Slide 17 of 24
Good Cures are important both aesthetically and practically - bubbles are unsightly and produce stress concentrations. These are especially problematic in flexures and other high stress applications. Bubbles tend to form along wall where unfortunately there is the most stress.
- Vacuum Polyurethane Twice: 1st in beaker after mixing (remember to use beaker 3x volume mixed because urethane triples volume while degasing first time); 2nd degas after poured into mold to remove bubbles trapped during pouring. Polyurethane is boiling at -28mmgh, so after material has visibly cleared more degasing is ineffective.
- Pre-Vacuum: vacuum palette before depositing material to remove excess moisture from the substrate.
- Over-Pour - approximately 2mm above desired surface: bubbles appear on top surface of cast even with best degasing method.
- Cure Times: polyurethanes list on slide 11 machinable after 6hours. Significantly harder and more machinable after 24hrs. (donít be too rough or machine too intricately within first few hrs. as ultimate cure times are roughly 7 days).