Although this topic is pertinent year-round, it is especially relevant at this time when most of the country is being plagued with unusually hot, humid weather that can wreak havoc on the liquid molding process. Thermoset polyurethane resins, which are by far the most widely used materials for this process, are particularly sensitive to moisture.
Isocyanate compounds, used as the reactive ingredients in these formulations, will actually begin to cure prematurely and “foam-up” if they become contaminated. This can result in the cured material being laden with tiny air bubbles throughout which, in most cases, will leave you with a cosmetically unacceptable part. A work environment temperature of between 20 – 25°C (68-77°F) with relative humidity between 45-50% is ideal.
Other precautionary steps to safeguard against moisture contamination include; purging opened material containers with dry nitrogen gas or warm air (hairdryer) prior to resealing; using metal or plastic containers and stirrers for mixing and pouring; and pre-heating masters, models and molds before processing to make certain they are moisture-free. Also, material containers should be stored off of cement floors and on shelving if possible.
Heat alone does not adversely affect thermoset materials. As a matter of fact, in many cases it can be used to a molder’s advantage. Be aware that it has two primary effects; it will thin the material’s viscosity and also speed up the reaction times (gelation & cure). A general rule-of-thumb is that for every 10°C you increase the temperature of the reactants, the reaction-rate will double. In other words, the gel time will be cut in half. Heat can be used effectively to allow a viscous material to flow more easily and also to increase throughput, however, if not properly controlled it can also cause problems, such as, sink marks and excessive shrinkage on your parts. Conversely, cold temperatures have the opposite effect; material viscosity thickens and gel/cure times will be lengthened. Again, this can be used to a molder’s advantage if the proper controls are put in place. It is a common practice in mold-making to incorporate heating or cooling elements within the structure of a tool to control a material’s exothermic reaction while it cures.
The bottom line is that temperature/humidity control is extremely important in order to produce high-quality parts. Ideally, your materials, molds, and any ancillary components used in the process should all be maintained at an even temperature. Learning how to tweak these elements to your advantage is part of the “magic and art” of Liquid Molding.