Injecting liquid silicones.

Injecting liquid silicones

There is a growing trend reported in the plastics industry towards the addition of liquid silicone rubber (LSR) injection molding to the normal plastics capability. It seems that the liquid silicone rubber materials can be run both efficiently and effectively on reciprocating screw type injection molding machines normally used for plastics. It is necessary to modify the machinery some, but the cost of this is low and it is relatively easy.

Silicones have found wide application in appliances, medical products as well as a variety of chemical and electrical applications. Its inherent flexibility, heat resistance, electrical insulation and chemical resistance continue to find increasingly wide areas of application.

Liquid silicone materials, RTVs etc., have been around for many years. However, in the 1970s, technology was developed that made it possible to manufacture injection moldable liquid silicone rubbers.

When use of these liquid rubbers in plastics machinery was originally thought of, there was concern that because of the low viscosity, plunger feed systems would be needed along with vacuum on the molds to eliminate air. These ideas were incorrect. Screws normally used with plastics can be used to move the liquid silicone materials. Likewise, with minor modification to the rear seals, the reciprocating screw can easily push the liquid silicone system into the mold.

What is liquid silicone rubber?

LSR is a low viscosity, two-part addition cure silicone. The curing reaction begins as soon as part A and B are mixed although it is quite slow until the temperature approaches 210 [degrees] F. Typically, it would have a pot life of 24 hours at room temperature. Once mixed, if stored at 0 [degrees] F, it can be kept for two weeks or more. At curing temperature, the reaction will take place in seconds. Unlike many conventional cure systems, no gasses are formed as a by-product of the cure.

Typically the mixed material has a viscosity of 200,000 to 300,000 centipoise - like thick honey. In fact, it can be poured.

Modification of plastics injection equipment is reasonably simple. First, most major equipment manufacturers offer conversion kits. Prices on these range from $1,500 to $5,000. Typically, they include the following items:

* revised seals for the back end of the screw to prevent backflow of the silicone in that area;

* modification for water cooling of the barrel;

* positive shut-off valve for the nozzle of the screw;

* water cooled nozzle to keep the silicone cool.

Actual modification time for the machines is around three days. Heaters and thermocouples that are used on the screw and barrel are normally removed and replaced with the water cooling system. Thermocouples thus removed may be used in molds which now must be heated rather than cooled as for plastics.

Liquid silicones are supplied as two part systems, A and B, which are normally mixed in a 1:1 ratio. These are fully compounded and ready for production.

The LSR must be metered and mixed prior to delivery to the molding machine. This can be done with a variety of mixing/metering equipment, all commercially available. If multiple machines are to be used for molding the same material, all machines can be set up to run off one set of mixing/metering equipment.

A pressure pot can be effectively used as a material hopper for the mixed silicone awaiting delivery to the machine. This pot can be effectively filled during the operation without any interruption of the molding operations. Follower plates used inside the pressure pot provide for uniform pressure across the entire elastomer surface and eliminate vortexing as it feeds to the molding machine.

Feed of the mixed silicone is done at the normal feed location for the screw.

What about the equipment?

Virtually any normal plastic screw will work. Probably the most difficult aspect of using plastics equipment is the fact that, with a thermoset material, it is necessary to keep it cool until it reaches the mold, then heat it to cause cross-linking. This is 180 [degrees] away from normal plastics procedures.

Sealing of the injection system on the molding machine is one of the most important aspects of the modification. Being a liquid, when the system is pressurized, it readily moves into any open area. Flow is much greater than with plastics.

Replacement of the seal in the aft end of the screw comes first. Normally, a flange cup seal is installed to provide sealing against both the rotational and reciprocal movement of the screw. In some cases, it may be necessary to lathe turn the aft end of the screw, chrome plate it and regrind it to its original dimension. A surface finish of about 7 RMS is reported as necessary in this area.

Next is the nozzle. A mechanical linkage, positive shut-off nozzle is required. Both pneumatic and hydraulic control shut-offs work well; spring loaded shut-offs are not recommended.

Since the nozzle is often placed directly against a heated mold surface, water cooling of the nozzle is very important. Inadequate cooling of the nozzle can result in pre-cure of the silicone which will block the nozzle or lodge in the runner system causing partial fills of the mold.

Injection pressures required for LSR are dramatically lower than for plastics. Normally, plastics require 2,000 to 3,000 psi injection pressure. LSRs on the other hand can be injected using 200-600 psi.

What about the mold?

The actual movement of rubber from the nozzle to the mold cavity proceeds just about the same as with any other material. Because of the easy flow of the material, however, runners can be much smaller than typical plastic runners - approx. 1/3 the size. Also, cold runner systems can be desirable to reduce material loss in the runner system.

Other than this change, there is no difference required in the design of runner systems, gating, cavities, overflows, etc. No special tricks are required.

One major difference between both plastics and other rubber materials is the heat of the material entering the mold. Typically, plastic materials are injected hot into the mold where they are cooled or "frozen" into the cavity. This results in thermal shrinkage of the product.

Rubber materials are normally kept cool when moving through the runner system. Upon injection into the cavity, however, through design of the sprues and gates, frictional and shear heat is generated that raises the rubber very close to the curing temperature. The cavity then just maintains the heat until the part is cured. Shrinkage occurs after the part is removed from the mold.

With LSR, the material is cool when it enters the cavity. The cavity then heats the material to accomplish the cure. As a result, there is a thermal expansion of the material in the cavity. If there is no allowance for this expansion and areas to bleed off overflow material, problems can occur.

While the possibility of heating the rubber with a secondary heater just as it enters the mold has been looked at, LSR has such a sharp cure rate at molding temperatures that, to date, this has not proved feasible.

Air trapping can also be a problem. This is not because of air in the material itself, but comes from injecting the rubber so fast that air in the cavity does not have a chance to escape. This can be corrected by multiple stage injection, rapid at first, slowing down as the cavity fills, allowing time for air to move out, then completing the fill.

How about molding?

For most parts, cure temperatures of 375 [degrees] to 410 [degrees] F work well. As the part thickness increases, lower temperatures should be used. For best results, cavities and areas that will be stripped of cured silicone after curing should be coated with a permanent or semi-permanent release film rather than relying on sprays or other agents applied by the machine operator. Surface treatments of nickel-TFE or titanium nitride surface coating are reported to have worked well. Another alternative is to add a release agent to the silicone itself. Zinc stearate has been used this way successfully in the past.

Normally, machines are dedicated to running LSR materials. It is just not feasible to switch back and forth from plastic or other rubber to LSR.

Since the mixed material has a pot-life of 24 hours at room temperature, it's possible for machines to be shut down at the end of a day and left overnight with material in them. Start-up the next day should proceed with little problem.

If the machine is to be shut down for a longer period, it may be possible to remove the filled chambers (screw and barrel, mold, etc.) and store them loaded with material in a freezer (at 0 [degrees] F).

Changing from one material to another can be done most effectively by purging with the new material.

When it's time to really clean out the system, a combination of trichloroethane and isopropanol works best. The trichloroethane will attach and "dissolve" any cured pieces of material present while the isopropanol will work well on any uncured materials or films. Normally, the unit would be washed first with trichloroethane, then given a final rinse with isopropanol. With the ever increasing problem with solvents, all solvent washing should be minimized.

Summary

I find it interesting that in the last several years, liquid silicone rubber molding has been picked up more by plastics processors than by the normal rubber manufacturers. In terms of processing, my only thought is that it is closer in processing philosophy to typical plastics than it is to rubber.

One comment that was made was that plastics processors are more innovative in their approaches than rubber processors.

In any case, it appears that this material and the relative ease of producing parts from it make it a very positive candidate for use in a variety of applications. In some cases, parts that are currently made from dense silicones in conventional molding techniques can probably be made cheaper using this technique. It's also possible that other products not currently being made from silicone can be made effectively and cheaply using these techniques.

I would hope that producers who are in the rubber industry can find the necessary means to make this a part of their portfolio rather than allow it to all move over to the plastics processors.