Melt fracture is an extrusion defect that produces rough, distorted, or irregular surfaces on plastic products. Depending on its severity, it may appear as fine “sharkskin” lines, repeating ridges, or severe twisting and distortion. These defects can reduce surface quality, affect dimensional consistency, and increase scrap rates.
This article explains the main causes of melt fracture and outlines practical solutions involving process settings, formulation adjustments, die design, and polymer processing aids.
What Causes Melt Fracture?
Melt fracture generally occurs when stress in the polymer melt becomes too high or is released unevenly as the material flows through and exits the die. Several factors can contribute to this instability.
1. Excessive Shear Rates and Extrusion Speeds
High screw speeds and throughput rates force more material through the die within a given time. This increases the shear rate and stress near the die wall. Once the stress exceeds the melt’s stable-flow limit, surface distortion can develop.
2. High Melt Viscosity
High-viscosity polymers resist deformation and require greater pressure to pass through the die. Low-melt-index resins, high-molecular-weight polymers, and some heavily filled formulations are more likely to experience melt fracture under demanding processing conditions.
3. Poor Die Design
Sharp die-entry angles, abrupt flow-path changes, narrow gaps, rough internal surfaces, and insufficient land length can create uneven velocity and localized stress. The polymer may then exit different areas of the die at inconsistent speeds, producing surface defects.
4. Low Extrusion Temperatures
When the processing temperature is too low, the polymer melt remains highly viscous. This raises pressure and shear stress throughout the extruder and die. Poor temperature distribution can also create regions with different viscosities, making the flow less stable.
How to Reduce or Eliminate Melt Fracture
The appropriate solution depends on the resin, formulation, equipment, and final product requirements. Processors should normally change one variable at a time and record the results.
1. Lower the Throughput or Screw Speed
Reducing output or screw speed lowers the shear rate in the die. This is often the quickest way to confirm whether excessive shear is causing the defect. However, it can also reduce production capacity, so further optimization may be required.
2. Increase Processing Temperatures
A moderate increase in barrel, adapter, or die temperature can lower melt viscosity and help the polymer flow more smoothly. But temperatures must remain within the recommended processing range to avoid degradation, discoloration, or dimensional problems.
3. Adjust the Formulation and Blend Ratio
Blending the base resin with a compatible grade that has a higher melt index may reduce viscosity. Processors can also review filler loading, pigment concentration, recycled material content, lubricants, and other additives. Any adjustment should be tested because it may influence mechanical properties, sealing, printing, or appearance.
4. Optimize Die Geometry
Improving the die entrance, flow channel, land length, gap uniformity, and internal surface finish can reduce localized stress. A streamlined transition and properly maintained die surface help the melt accelerate and exit more consistently.
5. Use Polymer Processing Aids
Polymer processing aids, commonly called PPAs, can help control melt fracture without requiring a major reduction in throughput. Fluoropolymer-based PPAs create a low-friction interface between the polymer melt and the die wall, reducing interfacial shear stress and supporting smoother extrusion.
PPAs are generally effective at very low concentrations, so they do not typically alter the bulk properties or overall performance of the base polymer. The appropriate grade and dosage should still be verified through production trials.
Where to Find Polymer Processing Aids
At Shine Polymer, we offer a range of polymer processing aids for thermoplastic extrusion. Our Rheopoly® polymer processing aids are white granular fluoropolymer-based additives. Used at low addition levels, these processing aids can help eliminate melt fracture, reduce die buildup, improve pigment dispersion, and support the processing of low-MI resins. They are suitable for applications such as blown film, cast film, wire and cable, pipe, profile, and high-viscosity resin extrusion.
Processors can review the available grades and discuss resin type, equipment conditions, current defects, and performance requirements with the supplier before selecting a product.
Conclusion
Melt fracture can result from processing conditions, material properties, and die design. Manufacturers can address it by optimizing extrusion settings, reviewing formulations and die geometry, and considering polymer processing aids. The right approach depends on the resin, equipment, and application, so controlled trials are recommended to identify the most suitable solution.
