Engineering plastics – The Manual - F.wood-supply.dk

Processing plastics
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-finished 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-specific 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 configured as a broad-nosed finishing
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 (Fig. 3).
Fig. 1
Broad-nosed finishing cutting edge
Secondary cutter
Fig. 3
Parting off flexible plastics
Lathe tool
Fig. 2
Grinding prevents burr formation
* We provide written and oral application advice designed to support you in your work.
This is offered in the form of a non-binding recommendation, also in respect of any
third-party industrial property rights. We are unable to accept any liability for damage
occurring during machining.
2. Milling For plane surfaces, end milling is more economical
than peripheral milling. During circumferential and
profile 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 finish 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
Stress development with blunt drill Stress development with sharpened drill
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 fillers 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.
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