Coordinate Measuring Machine Restraint Fixture Calibration for Thermoplastic Assemblies
Calibrate CMM restraint fixtures by matching datum locators to nominal CAD and regulating clamping forces to eliminate non-rigid part deflection.
Restraint Fixture
Meaning
Workholding apparatus holds a plastic part in a specific shape while it cools or during a measurement cycle. Use of a restraint fixture is a common method for controlling the warpage of large or complex mouldings. This device ensures that the component reaches its final dimensions while being physically constrained.
Warp Control
Internal stresses generated during the injection process can cause parts to twist or bow once they are ejected. When a restraint fixture is applied immediately after the moulding cycle, it prevents the part from deviating from its nominal shape. The cooling period in the fixture allows the polymer chains to stabilize in the desired orientation.
This technique is particularly useful for glass-filled resins that exhibit non-isostatic shrinkage.
Inspection Support
Accurate measurement of flexible parts requires them to be held in their functional state. Because the restraint fixture mimics the assembly conditions, it allows the quality team to verify the part against its intended fit. The locators and clamps must be positioned at the same points as the final mating hardware.
This ensures that the dimensional data is meaningful for the downstream assembly process. High quality fixtures are made from aluminum or heavy duty plastics to provide a stable reference over time. Each unit must be calibrated and checked for wear to maintain the integrity of the inspection.
Cooling Period
Time spent in the fixture is a variable in the production cycle. A well designed restraint fixture improves the yield of the moulding process. Stability is achieved through physical control of the resin’s transition.