Design Criteria for Liquid Molding with Thermoset Resins

Pattern in Mold Frame

The liquid molding, or the resin casting process as it is typically referred to, lets designers produce high-quality plastic parts economically in low volumes.  Any shape that can be machined or injection molded can be cast, however, liquid molding becomes most attractive when part geometry poses design restrictions for traditional processes.

Rick Boston Brace

Varying wall thicknesses, internal curved passageways, and zero draft are routinely designed in liquid molded parts.  The use of flexible rubber molds allow internal and external threads, O-ring grooves, or other modest undercuts to be easily cast in place without the need for costly secondary operations.

Mold Design

Combining multiple pieces in the same mold can save time and money.

Combining several parts into one is usually an easy way for a designer to get added value with this process.  Custom colors, textures and graphics can also be designed as cast-in details.  And  Insert-molding items or encapsulating delicate components and electronics can be achieved without damage because of the relatively mild processing conditions associate with liquid molding.  Many thermoset resin formulations will fully cure at ambient room temperature over the course of several days, and if post-curing is required or recommended, these temperatures typically do not exceed 80°C (175°F).

Hapflex Mold and Ultralloy Part

Standard tolerances on part geometry defined by rubber molds (soft tooling), are approximately ±.004-.006 in./in., and are referred to as “as-cast tolerances.”  With soft tooling, one needs to consider the harness of the flexible mold material which can range from putty-like to semi-rigid.  In general, a harder durometer material will allow for tighter tolerance control.  Using metal (hard tooling), rather than rubber to define critical part features will typically yield tolerances of ±.002 – .004 in./in..  A careful assessment of tolerance and dimensional requirements is essential at the onset of a project, and material shrink factors should be taken into consideration prior to building your mold or pattern.  However, bear in mind that many things affect shrinkage, such as part geometry, mass, and material gel times and cure temperatures.