Die galling, also known as die drainage, die bleed and scale build-up, is a common failure in the extrusion process. This can lead to a variety of product defects such as missing extrudate surfaces, process interruptions and complete cracking of the product. Processors often have to spend time and manpower troubleshooting such problems, but have no idea how to avoid them.

　　When the extrusion line is running, the simple shaped die may remain clean. Changing the angle at which the extrudate is pulled away from the die can change the shape of the layout of the die where ooze occurs and make it easier to clean. Die galling can also be consciously transferred to appear on the extruded product, moving it further away from the production equipment. If a mechanical automatic scraper is installed when processing extruded sheet material, this can prevent die bite failures.

However, most fabricators usually have to interrupt the production line to remove the galling problem, which is not economically viable. Applying a release agent or silicone to the outer surface of the die head after the mouth has been cleaned will reduce the incidence of galling and extend the interval between cleaning the die head. Slightly lowering the production rate may also reduce the occurrence of galling, but this will affect the yield. All of the above solutions are not critical. There is a better solution that will enable you to understand what is causing the galling.

　　The galling phenomenon is related to the amount of stress at the die opening, where the resin flows relatively slowly through the die head and then suddenly accelerates when extruded to the die opening. This acceleration creates a stress in the melt. Under the stress, the low molecular weight polymer components separate from the rest of the melt and accumulate at the die opening. The solution is either to reduce the stress at the die opening or to reduce the tendency for the various components of the melt to separate from each other. These solutions include changing the process conditions, the raw material and the mouthpiece.