A Technical Guide for Casting Polyurethanes
We often receive questions about how best to store, use and work with casting polyurethanes. We've put together a selection of the most popular questions below with our technical advice and responses.
My material has turned cloudy. What can I do?
Casting polyurethanes can be effected by the cold weather; an indicator of this is material being visually cloudy. The good news is that there is a simple remedy to this, just warm up the material and it should go right back to original clear state.
If your material has experienced chemical freezing it is recommended that to reconstitute you place it into an oven at a modest temperature (38-49°C) for 1 - 4 hours, with the cap being slightly opened. You will then want to re-blend by shaking or stirring with a metal or plastic spatula.
What is double mixing?
Double mixing is exactly as it sounds, and is a useful process when working with any 2 part material. The A&B components are mixed thoroughly in one container and then transferred into a new, clean container where they are mixed again. This process leaves much of the unmixed material that hides in the corners and sides of the cup behind, ensuring a more thorough mixture. Unmixed material can show up in the part as soft spots or unsightly swirls/streaks.
What happens if I need to demould early?
Casting materials needs time to chemically crosslink and to gain their physical properties; if you remove a part from the mould early or cast in cold temperatures it increases the risk of the part breaking due to brittleness or low tear strength. If you have no choice but to demould earlier than recommended, with some materials adding heat can speed up the demould time, providing the mould material can withstand the elevated temperature without deforming.
What is post curing? And how can it improve my finished parts?
Most casting polyurethanes from BJB are designed to cure at room temperature and their physical properties will continue to improve over several days; for most applications this may be sufficient however there are some instances where additional performance out of the same material is needed.
A post cure at elevated temperatures increases the cross-linking, when working with flexible materials this would typically increase tear and tensile strength. For more rigid polyureuthanes the main imporvement seen would be an increase in heat distortion temperature (HDT), however there is a drawback as you could see a reduction in the cold flow over time.
Post curing isn't a necessity, however it just works to ensure that you will get the optimum performance properties stated by the Technical Data Sheet (TDS).