In the last issue, we took a close look at first-stage injection. This month, we'll focus on second-stage injection, also referred to as pack and hold, hold/holding, and P2. Specific settings for hold vary from machine to machine, but the basic concept is pressure over time. The idea is to pack the part to replicate the steel cavity texture and shape, and to hold pressure on the plastic for a controlled period of time. Hold pressure influences surface finish, cosmetics, and part dimensions. It's critical that the molder understands the specifics of their controller, so keep things simple. In most applications, one pressure is sufficient. Optimize settings to minimize transition time and achieve a stable hold pressure quickly (within 0.1 seconds). Fluctuations, oscillations or extended stabilization times often indicate machine problems that must be addressed.
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