What Causes Die Buil-Up in Plastic Extrusion and How Can You Prevent It?

Die build-up, often referred to as die drool, die bleed, or plate-out, is a frequent challenge in plastic extrusion that can compromise product quality and hinder production efficiency. This phenomenon occurs when material accumulates on the die, particularly at or near the die exit, due to factors such as die design, material properties¹, or process conditions. Gaining insight into its causes and implementing effective prevention strategies are essential for ensuring smooth, high-quality extrusion processes.

Die build-up in plastic extrusion is primarily driven by die exit geometry², material properties, and process conditions like high pressure and velocity, but can be mitigated through optimized die design, process aids, and material control.

What Causes Die Build-Up in Plastic Extrusion?

Die build-up stems from a combination of factors that interact during the extrusion process. Here’s a closer look at the primary culprits:

• Die Exit Geometry: The shape and angle of the die exit significantly influence build-up. Research shows that sharp edges or converging die exits, such as a 90° angle, can increase die lip build-up (DLBU). In contrast, a beveled exit (e.g., 15° for HDPE) can reduce it by alleviating melt pressure and velocity. Even minor imperfections, like a small radius of 0.010 inches at the die exit, can trigger surface tearing and build-up with certain polymers Effect of die exit geometry on internal die drool phenomenon during linear HDPE melt extrusion; Sharper Dies Make for Smoother Extrusions | Plastics Technology.

• Material Properties: Polymers with additives, inorganics, or broad molecular weight distributions (MWD) are particularly susceptible. For instance, moisture in the material can accelerate component separation under high shear and temperature, leading to build-up. Materials like HDPE are prone to this issue due to thermally induced degradation that enhances melt elasticity NOVA-Chem-Tech-Bulletin-Die-Lip-Buildup-FINAL.pdf; Investigation of die drool phenomenon for HDPE polymer melt.

• Process Conditions: High melt pressure and velocity can create vacuums or vortices that draw lighter components to the die lips, exacerbating build-up. This is especially evident in high-speed extrusions or when processing materials with varying rheological profiles.

How Can You Prevent Die Build-Up?

Preventing die build-up requires a proactive, tailored approach. Here are some proven strategies:

• Optimize Die Design: Adjusting the die exit to a diverging shape, such as a 15° bevel for HDPE, reduces melt pressure and velocity, minimizing build-up. Testing these modifications on smaller equipment first and consulting machinery manufacturers is advisable NOVA-Chem-Tech-Bulletin-Die-Lip-Buildup-FINAL.pdf.

• Use Process Aids: Additives that reduce melt fracture can also mitigate die build-up, offering a flexible solution without major equipment changes.

• Control Material Moisture: Drying materials to below 0.05 wt% moisture prevents separation and build-up, particularly in polymer blends. Matching components with similar rheological profiles further reduces risks.

• Apply Die Coatings: Uniform metallic coatings on the die interior resist abrasion and polymer adhesion, though they may need periodic re-coating.

• Regular Maintenance and Cleaning: A routine schedule to inspect and clean the die prevents build-up from escalating, including checking for wear and ensuring proper alignment.

What Are the Common Materials Affected by Die Build-Up?

Some materials are more prone to die build-up³ due to their inherent characteristics. Here’s a breakdown:

• HDPE (High-Density Polyethylene): Highly susceptible due to die swell and thermal degradation effects on melt elasticity⁴.

• LDPE (Low-Density Polyethylene): Moderately affected, often linked to flow properties and moisture.

• PP (Polypropylene): Varies by grade, with broader MWD increasing susceptibility.

• Polymer Blends: High risk due to component separation under shear and temperature.

What Are the Steps to Troubleshoot Die Build-Up?

When die build-up occurs, a structured approach can pinpoint and resolve the issue:

1. Isolate the Source: Remove blend components individually to identify the culprit material.

2. Substitute the Primary Polymer: Test a different polymer to determine if the issue is material-specific.

3. Inspect the Die Exit: Check for wear, damage, or improper geometry contributing to build-up.

4. Evaluate a Wider Die Exit: Temporarily use a wider exit to assess geometry’s role.

5. Analyze Build-Up Composition: Examine the build-up to identify accumulating components and adjust the formulation.

Conclusion

Die build-up in plastic extrusion is a complex challenge that affects product quality and efficiency. By addressing its causes—die exit geometry, material properties, and process conditions—and applying solutions like optimized die design, process aids, and material control, manufacturers can keep it in check. Regular maintenance and systematic troubleshooting further ensure smooth operations.