from the 
internal volume of a mold to be used and the density of the foamed 
particles according to the desired density of an intended molded article. 
If foamed particles of, for example, an ethylene-propylene random 
copolymer, which have an internal pressure of about 0 kg/cm.sup.2 
.multidot.G, are weighed and filled into a mold having an internal volume 
of 15.times.10.sup.3 cm.sup.3 to attempt to obtain an intended molded 
article having a volume, V.sub.2 of (15.times.10.sup.3).times.0.985.sup.3 
supposing the shrinking of the molded article is 1.5%, and a desired 
density of 0.06 g/cm.sup.3, the weight of the foamed particles to be 
filled into the mold is preset to (15.times.10.sup.3).times.0.985.sup.3 
.times.0.06 =860 g. The weight of the foamed particles to be fill is 
preset in the above-described manner, and they are always filled in the 
thus-preset weight into the mold. 
The foamed particle are weighed to fill them into the mold in such a 
manner. When an intended molded article having a volume, V.sub.2 of 
(15.times.10.sup.3).times.0.985.sup.3 and a density of 0.06 g/cm.sup.3 is 
now attempted to obtain, the weight of the foamed particles is preset to 
860 g as described above. At this time, supposing the bulk volume of the 
foamed particles to be filled before filling and the volume of the 
intended molded article are V.sub.1 and V.sub.2, respectively, the foamed 
particle may be filled into the mold by the conventional filling method 
such as a cracking filling method, in which the mold is not completely 
closed to make the internal volume of the cavity greater by a certain 
volume (this increased volume is called cracking), the foamed particles 
are fully filled into the mold by a pneumatic conveying means making use 
of a gas of 1-3 kg/cm.sup.2 .multidot.G without substantially compressing 
the foamed particles, and the mold is then completely closed, when the 
V.sub.1 /V.sub.2 value is 1.50 or lower. When the V.sub.1 /V.sub.2 value 
exceeds 1.50, it is difficult to fill them by the cracking filling method. 
In such a case, it is preferable to use a special filling method as 
described in Japanese Patent Application Laid-Open No. 212131/1987, in 
which foamed particles are filled into a mold, which has been pressurized 
in advance, using a pressurized gas in a pressurizing tank. Such a method 
involves a potential problem connecting with increased cost because of, 
for example, use of particular equipments. On the other hand, when foamed 
particles are compressed and filled into a mold by pressurized gas (2-5 
kg/cm.sup.2 .multidot.G), but a mold is not pressurized, whereby the 
foamed particles are fully filled in the mold in a state that the foamed 
particles have been compressed (hereinafter referred to as pressurizing 
filling method), they take the form of forced filling if the V.sub.1 
/V.sub.2 value is higher than 1.50, so that there is a potential problem 
that scattering of sectional densities of the resultant molded article 
occurs. 
Secondly, from the viewpoint of the secondary expandability, when the 
V.sub.1 /V.sub.2 value is at least 0.94, it is possible to obtain a 
sufficiently good molded article without applying any internal pressure to 
the foamed particles in a pressurizing tank (internal pressure of the 
particles: about 0 kg/cm.sup.2 .multidot.G). On the other hand, when the 
V.sub.1 /V.sub.2 value is at least 0.80 but lower than 0.94, and exceeds 
the limit of the secondarily expanding ability of the foamed particle to 
be filled, there is a potential problem that a molded article conforming 
with a mold can not be obtained due to insufficient secondary expansion 
such as failure of fusion-bonding and occurrence of interstices even if 
the filling weight is correct. It is hence preferable to use, for example, 
a method in which the internal pressure of the foamed particles is made 
higher than 0 kg/cm.sup.2 .multidot.G (usually, 0.2-2.0 kg/cm.sup.2 
.multidot.G) in order to secure the secondary expanding ability of the 
foamed particles to a sufficient extent in a pressurizing tank. Further, 
when the V.sub.1 /V.sub.2 value is lower than 0.80, it is necessary to use 
special foamed particles obtained by applying an internal pressure to 
foamed particles to a considerably great extent, thereby highly increasing 
their expanding ability. The use of such foamed particles connects with 
increased cost and moreover, involves a potential problem that scattering 
of sectional densities of the resultant molded article occurs. 
From the reasons described above, when molded articles are attempted to 
obtain by using the conventional apparatus and heating foamed particles 
with steam of a usual pressure of 2.0-4.5 kg/cm.sup.2 .multidot.G to mold 
them, it is possible to always obtain good-quality molded articles having 
a fixed weight and a fixed density if a fixed weight of the foamed 
particles are always filled into a mold and a packing rate is 
0.8.ltoreq.V.sub.1 /V.sub.2 .ltoreq.1.50. 
By the way, when foamed particles applied with an internal pressure are 
weighed and filled, it is necessary to add the weight of the gas contained 
in the particles, .omega., which is found from the relationship of the 
equation of state of gas: PV=(.omega./M)RT, to the weight preset. 
One embodiment of the present invention will hereinafter be described with 
reference to the accompanying drawings. 
FIG. 1 illustrates a filling apparatus according to one embodiment of the 
present invention In FIG. 1, numeral 1 indicates a raw material hopper. 
Foamed particles of a thermoplastic resin in the raw material hopper 1 are 
transferred to a weigh hopper 3 by a foamed particle feeder 2. The foamed 
particle feeder 2 is constructed so as to be capable of controlling the 
amount of the foamed particles of the thermoplastic resin to be fed to the 
weigh hopper 3 according to the weight of the foamed particles present in 
the weigh hopper 3. The control of the amount of the foamed particles to 
be fed to the weigh hopper 3 can be conducted, for example, in the 
following manner. The foamed particle feeder 2 is made up so as to make it 
possible to preset the feeding speed to several stages, whereby the 
feeding speed of the foamed particles is reduced by stages as the weight 
of the foamed particles fed to the weigh hopper 3 comes near the intended 
weight. Alternatively, the foamed particle feeder 2 is made up so as to 
make it possible to preset the feeding speed steplessly, whereby the 
feeding speed of the foamed particles is reduced continuously as the 
weight of the foamed particles fed to the weigh hopper 3 is increased. Any 
feeders may be used as the foamed particle feeder 2 so long as they permit 
the control of the feeding speed. As examples of such a feeder, may be 
mentioned a belt feeder, helical screw feeder, table feeder, rotary 
feeder, injection feeder, etc. These feeders may be used in combination 
with each other. The helical screw feeder is particularly preferred. 
The weigh hopper 3 is provided with load cells 4 as a weighing means. The 
load cells 4 are attached, for example, by providing a collar 5 around the 
weigh hopper 3 and joining the load cells to and between this collar 5 and 
a hopper supporting frame 6 as illustrated in FIG. 2. In this invention, 
the foamed particles are weighed together with the weigh hopper 3 of a 
heavy weight. It is therefore preferable to provide a plurality of load 
cells. The provision of the plural load cells make it possible to decrease 
a load applied to each load cell. It is hence possible to measure the 
weight of the foamed particles light as compared with the weigh hopper 3 
with precision. The number of load cells varies with the weight of the 
weigh hopper 3, the weight of the foamed particles to be weighed, the 
required accuracy in weighing, etc. It is however preferable to provide 
2-5 load cells in general. As the weighing means, may be used, in addition 
to the load cell, any kinds of means, such as a direct-reading balance and 
spring balance so far as their accuracy in weighing allows. 
A fixed weight of the foamed particles weighed in the weigh hopper 3 are 
then fed to a particle feed hopper 7. At this time, whether the foamed 
particles remain in the weigh hopper 3 or not is judged by the load cells 
4 as the weighing means. If the foamed particles in the weigh hopper 3 are 
not fully fed to the particle feed hopper 7, but remain in the weigh 
hopper 3 (in particular, when the foamed particles are charged with static 
electricity, they tend to remain in the weigh hopper 3), it is necessary 
to completely transfer the foamed particles in the weigh hopper 3 to the 
particle feed hopper 7 by taking a measure such as air blowing 
(preferably, blowing of air electrified with both plus and minus charges). 
The foamed particles are once held in the particle feed hopper 7 and then 
fed therefrom to holding hoppers 8a-8d, for example, by compressed air 
supplied through a compressed-air supply pipe 13. In FIG. 1, an example 
making use of a multi-cavity mold 9 is illustrated. The fixed weight of 
the foamed particles, which have been weighed in the weigh hopper 3 and 
transferred to the particle feed hopper 7 to be held therein, are first of 
all transferred, for example, to the holding hopper 8a. On the other hand, 
another batch of foamed particles are successively weighed in the weigh 
hopper 3. The thus-weighed foamed particles are then transferred from the 
particle feed hopper 7 to the holding hopper 8b. In the similar manner, 
respective batches of foamed particles are in turn transferred to the 
holding hoppers 8c and 8d. By the way, the holding hoppers 8a through 8d 
possess a function as a pressurizing tank. The filling apparatus according 
to the present invention is constructed in such a manner that a fixed 
weight of the foamed particles are surely transferred from the particle 
feed hopper 7 to one of the holding hoppers 8a-8d by switching their 
destination to be transferred by change-over valves 10a-10d. The foamed 
particles transferred to the holding hopper 8a-8d are respectively filled 
into mold cavities 12a-12d of the multi-cavity mold 9 by respective 
filling machines 11a -11d connected to the holding hopper 8a-8d. In the 
above-described manner, the fixed weights of the foamed particles are 
respectively filled into the mold cavities 12a-12d. When a pressurizing 
and decompressing valve is provided in each of the holding hoppers 8a-8d 
to permit the increase and decrease of the pressure within each of the 
holding hoppers 8a-8d, filling process can be conducted speedily, for 
example, upon the compression of the foamed particles in the holding 
hopper and filling into the cavity by the pressurizing filling method. 
When the multi-cavity mold is used as described above, holding hoppers of 
the number corresponding to the number of the cavities in the mold are 
provided to feed the foamed particles from the holding hoppers to their 
corresponding cavities by respective filling machines. Such a construction 
can ensure that fixed weights of the foamed particles are fed to a 
plurality of the filling machines by only one weighing hopper. On the 
other hand, when a one-cavity mold is used, it is not always necessary to 
provide the holding hopper. 
The foamed particles filled into the mold cavities 12a-12d are heated with 
the conventionally-known heating means such as steams to mold them. 
EXAMPLES 
The present invention will hereinafter be described in further detail by 
the following examples. 
EXAMPLES 1-8 
Each of foamed particle samples shown in Table 1 was weighed, and the fixed 
weight of the foamed particle sample thus weighed was filled into a mold 
having an internal volume of 15 l by the cracking filling method, and 
heated with steam of its corresponding pressure shown in Table 1. In the 
above-described manner, each of the foamed particle samples was 
continuously molded 20 times. The desired weights and densities of 
intended molded articles, the weights and densities of molded articles 
practically obtained, and the densities and preset filling weights into 
the respective molds of the foamed particle samples used in the molding 
are shown in Table 1 and/or Table 2. Properties of the resulting molded 
articles are also shown in Table 2. 
Incidentally, in Examples 3 and 7, foamed particle samples whose internal 
pressures were increased to 1 kg/cm.sup.2 .multidot.G were used, while in 
Examples 4 and 8, the foamed particle samples were filled in respective 15 
l molds free of any cracking by a pressurizing filling method, thereby 
molding them. 
COMATIVE EXAMPLES 1 & 2 
Fixed volumes of foamed particle samples shown in Table 1 were separately 
filled into the same mold as those used in the examples, which had been 
provided with a cracking of 1 l, using a conventional feeder by cracking 
filling method, and then heated with steam of their corresponding 
pressures shown in Table 1, thereby continuously molding them 20 times. 
The desired weights and densities of intended molded articles, the weights 
and densities of molded articles practically obtained, and the densities 
and preset filling volumes into the respective molds of the foamed 
particle samples used in the molding are shown in Table 1 and/or Table 2. 
Properties of the resulting molded articles are also shown in Table 2. 
TABLE 1 
__________________________________________________________________________ 
Filling amount 
into mold of 
Properties of foamed particles used 
Pressure of 
foamed particles 
Average 
steam for 
(preset value) 
bulk density 
molding 
Weight 
Volume 
Kind of base resin 
(g/cm.sup.3) 
(kg/cm.sup.2 .multidot. G) 
(g) (l) 
__________________________________________________________________________ 
Ex. 1 
Ethylene-propylene 
0.053 3.5 873 .sup. 
-- 
random copolymer 
Ex. 2 
Ethylene-propylene 
0.060 " " -- 
random copolymer 
Ex. 3 
Ethylene-propylene 
0.068 " 883 *3 
-- 
random copolymer 
Ex. 4 
Ethylene-propylene 
0.043 " 873 *4 
-- 
random copolymer 
Ex. 5 
Ethylene-propylene 
0.041 " 675 .sup. 
-- 
random copolymer 
Ex. 6 
Ethylene-propylene 
0.047 " " -- 
random copolymer 
Ex. 7 
Ethylene-propylene 
0.053 " 685 *3 
-- 
random copolymer 
Ex. 8 
Ethylene-propylene 
0.033 " 675 *4 
-- 
random copolymer 
Comp. 
Ethylene-propylene 
0.053 " -- 16 
Ex. 1 
random copolymer 
Comp. 
Ethylene-propylene 
0.041 " -- " 
Ex. 2 
random copolymer 
__________________________________________________________________________ 
*3: Foamed particles applied with an internal pressure in a pressurizing 
tank. 
*4: Foamed particles were compressed and then filled by the pressurizing 
filling method. 
TABLE 2 
__________________________________________________________________________ 
Practically obtained 
Intended molded 
molded article 
article (20 articles) Properties of molded articles 
Weight Density 
Weight 
Density Scattering of *.sup.1 
Fusion bonding *.sup.2 
(g) (g/cm.sup.3) 
(g) (g/cm.sup.3) 
V.sub.1 /V.sub.2 value 
densities and weights 
property of particles 
__________________________________________________________________________ 
Ex. 1 
873 0.061 
865-884 
0.060-0.062 
1.15 .largecircle. 
.largecircle. 
Ex. 2 
" 0.061 
863-892 
0.060-0.062 
1.01 .largecircle. 
.largecircle. 
Ex. 3 
" " 869-894 
0.060-0.062 
0.91 .largecircle. 
.largecircle. 
Ex. 4 
" " 870-880 
0.061 1.42 .largecircle. 
.largecircle. 
Ex. 5 
675 0.047 
660-689 
0.046-0.048 
1.15 .largecircle. 
.largecircle. 
Ex. 6 
" " 660-688 
0.046-0.048 
1.00 .largecircle. 
.largecircle. 
Ex. 7 
" " 662-691 
0.046-0.048 
0.90 .largecircle. 
.largecircle. 
Ex. 8 
" " 659-685 
0.046-0.048 
1.43 .largecircle. 
.largecircle. 
Comp. 
873 0.061 
827-915 
0.058-0.064 
-- x .largecircle. 
Ex. 1 
Comp. 
675 0.047 
633-710 
0.044-0.050 
-- x .largecircle. 
Ex. 2 
__________________________________________________________________________ 
*1: The scattering of densities and weights of 20 molded articles in each 
example or comparative example was evaluated in accordance with the 
following standard: 
.largecircle.. . . Scattering smaller than .+-.3% as to all molded 
articles; 
.DELTA.. . . Scattering in a range of .+-.3-.+-.5 as to some of the molded 
articles; and 
x . . . Scattering greater than .+-.5 as to some of the molded articles. 
*2: The fusion bonding property of each molded article was evaluated in 
accordance with the following standard: 
.largecircle.. . . Non-interparticle breakage occurred on more than 80% of 
the molded article upon its destruction; 
.DELTA.. . . Non-interparticle breakage occurred on 50-80% of the molded 
article upon its destruction; 
x . . . Non-interparticle breakage occurred on less than 50% of the molded 
article upon its destruction. 
ADVANTAGES OF THE INVENTION 
According to the present invention, the following effects have been brought 
about. Since the weight of foamed particles is measured to always fill a 
fixed weight of the foamed particles into a mold on the basis of its 
preset value, thereby molding them, no scattering of volumes and weights 
of the resultant molded articles occurs even when the foamed particles 
somewhat scatter in expansion ratio, so that uniform molded articles can 
always be obtained. Since the foamed particles are also filled within a 
range of limited packing rates, it is possible to obtain excellent molded 
articles free of inferior in interparticle fusion bonding. Moreover, when 
a fixed weight of the foamed particles are filled and molded in such a 
manner that a V.sub.1 /V.sub.2 value becomes at least 0.94, it is possible 
to obtain sufficiently good molded articles without applying an internal 
pressure to the foamed particles by a pressurizing treatment in a 
pressurizing tank or the like. 
Besides, the filling apparatus according to this invention can always fill 
a fixed weight of foamed particles into a mold to mold them. Therefore, it 
is possible to provide the demanded constant weight and volume of molded 
articles. Even when a multi-cavity mold is used, fixed weights of foamed 
particles can be always fed to a plurality of cavities by only one 
weighing device. Furthermore, when holding hoppers of the number 
corresponding to the number of the cavities are provided between 
respective filling machines and particle feed hoppers, a fixed weight of 
the foamed particles can be fed into each of the cavities and moreover, an 
advantage is brought about, for example, upon compressing the foamed 
particles to fill them into the mold by the pressurizing filling method. 
In addition, when a helical screw feeder is used as a foamed particle 
feeder for feeding foamed particles to a weigh hopper, the fixed- weight 
feeding can be rapidly performed, and the specification of weighing from a 
large quantity to a small quantity can be enlarged. When a plurality of 
load cells are provided as a weighing means by joining them to the 
weighing hopper, the weight of foamed particles can be measured with high 
accuracy.