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Plastics


DESCRIPTION OF LIQUID RESIN CASTING TECHNOLOGY

Liquid Resin Casting (LRC) is a replication technique for the production of plastic parts from thermoset liquid resins, such as polyurethanes, epoxies and silicones. Liquid Resin Casting is unparalleled in its ability to accommodate complex design, and yet it offers unmatched flexibility for potential future design revisions. Complex geometry that includes undercuts, internal and external threads, compound curves, internal passages, and unusual shapes can be created inexpensively. Liquid Resin Casting is also not constrained by variations in mass and wall thickness. The broad variety of LRC resin systems produces a whole spectrum of polymers. Among these materials are soft pliable rubber-like elastomers with durometer hardness of 10-20 Shore A, and much harder materials similar to polypropylene or polystyrene with durometer hardness of 85-90 Shore D. Polymers can be water-clear, tinted in a variety of transparent tones or colored with conventional plastic dyes or pigments.

Advantages of Liquid Resin Casting technology include:

• The capital cost of the LRC-associated tooling is orders of magnitude less that that of other techniques, such as CM (Compression Molding), IM (Injection Molding) or even RIM (Reactive Injection Molding).

• LRC-associated tooling is not only much less expensive, but can also be manufactured much faster. The first finished parts can often be produced only 2-4 weeks after the drawings are ready.

• LRC molds are often easily modifiable. The manufacture of any product from polymeric resins always requires tooling that is specific for the produced parts. Practically any modification of products manufactured by CM, IM or RIM requires a completely new set of expensive tooling. On the other hand, LRC tooling often can accommodate "on the fly" engineering revisions rapidly and inexpensively. The lower mold fill pressures with LRC materials allow a designer an unparalleled ability to revise or modify an existing tool. Short of complete design revisions, the original tooling in most cases can be quickly salvaged for the next design revision.

• LRC molds can accommodate multiple and interchangeable inserts, which makes it possible to use one mold to produce several config- urations of the same product that differ in the geometry or physical shape of certain elements. Alternate mounting bosses, through-hole panels or alternate shapes are all possible within the same tooling set, if required. Limitations exist, but LRC's design flexibility is unique.

• LRC technology allows completely or partially encapsulate other objects into the body of the product. Foreign objects in a variety of configurations can be inserted into a mold cavity and encapsulated in the final finished form. The strength and isolation ability of cured plastic makes an encapsulated element impervious to external damage or contamination. Instead of relying on secondary bonding or attachment processes, the elements can be overmolded directly into a part, often with very close tolerances due to the low mold fill pres- sures. The low pressure of an LRC process allows easy control of weep or blow-out that causes the migration of encapsulated parts. This is significantly more user-friendly than encapsulations with higher pressure molding techniques, such as Injection Molding or RIM.

• The most common foreign objects routinely cast into LRC parts are threaded inserts and other fasteners, such as threaded nut plates and anchor plates. They are positioned inside a mold cavity and cast in-place within a finished part. Threaded metal screw inserts can also be added as a secondary operation when they are spun into the molded plastic. Metal threaded inserts are recommended over tapped threads when multiple re-assembly is anticipated.

• Other types of commonly imbedded objects include wiring, delicate electronics, fiber optics, structural reinforcement, tubing and other items that benefit from the protection of the plastic cured around them.

• LRC parts come out of production with a near perfect surface finish. Details on all surfaces of the products replicate the designed tex- tures. Cast part finishes can include a variety of machined and textured surfaces, such as a highly polished glass-like finish, light grain, aggressive grooving, graphics and heavily textured surfaces. Finished parts are created with no voids or porosity. LRC technology, unlike Pressure or Vacuum Forming, controls both sides of any product, as all the surfaces are contained.

• LRC includes techniques that allow the manufacture of products with complex internal passages and cavities, such as those found in manifolds and vascular models. Internal forms that are impossible to obtain with processes like RIM are easily created. Design possibilities with LRC processes include reverse draft openings, undercuts, through-hole geometry with increasing diameters larger than the initial opening hole size, through-part holes, internal and external threads, internal voids, irregular passage ways, and manifold chambers.

• LRC can combine thin and very thick walls without compromising the dimensions and finish of the product, whereas Injection Molding causes shrinkage or chill in similar parts.

• LRC plastic parts can be used in conjunction with other manufacturing techniques. For example, it is possible to use an LRC bezel as a sculpted control panel for an unattractive sheet metal housing. This combination of techniques allows a designer to be simultaneously cost-effective and creative in achieving the best final product.

Depending on technical considerations including the complexity of the product, ongoing design needs and the required volume of production, it is possible to economically justify using LRC to fabricate prototypes, unique parts and short runs of 5-500 units. In the majority of cases, LRC of 1,000 or more units is unfeasible, as other techniques allow the manufacture of large batches of products at much lower per unit costs. However, CM, IM or RIM process tooling and molds are extremely expensive, and any production using these techniques requires a significant capital investment and a long mold production time.

LRC RESINS

The main materials used for Liquid Resin Casting include polyurethanes, epoxies and silicones.

Resins are mixed with hardeners and transferred into a mold at an ambient temperature where they solidify (cure) due to a chemical reaction between the components. Often, in order to achieve the optimal properties of the solidified materials, it is necessary to post-cure them for several hours at an elevated (180-200?F) temperature. Usually the thermosets produced by Liquid Resin Casting do not contain any air inclusions, but, when necessary, it is possible to produce solid foamed thermosets.

POLYURETHANES

Polyurethanes range in hardness from a Shore A-10 (hardness of soft rubber) incrementally to the hardest, Shore D-85 (hardness of acrylic-like plastic). They are tough and abrasion resistant materials with a glass transition temperature below the 225?F. Polyurethanes can be compounded to achieve Underwriters Lab flame retardant specification of 94 VO.

There is a wide variety of polyurethane formulations, and the properties of the cured polyurethanes depend on their chemistry. The diversity of raw materials allows creative designers to generate a multitude of casted products, including instrument housings, bezels, keyboards, cable strain relief, electronic packaging, encapsulations, fluid manifolds, etc.

 



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