I reviewed the previous post where the blog reader asked whether electric machines are good for applications with long hold times and I wanted to add another comment…
Although some molders purchase shut-off nozzles to eliminate drooling, they are very helpful as part of an overall molding strategy. In the case of a process with a long hold time, a shut-off nozzle can be very helpful in reducing electricity consumption, improving melt capacity, and limiting strain on the hydraulic components or servo-motors.
Hold Time Strategy
In processes where the hold time is long, the shut-off nozzle can often be incorporated as an early hold time cut-off. Once the shut-off nozzle is activated, the material cannot flow back through the nozzle. In most applications with a long hold time, there is a point where a shut-off nozzle will hold the polymer in the mold cavity, allowing hold time to stop, eliminating as much as 30% of the hold time.
When implementing such a strategy, it is best to first determine the optimal hold time with a gate seal study. Once the optimal gate seal time is determined, begin a similar study, using the hut off nozzle in conjunction with a decrease in hold time. When using a cold runner system, you may need to reduce the sprue and runner thickness to help prevent back flow into the runner system. The overall reduction in hold time saves electricity, reduces long-term wear on the machine components, and allows the screw to start recovering early.