New trends in silicone elastomer technology.

Silicones have been interesting materials since Kipping developed a convenient method of producing organosilanes in 1904 (ref. 1). Silicones, or polyorganosiloxanes, possess a variety of material features not available in any other singular polymer family, due to their flexible, inert backbone consisting of alternating silicon and oxygen atoms. Table 1 lists the physical and chemical properties of various polymer systems.

Table 1 - properties of various rubbers

                                                SBR      Butyl
Property               Silicone     PTFE        rubber   rubber

Temp., ([degrees] C)   -40 to 200   250         75       90
rating

Mech. water            1.6          0.8         2.3      1.2
absorption
mg/[cm.sup.2]

Resistance             30,000       50,000      2,000    30,000
M ohm

Tensile, MPa           8.27         13.79       5.52     5.52

Elongation, %          >400         150         350      400

Heat age               Excellent    Excellent   Fails    Fails
5 days
@ 200 [degrees] C

Cold bend              Pass         Pass        Fail     Pass
@ 55 [degrees] C

Ozone resistance       Excellent    Good        Poor     Good

Chemical               Good         Excellent   Fair     Fair
resistance

Processing             Good         Poor        Good     Good

                       Oil based
Property               rubber      PVC         Polyethylene

Temp., ([degrees] C)   75          60 to 105   75
rating

Mech. water            3.9         1.4         0.08
absorption
mg/[cm.sup.2]

Resistance             21,000      2,000       50,000
M ohm

Tensile, MPa           8.27        10.34       10.34

Elongation, %          300         200         400

Heat age               Fails       Fails       Melts
5 days
@ 200 [degrees] C

Cold bend              Fail        Fail        Pass
@ 55 [degrees] C

Ozone resistance       Good        Very good   Good

Chemical               Poor        Very good   Very Good
resistance

Processing             Good        Very good   Very good

The free rotation of the silicon and oxygen atoms results in an extremely high degree of flexibility under harsh environmental conditions. The Si-O bond (at 369.3KJ/mol) is stronger than a typical C-C bond (347.9 KJ/mol), and longer (1.64 Angstroms vs. 1.53 A) (ref. 2). This combination of strength and flexibility make siloxane polymers a natural choice for sealing, dampening, deflecting or wherever an elastomeric material is required.

By varying the substituents on the silicon atom, as table 2 indicates, the properties of the resulting polymer can be influenced significantly.

Table 2 - substituent effects on siloxanes

Substituent group                  Imparted quality

[CH.sub.3] (methyl)                General purpose

[C.sub.6][H.sub.5] (phenyl)        Extreme low and high
                                   temperature resistance

CH = [CH.sub.2] (vinyl)            Network control, high
                                   durometer, strength

[CH.sub.2]-[CH.sub.2]-[CF.sub.3]   Fuel and solvent resistance
(fluoro)

Depending on the properties desired, a siloxane material could be designed by using polymers containing any of the above substituents attached. An example of this would be the use of fluoro-substituted siloxane polymers in applications, which must withstand exposure to solvents and corrosive oils. If extreme low temperature resistance were needed, the siloxane material would have some phenyl substituted siloxane polymers in place of or in addition to polydimethyl or polymethyl vinyl siloxane.

Silicone elastomers

When linear versions of the above polymers are compounded with a reinforcing filler (typically fumed silica), a special type of siloxane known as an elastomer is formed. Silicone elastomers are flexible materials possessing a large amount of internal physical strength when compared to other siloxane products such as pressure sensitive adhesives, resins or other systems. Table 3 illustrates the variety in the product line.

Table 3 - various siloxane products

Siloxane type          Duro.   Tensile (Mpa)

Heat cured rubber       45        4.8-7.4

Resin/glass laminate   n.a.       200-240

One part silicone
sealant                 28           2.93

Siloxane type          Elongation   Density

Heat cured rubber        100-400     1,180

Resin/glass laminate           2       990

One part silicone
sealant                      510     1,067

Siloxane elastomers can be classified into three distinct categories based on processing: pourable, pumpable and millable. The end application will determine the type of elastomer needed by taking into account the material requirements and the processing parameters. Only by coupling the correct processing with the correct material properties will a useful elastomeric system be obtained.

Pourable elastomers are made with low molecular weight siloxane polymers (10,000-100,000 g/mol), low amounts of reinforcing filler [is less than] 15 parts, and additional additives. Since the polymer molecular weights are that of a fluid and the loading of the filler is modest, it is necessary to add crosslinkers and other functional species to increase the physical strength of these materials. These elastomers are cured via either a condensation or addition reaction, depending on the formulation and catalyst system used. The pourable elastomers are usually sold as two part systems in a variety of ratios. In any case, the mixtures of starting components are such that the pre-cured material has a viscosity in the range of 100-1,000,000 [mm.sup.2]/s @ 25 [degrees] C, which is pourable (ref. 3). Pourable siloxane elastomers are typically applied as a mold in place material, or poured into a mold using low pressures. Once mixed and prior to molding, the materials need to be de-aired to remove any entrapped gases from mixing.

Independent of the mechanism, these materials are cured thermally at moderate temperatures. RTVs, or room temperature-vulcanized rubbers, undergo curing at room temperature.

Millable elastomers are composed of very high molecular weight organosiloxanes and large amounts of reinforcing filler (300,000-800,000 g/mol, and [is greater than] 20 parts of filler, respectively). The polymers used in this type of elastomer are linear in nature, and the crosslinking is typically accomplished through functional organic groups attached to the silicon atoms on the polymer backbone. Catalysts in these products are either peroxides or transition metals, depending on the formulation and end use of the product. The compounds are processed via milling the uncatalyzed rubber with the catalyst of choice, followed by subsequent compression molding or injection molding. Due to the high green strength of the gum-based system, extrusion is also a possible manufacturing process.

An alternative to millable and pourable systems is the pumpable siloxanes, commonly called liquid silicone rubbers. Liquid injection molding silicone elastomers were developed at General Electric Silicones in 1975 to fill the need for an easily processable alternative to millable, high consistency rubber (ref. 4). Liquid injection molded silicone elastomers are made from moderate viscosity siloxane polymers compounded with reinforcing filler, crosslinker and a curing catalyst. The curing system of choice in a liquid injection molding process is based on platinum catalyzed hydrosilylation (ref. 5). This type of catalysis has several inherent benefits including very quick cycle times, thick and thin section cure and no reaction by-products.

High productivity is the hallmark of a liquid injection molding system, and short cycle times are critical to a productive system. Typical cycle times for a liquid injection molded elastomer system are on the order of tens of seconds, as opposed to minutes for many other elastomer systems. The cure rate is determined by recording the torque as a function of time on a modulating disk rheometer. Typical torque/time curves are taken at 120 [degrees] C and 177 [degrees] C, and are shown in figure 1.

[Figure 1 ILLUSTRATION OMITTED]

The T02 is time to 2% cure and this represents the injection window of the material. In analogous fashion, the T90 is the time to 90% of the total curing of the material, and this represents complete cure of the sample. The max torque is a measure of the hardness of the material, and the peak rate tells how fast the curing is completed, once the process begins. The addition cure reaction is utilized in a liquid injection molding system due to the ability of the elastomer to simultaneously undergo both thick and thin section curing. Whereas a condensation process must remove the by-product to drive the reaction to completion, there is no small molecule evolution in an addition cure reaction. Therefore, as long as sufficient thermal energy is transferred to the bulk of the material, it will cure as quickly as the surface. This enables consistent curing in a part with unequal wall thickness.

The addition reaction is similar to metal catalyzed condensation in that the catalyst species stays present in the material after the curing has taken place. This differs with a radical or peroxide catalyzed system where the catalyst is destroyed during the molding process. In the condensation process, however, the remaining catalyst can take [H.sub.2]O from the atmosphere and re-equilibrate the cured polymer network. This process is known as reversion and can affect the functional life of the elastomer. The platinum remaining in an addition cure matrix does not catalyze the reversion, because there are no unsaturated groups with which to associate, and therefore no active site can be generated (ref. 6). Figure 2 illustrates platinum-vinyl interaction and the subsequent reaction to form an ethane linkage.

[Figure 2 ILLUSTRATION OMITTED]

The generation of the active site via a platinum/unsaturated carbon interaction is critical to the mechanism of hydrosilylation (ref. 6).

When compared to a millable silicone rubber, a liquid injection molded silicone elastomer has some inherent benefits which lead to increased productivity and lower the per part cost. The benefits of a liquid injection molded silicone elastomer system are illustrated in table 4.

Table 4 - liquid injection molded vs. heat cured silicone elastomers

                           Liquid injection molded
Heat cured silicones       silicones

One component              Two component
Many processing steps      One processing step
  * milling                No secondary operations
  * preforming               * no post-baking
  * molding                  * no deflashing
  * trimming                 * no catalyst quenching
High molding pressures     Molding pressures
  (34.47-137.89 Mpa)         (10.34-34.47 Mpa)
Long cycle times (>120s)   Short cycle times (10-40s)
Labor intensive process    Automated process
                             * consistent pumping
                               - consistent cure
                               - one to one A/B mix
                             * automatic mixing (static
                               mixer)
                             * computer-controlled cycle
                             * automatic de-molding

Depending on the part to be molded, the number of parts and the capital required, a liquid injection molded silicone elastomer system may be economically favorable. It is recommended that a detailed cost analysis be performed on the proposed process first using a liquid type system, then comparing it to a millable rubber system. Table 5 outlines a method for determining the true cost of a part. Although it is not as detailed as an actual analysis, it does illustrate the necessity of considering all of the costs in a processing system, and not just that of the material.

Table 5 - cost analysis

Liquid injection molded silicones cost analysis

                                 Liquid
                                injection
Part 1                           molded       HCE

Cycle time(s)                         30        330
Cycles/day                         2,640        238
Yield (%)                             98         95
Number of cavities                    24         40

Materials

Price/pound ($)                     4.85       2.50
Mass/part(g)                          10         10
Total material cost ($)          108,975     57,987

Labor

Cost/hour($)                          20         20
Hours/day                             22         22
Total labor cost                   7,086     48,733

Secondary operations

Inspection cost ($)                5,000     20,000
Deflashing cost ($)                    0     30,000
Total secondary ops. ($)           5,000     50,000

Part 2

Total material cost ($)          108,975     57,987
Total labor cost ($)               7,086     48,733
Total secondary ops. cost ($)      5,000     50,000

Total production costs ($)       121,061    156,720

Total equipment usage (days)        16.1      110.8

Number of machines needed
for job completion in 16.1
days                                   1          7

Liquid injection molded silicones vs. HCE cost analysis 1,000,000 parts

Experimental

The liquid injection molded silicone elastomer samples were press cured at 177 [degrees] C for 15 minutes and cooled to room temperature prior to testing the tensile, elongation, tear, durometer and modulus. Tensile strength, die-B tear strength, % elongation and 100% modulus were tested on a Monsanto Tensometer 10, at a rate of 50.8 cm/minute until failure. The durometer was measured on an Exacta Hardness Tester by NewAge using a Shore A stylus on press cured sheets. The application rate is a measure of the flowability of the material and was determined by extruding the material through a 3.17 mm orifice at 0.62 MPa and recording the amount in grams per minute. The cure rate of the materials was measured using a Monsanto MDR 2000E at 177 [degrees] C. The compression set was measured by compressing 1.27 mm plugs of material, which were cured for 30 minutes at 177 [degrees] C, to 75% of their original height in an air circulating oven at 177 [degrees] C for 22 hours. The samples were allowed to cool at ambient conditions prior to recording the final set. Adhesion tests were performed on lap shear specimens of the desired substrate using a 1.27 cm x 1.27 cm overlap with a 3.17 mm thick section of liquid injection molded silicone elastomer as an adhesive. The samples were cured in an air circulating oven for 30 minutes at 150 [degrees] C, and allowed to cool to room temperature prior to testing to failure on the tensometer.

Results and discussions

Standard LIM materials

Liquid injection molded silicone elastomers were developed as a processing alternative to millable and pourable siloxane elastomers. Since the time liquid injection molded silicone elastomers were invented in 1975, the materials have become stronger, easier to produce, and have been developed to achieve unique properties dependent on the target application. It is possible to trace the industrial acceptance in liquid injection molded silicone elastomers by looking at the material improvements made over the past 24 years. Liquid injection molded silicone was originally used for small parts under moderate conditions. Examples of such include o-rings, small grommets, small gaskets, etc. The first liquid injection molded silicone elastomers were opaque, addition cure materials with high tear strength when compared to standard RTV (pourable) systems. They had inherent solvent and abrasion resistance and were cured via an addition cure process in less than two minutes at 177 [degrees] C.

The performance characteristics of the initial liquid injection molded silicone material were sufficient enough to gain acceptance after their introduction, and the material is still in use today. There were, however, some drawbacks with this material. It was not clear, had a rather high specific gravity, and the cure rate of the material was almost two minutes. This cure cycle is too long to take full advantage of the productivity, which a liquid injection molding system has to offer.

The next improvement in liquid silicone elastomer chemistry was the introduction of a clear, fast curing product line. The material had the physical strength of the original liquid injection molded elastomer series, but was optically clear, had a low specific gravity, and cured in [is less than] 30 seconds at 177 [degrees] C.

This series has found uses in applications ranging from medical to electronics to consumer goods, and represents the liquid injection molded silicone elastomers with the highest physical strengths, the fastest cure rates and the lowest specific gravity. There are some applications which this line does not fill, including areas where low compression set is needed, where self-bonding is required or where the end use requires the siloxane to have a controlled force deflection.

New developments in LIM material performance

Controlled force deflection, primerless adhesion and low compression set as molded are three key technology features which the new generation of liquid injection molded silicone elastomers must have. Advances have been made in all of the above areas, and the resulting new materials can be used in applications where previous liquid silicone rubbers performed insufficiently.

Controlled force deflection

The need for an elastomer that exhibits controlled deflection has been identified. This property is useful in medical systems where the elastomer is used as a spring in a pumping system, or in the electronics industry where a keypad requires a controlled force to work effectively. A series of liquid injection molded silicone elastomers having a controlled force deflection has been developed offering a wide range of physical properties and processing alternatives.

All of these materials have an extremely high application rate, which means that they can fill complex geometries, such as those used for pumps or keypads, and cure in a very short cycle time. The material selection will depend on the degree of physical strength required, and the longevity of the part. Figure 3 illustrates the force deflection characteristics, and the life cycle of two of these materials.

[Figure 3 ILLUSTRATION OMITTED]

The controlled force deflection series of materials offers various physical strengths, application rates and degrees of longevity, thereby offering the design engineer a large degree of flexibility in platform materials, while maintaining a consistent deflection force.

Self-bonding

As a general rule, and partially by design, liquid injection molded silicone elastomers typically do not bond to substrates without priming and/or post-baking. LIM silicone elastomers are addition cure materials, and therefore do not have a large number of polar species at the surface of the material with which to interact with other materials. This type of non-functional surface is excellent for release properties, hence the reason why silicones are often used as release liners.

One reason siloxanes are so difficult to adhere to is that the surface energy of the cured siloxane is very low ~18 mN/m. The low surface energy prohibits other materials from wetting the surface of the siloxane. Wetting the surface means that the material must spread out in such a way that the two surface areas become contacted in an intimate fashion. A material with a lower surface energy will wet a material with a higher surface energy. This is because the intramolecular interactions of the lower surface energy material are low enough to be overcome by the higher energy, substrate surface.

To determine the wettability of a material, contact angle measurements are typically used. A drop of a material is placed on a surface of known energy, and the angle between a tangent to the drop and the surface is measured.

As figure 4 shows, if the sample does not wet at all, the angle is 180 [degrees]; if the sample completely wets, the angle is 0 [degrees]. Various degrees of non-wetting and wetting occur between the two extremes. Silicones usually have very high contact angles, corresponding to very low surface energies, and therefore are difficult to wet. A drop of water on a siloxane wax is a visual example of a non-wetting interaction.

[Figure 4 ILLUSTRATION OMITTED]

Recently, there have been developments that have enabled the formulation of siloxanes that bond readily to engineering thermoplastics, glass and metals. The ability of self-bonding or primerless adhesion, enables a design engineer to take full advantage of the productivity of a liquid injection molded elastomer system by eliminating the priming step of the process. Additionally, a self-bonding elastomer enables certain design flexibility in the part itself, as the need to mechanically lock the elastomer in place no longer exists. The first generation self-bonding liquid injection molded silicone elastomers are translucent, have good physical properties and mold in less than 30 seconds at 177 [degrees] C.

In addition to the fast cure and impressive physical profiles, the material has a low specific gravity, and a high application rate. The adhesive properties of this material were tested on many engineering thermoplastics and metals. The results of this test are given in table 6.

Table 6 - results of adhesion tests on various substrates with self-bonding liquid injection molded elastomer

Substrate                    Adhesion

ABS                          Good
Aluminum                     Excellent
PBT                          Excellent
Polyamide (unfilled)         Good to excellent
Polycarbonate (untreated)    Poor
Polycarbonate (UV treated)   Excellent
Polyphthalamide              Excellent
Polypropylene                Poor
PPO/nylon                    Excellent
PPO/PS                       Good
PVC                          Excellent
Steel                        Excellent

The values given in the table represent general materials categories, are qualitative, and do not represent absolutes. This is because the surface of the substrates can be affected by processing and contamination. Adhesion is purely a surface phenomenon, and therefore adhesion can be compromised by contamination, and impurities. Table 7 lists the results of a quantitative experiment on a single lot of 30% glass filled PBT.

Table 7 - quantitative adhesion test data with self-bonding 8040

Substrate   Treatment   Siloxane type   Grade

Aluminum    None        Non-bonding     LIM 6050
Aluminum    Primer      Non-bonding     LIM 6050
Aluminum    None        Bonding         LIM 8040
PBT(*)      None        Non-bonding     LIM 6050
PBT(*)      None        Bonding         LIM 8040

Substrate   Adhesion force   Failure mode

Aluminum    <0.69 MPa        100% adhesive
Aluminum    >1.72 MPa        100% cohesive
Aluminum     2.42 MPa        100% cohesive
PBT(*)      <0.69 MPa        100% adhesive
PBT(*)       2.38 MPa        100% cohesive

(*) 30% glass filled

The adhesive bond obtained through the use of this material is significant immediately out of the mold, and increases with time as the part stands at room temperature and pressure. To prevent the buildup of siloxane on the mold surface, the authors recommend that the mold undergo a semi-permanent Ni/PTFE coating prior to use. This enables easy removal of the part from the mold, and reduces the effort needed to perform routine cleaning on the mold.

Low compression set

In a typical addition cured siloxane elastomer, less than 100% of the functional groups are reacted. This is partially by design, as well as being a result of the vinyl and hydride being locked in the polymer matrix in positions which are not in close proximity to one another. This is sufficient for the material to perform as designed at standard temperatures and pressures. However, if the material is exposed to extreme pressure and temperature for extended periods of time, the unreacted functional groups present within the cured siloxane network can react such that the part takes on a new set of dimensions. This phenomenon is called compression set, and the susceptibility of liquid injection molded silicone elastomers has limited their use in applications in high temperature, high stress environments.

There are methods of reducing the compression set properties of a liquid silicone elastomer such that it is suitable for use in gaskets and seals for under the hood automotive applications, and environments where low compression set is needed. The first of these methods is a post bake to drive the possible reactions to completion. In this technique, the part is cooked in an oven for several hours at temperatures near 200 [degrees] C to drive the reaction of any remaining functional groups to completion. By post-baking one type of LIM, the compression set is reduced from a value of 45% to [is less than] 20%. An alternative method is to fill the liquid silicone rubber with precipitated silica filler. This results in a system with good, as molded, compression set values typically in the teens.

There are drawbacks to each method, which must be emphasized. In post-baking a part, the entire cycle time of subsystem manufacturing is lengthened by the amount of the post-bake. This severely detracts from the profitability of a liquid silicone system. When compared to a heat cured elastomer, liquid silicones are profitable when you can have fast molding cycles. If you are post-baking a part for four hours, the actual molding cycle is not the rate limiting step, therefore reducing the cycle time does not save a large percentage of the total production time. As far as the precipitated system is concerned, the high filler loading results in a low tear system, and a very high specific gravity.

A new method of achieving a low compression set in liquid injection molded silicone elastomer systems has been recently developed in which a standard liquid injection molded silicone elastomer system is combined with an additive package to yield a low compression set product. The resulting liquid injection molded silicone elastomer has excellent physical properties, a fast cure cycle, low specific gravity and an as molded compression set of [is less than] 20%.

This approach of creating a low compression set liquid injection molded silicone elastomer system has several advantages when compared to the aforementioned precipitated silica method. The specific gravity of the low CS liquid injection molded silicone elastomer is lower than the precipitated silica version. This results in a lower price per part, as parts are filled by volume (L) and the material is sold by weight (g). Figure 5 compares the specific gravity of various liquid elastomers.

[Figure 5 ILLUSTRATION OMITTED]

The tear strength of the material also suffers under the precipitated system, as figure 6 indicates. Even where tear strength is not a critical component of the part design, it is useful in part removal, and high tear strength will result in fewer parts which need to be scrapped due to tears during processing.

[Figure 6 ILLUSTRATION OMITTED]

Ultimate material benefits are obtained when the low compression set properties are combined with the self-bonding attributes to yield a liquid injection molded elastomer that has both. This material is perfectly suited for under-the-hood automotive applications.

LIM processing

The term liquid injection molded silicone elastomers refers to the method by which the siloxane is processed, via injection molding. When the overall process map of this method of producing siloxanes (thermosets) is compared to that of other injection molded elastomers and thermoplastics, many similarities can be seen.

The LIM material is pumped in a 1:1 ratio into a static mixer and fed into the barrel of the injection molding machine. The material is then fed into the injection nozzle by either a screw or a ram type unit, depending on the style of the machine. The screw is the most common method of transfer due to the similarity to thermoplastic molding, while the ram/plunger method is gaining in popularity due to the high accuracy and repeatability of each shot of material. Until this point, the material is maintained near room temperature to minimize the amount of pre-curing that takes place. This precaution must be taken because once the catalyst is present with the crosslinker (i.e., after the A and B are mixed in the static mixer) the material can cure at elevated temperatures. The material is injected through a chilled nozzle, into a heated mold. The molding takes place for anywhere from 10-40 seconds and then the mold opens and the part can be removed. The mold can be either side or parting line gated, and the runners can either be chilled (cold) or heated (hot). A cold runner system saves material, but the tooling cost is usually higher, while a hot runner mold is a less expensive capital investment, but the entire runner will have to be scrapped because liquid injection molded silicone elastomers are thermosets and cannot be reused. A comparison of the molding conditions between liquid injection molded silicone elastomers and thermoplastics is shown in table 8.

Table 8 - liquid injection molded silicon/ETP process comparison

Typical processing conditions             Silicone (LIM)

Material type                             Thermoset
Typical mold temperatures ([degrees] C)   149-204
Typical material processing               26.7-37.4
Temperatures ([degrees] C)
Typical injection pressures (MPa)         6.89-34.47
Typical cycle (sec.)                      30-60
Cure time (sec.)                          25-55

Typical processing conditions             Thermoplastic material

Material type                             Thermoplastic
Typical mold temperatures ([degrees] C)   26.7-107.2
Typical material processing               204-399
Temperatures ([degrees] C)
Typical injection pressures (MPa)         68.94-137.89
Typical cycle (sec.)                      10-40
Cure time (sec.)                          8-35

Liquid injection molded silicone elastomers are often used to make discrete parts such as baby bottle nipples, floppy gaskets, washers, septa, o-rings, etc. There are some applications, however, which require a more advanced form of molding in which the elastomers are molded over a thermoplastic or metal carrier. This is known as overmolding and eliminates the need to mold a discrete gasket and assemble it with the carrier of choice, thereby generating productivity and a lower overall cost per part.

When overmolding, several considerations must be taken into account, including the plastic used, the cycle time of the process, the injection pressures, the siloxane material used and the molding design. The molding conditions must be conducive to the engineering thermoplastic (ETP) substrate over which you are molding in that Tg, Tm of the material must be compatible with the mold temperature, and the injection pressures must not deform the part during the molding cycle. It is recommended that the parts be preheated prior to overmolding such that the substrate does not act as a heat sink and slow the curing process to an undesirable cycle time.

Two-shot or two component molding

Ultimate efficiency and design flexibility are made possible through the use of two component or two-shot molding, in which an ETP part is molded and a liquid injection molded silicone elastomer is molded over the plastic part in a single process. To achieve the goal of molding a thermoset and a thermoplastic in one process, it was necessary to break the overall procedure into three separate steps:

* The injection molding of the ETP initiates the whole process and is the fastest step, usually taking [is less than] 20 seconds.

* While the material is still hot, the mold opens and the part is transferred to the liquid injection molded silicone elastomer tool in step two.

* Then the liquid injection molded silicone elastomer is molded on top of the ETP to complete the cycle. The mold opens, a completed part is removed, and the entire process starts over.

The key to the entire process is the transfer of the ETP part to the silicone molding section, and there are three methods whereby this can be achieved: Rotary plate, retractable core and rotary platen.

Rotary plate

Perhaps the most versatile method of two component molding utilizes a rotating plate to transfer the ETP to the liquid injection molding section. In a rotating plate design, the thermoplastic is injected into the thermoplastic mold and is cooled in the mold to a certain degree. Once the structural integrity of the part has been sufficiently achieved, the mold opens and the part is removed from both sides of the mold. The part is held by a center plate, which is rotated such that the plastic part is now positioned in between the two heated halves of the liquid injection molded silicone elastomer mold. The mold closes, and the siloxane is molded atop the plastic part, while a new plastic part is being molded in the previous cavity at the same time. Once the elastomer molding cycle is complete, the mold opens again, separates from both sides of the part, the part is removed, the center plate rotates, the mold closes and the whole molding process is repeated. A schematic of this type of design is shown in figure 7.

[IFigure 7 LLUSTRATION OMITTED]

By having a complete and dedicated liquid injection molding side in addition to a complete thermoplastic side of the mold, it is possible to mold the elastomer onto both sides of the part. This ability offers significant design flexibility, however it also poses some technical challenges. The part must demold and remold on both sides in the middle of the molding process, which means that the tolerancing of the thermoplastic mold must be extremely tight for the part to consistently match the tolerancing in the liquid injection molded siloxane mold.

Sliding core mold

Another way of molding liquid silicone elastomers onto thermoplastics is via a sliding core mold. Instead of demolding the part and moving it to a new location, the mold has the ability to form additional cavitation for the liquid silicone by an internal sliding mechanism. The ETP is molded in the first portion of the process, then as it cools, the mold changes shape to create a void for the silicone to fill. The siloxane cavity can be formed via sliding pins or cores, and once this occurs, the liquid injection molded silicone elastomer is molded in place.

Although this eliminates the need to rotate a mold, the mold design becomes very complex, depending on the type and amount of cavities needed. Another drawback is that the two processes, thermoplastic molding and liquid silicone molding, must be carried out in series. This results in two separate molding activities that must take place sequentially during the molding cycle, thereby reducing the overall productivity of the process.

Rotating platen

The third mold design option available in a two component molding system is the rotating platen. In this design, three of the four mold cavities are of thermoplastic molding design, in that they are cooled to the temperature needed to mold the ETP portion of the part. Once the thermoplastic molding cycle is complete, mold opens and the part remains in half of the mold, then the entire half of the mold rotates to align the plastic part with the heated half of the liquid injection molded silicone elastomer. The mold closes for the liquid silicone molding, and at the same time another thermoplastic part is being molded. The liquid silicone mold uses the virgin thermoplastic as a shut off, and the entire thermoplastic cycle is completed while the LIM silicone elastomer is being molded.

The gain in cycle time comes at the cost of mold complexity. To take advantage of this method of two component molding, it is necessary to have the ability of sequential injections and gating. This, as well as the necessity of the entire mold to rotate, results in a more complex mold design. The rotating platen mold does not permit the molding of the liquid silicone elastomer onto both sides of the thermoplastic part, but only on the side which faces the heated silicone mold.

Independent of which type of mold system is utilized, there are other considerations which need to be taken into account when designing a two shot system. The gating type, either cold runner or hot runner, must be decided upon based on the mold complexity versus possible material loss. The process should have the flexibility of sequential gating. The location and size of vents must be identified. Even the geometry of the ETP part is of concern to the liquid injection molding puzzle, as it can be used as a shut-off, and its latent heat can be used to help thermally cure the liquid injection molded silicone rubber.

The proper material must be chosen based on the part design. As a rule, the physical properties remain constant across the entire liquid injection molded siloxane product lines. Typical values are: Tensile strength 6.2-8.96 MPa; elongation 300-700%; die B tear strength 2.62-5.25 J/[cm.sup.2]; and specific gravity 1,100-1,150 Kg/[m.sup.3]. The material should be selected based on the other factors needed, such as clarity, bondability and flowability. Table 9 is useful in determining the correct material to meet the needs of a particular design.

Table 9 - liquid injection molded selection guide

                  4200          6000          6746
Properties        Series        Series        Series

Duro. (Shore A)   30-70         10-70         45
Comp. set (%)
  As molded       <20           40-60         45
  Post baked      n/a           15-40         25-40
Self-bonding      No            No            No
FDA/Bio. Inert    No            Yes           Yes
Clarity           Translucent   Clear to      Translucent
                                translucent
Flowability       Medium        Medium        High

                  8040          9000
Properties        Series        Series

Duro. (Shore A)   40            70
Comp. set (%)
  As molded       67            <25
  Post baked      27            n/a
Self-bonding      Yes           Yes
FDA/Bio. Inert    No            In test
Clarity           Translucent   Translucent
Flowability       Medium        Medium

It is important to decide what type of bonding is necessary between the liquid injection molded silicone elastomer and the ETP substrate. There are three types of bonding available:

* In a static bond, the material is connected to the substrate by forces which are strong enough to hold the elastomer to the substrate for removal from the mold, but are weak enough such that the rubber can be removed without damage to the bulk material.

* The second adhesion option is a mechanical interlock. In this method, the ETP part is designed such that it has undercuts or flow channels which permit the flowing of the uncured liquid silicone elastomer, but prevent the cured material from being removed. This is a form of physical adhesion, and is useful where the part will see stresses that would remove a statically bonded elastomer. The undercuts can pose a technical challenge to the design of the ETP portion of the mold, additionally the bond is susceptible to leakage around the rubber portion at a microscopic level.

* Ultimate bonding occurs through the use of a chemical bond. This is possible through the use of the self-bonding liquid injection molded silicone elastomers. The bond achieved through this method results in [is greater than] 90% cohesive failure within the rubber.

The following data focus on the performance of the self-bonding liquid injection molded silicone elastomers when molded over various plastic substrates using a two shot molding process. The sample geometry was chosen to illustrate three distinct features commonly found in injection molded parts: o-ring gasket; a vertical interlocking seal; and variable nominal wall peal strip.

The parts were molded on a 150 ton two component ETP injection molding machine which was converted to a nETP/liquid silicone two shot machine. The part cycle times varied from several minutes to less than 40 seconds for the entire molding process, depending on the elastomer used. Adhesion data (table 10), are given in qualitative terms, corresponding to the amount of cohesive failure (excellent is [is greater than] 95% cohesive failure).

Table 10 - two-shot molding adhesion test using self-bonding grades

Sel-bonding LIM elastomer/substrate matrix

                            LIM 8040    LIM 90xx

PPO/PA (Noryl GTX)          Excellent   Excellent
PBT (Valox)                 Excellent   Excellent
PPA (Amodel)                Excellent   Excellent
PPO (Noryl)                 Good        Good
PA (Nylon 6 glass-filled)   Good        Good
ABS                         Good        Good
PPS (Supec)                 Marginal    Marginal
PC (Lexan)                  Poor        Poor
PC/ABS (Cycoloy)            Marginal    Marginal
PC (UV treated)             Excellent   Excellent
Aluminum                    Excellent   Excellent
Steel (carbon, stainless)   Excellent   Excellent
Steel (galvanized)          Marginal    Marginal

Conclusions

Siloxanes offer unique properties as elastomers, and therefore are being used in increasing amounts for such applications. Liquid injection molded silicone materials have excellent physical and chemical properties which make them suitable alternatives to standard millable and pourable siloxane elastomers. New advances in liquid injection molded siloxane materials, including self-bonding, low compression set and controlled force deflection, address some previous issues that have limited the use of these systems in certain applications. The technique of two-component molding is particularly suited to a liquid silicone system, and advances in mold design have enabled a two-shot ETP/liquid injection molded elastomer system to be developed. Such a system offers extreme productivity, and when combined with the material advances, paints a bright future for liquid injection molded silicones.

References

(1.) F.S. Kipping, Proc. Chem. Soc. 20, 15 (1904).

(2.) J. March. "Advanced organic chemistry" Wiley-Interscience (1985).

(3.) B.B. Hardman and A. Torkelson. "Encyclopedia of chemical technology" Wiley & Sons. pp. 922-962 (1982).

(4.) U.S. Patent #3884866 Jeram et al.

(5.) J.L. Speier, Webster J.A. and Barnes G.H., J. Am. Chem. Soc. 79, 974 (1957).

(6.) B. Marciniec. "Comprehensive handbook of hydrosilylation" Pergamon Press (1992).