Everyone involved in plastic molding knows that parts shrink as they cool. How much they shrink depends on a number of factors like the plastic make-up, (type and fillers), as well as processing, part geometry, and of course the design and manufacture of the mold used to make the part. All of these variables need to be considered and planned. They will work in harmony to arrive at the dimensions the part designer is looking for.
Once all of the factors have been weighed and planned for, when actually shooting the part you must know when to measure the part. This will determine if shrinkage has been managed. Shrinkage can be managed through the processing parameters, but knowing if they need to be modified is a function of correctly assessing the part. Measure before it has stabilized, you can wind up making bad product that is detected too late. If you wait excessively long, you may have been making bad product while waiting for stabilization.
As part of the initial sampling of the mold, an assessment of the dimensional shrinkage over time should be plotted. This will determine where the knee is, as the part transitions from significant change to minimal change. That way, each run will incorporate the most efficient assessment of dimensional shrinkage to maximize each production order for yield and run time.
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