Appearance Issues in Moldflow: Sink, Weldlines, Blush & more!
- CAE Services Corporation
- Aug 19
- 3 min read
Updated: 2 days ago
Plastic injection molding has revolutionized manufacturing, but appearance issues continue to be one of the biggest headaches for engineers, second only to warpage. These blemishes don’t just affect aesthetics—they can impact part strength, customer trust, and production timelines. In a recent session of Ask the Moldflow Experts, Tim Lanes, Director of Engineering at CAE Services, shared his deep expertise on how simulation tools like Moldflow can help prevent and solve these challenges.
Why Appearance Issues Matter
Even the smallest visual defect can escalate quickly. Knit lines, voids, sink marks, or gate blush might seem minor, but in industries like automotive and medical devices, they can trigger costly rework, delays, and strained supplier-customer relationships. As Tim explained, “Once production begins, fixing these problems becomes exponentially harder and more expensive. The best solution is to prevent them before the tool is built.”
Common Appearance Issues in Injection Molding
1. Knit Lines
Formed when two flow fronts meet, knit lines are weak points that often can’t be eliminated once a tool is built.
Solutions include adjusting gate placement, modifying part design with flow leaders/restrictors, or sequencing valve gates.
Tim shared case studies, like a riding mower component where part redesign successfully redirected knit lines to less visible areas.
2. Flow Lines
Caused by acceleration marks during valve gate sequencing, flow lines can mimic knit lines.
The fix often lies in velocity-controlled valve gate retraction, which smooths flow progression and avoids hesitation marks.
3. Air Traps and Voids
These occur when air cannot escape during filling, leading to burns, short shots, or blemishes.
Solutions include optimizing gate placement, using vacuum venting, or making part design changes early.
Core shift, often overlooked, can also create void-related blemishes when unsupported cores move under pressure.
4. Sink Marks
Visible depressions form when ribs or bosses intersect walls, or when wall thickness is inconsistent.
Best avoided through careful rib-to-wall ratios and early feasibility studies.
Once a tool is built, process tweaks like increasing pack pressure or adjusting gate placement may help, but fixes are limited.
5. Gate Blush
A blemish radiating from the gate, caused by excessive shear that breaks polymer chains.
Prevented by properly sizing gates, slowing fill rates, or distributing flow across more gates.
Since each material has a shear rate limit, simulation is crucial to ensure designs stay below it.
The Power of Simulation
Tim emphasized one theme repeatedly: simulation early and often. Moldflow can predict where knit lines, voids, or blush may form, giving engineers a chance to redesign parts, adjust gate strategies, or optimize wall thicknesses before cutting steel. “Processing tweaks are rarely a silver bullet,” he explained. “You want to hardwire solutions into the part and mold design upfront.”
CAE Services has completed over 28,000 projects since its founding in 1988, and Tim noted the shift over the years: where once Moldflow was mainly used to troubleshoot existing problems, now forward-looking companies demand simulation for every mold before production.
Key Takeaways
Avoidance is best: Fixing defects post-production is costly and often ineffective.
Ask the right questions early: Where are knit lines acceptable? Where can gates be placed?
Simulation pays for itself: Identifying just one critical flaw before tooling can offset the cost of dozens of trouble-free projects.
Expertise matters: A skilled Moldflow analyst knows when results are reliable and how to balance trade-offs.