Managing Anisotropic Shrinkage and Restraint Fixturing in Polymer Metrology
Polymer metrology requires force-controlled restraint fixturing and thermal conditioning to isolate mold geometry from anisotropic shrinkage and sag.
Non-Rigid GD&T
Meaning
Dimensioning and tolerancing standards for flexible parts describe the geometric requirements of a component while it is constrained in its specified functional state. Non-rigid GD&T is a specialized application of the ASME Y14.5 or ISO 1101 standards that addresses parts that deform under their own weight or during normal handling. It uses restraint notes to specify how a part should be held during inspection to simulate its final assembly condition.
This approach allows designers to define the tolerances that matter for the function of the part rather than its loose shape. The system applies to items such as gaskets, thin sheet metal and long plastic moldings. It stops being applicable if the part is rigid enough to maintain its shape without support.
Constraint Condition
Determining the correct way to measure a flexible part requires a clear definition of the forces that will be applied to it in the field. Non-rigid GD&T relies on a restraint specification that is included in the drawing’s notes. This note might specify the location and amount of force to be used by the clamping fixture.
For example, a large plastic dashboard might be held at several points with a specific torque on the bolts. Once the part is restrained, the geometric tolerances for features like holes and surfaces are measured. This ensures that the part will fit correctly when it is installed in the vehicle.
Without these notes, the inspection would be inconsistent and could lead to the rejection of good parts.
Measurement Procedure
Physical verification of non-rigid components involves the use of specialized checking fixtures that match the restraint notes. In the context of non-rigid GD&T, these fixtures are considered an extension of the measurement equipment. The quality team must ensure that the fixture itself is accurate and does not introduce its own errors.
They also need to follow the sequence of clamping specified in the drawing. If the clamps are closed in the wrong order, the part might be distorted in a way that is not representative of its functional state. This procedure is more time-consuming than inspecting a rigid part but is necessary for ensuring quality.
Automated systems can help by applying consistent pressure at each point.
Inspection Yield
Production readiness for flexible parts depends on the ability of the manufacturing process to produce parts that fit the restrained tolerances. Non-rigid GD&T allows for more variation in the free state, which can improve the yield of the molding or stamping process. However, it places more emphasis on the consistency of the assembly process.
If the parts are too far out of shape when loose, they may be difficult to load into the fixture or the assembly line. A supplier must demonstrate that the parts can be restrained without excessive force. The cost of a failed inspection is often related to the fixture design or the material properties of the part.
Proper use of these standards reduces the risk of assembly interference and improves the overall quality of the product.