Views: 0 Author: Site Editor Publish Time: 2026-03-09 Origin: Site
Polycarbonate (PC) is one of the most versatile engineering plastics, known for its exceptional impact resistance, optical clarity, and heat tolerance. Yet, for all its impressive properties, PC can be a formidable opponent on the injection molding floor. If you've ever struggled with flow marks, jetting, or short shots when molding PC, the culprit often isn't the machine—it's the gate and runner design.
A well-designed feeding system acts like a highway for molten plastic. A poorly designed one creates traffic jams. Here’s how to design gates for PC that give molders the widest possible processing window.
Unlike free-flowing materials like PP or PE, PC has a high melt viscosity. It doesn't like to be squeezed through tiny openings. When designing your runner and gate system, the core philosophy should be minimizing resistance.
If you force PC through thin, long gates, you will need:
Higher injection pressures.
Higher melt temperatures.
Faster injection speeds.
These three factors combined lead to shear stress, material degradation, and cosmetic defects.
To make life easy for the technician on the floor, stick to these proven numbers:
Length: Keep it under 40mm. The shorter the sprue, the less pressure drop.
Dimensions: The small end should be at least 3.0–4.0mm. The large end should be 10–12mm.
Why: A thick sprue ensures the pressure from the nozzle transfers efficiently into the mold. It also prevents the sprue from sticking.
Shape: Full-round runners are ideal because they offer the least surface area and lowest friction. Trapezoidal is a good second choice.
Size: Aim for a diameter of 8–12mm. This ensures the melt stays hot and flows freely to the gate.
Transitions: Never use sharp 90-degree corners. Always use generous radii to turn corners to prevent "dead spots" where material can stagnate and burn.
This is the most critical part.
Gate Land (Length): Keep it incredibly short—ideally 0.5mm to 1.0mm. A long gate land freezes too quickly and creates high stress.
Gate Depth (Thickness): This should be 70% to 100% of the part wall thickness. For a 2mm thick part, your gate should be at least 1.5mm thick.
Minimum Thickness: Never go below 1.2mm for a PC gate, or you risk excessive shear.
Not all gates are created equal. Here is how they rank for PC:
Why: These gates are essentially an extension of the runner. They allow the material to flow into the cavity naturally.
Benefit: Low pressure requirement. Easy to process. Minimal stress. Ideal for transparent parts.
The Risk: These gates are small. They force the material to accelerate rapidly, causing "jetting" (a snake-like flow mark) and high molecular orientation.
If You Must: Make the pinpoint gate as short and thick as possible. You will likely need to use high mold temperatures and multi-stage injection speeds to compensate.
If you struggle with silver streaks or dull surface marks on your PC parts, check your cold slug well.
When the screw retracts, the material at the very front of the nozzle cools slightly. If this cold material shoots directly into the cavity, it creates a visible defect.
The Fix: Always place a cold slug well at the end of the main runner and at the end of any secondary runners.
Size: Make it slightly larger in diameter than the runner itself. This traps the cold material so it never enters the part.
Even with perfect gates, the machine setup matters. Here are two tips for processing PC:
Do not use decompression (suckback) after plastication.
Why: Suckback pulls air into the nozzle. At high temperatures, PC mixes with this air. The result? A silver streak disaster on the next shot.
Exception: If the material drools, use the minimum slow suckback possible—just enough to break the vacuum, not enough to inhale air.
Use a slow-fast-slow profile:
Slow through the gate area (to prevent jetting).
Fast through the middle of the cavity.
Slow at the end (to prevent flashing and burning).
