Apparatus for molding particulate expandable thermoplastic resin material using microwave heating

Molding of expanded plastic material such as dielectric material by microwave radiation is effected in a mold in which the part of the mold which contacts the material to be molded is of a material having high dielectric losses and capable of absorbing microwave radiation. Such mold portion preferably comprises a resin containing a small amount of carbon black.

BACKGROUND OF THE INVENTION 
In recent times there has been considerable development and increase in the 
use of plastic materials for example in the building industry, the 
refrigerating industry, in the production of unsinkable members e.g. for 
boats, in packaging and other product conditioning, and the aeronautical 
and automobile industry. One consequence of this increased use has been a 
certain amount of progress in improving apparatus and processes for 
producing expanded plastic material. For example, one step along this 
route was the use of a high-frequency field to melt the synthetic resin, 
as disclosed for example in French Pat. Nos. 2 149 529, 1 217 351, 2 045 
888 and 2 186 344, and U.S. Pat. No. 3,377,653. 
A further improvement lay in the use of ultra-high frequency or microwave 
radiation (generally 500 to 300,000 MHz) which made it possible to produce 
plastics foams in an economical manner. This process does not require a 
capacative circuit and does not cause voltage cracking, the ultra-high 
frequency (referred to herein as U.H.F.) field energizing a resonant 
cavity or a resonant waveguide. A process of this nature is disclosed in 
French Patent applications Nos. 76 01049 and 76 31899 in the name of the 
assignees of the present application, which set forth an operating 
procedure of simultaneously introducing beads or pearls of plastic 
materials which had first been pre-expanded, and water, into a mold 
disposed in a resonant cavity. This mode of operation produces conditions 
which permit the formation of vapour in situ, and excellent welding of the 
beads or pearls to each other, with a good degree of distribution of the 
heat involved in melting the material. 
However, this process suffers from the disadvantage that the molds used are 
of a material which is transparent to or transmissive of the U.H.F. 
radiation or wich has a low absorption capacity in respect of such 
radiation. A consequence of this is a certain lack of uniformity in the 
heating action and an increase in the amount of power which has to be 
employed due to the fact that it is necessary to heat the walls of the 
mold in each molding cycle to achieve heating uniformity. This process 
therefore not only suffers from poor power efficiency but also tends to 
suffer from an extended operating cycle due to the need for additional 
heating of the mold. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a mold which better fulfills the 
practical requirements of microwave molding operation. 
A further object of the invention is to provide a mold for molding expanded 
plastics materials by microwave radiation, which provides an excellent 
temperature gradient and heat distribution, and which can achieve a 
substantial saving in the amount of power required for melting the surface 
of the plastic material in the mold. 
According to the present invention, these and other objects are achieved by 
a mold and a molding apparatus for molding expanded plastic materials, 
such as dielectric materials, by microwave radiation wherein the part of 
the mold cavity which contacts the plastic material to be molded comprises 
a material having high dielectric losses, and capable of absorbing 
microwave radiation. The material is preferably a resin containing a small 
amount of microwave radiation-conducting carbon black. The amount of 
carbon black in the resin is preferably from about 1 to 5%. 
The body of the mold may be made of a material which does not absorb 
microwave radiation, preferably a resin with a filler or reinforcement of 
fibres of pure silica and/or glass with a low loss angle. 
A circuit for the circulation of a heat-exchange fluid is associated with 
the body of the mold, such fluid preferably having low dielectric losses 
and a high heat capacity per unit of weight. The heat-exchange fluid 
circulation circuit is preferably disposed in the body of the mold itself, 
but the fluid circulation circuit may additionally or alternatively be in 
the form of a jacket or sleeve assembly on the mold body. 
The mold may be cooled by forced air circulation around its outside walls, 
and such circulation may be achieved by means of blades or vanes on the 
outside walls of the mold. 
Furthermore, the mechanical parts of the mold, such as injectors, vents, or 
risers, or air-gates and the like can be made e.g. machined of 
polytetrafluoroethylene (known under the Trade Name of TEFLON) and/or 
vitroceramics. 
In the molding apparatus of the invention, the mold is disposed in a 
resonant cavity mounted in a press of which one half is fixed and the 
other half is movable. Such installations are described in the abovequoted 
French patent applications Nos. 76 01049 and 76 31899. In a preferred form 
of the apparatus, a portion of the fixed part of the press is excited by a 
waveguide which is fed by an microwave generator, while the movable part 
of the press carries the mold assembly. It will be apparent however that 
the reverse arrangement may be employed without disadvantage. 
In the apparatus according to the invention, a minimum distance must be 
maintained between the mold and the wall of the resonant cavity, and this 
distance is preferably not less than 65 mm. An electromagnetic field 
agitating means may be disposed at the outlet of the waveguide into the 
resonant cavity, the effect of which agitating means is to prevent the 
formation of standing waves. 
A process for molding articles of plastic material in molds in accordance 
with the invention provides that beads of plastic materials, which may 
possibly contain a swelling agent or a volatile liquid blowing agent and 
which may possibly have been pre-expanded before being used in the mold, 
are introduced into the mold according to the invention, possibly at the 
same time as a polar liquid such as water. The mold may be a two-part 
mold, in a suitable form of press. The beads of plastic material in the 
mold are subjected to the action of microwave radiation for a period of 
from 5 to 1,800 seconds, in order to produce molded articles of expanded 
plastic materials having walls of any desired thicknesses, in particular 
thin-walled articles. 
The invention is concerned only with molds for producing expanded plastic 
material articles as set out above, but other installations, processes and 
production lines for producing molded articles of expansible plastic 
material, which include the principles of the present invention, together 
with the molded articles produced by such molds and installations 
according to the invention are not excluded.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will first be made to FIG. 1 which shows a sectional view of part 
of a mold 4 for molding an expanded plastic material article. The mold 4 
has a mold cavity defined at least in part by a wall portion 1 comprising 
a material which has high dielectric losses and which is capable of 
absorbing microwave radiation. The wall portion 1 is preferably formed by 
a surface coating or layer of epoxy resin containing microwave 
radiation-conducting carbon black (for example "Vulcan XC 72 R" by CABOT). 
The resin contains a small proportion of carbon black relative to the 
amount of resin, for example preferably from about 1 to 5% by weight. The 
wall portion 1 of microwave radiation-absorbing material will generally be 
provided where the material to be molded comes into contact with the mold, 
i.e. generally on the whole inside surface defining the mold cavity. 
The thickness of the mold wall portion 1 is determined by the desired 
amount of heat to be applied to the plastic material in the mold, and thus 
the thickness of the wall of the molded article. It is generally 
advantageous for the thickness of the coating which forms the mold cavity 
wall portion 1 to be from about 0.5 to 2 mm, although it may vary 
according to requirements. 
The body 3 of the mold 4 is advantageously made of an epoxy resin with a 
filler of fibres of pure silica and/or glass with a low loss angle 
(tg.sup..delta. .ltoreq.1.10.sup.-4 for a frequency f=2.45 GHz), which 
does not absorb microwave radiation. The thickness of the silica and/or 
glass fibre-reinforced resin body 3 of the mold is advantageously from 3 
to 5 mm, taking into account the molding pressures which will generally be 
encountered in operation. 
The mold 4 also has means for the circulation of a heat-exchange fluid, 
including for example a silicone oil as produced by Rhone-Poulenc and 
known as 47 V 10 RHODORSIL. The fluid-circulation means of the FIG. 1 mold 
comprises a passage arrangement 2 within the mold body 3, to achieve 
efficiency in the heat-exchange action, by being adjacent to the wall 
portion 1 as shown. It will be appreciated however that a passage 
arrangement for such a heat-exchange fluid circulation may additionally or 
alternatively be provided in an external jacket or sleeve assembly on the 
mold body 3, depending inter alia on the nature of the molding operation 
and the article to be molded. 
The heat-exchange fluid preferably has low dielectric loss, and a high heat 
capacity relative to its weight. 
External cooling of the mold may advantageously be effected by means of a 
forced air draught, for example by mounting blades or vanes on the outside 
surface of the mold. 
Reference will now be made to FIG. 2 which shows a diagrammatic view of an 
apparatus for molding expanded plastic materials, in particular dielectric 
materials. This apparatus includes a mold assembly providing a plurality 
of molds 4 as shown in FIG. 1, and a press having a movable portion of 
plate 6 and a fixed portion or plate 7. The movable portion 6 may be moved 
for example by a pneumatic jack, or any other suitable means. 
The mold assembly 4 is disposed in a resonant cavity 11 mounted in the 
press between the two portions 6 and 7 of which the movable portion 6 
carries part of the cavity 11 and one part of the two-part mold assembly, 
providing the mold cavities. The part of the cavity 11 on the fixed 
portion 7 of the press is connected to a microwave waveguide 9 which is 
fed by a microwave generator (not shown), to energize or excite the 
portion 7. 
The two parts of the mold assembly 4 disposed in the resonant cavity 11, as 
shown clearly in FIG. 2, are supported on respective support assembly 12, 
and it will be understood that the support assemblies 12 are also 
effective to provide an adjustment in respect of the position of the molds 
4, relative to the movable part 6 of the press, to ensure proper closure 
thereof. 
Mounted within the resonant cavity 11 adjacent to the outlet end opening of 
the microwave waveguide 9 is a means 10, shown as a rotary blade assembly 
driven by a motor, for agitating the microwave field, to thereby prevent 
the formation of stationary or standing waves. Disposed adjacent to the 
field agitating means 10 is a deflector 8 illustrated as a plate inclined 
downwardly towards the means 10 which is disposed below the waveguide 9 
outlet, to direct the waves towards the means 10 thereby to enhance the 
efficiency of the agitation action. 
The apparatus of FIG. 2 also has an injector 5 which is in communication 
with the mold cavities of the molds 4, for injecting into the mold 
cavities plastic material in bead or pearl form, which may have been 
subjected to a pre-expansion operation before being injected and which may 
possibly contain the swelling agent or volatile liquid blowing agent. The 
injector 5 is preferably made of a material having low dielectric losses. 
The molds 4 and the wall of the resonant cavity 11 must be spaced from each 
other by a given minimum distance, and it has been found that this minimum 
distance is advantageously about 65 mm. 
Still referring to FIG. 2, reference numeral 13 denotes flow means for 
introducing a heat-exchange fluid which is to circulate in the internal 
passage arrangement 2 of the molds 4 and/or in the external jacket or 
sleeve assembly, where employed, as mentioned in relation to FIG. 1. The 
flow means 13 are preferably also made of a material with low dielectric 
losses. Reference numeral 14 denotes flow means for a flow of e.g. 
compressed air, which is provided for ejecting the molded articles, the 
means 14 also preferably comprising a material with low dielectric losses. 
Additionally, the flow means 14 may also be used to direct a stream of 
compressed air over the mold for cooling by forced air circulation. 
It should be noted here that the apparatus of FIG. 2 may be used for 
molding any expansible plastic material, non-limiting examples of which 
include expansible polystyrene, polyethylene and polyurethane. 
Reference will now be made to FIG. 3 which shows a non-limiting example of 
use of the mold according to the invention, for producing articles with 
thin walls, illustrated in the form of beakers or cups. Beads or pearls of 
expansible plastic materials are introduced by an injector assembly 15 
into the mold cavities. Each mold is in two parts, a female part 16 and a 
male part 17, and the inside surfaces which define the mold cavities are 
provided with surface coatings to form the wall portions 1. The surface 
coatings 1 are of a material with high dielectric loss and capable of 
absorbing microwave reduction, as described above with reference to FIG. 
1. As in the FIG. 2 apparatus, the mold assembly of FIG. 3 is disposed in 
a press within a resonant cavity to which microwave radiation is fed. 
In the arrangement shown in FIG. 3, the high-loss, radiation-absorbent wall 
portions 1 of the molds are about 0.8 mm in thickness. When the mold 
cavities are also to be supplied with polar liquid, for example water, at 
the same time as the plastic material is to be introduced into the mold 
cavities, this liquid injection operation may be effected by movable 
injectors (not shown). It will be noted that the operation of molding 
plastic materials according to the present invention can be effected 
without introducing a polar liquid, when molding thin-walled articles. The 
mold is cooled by convection of forced air draught, which can 
advantageously also be introduced by movable injectors (also not shown in 
FIG. 3). Blades or vanes 18 may be mounted on the outside surface of the 
mold to aid cooling. 
By way of example only, with an apparatus comprising eight mold assemblies 
mounted in a carrousel arrangement and each comprising six mold cavities, 
operated in a molding cycle of a duration of 45 seconds, it is possible to 
produce 3, 840 cups per hour, each cup weighing about 2 grams, for 7.6 kg 
of input material for molding. The total microwave power consumption of 
the press plus the pre-expansion arrangement is about 2.5 kw. 
In operation of the above-described apparatus, bead or pearls of plastic 
materials are introduced into molds 4 (FIGS. 1 and 2) or 16, 17 (FIG. 3). 
The plastic materials may contain a swelling agent and it may have been 
subjected to a pre-expansion action before being introduced into the mold 
cavity. The plastic material may be introduced simultaneously with a polar 
liquid, for example water, to cause the formation of vapour in situ, which 
is required for welding the beads of plastic material together. When 
microwave radiation is introduced into the resonant cavity 11, the wall 
part 1 of the mold which is in contact with the beads of plastic material, 
such part 1 being made as mentoned above of a material having relatively 
high dielectric losses, absorbs a part of the microwave energy and applies 
it in the form of heat by heat conduction to the plastic material 
contained in the mold cavity or cavities. In this way the apparatus 
provides for good heat distribution. 
The material in the mold may be subjected to microwave radiation for a 
period of e.g. from 5 to 1,800 seconds, to produce the molded article. 
Accordingly, as the mold cavity wall is capable of absorbing microwave 
radiation, it can avoid the formation of vapour condensate on the walls. 
This fact, together with the fact that the mold cavity wall portions are 
very thin and absorb microwave radiation, can shorten the mold cooling 
cycle, and also provide a considerable saving in regard to the power used. 
Furthermore, molds according to the invention can be of very low industrial 
cost price, as they can be made by a simple casting operation, and an 
apparatus of the carrousel type can often produce approximately twice as 
much for the same cost price (i.e. equipment and mold), relative to 
conventional prior art processes which employ a steam boiler and a 
machined mold. Mechanical parts of the mold such as injectors or vent 
means may be made of polytetrafluoroethylene and/or vitroceramics. 
Yet another advantage of the apparatus is that it can provide for a 
considerable reduction in the time required for changing the mold, by 
virtue of the simplicity of its construction. The apparatus also makes it 
possible to mold thin-walled articles of high quality, by a process 
referred to as a "dry wall process", i.e. without using a polar liquid in 
the mold. 
It will be understood that the invention has been described above with 
reference to embodiments which are given only by way of example, and that 
various modifications and variations may be made within the scope and 
spirit of the present invention.