Views: 0 Author: Site Editor Publish Time: 2026-04-01 Origin: Site
In injection molding production, dealing with flash—also known as burrs—or trimming excess plastic is an everyday challenge. Uncontrolled flash not only increases labor costs for post-processing but also affects product appearance, dimensional accuracy, and assembly performance. So what exactly causes flash, and how can we prevent and fix it? In this post, we break down the key reasons and practical solutions.
Flash occurs when molten plastic is forced into small gaps in the mold and solidifies into thin, unwanted edges. The root causes fall into three main categories: mold precision, material properties, and processing parameters.
Mold conditions directly determine whether flash forms.
Flash along the parting line often comes from excessive molding pressure pushing the mold halves open.
Flash near the gate area is typically caused by overly high holding pressure.
Flash in other areas often signals poor mold fitting: uneven parting surfaces, excessive clearance in slides, inserts, or ejector pins, and mold deformation.
Material flow and shrinkage play a big role in flash formation.
High-flow materials such as polypropylene (PP) flow easily into tiny mold gaps. Improper settings like high injection pressure or high temperature make this even worse.
High-shrinkage materials contract noticeably when cooling. This can create small gaps between the mold and the part, allowing more melt to flow in and form flash.
Incorrect process settings are the most common on-site cause of flash:
Excessively high injection pressure or speed
High barrel or mold temperature reducing melt viscosity
Overly high holding pressure or long holding time
Excess injection volume overfilling the cavity
Controlling flash requires improvements in mold design, processing, part design, and post-processing.
Simplify parting line design and avoid complex gaps or sharp angles.
Add properly sized venting grooves to release trapped air and prevent counter-pressure that causes overflow.
Improve fitting precision for slides, inserts, and ejector pins.
Lower injection pressure, injection speed, and shot volume.
Control barrel and mold temperatures to maintain proper melt viscosity.
Reduce holding pressure and holding time to avoid overpacking.
Slow down injection speed for smoother cavity filling.
Maintain uniform wall thickness and avoid extremely thin sections.
Replace sharp corners with rounded radii to reduce material accumulation.
When flash still occurs, efficient trimming is essential:
Manual trimming: Using blades or sandpaper, ideal for small-batch or high-precision parts.
Cryogenic deflashing: Using liquid nitrogen to embrittle flash, then removing it via shot blasting — suitable for rigid plastics and mass production.
Stable parameters mean consistent quality and less flash.
Use high-precision injection molding machines with advanced closed-loop control.
Regularly calibrate sensors, temperature controllers, and pressure gauges.
Apply zone-based temperature control for large or complex molds.
Install high-precision sensors for real-time pressure and speed monitoring.
Use data logging and analysis software to track parameter stability.
Small changes in processing conditions directly influence defects like flash, shrinkage, warpage, and poor surface finish.
High barrel temperature: Causes material degradation, bubbles, discoloration, and severe flash.
Low barrel temperature: Leads to poor melting, short shots, and rough surfaces.
High mold temperature: Extends cycle time, causes warping, and increases flash risk.
Low mold temperature: Creates internal stress, weld lines, and cracking.
High injection pressure: Creates flash, high internal stress, and deformation.
Low injection pressure: Results in incomplete filling and weak part strength.
Excessive holding pressure: Causes flash, stress buildup, and difficult demolding.
Fast injection speed: Leads to jetting, flow marks, and flash.
Slow injection speed: Increases weld lines and insufficient filling.
Long holding or cooling time: Wastes production time and may cause flash.
Short cooling time: Causes deformation and unstable dimensions.
Insufficient clamping force: Directly causes large-scale parting-line flash.
Excess shot volume: Creates waste and flash; insufficient volume causes short shots.
Flash is one of the most common yet avoidable defects in injection molding. By improving mold precision, selecting suitable materials, fine-tuning process parameters, and optimizing part design, manufacturers can greatly reduce flash and lower post-processing costs. Stable parameter control and systematic troubleshooting will further improve product consistency and production efficiency.
