Improving TPE Wire Extrusion Quality: Addressing Material Buildup and Surface Roughness Issues

During TPE wire extrusion, material buildup in the die and surface roughness are common issues that recur frequently in mass production. The causes of these problems are often multifaceted, involving an interplay of material, process, and mold factors. Adjusting a single aspect may provide temporary relief, but it is difficult to eliminate the problem completely. Why do these issues occur? The editors at Zhongsu Wang provide the following explanation.

First, it is prudent to begin troubleshooting by examining the material. If the formulation contains a high proportion of low-molecular-weight additives—such as insufficiently refined white oil or excessive lubricants—these low-molecular-weight components will precipitate during high-temperature extrusion. They accumulate at the die exit, forming buildup; when carried away by the melt, they leave pitting or irregular rough textures on the surface of the TPE wire. Insufficient material flow can also cause problems. Raw materials with a low melt flow index do not flow smoothly through the die, making it easy for a “shark skin” texture to form upon extrusion—a problem that is particularly pronounced in thin-wall overmolding or high-speed extrusion applications. Additionally, if raw materials have not been sufficiently dried to remove moisture and volatiles, microbubbles formed by vaporization during extrusion can leave fine, dense air bubbles on the surface of the TPE wire.

Second, proper matching of process parameters is equally critical. If the barrel temperature is too low, the material will not be sufficiently plastified, resulting in partially unmelted particles in the melt. These particles become the starting point for material buildup at the die. Conversely, if the temperature is too high, the material may undergo slight degradation, and the degradation products will also accumulate on the die. If the die temperature is set too low, the surface layer of the melt will cool prematurely, causing cracks or a rough surface when the material exits the die. Careful attention must also be paid to the coordination between screw speed and pull-off speed. Excessively fast pull-off can cause the melt to be overstretched, making the surface of the TPE wire prone to waviness. When the extrusion rate exceeds the pull-off rate, material accumulates at the die, further exacerbating the build-up problem. Finding the appropriate speed ratio to maintain a uniform and stable state of stretching as the melt exits the die can effectively improve the surface quality of TPE wire.

Additionally, the condition of the die must not be overlooked. An improper runner design or insufficient inner wall finish can cause uneven shear stress on the melt at the exit, leaving marks on the surface of the TPE filament. Dies that have become worn or carbonized from long-term use are more prone to material buildup and must be thoroughly cleaned before further troubleshooting. The temperature and position of the cooling water channels also affect the final surface finish of TPE wire. Excessively rapid cooling causes the surface to contract abruptly, resulting in a hazy and rough appearance, while insufficient cooling makes the wire prone to deformation during subsequent drawing.

In summary, experienced manufacturers typically conduct a dry-run extrusion test first when troubleshooting TPE wire extrusion issues. By observing the surface condition of the melt as it exits the die under stable process conditions, if roughness or material buildup is already present during the empty-machine test, it can generally be concluded that there is an issue with the material formulation or temperature settings. If the empty-machine test is normal but problems arise as soon as the coating material is applied, the focus of the troubleshooting shifts to die compatibility and the coordination of the draw-off speed ratio. This approach—first narrowing down the problem area and then troubleshooting item by item—is far more efficient than blindly adjusting parameters through trial and error.