Views: 0 Author: Site Editor Publish Time: 2026-07-20 Origin: Site
When molding thick-wall polycarbonate (PC) parts, two of the most common—and frustrating—defects often appear together:
Sink marks on the surface
Burn marks or short shots caused by trapped air
Many process technicians instinctively try to solve both by tweaking the same parameter. That’s where things go wrong.
The truth is: sink marks and venting issues stem from different stages of the injection cycle and require separate, targeted adjustments. In this post, I’ll walk through a practical strategy to tackle both without falling into the trap of overcomplicating your process setup.
PC has high viscosity and low thermal conductivity. In thick sections, the surface skin solidifies while the core is still molten. As the core shrinks during cooling, if there isn’t enough melt being pushed back into the cavity, the surface collapses inward—that’s your sink mark.
The primary lever: Packing (hold) pressure and, more importantly, packing time.
Poor venting occurs during the filling stage, not during packing. When the melt front advances too quickly, the air ahead of it doesn’t have enough time to escape through the vents. The compressed air heats up and can cause burns, or it can block the flow entirely, leading to short shots.
The primary lever: Injection velocity profile, especially at the end of fill.
No. And here’s why:
Longer packing time does not help trapped air escape—the air is already trapped before packing begins.
Increasing injection speed to improve melt flow for sink-mark compensation will worsen venting issues.
The two parameters operate in different phases of the cycle. You need to separate them and optimize each independently.
For thick-wall PC, extending packing time is far more effective than increasing packing pressure—and it introduces less internal stress.
Procedure:
Start with a conservative packing time (e.g., 5–10 seconds).
Gradually increase it in 0.5–1 second steps.
Weigh each molded part. When the part weight stops increasing, you’ve reached the gate-freeze-off point.
Add 10–20% to that value as your final packing time.
⚠️ Avoid cranking up packing pressure above 80% of injection pressure—it adds stress and may cause cracking or warping near the gate.
Venting defects are a filling-phase problem. Address them by:
Reducing final-stage injection velocity – use a multi-stage speed profile, such as slow → fast → slow.
Keeping overall injection speed moderate (e.g., 20–60 mm/s for most PC grades).
Checking physical vent depth—PC typically requires vents of 0.02–0.03 mm to avoid flash while allowing air release.
Remember: slowing down at the very end of fill gives air a chance to escape through the vents without causing burns or hesitation marks.
For thick-wall PC, mold temperature is the foundation that makes both fixes work better.
Parameter | Recommended Range | Why It Matters |
|---|---|---|
Mold Temperature | 100–120°C | Delays skin solidification, extends the effective packing window, and reduces flow resistance—which also lowers the risk of trapped air. |
Packing Time | Extend until gate freeze | Prevents cavity pressure drop before solidification. |
Injection Speed | Moderate, with end-of-fill deceleration | Gives air time to escape; avoids jetting and shear heating. |
Packing Pressure | 50–80% of injection pressure | Supports cavity filling without overpacking. |
Problem | Primary Adjustment | Secondary Check |
|---|---|---|
Sink marks | ✅ Increase packing time (until weight stabilizes) | Raise mold temperature to widen the processing window |
Venting defects | ✅ Reduce speed at the end of fill | Verify vent depth and position |
Both issues present | Separate filling and packing phases in your parameter settings | Avoid high injection speed and high packing pressure simultaneously |
PC thick parts demand a disciplined, phase-separated approach. Don’t mix up filling and packing—they serve different purposes. By extending packing time to combat sink marks and tuning injection velocity to handle trapped air, you can resolve both defects without compromising cycle time or part quality.
And always, always start with mold temperature. For PC, 100°C is not a suggestion—it’s a prerequisite.
