Originally published April 1996
James E. Snyder
Polymer Design Corp., Rockland, MA
Pouring liquid rubber or plastic into molds, then allowing it to cure to solid form, describes the fundamentals of liquid resin casting. A technically refined version of this decades-old process is a reliable and cost-effective choice for manufacturers of sophisticated medical devices. With new developments in materials and process controls, liquid resin casting lends itself to many demanding medical ap- plications, such as cardiac pacemaker encapsulations, handheld electro-optical surgical devices, and key components of medical imagers and scanners.
Medical product developers use liquid resin casting in two principal ways: for prototyping prior to committing to high-volume production, and for ongoing low-volume production of 252000 units per year. The principal advantages of liquid resin casting--comparatively inexpensive tooling, short lead times for tooling and parts, mild processing conditions, and design flexibility--enable the manufacture of highly complex parts with specialized performance characteristics difficult for other technologies to duplicate. For example, unlike molding or machining, liquid resin casting is associated with mild processing conditions that allow delicate components, such as fiber optics or electronics, to be encapsulated directly into the final or near-net shape required.
New tooling starts with a model. A castable material such as silicone, epoxy, or polyurethane is poured over the model in one or more steps. The material then cures, creating a mold. (Molds may also be machined directly out of aluminum or another suitable material.) Tooling lead times are generally three to six weeks. Once the mold is finished, parts are produced by pouring a resin into it and allowing the material to cure.
For prototyping requirements, plastic patterns can often reduce initial costs and lead time when compared to metal patterns. However, manufacturers who use plastic patterns must be prepared to accept limited options for surface finish and less tolerance control. In addition, if significant design modifications are necessary, a new generation of tooling will probably be required. Production programs are better accomplished with metal patterns, which enable the manufacturer to achieve the best possible results and avoid the limitations of plastic patterns.
Equipment. Special equipment for liquid resin casting includes mixing and dispensing equipment for handling resins, degassing equipment for removing entrapped air within the resin, and ovens for curing materials. The specific equipment needed depends on the kinds of materials being processed and whether they are for a prototype or production activity. The more demanding the application, the more sophisticated the equipment required.
Materials. Thermoset resins most commonly used in liquid resin casting--epoxies, polyurethanes, and silicones--must be in liquid form at approximately room temperature for successful processing. Formulations that will satisfy virtually any application can be developed from these three basic materials types. An expanded selection of formulations also is generally available from manufacturers with a production, rather than a prototype, focus.
In general, each type of resin has its own distinct advantages. Epoxies are ideal for high temperatures (up to 450°F), or for highly corrosive applications. For example, formulations of epoxy are used when steam sterilization is essential. Polyurethanes are an excellent general-purpose material for both soft-rubber and hard-plastic applications where exceptional toughness and wear resistance are important. They are used routinely in devices where blood and patient contact is expected. Silicones are best for product applications that require rubber that is soft or of medium hardness over a broad temperature range.
The various casting resins can be compounded with fillers or reinforcements to heighten specific qualities, such as impact strength, chemical resistance, or thermal conductivity. With rare exceptions, thermoplastics are available only in injection molding or extrusion grades and cannot be processed by liquid resin casting. Other materials that cannot be processed include ABS, polycarbonate, polyethylene, and acetyl. Their physical characteristics, however, can generally be matched with elevated cure formulations of castable resins.
Processing Parameters. Little or no pressure occurs within the liquid resin casting process, but humidity should be controlled during material handling. Polyurethanes and selected curing agents for epoxies are sensitive to moisture and will react to the presence of water in the mold.
Release agents can be used on mold surfaces to facilitate part removal and are available in silicone and water-soluble formulations. If painting of parts is anticipated, water-based release agents are recommended.
Curing can occur at room temperature or at elevated temperatures, and can take anywhere from a few minutes to several days. Fast-curing compounds are generally highly reactive and generate a large amount of heat. This can cause parts to distort or discolor, and may impart residual stress if not controlled. Longer cures, which are generally performed at elevated temperatures using precisely controlled ovens, require a 618-hour cure schedule. This ensures superior physical characteristics for the materials processed and more predictable results.
Design Considerations. Complex parts with highly contoured surfaces and critical finish requirements are well suited to liquid resin casting. Manufacturers can achieve tolerance control of ±0.004 in./in. with silicone molds and ±0.002 in./in. using metal molds or cores to define part features. Still greater tolerance control can be achieved through the use of machining as a secondary operation.
Aesthetic effects such as cast-in color and surface finish are readily accommodated. Color control during casting must be carefully managed owing to the nature of batch processing. A slightly higher part-to-part variation should be expected compared to painting. Except for silicone rubber, all materials used for casting can be painted, even low-durometer polyurethanes.
There are no manufacturing restrictions on part size, which can range from a few grams to hundreds of kilograms in weight. There is no need to maintain uniform wall thickness. When using flexible silicone molds, modest undercuts can be cast in place without splitting the mold or creating side action.
Finally, mild processing conditions are especially well suited to component encapsulation, a technique that typifies the flexibility developers of precision medical devices can expect from liquid resin casting.