Meaning
Position control for molded components during secondary machining or assembly operations relies on direct alignment with the mold cavity coordinates. Utilizing the original mold features as datum points prevents indexing errors caused by part shrinkage or warping. This method ensures that secondary cuts, drilling, or insert installations align perfectly with the molded features regardless of run-to-run material variation.
Workholder Design
Rigid nests are machined to match the negative profile of the specific cavity that produced the part. When cavity-indexed workholding is used, it uses multi-point clamping or dedicated locating pins that register in pre-molded recesses to restrict the part from shifting. Molders must maintain a master cavity file to machine these nests to ensure they replicate the exact contours of the molding steel.
This approach avoids the compounding tolerances that occur when locating from non-molded or sheared edges.
Shrinkage Compensation
Variation in plastic shrinkage rates during cooling can shift critical part dimensions away from the theoretical nominal values. Because the nest matches the molded shape, it accommodates these uniform volumetric changes without pinching or distorting the part walls. Uncontrolled mold temperature shifts or changes in holding pressure will alter the final dimensions, causing fitment issues in the workholder.
Quality Benefit
Reject rates during secondary milling drop when the machining datum matches the molding datum. The workholder stabilizes flexible polymer walls, preventing chatter and surface defects during tool engagement. Parts that undergo this precise alignment demonstrate superior dimensional consistency in final assemblies.
