Melt Fracture

Flow Instability

The root cause of the problem lies in the way the polymer chains entangle and resist the deformation imposed by the machinery. As the melt exit the die, the sudden release of pressure causes the material to snap back and create the characteristic rough texture of melt fracture. This is most common in resins with a narrow molecular weight distribution or those with a high degree of long chain branching.

The instability is often localized at the surface of the part where the shear rates are the highest. It can be monitored by observing the pressure fluctuations in the extruder barrel or by inspecting the surface of the extrudate. Understanding this behavior is necessary for optimizing the design of the dies and the processing conditions.

Processing Control

Engineers use several techniques to prevent the defect from ruining the quality of the finished product. Increasing the temperature of the die and the barrel reduces the viscosity and the shear stress, which helps to eliminate melt fracture. Another common approach is to add processing aids like fluoropolymers that coat the internal surfaces of the die and allow the melt to slip more easily.

This reduces the friction at the wall and allows for higher production speeds without sacrificing the surface finish. Changes to the geometry of the die, such as increasing the land length or adding a taper, can also help to stabilize the flow. These adjustments must be carefully balanced to avoid degrading the polymer or increasing the cycle time.

Visual Defect

The presence of surface roughness not only ruins the aesthetics of the part but can also affect its mechanical performance and functional properties. In film production, melt fracture can lead to poor clarity and reduced tear strength, making the material unsuitable for packaging. For injection moulded parts, the rough surface can interfere with the assembly of the components or cause issues with printing and coating.

The defect is a clear sign that the process is operating outside of its stable window and that the material is being overstressed. Consistent monitoring of the part surface allows the operator to detect the onset of the problem and take corrective action. It ensures that the final product meets the high standards required by the customer.

Precise control of the extrusion speed and the melt temperature is the most effective way to prevent the occurrence of this defect.