MACHINING GUIDELINES

Machining of Engineering Plastics

Engineering plastics have opened new horizons for machinery builders and design engineers. Aside from mechanical property limitations, it has often been the case that manufacturing methods were the limiting factor for using engineering plastics. This was especially true for large volume parts made of cast nylon, acetal or PET, where other manufacturing methods such as injection molding could not be used. It was also true for complex parts, which needed machining on all sides for close tolerances. Both high precision and large volume parts can be machined economically in small and medium sized lots. For the machining of quality, high value products, specific characteristics of the plastics must be taken into account when choosing which machinery and tooling to use and how to use it. 

Machines and Tooling 

No special machines are required for machining. Normal woodworking or metalworking machinery can be used with tools made with high speed steel. Saw cutting of plastic with a circular saw requires the use of carbide toothed saw blades. An exception to this is the group of glass filled plastics. Machining with carbide tools is possible, but the short tool life does not make it economical. For glass filled plastics, we recommend the use of diamond tipped tooling which, although much more expensive than conventional tooling, have significantly longer life. 

Machining and clamping the part 

In comparison to metals, plastics are poor thermal conductors and have a low modulus of elasticity. If machined inappropriately, the part can heat up and thermal distortions can occur. High clamping pressure and dull tooling also cause deformation of the part during machining. The result can be dimensional variations outside of the tolerance range. Satisfactory results can only be achieved if certain guidelines are followed during machining. These are:

• The feed rate should be as high as possible.
• An optimal chip removal path should be established so that the chips do not come into contact with the part.
• The tooling should produce very sharp cuts. Dull cuts can produce heat, which can produce deformation and thermal expansion.
• The clamping pressure should not be too high, otherwise the part may deform and / or have indentations from the clamping tools.
• Because of the material's flexibility, the part must supported as fully as possible on the machine table.
• Smooth, high quality surfaces can only be achieved if the machines are vibration free.

Drilling large diameter holes in round discs 

Drilling large holes in high crystalline plastics such as cast nylon creates high temperatures on the drill. Plastics are poor thermal conductors, and the heat cannot be conducted away fast enough. The heat expands the material, which can lead to inner stresses in the material. The stresses can get so high that the work piece could crack. Proper machining of the part can significantly reduce this possibility. It is recommended to rough drill the hole and finish drill it with two different tools. Rough drilled holes should have a diameter of > 1.375". With very long work pieces, holes should be drilled only from one side. When drilling from both sides, a high-stress point develops where the two holes meet, and this can cause cracking. In extreme cases, it may be necessary to heat up the entire work piece to about 250 - 300 ° F and rough drill the hole in this condition. The finishing of the hole and the rest of the part can then take place after the piece has cooled off completely and an equal temperature throughout the material has been achieved. If these machining guidelines are followed, complex parts made of engineering plastics can be finish machined to the highest quality standards. 

Annealing guidelines of different thermoplastics

Despite all precautionary measures an uneven cooling speed in the production process of the semi-finished material might not be avoidable; in this case internal tensions might occur. Likewise tensions can be conferred into the part by the machining process into the part. These tensions can lead to the distortion and in the worst case even to the breaking of the part. To reduce the danger from distortion or breaking an annealing e.g. in air or in nitrogen is recommended, with an annealing time of min. 2 hours (4 hours are better) for each 10 mm wall thickness. To avoid additional tensions in the part by heating and/or cooling, these processes which must be added to the annealing time and should be persecuted slowly.