In the past, we’ve discussed the importance of temperature control when casting with thermoset resins. In this article, we will focus on the post cure process for molds or parts made with thermoset resins.
What is post-curing?
Post curing is the process of exposing a part or mold to elevated temperatures to speed up the curing process and to maximize some of the material’s physical properties. This is usually done after the material has cured at room temperature for at least 12 hours. In general, thermoset materials will achieve full cure at room temperature over a period of 7-10 days. After a full cure is achieved at room temperature, post curing will have no effect on the material’s properties.
Why is post curing necessary?
Post curing will expedite the cross-linking process and properly align the polymer’s molecules. Much like tempering steel, post curing thermosets can increase physical properties (e.g., tensile strength, flexural strength, and heat distortion temperature) above what the material would normally achieve at room temperature.
Post curing is extremely important when an application requires secondary machining.
Fred DeSimone of Hapco says, “Post curing your parts prior to machining is critical to ensure dimensional stability, particularly when trying to maintain tight tolerances. Elevated temperature acts as a catalyst to complete the cross-linking process and stabilizes the cured plastic so that it does not continue to creep over time. Although most thermosets will appear to be cured after several hours at ambient room temperature, the reality is that it can take up to two weeks for the material to fully cure. If secondary machining is completed during this time on non-post cured parts, they can either shrink or grow out of specification while the polymerization process is still occurring.”
How should I post cure my parts?
An oven is best for applying uniform heating; however, we recommend not using the one in your kitchen. Applying too much heat to some materials may result in dangerous fumes being emitted or a material may melt, ruining your oven.Digital, vented lab ovens are ideal for post curing parts and molds; however, this can also be done in an X-Series Molding Chamber.
During elevated temperatures, thin-walled parts may bubble or deform. Keeping them in the mold or using a fixture during post cure is recommended. Make sure that anything you place in an oven can take the heat.
Is there an ideal post cure temperature?
Generally speaking, rigid materials are post cured at 175F for 8-24 hours and flexible materials at 140F for 8-24 hrs. Be aware that post cure temperatures vary for different materials. Please review the Material Handling & Safety Notes supplied with your specific Hapco products, or contact Hapco’s Technical Support for more information.