Anisotropic Shrinkage Variance in Glass-Filled Polyamide Secondary Tooling Nests
Secondary tooling nests for glass-filled polyamide accommodate anisotropic shrinkage through floating datum pins aligned with transverse contraction vectors.
3-2-1 Positioning Scheme
Meaning
Geometric design methods establish the necessary constraints to lock a solid body in three-dimensional space by restricting six degrees of freedom. The 3-2-1 positioning scheme accomplishes this by placing three points on a primary datum plane, two on a secondary plane, and one on a tertiary plane. Molded polymer parts rely on these reference points for dimensional inspection to ensure repeatable measurements despite inherent part warpage.
Kinematic Constraint
Orthogonal placement of the six locator contacts prevents translation along the three axes and rotation about them. This rigid arrangement guarantees that the workpiece settles into the exact same spatial coordinates during every measurement cycle.
Tooling Application
Secondary fixture components like clamps and spring-loaded pins hold the workpiece firmly against the six primary locators. In plastic injection molding quality control, the 3-2-1 positioning scheme prevents the flexible molded part from being distorted by clamping forces. Solid steel rests support the datum surfaces while light clamping holds the part without causing deflection.
Defect Prevention
Part deformation from excessive clamping forces creates false measurement readings that do not match the free-state dimensions of the polymer component. Rigid alignment fixtures that do not distribute forces correctly can bend the part, masking mold shrinkage variations or warp. Using this structured locating layout eliminates operator influence on the metrology results, allowing true process variations to be tracked accurately.
If a flexible plastic cover is clamped on non-datum surfaces, the resulting strain causes the part to appear within tolerance on the fixture but fail in final assembly.