Polypropylene is what many people mean when they say “plastic” because it is the base of many products and capable of being fabricated by many different processes. From fiber to film, from injection molding to thermoformed sheet polypropylene is as versatile as it is varied. It can be formulated to result in a wide range of melt points, weights, stiffness and machineability. Formulations can provide a substance somewhere between traditional rubber and conventional plastics. Other possibilities may be filled or reinforced grades which offer good stiffness and stability.
Most Polypropylene products are machined on CNC routers. Handheld electric or air routers do not normally give adequate results. Polypropylene can be a difficult material to work with because of its gummy nature. Feed rates are critical for productivity and cutting tool selection is critical for the best results.
Machining Polypropylene is an ongoing improvement process often established by a basic trial and error process. There are a few principals to make use of when machining polypropylene. Every attempt should be made to cut large chips. This can be accomplished using slow helix tools; these tools tend to take a larger chip than conventional helix tools and are available in single or double flutes in both upcut and down-cut spirals. Here is where some trial cuts should be made to determine whether single or double flutes and up or down spiral works better in a specific application.
Slow helix tools are also available with a bearing pressed on the end of the cutting edge for guided trimming operations if a CNC router is not available. Because of the gummy nature of Polypropylene and the heat generated by the cutting action, high-speed steel tools are not recommended. Solid carbide bits will outperform high-speed steel, carbide tipped or diamond tools and are the only type recommended for cutting Polypropylene. High feed rates should be employed along with lower spindle speeds. This will tend to subside re-weldment behind the cut and waste wrap around.
Feed rates should be increased until such time as the finish is unacceptable. Spindle speed should then be reduced until the finish is once again acceptable. The process can then be repeated until the best result is achieved. This process should be repeated, and catalogued, for each unique set up.
One may want to consider a two-pass process to get the most out of both feed rate and piece part finish. If a tool changer is available, the second pass can be taken with a finishing tool. When the depth of cut exceeds the cutting-edge diameter of the tool by more than a factor of three, multiple passes should be taken. If this is the case, the second pass should be taken with the same tool as the first cut.