Flow molding surface of plastic and conductive particles

Thermosetting molds are formed with electrically conductive surfaces for use in high frequency energy flow molding of sheet materials. Preferably the mold of this invention has a highly electrically conductive layer preferably in an epoxy gel coat carrying electrically conductive particles of gold, silver or platinum and positioned over a compatible epoxy mold body. A vacuum molding technique is used to obtain good surface definition in surface molding.

BACKGROUND OF THE INVENTION 
Flow molding with the use of high frequency energy such as radio frequency 
energy has come into widespread use in recent times particularly in the 
surface molding of shoe uppers. As well-known, in such procedures, a 
negative RTV silicone rubber mold is produced, positioned on a ground 
electrode and a sheet stock such as vinyl sheet stock to be surfaced is 
laid over the mold with a top electrode pressed thereon and radio 
frequency energy flowed through the mold to thermally soften the vinyl and 
allow it to take on the surface characteristics of the negative mold. 
The molds used are frequently made from RTV silicone rubber in known flow 
mold making procedures. However, such RTV silicone rubber mold masters 
have limited life spans in high frequency flow molding. The RTV silicone 
rubber molds act to absorb heat causing increased dwell times in the mold 
and in some cases, absorb plasticizers and secondary plasticizers of the 
vinyl thereby causing weakening, swelling and dimensional change of the 
silicone rubber molds. The definition of the surface configuration to be 
transferred is sometimes lost after few molding operations. The silicone 
rubber in some cases tears or permanently distorts. These defects in the 
silicone rubber molds used in flowmolding flow molding well-known. 
It has been suggested that more durable molds be formed. For example, epoxy 
molds have been suggested. However, it is found that when such epoxy molds 
are used in radio frequency flow molding, the epoxy tends to absorb heat 
at a rate 3 to 8 times greater than absorbed by silicone rubber thereby 
lengthening dwell times in the mold to an unacceptable degree. Moreover, 
detail of the surface configuration to be transferred is sometimes lost. 
The removal of heat by the epoxy is such that in some cases the vinyl 
sheet being molded never reaches a molten state to allow detail to be 
transferred. 
Probably because of the foregoing problems, the art has in most cases 
continued to use RTV silicone mold masters in high frequency flow molding 
procedures. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a long life thermosetting mold 
for use in high frequency flow molding which mold does not absorb heat of 
molding in an amount to destroy or seriously affect the molding operation. 
Another object of this invention is to provide a thermosetting plastic mold 
in accordance with the preceding object which has an electrically 
conductive molding surface. 
It is still another object of this invention to provide a thermosetting 
plastic mold in accordance with the preceding objects which is resistant 
to adverse interaction with sheets being molded, retains transfer of 
definition for long time periods, is non-self-destructive under normal 
operation pressures and temperatures and has many of the advantageous 
characteristics of a metal mold including long life and high definition. 
Still another object of this invention is to provide a method of flow 
molding incorporating the molds of this invention. 
Still another object of this invention is to provide an improved vacuum 
assist step for use in obtaining good surface definition in flow molding. 
According to the invention, a thermosetting plastic production mold is 
formed with an electrically conductive molding surface layer. Preferably 
the mold is formed with a surface layer of conductive particle filled 
resin, although in some cases the entire body of the mold can be 
conductive. 
In a preferred method a vacuum is created between a sheet to be surface 
flow molded and the forming mold to enhance surface definition of the so 
molded sheet. 
The electrically conductive surface layer is suitable for grounding in an 
RF flow molding machine. When used in this manner, no deleterious heat 
buildup occurs in the thermosetting plastic mold. High surface definition 
can be obtained in sheet materials molded over long time periods without 
cracking or shattering of the mold. Because of the high thermal 
conductivity as well as electrical conductivity of the surface layer, 
short molding cooling cycles and shorter dwell times than customarily used 
are obtained. Because the mold is rigid rather than flexible as with 
previous silicone rubber master molds, better detail such as detail of 
thread twist, needle perforation and the like is easily obtained. The 
conductive surface is preferably, continuous, liquid impervious and 
non-porous thus allowing for good surface definition in molding. The molds 
are rigid at temperatures and pressures used in conventional flow molding.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The high frequency flow molding conditions and apparatus used are as 
well-known in the art. For example, high frequency molding can be carried 
out at the following conditions: 
______________________________________ 
pressure (lbs/sq.in.on 
0.5 to 1000 lbs./sq.in. 
sheet molded) 
dwell time 1-20 seconds 
temperature 70.degree. F-400.degree. F 
frequency 20-54 megacycles 
Wattage 7.0 to 70 kilowatts 
cooling time under pressure 
after dwell time under 
electrical energy 1 to 60 seconds 
______________________________________ 
In such molding as known, a sheet material such as a vinyl sheet for 
example having a thickness of from 0.001 inch to 0.200 inch is placed over 
a production mold which is in turn placed on a ground electrode and a top 
electrode brought into contact with the vinyl sheet to press it against 
the production mold while the high frequency energy is passed 
therethrough. Slight flow molding occurs causing the sheet material to 
take on the surface contour and definition of the production mold. For 
example, as known in the shoe art, surface textures including stitching, 
perforations and the like can be imparted to a vinyl sheet during such 
flow molding. The surface contour and texture of a variety of shoe upper 
materials including stitched and embossed leather can be simulated. In 
some cases, the sheet to be molded need not be vinyl but other heat 
softening plastics and even surface treated leather can be surface flow 
molded by standard procedures. 
The particular production mold used in flow molding is an important feature 
of the present invention. It has now been found that by using a 
thermosetting rigid plastic production mold having an electrically 
conductive surface, long life, high definition molds can be rapidly and 
inexpensively made for use in flow molding. 
In an example of making a negative thermosetting plastic negative 
production mold of this invention, a conventional three-step process is 
used with the third step incorporating the conductive layer of this 
invention. In a first stage, a conventional flow molding mold box as 
diagrammatically shown at 10 in FIG. 2 has a rectangular frame 11 
thereover with a pattern or master 12 positioned therein. The pattern 12 
as best shown in FIG. 1, can be a leather-shoe upper having an embossed 
toe portion 13 with a line of stitching 14 therein and a series of 
perforations 15 with a leather grain 16 thereover. The object is to 
produce a mold which will reproduce the embossed, perforated and surface 
definition of the shoe master 12. The first stage is carried out by 
bonding the master 12 to a flat planar lower surface of the mold box as by 
the use of a double surface sticky tape which is compatible with the 
resins later used and which may be Minnesota Mining and Manufacturing 
Company two-way tape No. 419. A mold release agent containing an 
isopropyl-alcohol-water solution of polyvinyl alcohol is then brushed or 
sprayed onto all surfaces and allowed to dry for 30 minutes. Uncoated 
vinyl masters do not require the mold release. Four parts of Acron epoxy 
T4051A are then mixed with 1 part Arcon T4051B by weight and degassed in a 
vacuum chamber having about 4 times the volume of the epoxy in accordance 
with known procedures. Arcon epoxy T4051A is a product of Allied Resin 
Corporation of East Weymouth, Mass. formed of a bisphenol 
A/epichlorohydrin epoxy resin filled with 50% by weight CaCO.sub.3 of a 
particle size ranging from 0.1 to 25 microns with an apparent epoxy 
equivalent weight of 360 .+-. 10. Arcon epoxy T4051B is a product of 
Allied Resin Corporation of East Weymouth, Mass. formed of 
polyoxypropyleneamine having an average molecular weight of 340 .+-. 50. 
The epoxy mixture is then poured slowly over the master 12 and cured for 24 
to 36 hours at room temperature to produce a negative first stage epoxy 
mold 20. Curing can be carried out at 110.degree. F for 12 hours if 
desired. 
As illustrated in FIG. 3, using the first stage mold 20, the general 
procedure illustrated in FIG. 2 is repeated in a second stage. In this 
second stage, the first stage epoxy mold is bonded to the mold box 10 with 
a two-surface stick tape. Ten parts by weight of a silicone rubber RTV 
664A, a trademarked product of General Electric Co. of Waterford, N.Y., 
containing a high strength methyl silane, addition curing, room 
temperature vulcanizing silicone rubber is mixed with 1 part by weight RTV 
664B, a trademarked product of General Electric Co. of Waterford, N.Y. and 
containing a low molecular weight silicone rubber curing agent. The 
mixture is degassed in an area approximately 5 times the volume of the 
mixture after which the mixture is poured into the mold box and cured for 
24 hours at room temperature or until tack free. The resulting silicone 
rubber positive second stage mold 21 is then thus formed and may be 
postcured for 2 to 3 hours at 300.degree. F or until all surfaces are tack 
free. This results in a positive RTV silicone rubber mold 21, which is 
useful for forming the electrically conductive production mold of this 
invention. 
In a third stage, a production mold of this invention is formed. Three 
parts by weight of T4083A, a product of Allied Resin Corporation of East 
Weymouth, Mass., formed of 24% by weight of bisphenol A/epichlorohydrin 
epoxy resin (molecular weight 360 .+-. 10) filled with 76% by weight of 
silver flake of a particle size ranging from 2 to 25 microns with an 
apparent epoxy equivalent weight of 700 .+-. 50 are mixed with 1 part by 
weight T4083B curing agent, a product of Allied Resin Corporation of East 
Weymouth, Mass., formed of methylethyl ketone solvent solution of two 
isomers 2,2,4 and 2,4,4 trimethylhexamethylenediamine and N-2 
hydroxypropylimidizol as follows: 
2.79% by weight: 2,2,4 trimethylhexamethylenediamine 
2.79% by weight: 2,4,4 trimethylhexamethylenediamine 
1.72% by weight: N-2 hydroxypropylimidizol 
92.7% by weight: methylethyl ketone 
The mixture is then sprayed onto the silicone rubber mold 21 in a mold box 
10 using a Paache #3 air brush to develop a film thickness of 0.010 inch. 
The mold box 10 is then heated at 140.degree. F for 2 hours in an air 
circulating oven. This forms a gel coat of an epoxy over the silicone 
rubber positive mold 21. 100 parts of Arcon epoxy T4046A, a product of 
Allied Resin Corporation of East Weymouth, Mass., formed of an admixture 
of: 
26.7% by weight bisphenol A/epichlorohydrin epoxy resin molecular weight of 
360 .+-. 10 
5.4% by weight milled glass fibers (1/32 wide screen size) 
14.6% by weight Al.sub.2 O.sub.3 particle size 1 to 8 microns 
53.3% by weight Iron powder particle size 5-125 microns. 
The above admixture, having an apparent epoxy equivalent weight of 675 .+-. 
10, is mixed together with 3 parts by weight Arcon T4046B formed of: 
38.3% by weight 2,2,4 trimethylhexamethylenediamine 
38.3% by weight 2,4,4 trimethylhexamethylenediamine 
23.4% by weight N-2 hydroxypropylimidizol 
The mixture is degassed in a mixing container having a volume approximately 
5 times the volume of the mixture. The mixture is then slowly poured into 
the mold over the gel coating and cured for 2 hours at 200.degree. F and 
then 3 hours at 350.degree. F. The completed third stage negative 
production mold is then cooled to room temperature and demolded slowly. 
Additional gel coating formed of T4083A and T4083B in the proportions 
previously described are sprayed onto the sides and bottom of the mold so 
that all exterior surfaces have a volume resistivity of 1 .times. 
10.sup.-3 ohms-cm with minimum film thickness of 0.005 inch. This 
additional coating is heated to 140.degree. F for 2 hours and post-cured 
for 2 hours at 350.degree. F. 
The resultant production mold has a surface coating which is uniformly 
electrically conductive and which is eminently suitable for use in high 
frequency flow molding. 
The resultant third stage mold 30 (FIGS. 6 and 7) has a rigid body 31 with 
an electrically conductive surface 32 preferably extending completely 
therearound and with a negative surface configuration 33 conforming to the 
surface configuration of the original 12. When placed in a high frequency 
flow molding apparatus diagrammatically illustrated in FIG. 5 between a 
planar surfaced top electrode 36 and a bottom planar surfaced electrode 37 
along with a vinyl sheet 38 to be molded, the surface configuration can 
easily be produced in the vinyl sheet at conventional pressures and 
frequency ranges. In fact, somewhat lower amounts of electrical energy are 
necessary. For example, a flow molding machine can be used for molding 
vinyl sheets having a thickness of 0.068 inch in the form of a vamp 
pattern having a surface area of 100 sq. inches. The mold 30 is used and 
the machine operated at a pressure of 3 psi, dwell time of 7 seconds, room 
temperature, frequency 48 megacycles, wattage 15 kilowatts drawing 1500 
volts R.F. and cooling time under pressure 10 secones. Good surface 
texture is produced on a surface of the vamp. The mold 30 has a long life 
span with relatively inexpensive cost as compared with metal molds. The 
mold does not absorb heat because of its surface coating which is heat 
conductive as well as electrically conductive causing substantially all 
the heat to be absorbed in the vinyl resulting in short dwell times. 
Moreover, the surface coating of the epoxy prevents absorbing of dioctyl 
phthalate and secondary plasticizers from the vinyl. Shorter cooling 
cycles in the mold are possible because of the high thermal conductivity. 
The rigidity and wear resistance of the molds are excellent giving high 
surface definition and extremely good detail in the surface coating 
formed. 
The epoxy mold of this invention essentially has all the characteristics of 
a metal mold without the high cost of producing a metal mold. 
In an alternate form of the method of this invention, in some cases even 
better surface definition can be obtained in flow molding by removing 
gases from the area between the sheet to be molded and the mold during the 
molding operation. This can be easily carried out by evacuating the area 
between the platens after first forming a gas seal therebetween. Removal 
of air along with water vapor from the molding area enables more uniform 
control by the molding operation, removes ionizable gases and water vapor 
which could cause problems and prevents gas pocketing to allow free flow 
of material without resistance into deep cavities in the mold surface. 
FIG. 8 illustrates the use of vacuum and removal of gases during flow 
molding. In this figure, the mold 30 previously described is positioned 
between platens 36 and 37 as previously described with a vinyl sheet 38 
interposed. The molding arrangement includes an encircling resilient 
gasket 50 attached to preferably the upper platen 36 and a solenoid flip 
valve 51 connecting a vacuum line 52 to the mold area. High frequency 
transmission lines 53 and 54 are attached to the platens as known in the 
art and a conventional hydraulic press 55 is used. Using the conditions of 
the above-noted example, the vinyl sheet 38 is positioned in the mold area 
with the mold attached to the lower platen 37. The platens are then 
partially closed to form a hermetic seal about the mold and vinyl sheet 
due to the contact of the encircling gasket 50 with both platens. The 
vacuum line 52 is then opened to create a vacuum in the molding area. 
Power is applied and molding carried out using conventional procedure 
after which the mold is allowed to cool. The solenoid flip valve is then 
opened to allow air to reenter the molding area whereby the chamber can be 
easily opened. 
It should be understood that the creation of a vacuum which removes gases 
including water vapor from the molding area is useful in high frequency 
flow molding even where the electrically conductive molds of this 
invention are not used. Thus, the vacuum step has advantages even where 
metal or other molds in place of mold 30 are used. While it is preferred 
to obtain a high vacuum in the molding area during molding, any vacuum 
provides some advantages. Preferably a vacuum of 29" of mercury is used. 
The vacuum can be applied only to the area between the sheet to be molded 
and the surface textured area of the mold if desired rather than to the 
entire mold area. However, application of the vacuum to the entire mold 
area between the platens and surrounding the sheet is preferred. 
While specific examples of this invention have been shown and described, it 
should be understood that many variations are possible. The plastic 
thermosetting mold need not be formed of an epoxy material and in some 
cases other thermosetting materials which have heat distortion rates above 
250.degree. F can be used. Such materials include other epoxys, 
polyesters, phenolics, silicones and combinations of these and the like. 
The specific fillers used to provide electrical conductivity can vary with 
silver, gold or platinum particles being preferred because the oxides 
thereof are highly electrically conductive while other metal oxides are 
non-conductive. The fillers can be in conventional finely divided particle 
forms such as powder, flakes, spheres or irregular shapes. In some cases, 
the particles can be formed with cores of non-conductive or conductive 
materials and carry desired outer coatings of metals such as silver, gold 
or platinum in order to reduce cost while maintaining desired properties. 
For example, known silver coated copper particles are suitable for use in 
the molds of this invention. Preferably the electrically conductive 
surface layer of the mold has a thickness of at least 0.001 inch. The 
thickness can vary greatly depending on the frequency of R.F. energy used 
in the molding operation. Preferably the surface layer and mold body are 
of the same general family of plastics such as in the example given; 
however, different plastic materials can be used for each so long as they 
are compatible and evidence the desired characteristics of this invention. 
In all cases it is preferred that the highly conductive surface layer of 
the mold have a volume resistivity no higher than 1 .times. 10.sup.-1 
ohms-cm at 20.degree. C and more preferably be in the range of from 1 
.times. 10.sup.-3 to 1 .times. 10.sup.-5 ohms-cm at 20.degree. C. 
Preferably the conductive layer has a thickness of from 0.003 to 0.1 inch. 
The conductivity should be sufficient to avoid RF energy absorption and 
heat buildup in the mold. In some cases, the entire mold can be 
electrically conductive although surface layers as described are 
sufficient to achieve the results desired. Preferably, the molds have 
hardness values of at least 45 Shore D with minimum coating thicknesses of 
at least 25 microns. 
In some cases, the entire mold can be electrically conductive although this 
does not significantly increase the advantageous properties of the mold. 
When the entire mold is electrically conductive, it is formed integrally 
of a thermosetting plastic material as described above. Thus epoxys, 
polyesters, phenolics, silicones and the like used for either the coating 
layer or base of the mold such as 30 is used for the entire mold body and 
has uniformly incorporated therein the conductive fillers of this 
invention as previously described. For example, a uniformly electrically 
conductive mold throughout can be formed using a silicone rubber positive 
mold such as 21, by pouring into the mold 21 a mixture comprising 100 
parts of Arcon epoxy T4046A and three parts by weight Arcon T4046B having 
uniformly incorporated therein 76% by weight of silver flakes of a 
particle size ranging from 2 to 25 microns. The molding material is mixed 
together and molded as previously described in the above example and a 
fully uniformly electrically conductive mold identical to mold 30 is 
formed except that the entire mold is electrically conductive. In this 
case, the entire mold has the uniform electrically conductive properties 
of the surface layer of mold 30. 
While sheet molding has been described for use on shoe uppers, sheet 
molding of various types can be carried out with high frequency energy for 
various uses including producing of surface effects for handbags, shoes, 
clothing, automotive dashboards, place mats and other uses.