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Injection Molding Cooling Time

It's old school to try and use the process to push the plastic part around and compensate for injection mold or plastic part design deficiencies. Scientific Molding defines the process capability giving you an optimum window for a stable and repeatable process. Using processing gimmicks will tend to give you grief as it does not compensate for the natural variations all injection molding has and instead you are adding an un-natural variation making it difficult for future processing. The trend in process development is to find and fix these deficiencies.

The caveat here is that if the production requirements are low then biasing the process in an unbalanced method can satisfy the needs. Still, not a best practice.

Injection molding cooling time is often the forgotten child in a lot of ways. The plastic part stability is a good way to look at the problem. Given the rest of the process is correct and stable then releasing the plastic part prematurely will result in fairly unpredictable dimensional and form changes from the volumetric differential shrinkage. Every short is different in some may. Processing is the attempt to control that natural variation to produce acceptable plastic parts. If you neglect this and "muck" around with once confirmed stable settings, especially if you do not apply scientific molding, you will get trouble.

Engineer your injection molds and process- don't design these. Part of answering your cooling time question is answered in what the cooling part of the process is supposed to do. Thermal management of which cooling is a component is non-trivial. The goal of thermal management is not to just cool the plastic part down enough to allow for ejection but instead consider it to have the objective of managing the thermal load in the most advantageous way to remove heat while keeping residual stresses under control and orientation optimized. The polymer starts to cool the instant it leaves the nozzle (or manifold). The first requirement is to keep flow going then transition to heat removal. You can simulate this fairly well with CFD transient thermal analysis and confirm with thermocouples in the injection mold. Be cautious of steady state methods as these tend to not model the thermal energy accurately. A good analyst can model this during injection mold design and then do confirmation after.

Again, the level of engineering effort needs to be balanced with production costs. Striking the balance between doing the correct amount of injection mold and process engineering without going overboard is the challenge. My advice is always to apply best process engineering practices, in this case adoption of scientific molding methods.

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