Nondestructive Computed Tomography Volumetric Void Acceptance Criteria for Heavy Polymer Sections
Computed tomography volumetric void acceptance requires zoning heavy polymer walls by stress field, setting pore limits below 0.1% in tensile skins.
Heavy Polymer Sections
Meaning
Heavy polymer sections identify zones within an injection moulded part characterized by a wall thickness that exceeds the standard nominal dimension of the geometry by fifty percent or more. These thick regions pose challenges for cooling and material flow during the manufacturing cycle. Engineers calculate these areas by comparing the local cross section against the primary flow wall thickness defined in the technical design.
The cooling rate for such volumes remains lower than the surrounding material because heat dissipation occurs slowly through the plastic mass.
Dimensional Stability
Variations in cooling lead to internal stresses that manifest as sink marks or surface depressions on the exterior. Molten material shrinks as it transitions from a liquid state to a solid, and a heavy polymer section draws mass from surrounding areas to compensate for this density change. Moulders counteract these defects by extending the hold time of the injection process to force additional resin into the cavity.
Excessive hold pressure increases the risk of part sticking or flashing at the parting line.
Regrind Economics
Recycled resin often exhibits different thermal characteristics that exacerbate the cooling discrepancy inside thick zones. Process control becomes harder when the batch contains high percentages of regrind because the melt viscosity fluctuates. virgin material provides predictable heat transfer properties which stabilize the solidification window. Small increments of regrind modify the crystallization kinetics and often require a corresponding increase in the total cooling cycle length to prevent internal voids.
Structural Performance
Thick walls provide high local rigidity yet introduce weak points if the curing remains incomplete. Residual heat trapped deep within the cross section prevents the formation of a homogeneous molecular structure during the transition. Voids form at the center of a heavy polymer section whenever the pressure drop during cooling exceeds the capacity of the gate to feed the volume.
Proper design relies on uniform wall thickness to maintain consistent mechanical performance across the entire component.