7 Tips about short molding in 2025
Introduction: Understanding and Preventing the Dreaded Short Shot
Short molding, often referred to as a short shot, is one of the most frustrating and costly defects in injection molding. It occurs when the molten plastic material fails to completely fill the mold cavity, resulting in an incomplete or partially formed plastic part. As molding technology advances into 2025, maximizing efficiency and minimizing scrap is paramount.
Here are 7 essential, up-to-date tips and strategies to help you conquer short molding defects and ensure 100% cavity fill.
1. Leverage Advanced Mold Flow Analysis (Simulation)
Don’t wait for production failures. The number one modern tip is to utilize advanced simulation software before cutting steel.
Actionable Insight: Use sophisticated Mold Flow Analysis to predict flow patterns, pressure drops, and potential short shots.
SEO Focus: Highlighting mold flow analysis as a preventative measure directly addresses user queries about eliminating defects proactively.
Result: Identify and optimize the gate location, runner system balance, and wall thickness uniformity to ensure complete filling under projected process conditions.
2. Increase Melt and Mold Temperatures (Optimize Viscosity)
The viscosity of the plastic material is a major factor in flowability. A higher temperature generally lowers the material’s viscosity, allowing it to flow more easily into the thin and distant sections of the mold.
Melt Temp: Increase the barrel temperature within the supplier’s recommended range to reduce viscosity.
Mold Temp: A warmer mold keeps the material fluid for longer, preventing premature solidification (freeze-off) that causes short molding.
3. Adjust Injection Speed and Increase Pressure
While increasing pressure seems obvious, the speed at which the plastic is injected is often the critical adjustment.
High Speed: Use a faster injection speed, especially during the initial stages, to push the material quickly past restrictive areas before it cools.
Injection Pressure: If speed is maximized, increase the injection pressure. This is the driving force that overcomes flow resistance. Caution: Do not exceed the machine’s clamping force or material stress limits.
4. Ensure Proper Venting and Decompression (Air Traps)
Air trapped inside the cavity acts as a cushion, physically blocking the plastic flow and leading to short shot defects.
Venting Strategy: Ensure your precision molds have adequate venting ($0.01$ to $0.05$ mm depth).
Decompression: Consider utilizing a “decompression” or “suck-back” step after screw recovery to prevent air from being dragged into the flow front.
5. Optimize Part Design and Wall Thickness
A poorly designed part with drastically varying wall thicknesses is a primary suspect for short molding.
Design Rule: Maintain uniform wall thickness across the entire part to allow for consistent flow and cooling.
Flow Path: Minimize the length of the flow path and avoid sharp corners or sudden transitions that increase shear stress and flow resistance.
6. Check Material Feed and Moisture Content
Inconsistent material feeding or contamination can starve the shot, directly causing a short molding defect.
Drying: Ensure hygroscopic materials (like Nylon or PC) are dried to the manufacturer’s specification. Excess moisture turns to steam, creating gas pockets that impede flow and weaken the part.
Screw Recovery: Verify that the screw recovery (plasticizing) time is consistent and that the machine is delivering the correct, stable shot size (cushion).
7. Inspect the Non-Return Valve and Screw
Wear and tear on the machine’s mechanical components can cause backflow, leading to an effective reduction in the delivered shot size—a mechanical cause of a short shot.
Check Valve: Regularly inspect the non-return valve (check ring assembly) on the screw tip. If it is worn, it will allow melt to leak backward during the injection and holding phases, resulting in a true short shot even if the programmed shot size is correct.
Maintenance: A proactive maintenance schedule for the barrel and screw components is key to maintaining metering precision in 2025.
Conclusion
Preventing short molding defects in 2025 relies on a holistic approach: combining predictive software (Mold Flow), optimizing process parameters (temperature, speed), ensuring mechanical integrity (venting, screw), and refining part and mold design. By systematically applying these 7 tips, you can drastically reduce scrap and achieve perfect part consistency.
