Liquid silicones - take two.

Liquid silicones - take two

Last month's article on liquid silicone injection molding discussed some basic facts about liquid silicone rubber - LSR. This month's article will review these, talk about one company that has gotten into the business and give a little more information about molding procedures.

Review

As noted last month, liquid silicone rubber is supplied as a two part system, A and B, designed to be mixed on a 1:1 basis. Platinum catalyzed cures are used, eliminating problems of outgassing. Also, all materials currently being manufactured fit into the "high performance" or "high strength" arena.

Although referred to as liquid, they are actually very thick, heavy fluids, at best having the consistency of thick honey. Viscosities are typically 200,000 to 300,000 centipoise.

Curing reactions begin as soon as the two parts are mixed. At room temperature, a "pot life" of around 24 hours can be expected. However, when the material is heated to normal curing temperatures (260 [degrees to 500 [degrees] F complete cures can be achieved in seconds.

Liquid silicone rubber systems are simple systems. Current materials on the market are all fully formulated. No further compounding or "tweaking" is required once the material is received. In this respect, they are more like plastics or castable urethanes than rubber. Materials are fed directly from their original containers into the mixing/molding equipment.

Recent history

Liquid silicone rubber materials were originally commercialized in the mid- to late-1970s. At that time, major producers of the materials established customer service centers where potential customers for the products could get some real-time knowledge of how the materials could be used and processed.

However, there were drawbacks. Most customers were already established with normal gum silicone processes. Using the newer liquid process would require heavy investment in new equipment. The new material was not seen as a product that would add either markets or market share. On top of this, the raw material was significantly more expensive than the traditional gum stocks and the technology was both new and untried. And, as we all know, the rubber business is not one that moves quickly with new and untried technologies.

As a result, what looked to be a promising market potential for the new liquid materials, did not start off with the bang expected.

It was in this environment that Liquid Silicone Molding Laboratories - LSM Labs came into being. The president of that company, Adrien Larrivee, had extensive experience in plastics molding and had spent several years directing activities of the liquid silicone customer service laboratory for one of the major manufacturers. Based on experiences in the customer service laboratory, Larrivee realized the significant business potential of liquid silicones. So, in 1980, he left the supplier and started LSM Labs.

During the first five years of operation, the principal objective of the company was to help new potential users of these products develop the systems and processes necessary for manufacturing. Fabrication during these years was limited to prototyping for other customers. Once complete, production was turned over to an outside molder.

Then, in 1985, they began fabrication of products on a more earnest basis. Rather than just prototype work, production work on finished products became a significant part of the business.

In 1990, a further change was made. Process and product development for outside manufacturers was discontinued. The main emphasis of the company became fabrication of final products. It was no longer prudent to pass off work to what had become competitors.

Part of the reason for the shift was that more information was becoming available on liquid silicone processing. However, the more important reason was that the market for finished parts was beginning to swing up - dramatically. The advantages of products made using this system were finally becoming known.

What advantages?

Anyone who has not had their head buried in the sand for the last five years understands that there has been a major shift in manufacturing across the country. Key terms in today's business world are "consistency," "total-quality," "just-in-time-delivery," "statistical control" and "uniformity-of-product." Liquid silicone fits very well with these concepts. Because of the simplicity of manufacturing, there is much less potential for error and variation.

Typical gum silicones, as well as other rubber products, start out as unreinforced gums. These are formulated and compounded with a variety of ingredients and catalysts to achieve the desired physical and cured properties. Most molding and fabrication methods use techniques that have been used for the last 30-50 years and, while functional, are not necessarily as simple and uncomplicated as could be desired. And, because of the many potential sources of variation, uniformity tends to be lower.

Since liquid silicones are rarely ever exposed to air or the environment outside the receiving container and the pumping systems, the potential for contamination is much lower. As a result, the material runs cleaner and losses from contamination are significantly lower. This is a critical factor in the medical area.

Lot to lot variation in the liquid systems is reported (by users) to be quite low. This is an increasingly important factor in many industries, including automotive, electronics, aircraft, aerospace, medical, etc.

Liquid silicone rubber can also be molded at much lower pressures than conventional gum rubbers. They also flow much more and are able to pick up much greater detail and be molded into configurations that would be either very difficult or impossible using gums. Since mold filling can he accomplished volumetrically and very precisely, combined with the consistency, it is possible in many cases to mold flashless parts, reducing secondary labor and trimming operations. Parts that are produced without flash are reported to have more precise edges and, again, increased dimensional uniformity.

Another aspect of the low viscosity and lower molding pressures required is the ability to produce very thin walled sections. One part that is being manufactured requires that the rubber flows into a wall, 0.020" thick and four inches long. This type of a membrane would be very difficult to produce using conventional gum materials.

Again, because of the low pressures required for molding, very delicate electronics and instruments can be encapsulated with little danger of damage.

Cure times for liquid injection molded parts are far lower than for normal gum products. Cure times typically range from 10 to 90 seconds. This is much faster than anything possible with gums. As a result, productivity off a given piece of equipment can be substantially greater.

Some more technology

Last month, a number of details regarding injection molding were discussed. These included the following:

* Injection pressures normally range from 200 to 700 psi. In some cases, injection pressures as low as 50 psi are feasible.

* Bleeding air out of the mold cavity can be a problem, in some cases requiring multiple stage injection to eliminate air entrapment.

* Runners can be about 1/3 the size of typical plastics runners.

* Semi-permanent release coatings should be used on molds rather than sprays, etc.

Molds manufactured for liquid silicone injection are of necessity made to much closer tolerances than many "standard" rubber or plastics tools. Plastics tend to be very forgiving as far as fit and open space for air to move through. In most cases, plastics tools can have openings as large as 0.001 inch and no flashing will occur.

As we all know, gum rubber will force its way into any opening in a mold. The same is true for liquid silicone. As a result, and to avoid flash, open spaces in tooling must be very small, in fact, less than 0.0002 inch (two ten-thousandths). Otherwise, flashing will occur.

Air entrapment, cause by not having enough time to get all the air out of the mold can be a problem. If flashing and trimming are to be avoided, vents should be kept less than 0.0002 inch in diameter. Real small.

If air is not adequately removed as the mold is filling, compressive heating of the air in the cavity can occur. This can result in a scorched or burnt spot forming on the part. Additional venting or modification of the injection profile is the only way to correct the problem. A good dimension for a vent on a parting line is 0.0002 inch thick and 0.005 inch wide.

Air carried with the polymer is very rare and, according to Larrivee, usually only occurs when containers are shipped from the producer with air trapped in them. Because of the high viscosity and surface tension of the material, the air bubbles will not release or even float to the surface.

Mixing can be done using external mixing equipment which then feeds the screw of the extruder on the molding machine (or extruder). However, by using a reasonably high compression screw (e.g. 3:1 compression), it is possible to achieve enough mixing in the barrel of the extruder to thoroughly blend the two parts.

How about problems?

Several of the potential problems in using the material to fabricate parts have already been mentioned. In addition, there are some others.

Economics are a factor that weighs against liquid silicone for larger parts. On strictly economic terms, the balancing point between cost savings achieved through reduced handling, faster cycling and less secondary operation vs. the higher price of the material currently occurs at a part size of about 4 oz. For parts smaller than this, liquid injection will normally produce a less expensive part. For larger parts, the price will be higher.

There are expections. In some cases, the additional material cost is outweighed by the required quality and consistency of the product or by the manufacturing efficiency obtained using the liquids.

While there is high consistency of product from lot to lot with a given manufacturer, the possibility of dual sourcing of materials is virtually non-existent. Each manufacturer's products are unique. In most cases, one manufacturer's product cannot be easily substituted for another's. This establishes some degree of risk.

Summary

While liquid silicone rubber is not be answer to all problems in silicone manufacturing, it certainly appears to be the primary direction in which new critical parts manufacture will move. Competition between suppliers is increasing and product availability is increasing. It is definitely a growing niche in what, overall, is a shrinking marketplace.

LSM Labs is certainly one of the companies looking forward to capitalizing on this expanding market. And, as the market further develops, I'm sure other rubber manufacturers will join them.