How Can You Solve Surface Burning Issues in Molds?

In the high-speed, high-pressure world of plastic injection molding, achieving a flawless surface finish on your parts is non-negotiable. One of the most persistent and troubling defects that can mar this finish is surface burning, often identified by black or brown streaks, discolorations, or a burnt appearance on the molded component. At GV MOLD, our expertise in precision mold manufacturing and process optimization allows us to not only identify the root causes of this issue but to implement lasting solutions. Let's explore the mechanics of surface burning and the systematic approach to solving it.

What is Surface Burning and Why Does It Occur?

Surface burning, also known as "dieseling," is a thermal degradation of the plastic resin. It happens when compressed air trapped within the mold cavity ignites due to the extreme heat and pressure of the injected melt. The rapid compression heats the air to a point where it literally burns the plastic, leaving behind unsightly and often structurally weakened marks.

The primary culprit is almost always inadequate venting. However, the path to a solution requires a holistic analysis of the mold, material, and machine.

A Systematic Approach to Diagnosing and Solving Surface Burning

Solving surface burning is not about applying a single fix but conducting a targeted investigation. At GV MOLD, we follow this structured diagnostic framework:

1. Investigate and Optimize Mold Venting

This is the first and most critical area to address.

• Assess Vent Design: Vents must be deep enough to allow air to escape but shallow enough to prevent plastic flash. They should be strategically located at the end of fill and where air traps are predicted by mold flow analysis.

• Check Vent Condition: Over time and production runs, vents can become clogged with residue from mold release agents or degraded plastic. Regular mold maintenance and cleaning are essential.

• Implement Active Venting Solutions: For deep ribs or problematic areas, consider ejector pin vents or specialized venting inserts. Ensuring the entire venting path—from the cavity to the atmosphere—is clear is crucial.

2. Review and Adjust Injection Molding Process Parameters

Machine settings can often induce or exacerbate the problem.

• Reduce Injection Speed: Excessively high injection speed can trap air by not allowing it time to escape through the vents before the melt front seals them off. A slower initial fill can be highly effective.

• Optimize Melt and Mold Temperature: While a high melt temperature can contribute to degradation, an excessively low mold temperature can cause the melt front to cool and solidify too quickly, blocking vent paths. Fine-tuning this balance is key.

• Verify Back Pressure and Screw RPM: High back pressure or excessive screw rotation speed during plasticization can introduce air into the melt itself, which then enters the cavity.

3. Evaluate Material Handling and Selection

• Ensure Proper Drying: Some hygroscopic resins (like PA/Nylon, PC, PET) must be thoroughly dried before processing. Moisture turns to steam in the barrel, acting as a source of trapped gas that can cause burning and bubble issues.

• Check for Contamination or Regrind: Contaminated material or using too high a percentage of regrind can lower the resin's thermal stability, making it more prone to degradation.

4. Examine the Mold Design and Condition

• Conduct Mold Flow Analysis: A proactive mold flow analysis during the mold design phase is invaluable for predicting air trap locations and optimizing gate design and venting layout before the mold steel is ever cut.

• Verify Gate and Runner Design: A poorly sized or located gate can cause turbulent filling, which entraps air. Similarly, an unbalanced runner system in a multi-cavity mold can lead to over-packing in some cavities, sealing vents prematurely.

• Inspect for Damage: Check for damaged or worn seals, which can allow oil or other contaminants from the mold base or hydraulic system to enter the cavity and burn.

The GV MOLD Advantage: Preventing Surface Burning from the Start

While troubleshooting in production is necessary, the most cost-effective strategy is prevention through expert design and manufacturing.

• DFM with Venting in Mind: Our mold design process prioritizes venting strategy. We specify optimal vent locations, depths, and land lengths based on the specific material and part geometry.

• Precision Manufacturing for Perfect Seals: Our state-of-the-art mold factory ensures that mold components—cavities, cores, slides, and inserts—fit together with precision. This minimizes flash risk while allowing for effective, controlled venting at the parting surface and other interfaces.

• Advanced Mold Steels and Surface Treatments: We select mold steels with appropriate hardness and toughness, and apply targeted surface treatments that can enhance polishability and reduce the tendency for material sticking, which can contribute to burn marks.

• Comprehensive Mold Trials: During our rigorous mold trial process, we meticulously test for surface defects. We systematically adjust process parameters and, if needed, implement on-the-spot venting modifications to eradicate burning before the tool is delivered.

Don't let surface burning defects compromise your product quality or slow down your production. Partner with GV MOLD for molds engineered to run cleanly and efficiently from the very first shot. Contact us today for a consultation on your next project.

GV MOLD – Engineering Flawless Surfaces, Delivering Impeccable Quality.