Injection Speed Settings in Plastic Mold Production

Burning is a common quality defect in injection molding, characterized by local yellowing, dark spots, charring, brittleness, or even carbonized marks on plastic parts. It mostly occurs at sharp corners, deep ribs, flow ends, narrow gaps, and areas far from the gate. Burning not only destroys appearance but also reduces mechanical strength and sealing performance, leading to high rejection rates. This document systematically summarizes the root causes of burning and provides practical, repeatable solutions for industrial production, mold design, and process debugging.

Mechanism of Burning Defects

Burning in injection molding is mainly caused by two types of factors:One is compression combustion of trapped air inside the cavity. When air cannot be vented in a timely manner, it is rapidly compressed by high-speed melt, generating localized high temperature that ignites or scorches the plastic.The other is thermal decomposition of plastic melt due to excessive temperature, excessive shear heating, or prolonged residence time. Both types result in molecular degradation, discoloration, carbon deposition, and surface burning marks.

Causes and Solutions Related to Molding Process

Excessively High Injection Speed

Excessively high injection speed causes the melt to rush into narrow spaces, compressing air before it can escape. Air compression produces instantaneous high temperature and burns the plastic.Reduce the injection speed at flow ends, especially in the last stage of filling. Use a multi-stage injection profile: slow at the start, moderate in the middle, and slow at the end.

Excessively High Melt Temperature

When the barrel or nozzle temperature exceeds the plastic’s thermal stability limit, the material decomposes, discolors, and forms burning marks.Lower the barrel and nozzle temperature according to the material’s recommended range. Reduce the rear and middle barrel temperatures appropriately to shorten residence time.

Excessive Back Pressure and Screw Speed

High back pressure and high screw speed cause strong shearing during plasticizing, leading to internal overheating and decomposition.Reduce back pressure and lower screw speed. Ensure uniform melting without excessive shear heat.

Long Residence Time of Melt

Small shots or low cycle speed cause the plastic to stay in the barrel for too long, resulting in thermal degradation.Increase production tempo, reduce barrel temperature, or use a smaller barrel if possible. Clean the barrel before long-term shutdown.

Unreasonable V/P Switching

Late switching leads to over-packing and intense compression at the flow end, promoting air entrapment and burning.Adjust the V/P switching position earlier, at 95–98% filling volume, to avoid overcharging.

Causes and Solutions Related to Mold Design

Insufficient or Blocked Venting

Poor venting is the leading cause of burning. Venting grooves that are too shallow, too narrow, blocked, or misplaced prevent air from escaping.Clean existing venting grooves to ensure they are unobstructed. Increase venting at flow ends, ribs, and dead corners. Standard vent depth is 0.015–0.03 mm for most plastics, width 5–12 mm.

Unreasonable Gate Location

Gates positioned far from venting areas or pointing directly into narrow gaps cause air entrapment.Redesign the gate to direct melt toward venting areas. Place gates at thick sections and ensure flow paths lead to open spaces.

Mold Structure with Dead Corners and Deep Ribs

Deep, narrow ribs, closed cavities, and sharp corners easily trap air and cause burning.Add venting inserts or porous steel inserts at trapped-air locations. Increase fillets to reduce sharp corners. Optimize rib thickness and height to improve flow and venting.

Overly Small Runner or Gate System

Small runners and gates increase flow resistance and shear heating, raising melt temperature locally.Enlarge runners and gates appropriately to reduce flow resistance and shear heat.

Mold Temperature Too High

High mold temperature slows cooling and increases the risk of thermal degradation on the part surface.Lower mold temperature or strengthen cooling to stabilize the surface temperature.

Causes and Solutions Related to Plastic Materials

Moisture or Volatiles in the Material

Moisture, additives, or residual solvents evaporate during molding, forming gas that contributes to burning and silver streaks.Dry the material strictly according to requirements. Use hot air drying or dehumidifying drying. Check for material contamination.

Poor Thermal Stability of the Plastic

Low-grade or recycled materials have poor stability and decompose easily at normal processing temperatures.Use virgin materials with stable thermal stability. Reduce processing temperature and residence time.

Excessive Release Agent or Contamination

Excessive release agent or oil contamination decomposes under heat and causes discoloration similar to burning.Reduce the use of release agent. Keep the mold surface and material clean.

Causes and Solutions Related to Equipment and Operation

Barrel, Screw or Nozzle Damage

Worn or rough surfaces cause dead spaces where melt decomposes over time.Maintain or replace the screw, barrel, and nozzle. Clean the plasticizing system regularly.

Unstable Injection Speed or Pressure

Fluctuating process parameters lead to inconsistent venting and occasional burning.Calibrate the injection unit. Lock stable process parameters after debugging.

General Quick Debugging Procedure for Burning Defects

First, reduce the final-stage injection speed to verify whether burning is reduced.Second, check and clean venting grooves to ensure effective venting.Third, lower barrel temperature to eliminate material decomposition.Fourth, optimize gate position to reduce air entrapment.Fifth, modify mold structure if necessary, such as adding venting inserts or enlarging vents.

Summary

Burning in injection-molded parts is primarily a venting problem, followed by process temperature and shear issues. Stable production requires comprehensive control over mold venting design, injection speed profiling, temperature management, and material preparation. By systematically eliminating trapped air and controlling thermal degradation, burning defects can be completely eliminated, ensuring consistent appearance and performance of plastic parts.