Question: Why do you have to clean the barrel and feeding system in medical silicone injection molding when changing the material?

Answer: Cleaning the barrel and feeding system is essential in medical silicone injection molding because it significantly reduces the risk of contamination during the molding process. In thermoplastics injection molding, the barrel is simply purged a few times when changing from one material to another. However, this approach doesn’t completely remove all the material in the barrel and feeding system in silicone injection molding; varying amounts of mixed silicone and purging agent (typically part B of the silicone) will remain in the barrel and feeding system due to their tackiness and low viscosity. As a result, the residual can ultimately get mixed in with the desired material during the subsequent molding process. In addition to cleaning the barrel and feeding system, they should be purged through at least once to remove the cleaning agents.

Although cleaning the barrel and feeding system is a time consuming processing, it helps to reduce variation in the molding process. Relying on purging the barrel and feeding system without cleaning could be a costly mistake because the resulting contamination is often not visibly detectable. For example, suppose that two different materials are molded in succession with only purging during the transition; the only difference between the two materials is their durometers (hardness). In this case, the contamination from the residual is often visibly undetectable in the finished parts because the residual can simply blend in with the desired material to form a homogenous phase unless the residual is cured before mixing. Imagine that the contamination is eventually found. All the previous molded parts would become suspect and excessive tests would be needed to insure their quality. In addition, the barrel and feeding system would need to be purged again or cleaned. As a result, the time and resource are not effectively used, and a simple procedure becomes longer and more labor-intensive. Therefore, the cleaning the barrel and feeding system is highly recommended when changing material.