Meaning
Deviation from a defined geometric boundary signifies a profile form error, which measures the local displacement of a molded surface relative to the nominal CAD geometry. This profile form error quantifies how far the actual plastic wall sits from the intended design coordinate system. It remains independent of orientation or positional displacement, focusing solely on the deviation in shape across the part surface.
High values indicate improper cooling rates or excessive injection pressure, resulting in uneven shrinkage or warpage during the solidification phase of thermoplastic components.
Process Drift
Variable cooling temperatures across different cavities during high speed injection molding cause specific regions to shrink at unequal rates. Thermal gradients within the tool steel often create this profile form error by preventing uniform wall thickness. Operators maintain process stability by monitoring melt temperature and packing pressure to ensure the resin fills the tool geometry before the gate freezes.
Consistent gate vestige removal and uniform cooling circuit performance mitigate the potential for these geometric distortions to occur.
Material Specification
Resin suppliers provide a melt flow index to characterize the rheological behavior of a grade, yet the actual flow through a mold depends heavily on the mold geometry and the processing window. Moulders often compare incoming virgin resin pellets against previous lots to verify consistent viscosity, which remains a primary control against unexpected shrinkage patterns. While a datasheet provides a reference for property stability under controlled conditions, the production environment requires a tightened range for mold shrinkage to prevent part rejection.
Cost Impact
Excessive geometric distortion necessitates additional secondary operations, such as manual grinding or fixture correction, which add labor and increase the unit cost. Scrapped parts represent a total loss of material and machine time, occurring when the profile form error exceeds the allowable tolerance established by the engineering drawing. Quality control systems quantify these losses by calculating the percentage of parts failing coordinate measuring machine inspections during standard batch validation.
