Outside of its design, precisely how your product or part is molded has the greatest bearing on its cost and capabilities. Great ideas can founder taking the wrong course. So it is prudent to consider your options early in design. How do you know when liquid molding ends and injection molding begins?

Thermoset vs Thermoplastic

One of the first—but not always the most important—questions to ask is: How many pieces do I need to make?  High volume generally requires high production speed, which is the primary benefit of injection molding. However, thermoplastic molds are expensive to make and production runs in the thousands or millions are typically required to offset that initial mold investment. Conversely, several tooling techniques can be used for liquid molding; such as, plastic or composite molds or soft rubber molds that can significantly minimize up-front expenses, yet still produce high-quality plastic parts for both prototyping and low volume production. (1-5,000 parts)

The three main families of thermoset resins are polyurethanes, epoxies and silicones. Of these, polyurethanes are by far the most widely used and are available in both elastomeric and rigid formulations. Epoxies are usually the materials of choice for high temperature and corrosion resistance, but their cross-link density also results in a tendency towards brittleness. Silicones should be considered where continuous flexibility over a broad temperature range is required. Their natural self-releasing(non-stick) attributes can be used to a molder’s advantage.

Thermoplastics are available in pellet or sheet form and must be melted and formed utilizing specialized molding equipment. A major benefit to using these materials is that they can be re-melted and re-formed over and over, unlike thermosets which are chemically inert once cured.

Thermoplastic Thermoset

Another major advantage of liquid molding is that the process imparts much more design freedom over conventional injection molding. Since the casting does not involve the use of high heat and pressures to initially melt the material so that it can flow evenly into the mold, the designer is not limited to maintaining uniform part geometry.

Liquid molding lends itself well to producing highly complex parts at lower volumes and may always win in the early stages of production. It can also have advantages over injection molding in the manufacturing of medical and surgical equipment, scientific instrumentation, and electronics just to name a few. When part volumes exceed a few thousand; however, injection molding is usually the best option.

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