Machinability Characters of Plastics

Machinability Characters of Plastics

Machinability Characters of Plastics - Even though plastics do not possess a crystalline structure of the kind that one finds in metals, they can be separated by shear stresses just as metals are. Therefore chip forming processes can be applied, if allowance is made for the property differences. Compared with metals, plastics have a low elastic modulus, and deflect easily under the cutting forces; therefore they must be carefully supported.

Because of viscoelastic behaviour, some of the local elastic deformation induced by the cutting edge is regained when the load is removed. This elastic recovery calls for large relief angles, and for a tool setting closer than the finished size of the part. In general, plastics have low thermal conductivity; therefore the heat buildup in the cutting zone is not distributed over the body and the cut surface may over heat the tool.

In a thermoplastic resin, the glass transition temperature may be reached and the surface smeared or damaged, while thermal break down and cracking may occur in thermosetting resins. Therefore friction must be reduced by polishing and honing the active tool faces and by applying a blast of air or a liquid coolant (preferably water based, unless the plastic is attacked by it).

Since the shear zone is shortened and the cutting energy is reduced with a large rake angle, cutting tools are made with a large a rake angle as possible without changing the cutting mechanism into cleaving, in which coarse, disjointed fragments are lifted up and a very poor surface is produced.

Turning Tool

Cutting Tool

Plastics can be surprisingly difficult to machine when rein forced by fillers. Glass fibres are particularly hard on the tool and it is not uncommon that only tungsten carbide or diamond tools can stand up. Non contacting techniques such as laser beam drilling or cutting offer promise but have not found extensive application yet.

Twist drills for drilling plastics should have wide, polished flutes, a low (<30°) helix angle and a 60-90° point angle, particularly for the softer plastics. Acrylic and polystyrene plastics can be machined to tolerances of +- 0.025 mm. However, to maintain close to lerances, it is necessary to anneal at the time of machining.

This is especially true when considerable material is to be removed and there is likeiihood of stress build up. Annealing can be accomplished by heating the plastic to about a few °C below the practical heat distortion temperature. Because of the poor heat conductivity, the time required for the anneal may be as long as 24 hours.

Drill point recommended for drilling teflon

In general, rotary files and burrs are run at higher speeds when machining plastics than when cutting metals in order to prevent edge chipping. Speeds of 30 to 100 meters per minute are ordinarily) used for machining. If abrasive fillers are used in the plastics, the speed should be cut to about one half this figures.

Laminated plastics can be milled with standard tools at speeds and feeds similar to these used with bronze and soft steel. The feed rate is determined by the desired finish.

The finish on some plastic articles may be improved by polishing or buffing using only light pressure. Since the lustrous surface on most molded plastic products may be destroyed by machining, wherever possible, the plastic products may be used in the as molded condition with out machining, If, however, the machining can not be avoided, the designer should try to specify machining operations only in locations not visible when the product is in use.