Machining Engineering Plastics

Supply Services Limited has manual and CNC machine shops located in Auckland, Mount Maunganui and Christchurch. We are specialists in machining semi-finished plastics into finished plastic components for a wide range of customers New Zealand wide. Our expertise allows us to produce machined finished parts that are dimensionally accurate without introducing stresses and distortion to the part.

With decades of experience in machining engineering and high-performance plastics, let us produce your precise components from our vast range of plastics using our in-house machining capabilities.

Contact us today to discuss your requirements. 0800 102 112

Differences between plastics and metals

Compared to metals, plastics have a wide range of benefits to offer, although a number of restrictions must also be kept in mind. The use of plastics is possible in those areas where a favorable ratio between weight and strength is required.

βž• Benefits over metal

  • Low density
  • Noise and vibration damping
  • Electrical insulation
  • Chemical resistance
  • Corrosion resistance
  • Thermal insulation
  • Application specific modification possible

βž– Limitations compared to metal

  • Low thermal resistance
  • Greater thermal expansion
  • Lower mechanical properties
  • Poorer creep resistance

❗ Possible consequences, if not observed

  • Too much heat input in the component can lead to high stress levels and thus warping or fracture
  • Excessive heat input causes expansion of the plastic – the required tolerances of machined parts cannot be maintained
  • Inadequate fixation lead to deformation during machining

πŸ’‘ Recommendations

  • Good heat dissipation – best via the material chips, as well as adequate fixation

This approach needs to be adapted depending on the plastic, in order to establish the optimum cutting tools and parameters for all thermoplastic materials.

Only in this way can optimum components be made. Detailed information of machining plastic materials is available below.

plastic machining near me
high performance plastics nz

Tools for machining plastics

For the machining of plastics and semi-finished products, normal commercially available machines from the wood and metal working industries can be used with tools made of high-speed steel (HSS).

In principle, tools with cutting edge angles like those used with aluminium are suitable. But it is recommended to use special tools for plastic with a sharper wedge angle.

Hardened steel tools should not be used for processing reinforced plastics, due to the low holding times and the long processing times. In this case, the use of tungsten carbide, ceramic or diamond tipped tools is advisable. Circular saws fitted with carbide tipped saw blades are ideal for cutting plastics.

πŸ’‘ Recommendations

  • Use tools which are specific for plastics
  • Have a suitable cutting geometry
  • Have very sharp tools
Tooling in-place on a change parts trolley
Change part trolley nz with tooling manufactured in 316 stainless steel with HDPE tops

Download the full Ensinger Plastics, Machining Recommendations for Semi-Finished Engineering Plastics at the bottom of this article.

Machining Plastics

Machining is the fastest and most economical way to produce precise components, especially in small volumes. Very narrow tolerances can be achieved using machining procedures.

Cutting

Plastics can be cut using a band saw or circular saw. The choice depends on the shape of the stock shape. Heat generated by tooling when processing plastics can damage the material. For this reason, the right saw blade must be used for every shape and material.

βž• Band Saws

  • Suitable for cutting to size round rods and tubes
  • Support wedges must be used
  • Sharp and sufficiently set saw blades should be used
    • Good chip removal
    • Avoidance of high friction between blade and material to avoid excessive thermal build-up
    • Avoids saw blade blocking

βž• Circular Saws

  • Suitable for cutting sheets to size with straight edges
  • Use tungsten carbide tipped saw blades
  • Use high enough feed rate and adequate offset
    • Leads to good chip deflection
    • Avoids sticking to saw blade
    • Avoids overheating the plastic
    • Leads to good cutting edge quality

πŸ’‘ Recommendations for cutting plastics

  • Use a corresponding tensioning device
    • Avoidance of vibrations and unclean cutting edges
  • Warm cutting of very hard and fibre reinforced materials
    • pre-heat to 80-120Β°C
  • Tungsten carbide saw blades wear well and provide an optimum surface finish
Cutting engineering plastics and industrial composites

How are plastics best machined on a lathe? (turning)

Plastics can be processed on commercially available lathes. For optimal results, however, specific plastic cutters should be used.

βž• Cutting Tools

  • Use tools with small cutting radii
  • Broad-nosed finishing cutting edge for high-quality finish
  • Knife like cutting geometries for machining flexible parts
  • Use favourable geometries for fixing
  • Special chisel geometry for parting off
  • Cut circumferences and polished surfaces

πŸ’‘ Recommendations for lathe turning

  • Select a high cutting speed
  • Use a cutting depth of at least 0.5mm
  • Compressed air is well suited for cooling
  • Use of a steady due to reduced rigidity of plastics
    • Stabilise the component
    • Avoidance of deformation
machining plastics in lathe

Milling recommendations

Plastics can be milled using customary machining centres. This should be done using tools with adequate chip space in order to guarantee reliable discharge of chips and prevent overheating.

βž• Tools

  • Suitable for thermoplastics
    • Slot milling cutter
    • Face milling cutter
    • Cylindrical milling cutter
    • Single cutter tools
    • Fly cutter

πŸ’‘ Recommendations for milling

  • High cutting speeds and medium feed rates
  • Ensure good attachment
    • Rapid feed rate for the table and a high spindle speed coupled with correct fixture alignment lead to a higher quality machined finish
  • Thin work-pieces can be secured using a suction fixture or dual-sided adhesive tape on the router table
  • For flat surfaces, end milling is more economical than peripheral milling
  • During peripheral milling, tools should not have more than two cutting edges in order to minimise vibrations caused by a high number of cutting edges, and chip spaces should be adequately dimensioned

πŸ’‘ How better milling surfaces can be achieved

  • For surface milling, choose a low chip angle
  • Optimal cutting performance and surface qualities result from single cutter tools
  • Down milling should be used in preference to conventional milling

Drilling engineering plastics

When drilling plastic components, select a method suitable for plastic materials in order to avoid defects. Otherwise, there is a danger of breaking, tearing, overheating or dimensional deviations of the drill holes.

When drilling, attention must be paid to the insulating characteristics of plastic. These can cause plastics (especially semi-crystalline ones) to quickly build up heat during the drilling process, particularly if the drilling depth is more than twice the diameter. This can lead to β€œsmearing” of the drill and an inner expansion arising in the component, which can lead to compressive stress in the part (especially when drilling into the centre of round rod sections). The stress levels can be high enough to cause a
high level of warping, dimensional inaccuracy, or even cracks, fractures and bursting open of the finished component or blank. Appropriate processing for the material will prevent this.

βž• Tools

  • Well sharpened commercially available HSS drills
  • Use drills with a narrow bridge
    • Reduces friction and avoidance of heat build-up

πŸ’‘ General drilling recommendations

  • Use a coolant
  • Frequent withdraw of the drill
    • Chip removal
    • Additional cooling
  • Avoid the use of a manual feed
    • Ensures the drill does not become caught
    • Prevents cracking

πŸ’‘ Recommendations for drilling small diameter holes (<25mm)

  • Use of HSS drills
  • Use a spiral drill
  • Twist angle of 12-25Β°
    • Very smooth spiral grooves
    • Favours chip deflection
  • Frequent removal of drill to avoid thermal build up
  • In the case of thin-walled components
    • high cutting rates
    • Neutral chipping angle in order to avoid drill catching in the component and thus tearing or lifting the work piece by the drill

πŸ’‘ Recommendations for drilling large diameter holes (>25mm)

  • Carry out trial drilling with large drill holes
  • Select pre-drilling diameter which is no larger than 25mm
  • Carry out finishing with an inner cutting chisel
  • Introduce drilling into long rod sections only from one side
    • Bilateral drilling can cause unfavourable tension or tearing of the material
  • In extreme cases, it may be advisable to carry out the drilling on a pre-warmed component at approx. 120Β°C (heating time approx. 1 hour per 10mm cross-section)
    • To ensure dimensional accuracy, finish machining then takes place after the blank has cooled down completely
machining plastics with manual lathe

Cutting threads

Threads are best produced in engineering plastics using full form tips for male and full form tips or thread milling cutters for female threads

βž• Tools

  • Use full form tips
  • Dies are not recommended

πŸ’‘ Recommendations for cutting threads

  • Taps often have to be provided with an allowance
  • Do not select a pre-setting which is too high, in order to avoid squashing the thread

Planing/plane milling

Planing and plane milling are chip production methods with geometrically determined cutting for the manufacture of certain cuts to produce equal surfaces, grooves or profiles (using shaping milling).

Both procedures differ only in that with planing a straight line of material removal is made across the surface using a planing machine cutting tool. In the case of plane milling, on the other hand, the surface is processed using a milling head. Both processes are well-suited to produce even and/or equalised surfaces on semi-finished goods. The main difference is that optically different surfaces arise (surface structure, gloss).

Grinding

In grinding, the overall effect of cutting, workpiece, delivery and feed movements results in a continuous chip removal from the surface being processed. The grinding result is influenced by:

  • Grinding machine
  • Tools being used
  • Grinding medium
  • Working parameters of the grinding process
  • Material to be processed
  • Roundness/straightness of the semi-finished product

Particularly decisive working parameters are:

  • Cutting speed
  • Forward rate of advance
  • Delivery
  • Cross-sectional advance rate
machining engineering plastics

Surface quality, reworking and de-burring

To obtain a good surface quality, the following guidelines should be followed:

βž• Tools

  • Suitable for plastics
  • Tools well sharpened and smooth
  • Tools adequately spaced to ensure that only the cutting edge comes into contact with the plastic

βž• Processing Machines

  • Flawless, high-quality finished surfaces can only be achieved with low-vibration machines

βž• Materials

  • Use low-tensioned annealed materials
  • Note the properties of the plastic (thermal expansion, low strength, poor heat conduction … )
  • Due to the minimal rigidity of the material, the work piece must be adequately supported and lie as flat as possible on the supporting surface in order to avoid deflection and out-of-tolerance results

βž• Cooling

  • Use coolants for processes involving the generation of high levels of heat (such as drilling)
  • Use suitable coolants

πŸ’‘ Recommendations

  • Tension pressures must not be too high, which can lead to deformation and impression marks on the material
  • Keep feed rate to a moderate level
  • Select a high cutting speed
  • Good removal of chips must be guaranteed in order to prevent tool congestion
  • Ensure that chip removal is equal on all sides in order to prevent warping

βž• Typical de-burring methods for engineering plastics

  • Manual de-burring
  • Jet de-burring
  • Cryogenic be-burring
  • Flame de-burring
  • Hot air de-burring
  • Infrared de-burring
  • Rumbling/Trovalising

Cooling and cooling lubricants

Currently there is a trend towards using dry machining with engineering plastics. As there is now sufficient experience available in this area, it is frequently possible to do without the use of cooling lubricants. Exceptions for thermoplastic machining processes are:

  • Deep drill holes
  • Thread cutting
  • Sawing reinforced materials

It is possible to use a cooled cutting surface to improve both the surface quality and tolerances of the machined plastics parts, and also to allow the faster feed rates and consequently reduced running times.

βž• Machining with coolants

If cooling is required, it is recommended to cool via:

  • The chippings
  • Using compressed air
  • Use of water-soluble coolants
  • Commercially available drilling emulsions and cutting oils can also be used

βž• Machining amorphous plastics

  • Avoid using coolants – materials are liable to develop tension tearing
  • If cooling is imperative
    • Parts should be rinsed in pure water or isopropanol right after machining
    • Use suitable coolants
  • Pure water
  • Compressed air
  • Special lubricants

What are amorphous plastics?

πŸ’‘ Advantages of dry machining

  • No media residues on the components
  • Advantageous for components used in the medical device and food technologies
  • Influence of cooling lubricants on the material can be excluded (swelling, dimensional changes, tension, tearing etc)
  • No interaction with the material

❗ Note about dry machining

  • Cooling is essential to achieve a good dissipation of heat!

Machining errors – causes and solutions

Cutting and Sawing Turning and Milling 
DifficultiesRoot CausesDifficultiesRoot Causes
Surface has started to melt- Blunt Tool
- Insufficient lateral play / clearance
- Insufficient coolant feed
Surface has started to melt- Blunt tool or shoulder friction
- Insufficient lateral play / clearance
- Feed rate too low
- Spindle speed too high
Rough surface- Feed rate to high
- Tool unprofessionally sharpened
- Cutting edge not honed
Rough surface- Feed rate too high
- Incorrect clearance
- Sharp point at the tool (slight radius on point of milling cutter required)
- Tool not centrally mounted
Spiral marks- Tool friction during withdrawal
- Burr on the tool
Burr on corners of cutting edge- No space in front of the cutting diameter
- Blunt tool
- Insufficient lateral play / clearance
- No lead angle at the tool
Concave and convex surfaces- Point angle too great
- Tool not vertical relative to the spindle
- Tool is deflected
- Feed rate too high
- Too mounted above or below the centre
Cracks or flaking at the corners- Too much positive inclination at the tool
- Tools not sufficiently run-in (action of tool is too hard on
the material)
- Blunt tool
- Tool mounted under the centre
- Sharp point at the tool (slight radius on point of
milling cutter required)
"Stumps" or burr
at the end of the cutting surface
- Point angle not large enough
- Blunt tool
- Feed rate too high
Chatter marks- Excessive radius on point of milling cutter at the tool
- Tool not sufficiently firmly mounted
- Insufficient material guidance
- Cutting edge width too great (use 2 cuts)
Burr on the outside diameter
- Blunt tool
- No space in front of the cutting diameter
machining plastics without correct tools
machining plastics with the correct tools

Download Ensinger Plastics “Machining Guidelines for Semi-Finished Plastics”

Download PDF
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Thank you to our supplier Ensinger for allowing us to reproduce this material and the use of their images.

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