Processing plastics

- Feb 18, 2019-

General remarks*
Unreinforced thermoplastics can be machined using highspeed steel tools. Machining reinforced materials calls for the use of carbide tools. In either case, only flawlessly sharpened tools should be used. Due to the poor thermal conductivity of plastics, steps must be taken to ensure good
heat dissipation. The best form of cooling is heat dissipation through the produced chips.

Dimensional stability
Dimensionally precise parts can only be made from stressannealed semi-fnished products. Otherwise, the heat generated by machining will inevitably lead to release of processing tension and component warping. If high stock removal volumes occur, intermediate annealing may be advisable after the main machining process in order to dissipate any build-up of thermal tension. We can provide information on the necessary temperatures and timings on a material-specifc basis. Materials with excessive moisture absorption (e.g. polyamides) must be conditioned before machining where applicable. Plastics require greater production tolerances than metals. In addition, it is important to bear in mind that thermal expansion is many times greater than with metal

Machining methods
1. Turning
Guideline values for tool geometry are given in the table (S. 77). For surfaces of particularly high quality, the cutting edge must be confgured as a broad-nosed fnishing cutting edge as illustrated in Fig. 1. For parting-off operations, the lathe tool should be ground as illustrated in Fig. 2, in order to prevent the formation of burrs. When working with thin-walled and particularly flexible workpieces, however, it is more advantageous to work with tools which have a knife-like cutting geometry.

2. Milling For plane surfaces, end milling is more economical than peripheral milling. During circumferential and profle milling, tools should not have more than two cutting edges in order to minimize vibrations caused by a high number of cutting edges, and chip spaces should be adequately dimensioned. Optimum cutting performance and surface fnish quality are achieved with single-cutter tools.

3. Drilling Generally speaking, twist drills can be used;
These should have a twist angle of 12° to 16° and very smooth spiral grooves for optimum chip removal. Larger diameters should be pre-drilled or should be produced using hollow drills or by cutting out. When drilling into solid material, particular attention should be paid to properly sharpened drills, as otherwise the resulting compressive stress can increase to the point that the material can split.

Reinforced plastics have higher residual processing stresses with lower impact strength than unreinforced plastics, and are consequently particularly susceptible to cracking. Where possible they should be heated prior to drilling to a temperature of around 120 °C (heating time appr. 1 hour
per 10 mm cross-section). This method is also advisable when machining polyamide 66 and polyester.

4. Sawing It is important to avoid unnecessary heat generation due to friction as, when sawing mostly thick-walled parts, relatively thin tools are used. Well sharpened saw blades with large tooth offsets are therefore recommended for sawing.

5. Thread cutting The best way to cut threads is using thread chasers; the formation of burr can be avoided by using twin-toothed chasers. Die nuts are not advisable, as these can cause additional cutting when withdrawing the nut. When using tap drills, a machining allowance (dependent on the material and diameter, guideline value: 0.1 mm) is frequently required.

6. Safety precautions Failure to observe the machining guidelines can result in localized overheating which can lead to material degradation. The decomposition products released from materials such as PTFE fllers must be captured by an extraction device. In this context, tobacco products must be kept out of the production area due to the risk of toxic effects