Casting Process Eliminates Plastics Outgassing

By Susan English

Rockland, MA–A new plastics casting process, developed by Polymer Design Corp., allows designers to produce a polyurethane plastic that`s free of outgassing. Designers of production-related devices for the semiconductor industry are already taking advantage of the Liquid Resin Casting (LRC) process. The LRC process allows suppliers to produce complex, low-volume parts not well-suited for machining or other molding processes.

Most recently, Polymer Design used LRC to produce a material with no contaminants or outgassing properties in the manufacture of a portable carrying device that allows wafers to be stored or ported under cleanroom conditions. Working with Portable Clean Rooms (Wayland, MA), a manufacturer of a nitrogen-purged Class M-1.5 microenvironment, engineers at Polymer Design applied the LRC process to two different kinds of cast polyurethane plastic to cost-effectively produce a carrier that was both lightweight and free of outgassing that could contaminate and destroy the expensive wafers.

The most difficult aspect of the project was the casting of the unit`s clear, tinted cover. Residual solvents from bond lines proved to be a significant source of outgassing destructive to components in the fabricated-prototype stage. The lid was redesigned by Polymer as one piece to eliminate bond lines, leaving no residual bonding solvent to be baked out. According to Portable Clean Rooms president David Elliott, the LRC method enabled his company to make changes as needed while keeping costs down. Polymer Design created a tinted transparent housing out of cast, UV-blocking poly urethane. Also included in the carrying device`s transport system is a small nitrogen tank and diagnostic electronics. “It would have been cost prohibitive for us to constantly change the device using injection molding,” said Elliott. Turnaround time was about eight weeks.

Gas chromatographs and mass spec analysis proved that the material, comparable to the polycarbonate material used in SMIF pods, showed “no detectable hydrocarbons.” Says Elliott: “They provided me with a molded portable cleanroom that passed all the outgassing tests and very specific quantitative results to prove that the product does not, in fact, outgas down to the level of current plastics being used in fabs–in the ppm/ppb range.”

LRC is not unique to Polymer Design. Other companies do use the process, which is the formation of plastic parts by pouring resins in the form of liquids at or near room temperature into various types of molds. Heat is applied and the resins allowed to cure (solidify). The materials, called “thermoset resins” are plastic compounds that “set” or cross-link when heated. (Cross-linking is the formation of chemical bonds between long carbon chains. The additional chemical bonds of the cross-linked thermosetting plastic allow it to absorb more thermal energy before the carbon chain is broken.) Like metal casting, LRC usually starts with a male pattern, often produced from aluminum, which is fastened to a flask. A liquid mold material such as epoxy, urethane or RTV silicone is poured into the pattern to form the mold. Liquid resin, such as polyurethane, is then poured into the mold to produce a part. Because they are able to perform at higher temperatures, thermosetting plastic compounds offer the plastic part designer a material with outstanding chemical and electrical resistance.

Tooling for LRC is comparable in cost to metal-sand casting, but far less expensive than injection molding or metal die casting. The process is especially economical when the part design is very complex and the quantities too small for molding processes (generally from 25-50 pieces up to a maximum of 2,000) and too large for machining.

Unlike metal casting, liquid resin casting captures significantly more detail, particularly when silicone is used as a mold material. Fine detail such as threads, surface finishes and material properties, including textures or high-gloss, can be readily achieved straight from the mold without secondary operations, says the company. Accuracy of detail was crucial to Progressive Technologies Inc. (Tewksbury, MA) in the production of 6-in. and 8-in. pistons used in its Sentry air flow control systems. Extremely tight tolerances and a caustic environment were two major challenges.

According to Progressive Technologies` president David Palmer, Polymer worked on the prototyping in changing materials from metals to much more chemical-resistant plastics to solve corrosion problems in the company`s exhaust systems.n

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The portable wafer carrier with nitrogen-purged Class M-1.5 microenvironment by Portable Clean Rooms (Wayland, MA). Polymer Design (Rockland, MA) used liquid resin casting (LRC) to produce a tinted transparent housing.