Questions: If quoted a tolerance of +/- x, what is the piece-to-piece tolerance? i.e. does tolerance change within a batch? I assume that since the mold is the same, piece to piece variation should be negligible, but maybe there is something to do with the silicone handling/curing that may affect different pieces from the same batch?
Answer: Assuming the processing parameters are maintained throughout the batch, the medical silicone parts should all have consistent dimensions. However, if temperature and pressure are permitted to fluctuate (significantly) during the curing cycle, the parts will exhibit different overall dimensions. These deviations will be slight and typically difficult to measure (especially in micro parts).
The main factor in this is shrink, Liquid Silicone Rubber typically shrinks 1-3%, depending upon the material and the processing parameters (particularly operating temperature). If shrink of the medical silicone isn’t properly accounted for, and processing parameters are not properly controlled, you could theoretically see a swing as drastic as an eighth inch over a twelve inch diameter gasket.
To ensure that you are creating liquid silicone parts repeatably, fine tune your processing parameters, and keep them tight, try not to fluctuate on temperature by more than a few degrees Fahrenheit, and try to keep your pressure within a few hundred PSI. Postbaking medical silicone parts is also crucial in assuring that part dimensions repeat. Most silicone distributors will recommend a postbaking cycle for completed silicone parts, this cycle helps to ensure that the molecular matrix of the medical silicone is fully cross-linked. While it will vary based on material, it is typically a 2 – 4 hour period of cooking at around 350°F – 450°F.
If you have any other questions, please email Kevin Franzino directly at kevin@albright1.com.