Tolerance Grades a Toolroom Signs under DIN 16742
DIN 16742 defines nine tolerance grades based on resin shrinkage variance, requiring toolmakers to split tool-bound and parting-line tolerances during sign-off.
Mold Produced Dimensions
Meaning
Injection moulding parameters govern mold produced dimensions by translating polymer volumetric shrinkage into physical part boundaries inside the cavity. Crystallisation kinetics and thermal contraction dictate the final geometry after ejection. These physical limits separate raw material datasheets from shop floor realities.
A resin supplier quotes shrinkage rates derived from standard plaques, but actual cavities diverge due to gate freeze-off and packing pressure profiles.
Shrinkage Variance
Virgin resins exhibit predictable volumetric reduction curves based on molecular weight distribution, whereas regrind batches introduce variable melt flow rates that shift the boundary outwards. Operators adjust barrel temperatures and holding times to compensate for viscosity changes, yet thermal drift alters the cooling rate across multi-cavity plates. Part specifications tolerate limited variance before assembly interference occurs, unlike raw material datasheets that assume uniform wall thickness.
Tooling Allowance
Machinists cut steel cavities larger than the nominal blueprint value to counteract thermal shrinkage upon cooling. Tooling designers calculate this metal-cutting offset by applying specific volumetric contraction coefficients for engineering thermoplastics. Tool temperature control circuits stabilise this offset during long production runs, preventing dimensional creep caused by uneven coolant flow.
Cavity Pressure
Transducers mounted behind the ejector pins measure real-time pressure decay during the packing phase, linking hydraulic input directly to finished part dimensions. Underpacking leaves voids and sink marks along heavy rib junctions, while overpacking causes flash and crushes core pins. Dimensional stability depends on maintaining consistent melt density inside the closed tool until the gate solidifies completely.