It was waaaay back in May 2013 when we first published a blogpost about polishing plugs and why it is such an important concept. After some recent discussions with toolmakers and thermoformers about different plug materials and geometries, we figured this would be a good time to revisit the topic of polishing. Looking back in the archives, our original technical guide was titled “Polishing Guide.” The updated version is more precise: “Tips for Modifying Plug Surface“. The distinction is important as we shall see.
Temperature, substrate, surface finish, motion, dust, and many other factors affect the plug/sheet interface. Different surface conditions due to the machining or polishing of a plug material, or even a few degrees of temperature variation in a mold will affect the coefficient of friction (CoF) and sheet stretch values. The value changes for each specific plastic relative to plug material and even from roll to roll of plastic. There is both a static (objects at rest) value and a kinetic (objects in motion) value for the frictional coefficient. The only suitable method for determining a CoF value is experimentation with the actual plug and sheet.
Because there is no specific, descriptive guide on the ideal plug surface finish, we often encounter discrepancies between what a toolmaker provides and what a thermoformer needs. (This can also happen internally where a thermoformer has an in-house tool shop.) Time, of course, can factor into the decision about how much work is required to achieve a certain finish. And machining the plugs at the right speeds and feeds with sharp tools to achieve the best possible surface is not always a given. Still, it should always be assumed that the plugs will require some level of polish before they are first put into service.
The HYTAC Selector Guide provides a good overview of material properties, including surface roughness. Using the guide and recalling that HYTAC syntactics fall into three categories will help you choose the right material and set the right expectations for the surface finish.
Epoxies: W, WF, WFT – typically the most brittle materials with relatively high Ra values, with the exception of WFT which contains PTFE.
Copolymers: FLX, FLXT, C1R – these offer the best possible surface finish due to the combination of microballoons and a resin formulation which improves machinability. FLXT also contains PTFE.
Thermoplastics: B1X, XTL – the toughest materials are typically chosen for their durability and overall performance, but there is a clear difference in surface roughness between the two, with XTL offering a smoother surface.
When forming multilayer sheet, for example, a PTFE-impregnated syntactic like FLXT is typically the best choice because the surface can be polished to a high lustre. The added release properties from the PTFE reduce sticking. (Of course, sheet temperature control is always criticial.) But for other materials, the plugs do not always have to be highly polished. In some cases, roughing up a plug with sandpaper can help with material distribution and final part performance. As we’ve said before, it’s important to choose the right plug material in the context of the job.