Troubleshooting Common Thin-Wall Injection Molding Defects

Short shots occur when the mold cavity does not fill completely, producing parts with missing sections, typically at the farthest points from the gate. In thin-wall molding with wall thicknesses of 0.4-0.7 mm, the melt freezes rapidly against the cold mold surface, and any delay or restriction in flow leads to premature solidification before the cavity fills. The primary causes include insufficient injection speed, allowing freeze-off before fill completion, and inadequate injection pressure to push melt through the narrowing flow channels. Troubleshooting should begin with verifying that injection speed is within the 300-500 mm/s range required for thin-wall PP. If speed is adequate, check melt temperature, which should be 220-260 degrees C for PP food packaging grades. Increasing melt temperature by 5-10 degrees C reduces viscosity and extends flow length. Examine mold venting next, as trapped air at flow ends creates back-pressure preventing complete filling. Vents should be 0.02-0.03 mm deep for PP. For multi-cavity molds on HWAMDA SPV5 machines, verify hot runner balance by collecting and weighing individual cavity shots.

While sink marks are less common in thin-wall parts than conventional molding due to uniformly thin sections, they can still appear at rib intersections, stacking ledges, and gate areas where local wall thickness increases. Sink marks form when the outer skin solidifies and the interior continues to shrink, pulling the surface inward. In thin-wall food containers, even shallow sink marks of 0.05-0.1 mm depth can be visible on high-gloss surfaces. The primary solution is optimizing packing pressure magnitude and duration to compensate for volumetric shrinkage during solidification. For ribbed sections in thin-wall containers, design guidelines recommend rib thickness at 50-60% of the nominal wall thickness rather than the conventional 60-80% rule used for thicker parts. Surface defects beyond sink marks include burn marks from trapped gas, peeling or delamination from contaminated material, and surface roughness from mold wear. HWAMDA's thin-wall molds use high-quality P20 or NAK80 steel with mirror-polished cavity surfaces to maintain consistent surface finish quality throughout the mold's production life.

Flash occurs when molten plastic escapes from the cavity through the parting line, creating thin projections of excess material on the finished part. In thin-wall molding, the high injection pressures of 180-280 MPa make flash particularly challenging to control. The mold's clamping force must exceed the total opening force generated by cavity pressure acting over the projected area. For a 16-cavity sauce cup mold running on an HWAMDA SPV5-270 machine, the required clamping force calculation considers 250 MPa cavity pressure across the total projected area. Parting line precision is critical. The mold steel surfaces at the parting line must maintain flatness within 0.01-0.02 mm across the entire mold face. Any damage, wear, or contamination on the parting line surfaces allows flash to form. Regular parting line maintenance, including inspection every 100,000 cycles and re-polishing as needed, prevents flash from becoming chronic. If flash appears on a previously clean-running mold, check for foreign material on the parting line, verify clamping force has not decreased, and confirm process parameters have not drifted from validated settings.

Effective thin-wall troubleshooting requires a systematic approach rather than random parameter adjustments. Begin with defect identification and classification: photograph the defect, note its location on the part, and determine whether it appears on all cavities or specific ones. All-cavity defects point to machine or material issues, while single-cavity defects indicate mold-specific problems. Document the current process parameters as a baseline before making any changes. Follow a one-variable-at-a-time methodology. Change one parameter, run 10-20 shots to stabilize, then evaluate the result before adjusting anything else. For HWAMDA SPV5 machines, the controller's trend analysis function helps track how each parameter change affects quality metrics. Maintain a troubleshooting log that records each change, its effect, and the resulting part quality. When multiple defects appear simultaneously, address the most fundamental issue first. Short shots take priority because they indicate the process cannot fill the cavity. After achieving complete filling, address dimensional issues through packing optimization, then refine surface quality through temperature and speed adjustments.