Coordinate Measuring Machine Layout Protocols for Flexible Plastic Part Boundaries
Flexible plastic part boundary metrology requires ISO 10579 restrained datum targets and controlled clamping forces to yield repeatable CMM dimensional data.
Free State Deviation
Meaning
Dimensional variation of a flexible part occurs when it is released from its holding fixtures and allowed to relax. This free state deviation arises from internal molding stresses that cause the plastic part to warp or twist. Measuring this effect determines how much the part deforms under its own weight and internal stress.
Warpage Analysis
Predicting how a molded part behaves after it is ejected from the tool is essential for assembly planning. Analyzing the free state deviation helps engineers identify areas of high residual stress that need cooling adjustment. This evaluation guide is used to modify the injection cycle to produce flatter parts.
Fixture Design
Automotive bumpers and trim pieces are rarely flat when sitting unconstrained on a workbench. To measure the free state deviation, the part is scanned first in its restrained state and then in its free state. This comparison reveals the magnitude of the spring-back effect and assists in the design of mating assemblies.
Control Strategy
Molding parameters like melt temperature and holding pressure have a profound impact on the final shape of the part. Reducing free state deviation requires optimization of the cooling line layout to ensure uniform temperature reduction across the entire part. When cooling is uneven, different shrinkage rates across the part generate the bending moments that drive the deviation.
Fine-tuning these process variables minimizes assembly issues in downstream operations.