Hot runners, or hot drops as they are sometimes called, allow molders to minimize both scrap and cycle time in the increasingly competitive injection molding market. They present some unique problems, though, when it comes to cleaning. Unlike molds and even screws, it is difficult to know what exactly is going on inside a hot runner. Many people focus on cleaning the screw and barrel to prevent contamination, but neglect the hot runner because they cannot see what deposits forming within the channels or at the valve gate. Furthermore, there are often dead spots where material does not readily flow. These areas are prone to burning because material is not flushed out with each successive shot, and all resins will degrade when under heat for a long enough duration. These challenges make the use of commercial purging compounds a necessity if you want to keep reject rates low and protect part integrity.
Precautions Since hot runners are closed systems that need to hold tight tolerances, there are several things to watch out for:
- Make sure that any clearances are not tighter than the purging compound grade can flow through – check with supplier for a hot runner-friendly grade.
- Glass filled grades are not typically recommended for hot runner applications.
- Raise the manifold 68°F – 86°F (20-30°C), but you must remain in the SAFE operating temperature ranges of both the previous resin AND the purging compound.
- If the screw and barrel are not cleaned prior to cleaning the hot runner, contamination from the screw could dislodge from the screw and become trapped in the hot runner. Always clean the screw and barrel first.