Plastic extrusion is a core technology in the production of films, sheets, pipes, and fibers. This process enables continuous shaping, high productivity, and stable quality. However, in reality, the surface of products does not always meet the desired standard. A common defect is cracking or roughness that reduces aesthetics and mechanical performance. This phenomenon is generally known as melt fracture. To maintain production efficiency, it is important to identify its causes and apply proper solutions.
MELT FRACTURE IN PLASTIC EXTRUSION
Melt fracture occurs when molten plastic flows through a die under non-optimal conditions. The flow becomes unstable, creating different surface defects. At a mild level, sharkskin appears as fine wrinkles resembling shark skin. When more severe, melt rupture forms deep cracks that deform the product. Another type is bambooing, which produces bamboo-like patterns due to pressure fluctuations during extrusion.
These deformations not only reduce surface quality but also compromise strength and durability. In high-end applications such as packaging films, technical pipes, or fibers, even slight surface defects can disqualify an entire batch. For this reason, melt fracture remains a critical challenge that producers seek to control.
CAUSES OF MELT FRACTURE
Operating conditions
The most common cause is an excessively high shear rate. When the melt exits the die too fast, the flow becomes unstable and cracks appear. Low extrusion temperature also contributes, as the material is not soft enough to flow smoothly and surface ruptures occur.
Material characteristics
High or inconsistent viscosity strongly influences the phenomenon. Greater viscosity increases friction with die walls, creating strong pressure variations. Contaminated resins or degraded polymers also alter flow properties, intensifying melt fracture.
Equipment design
Narrow channels or sharp turns in the die often create pressure build-up points. The melt then undergoes stick-slip, meaning it alternately adheres and slips on the die wall. This cyclical behavior leads to irregular surface patterns and even cracks.
IMPACT ON PRODUCTS AND PRODUCTION
Melt fracture causes severe aesthetic issues, preventing products from meeting commercial standards. Rough or patterned surfaces reduce market competitiveness. Mechanically, small surface cracks may expand during use, reducing lifespan and reliability.
The impact extends beyond quality to overall production cost. Higher scrap rates waste raw materials and energy. Frequent machine stops for adjustments lower productivity and increase operational costs. Clearly, melt fracture is both a technical and economic problem.
SOLUTIONS TO MELT FRACTURE
Process adjustments
A basic measure is raising barrel or die temperatures to lower viscosity and improve flow. At the same time, reducing screw speed lowers shear rate and avoids exceeding melt fracture thresholds. Proper parameter adjustments can deliver immediate improvements.
Die design optimization
Engineers can improve flow by enlarging radii in flow channels and removing dead corners. Such changes distribute melt more evenly and reduce pressure accumulation. Selecting the proper L/D ratio of the die is also essential for stable extrusion.
Additives and material selection
Lubricant additives such as fluoroelastomers or PTFE-based agents reduce die wall friction. Choosing resins with higher melt flow index ensures smoother extrusion. For recycled resins, drying, filtering, and stabilizing are crucial steps to prevent flow irregularities.
VICHEM – ĐẠI LÝ PHÂN PHỐI PHỤ GIA HOÁ CHẤT ĐẠT CHUẨN CHÍNH HÃNG TẠI VIỆT NAM
Vichem is an authorized distributor of chemical additives with full legal compliance in Vietnam. We not only supply high-quality products but also provide after-sales services that meet international standards. Our services include:
- Technical consulting: Supporting customers in selecting the right product samples for their intended applications.
- Color matching tests: Vichem’s expert team conducts color tests to ensure results meet design standards.
- Application support: Assisting customers in integrating products into their production lines, ensuring optimal performance.
For more information about our products, please contact us through the following channels:
📞 Hotline: 08 1790 1790
📧 Email: contact@vichem.vn
🔎 LinkedIn: Vichem JSC
🌐 Website: www.vichem.vn
📌 Facebook: Vichem JSC
