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Plastic Machining Tips

General machining tips

 
Engineering Plastics in stock shapes can be easily machined on ordinary metalworking and in some cases on woodworking machines. However, there are some points, which are worth noting to obtain optimum results. Due to the poor thermal conductivity, relatively low melting temperatures of thermoplastics, generated heat must be kept to a minimum level and heat buildup in the plastics part during machining should be avoided. This is to prevent deformations, stresses, color changes or even melting. As such, the following general machining guidelines need to be observed.
 
-  Tools must be kept sharp and smooth at all times
-  Feed rates should be as high as possible
-  Tools must have sufficient clearance so that the cutting edge only comes in contact with the plastics material
-  Swarf must be removed constantly
-  Coolants should be applied for operations where plenty of heat is generated (e.g. drilling).
 

Machining forces / clamping

 
Since engineering plastics are generally softer than metals, machining forces need to be lower and plastics may deform if clamped too tightly. However, the clamping pressure should be high enough to avoid any unsafe condition where the plastics part could come out of the clamping tools. As engineering plastics are not as rigid as metals, it is important to support the work adequately during machining in order to prevent deflection or deformation, e.g. thin walled tubes often require the use of an internal plug at the chucked end in order to allow accurate machining of bushings with respect to roundness and tolerances.
 

Tools


High speed steel (HSS) tools work well with many plastics. However, tungsten carbide (ground cutting edges!), ceramic or polycrystalline diamond (PCD) tooling is preferred for long production runs. This is essential when machining glass and/or carbon fiber reinforced or graphite filled materials (long tool life and good surface finish). When machining PBI or PAI, diamond coated or polycrystalline diamond tooling provides optimum results, but carbide tipped tools can be used in case of very short production runs.
 

Coolants


Keeping the cutting area cool generally improves surface finish and tolerances. When coolants are required, water-soluble coolants generally do very well. They should, however, not be used when machining amorphous thermoplastics, such as PC, PPSU, PEI, and PSU because these materials are susceptible to environmental stress-cracking. The most suitable coolants for these materials are pure water or compressed air. When the use of water-soluble coolants or general-purpose petroleum-based cutting fluids cannot be avoided during the machining of amorphous thermoplastics (e.g. during drilling of large diameters and/or deep holes or during tapping operations), the parts should immediately after machining be thoroughly cleaned with isopropyl alcohol first and rinsed with pure water afterwards in order to reduce the risk of stress-cracking. A strong jet of compressed air or coolant also directs or removes chips from the work area, avoiding them to interfere with the cutting tool and the workpiece.operations. Apart from drilling and parting, coolants are not typically necessary for thermoplastic machining
 

Machining tolerances


The machining tolerances that are required for thermoplastic parts are in general considerably larger than those normally applied to metal parts. This is because of the higher coefficient of thermal expansion, lower stiffness and higher elasticity, eventual swelling due to moisture absorption (mainly with nylons) and possible deformations caused by internal stress-relieving during and after machining. The latter phenomenon mainly occurs on parts where machining causes asymmetric and / or heavy section changes, e.g. when machining a U-shape from a plate or a bush from the solid rod. In such cases, a “balanced” machining on both sides of the stock shape’s centreline, reducing warpage, or a thermal treatment (stress-relieving) after pre-machining and prior to final machining of the part may prove advantageous. As a guideline, for turned or milled parts, a machining tolerance of 0.1-0.2 mm can be applied without taking special precautions (min. tolerance for small sizes is 0.05 mm). However, tighter tolerances are possible with very stable Advanced Engineering Plastic Products’ such as PBI, PAI, PEEK, and PPS.
 

Turning


The continuous chip stream produced when turning and boring many thermoplastics can be handled well using a compressed air powered suction system (directly disposing of the swarf onto a container), in this way avoiding the chip wrapping around the chuck, the tool or the workpiece.
 

Milling


Two flute end mills, face mills and shell mills with inserts as well as fly cutters can be used. Climb milling is normally recommended to help reduce heat by dissipating it into the chip, and melting and poor surface finishes are minimized.
 

Drilling


High speed steel twist drills work well, but plenty of heat is generated so that a cooling liquid should be applied, especially when hole depths are more than twice the diameter. In order to improve heat and swarf removal, frequent pull-outs (peck drilling) are necessary, especially for deep holes (pull-out the drill every time a depth ≈ 1.5 x the diameter is attained). For large diameter holes, it is advisable to use drills with a thinned web (dubbed drill) in order to reduce friction (shorter chisel edge) and consequently heat generation. It is also recommended for large holes to drill stepwise; e.g. a bore diameter of 50 mm should be made by drilling successively with Ø 12 and Ø 25 mm, then by expanding the hole further with larger diameter drills or with a single point boring tool. For PBI, PAI, PEEK, PPS rods over 100 mm diameter, as well as for PA rods over 200 mm diameter, it is even recommended not to use high speed twist drills at all in order to avoid cracks, but to "bore" the holes on a lathe using “insert drills” or a rigid, flat boring tool with its cutting edge perfectly set on centre-height. For these materials, some machinists prefer to heat the stock shapes up to about 120-150°C prior to drilling. However, care has then to be taken that after drilling and before starting the finishing operations, the plastics piece is completely cooled off to room temperature (uniform temperature all over the section prior to drilling as well as prior to finishing) When drilling or boring through-holes, feed should be reduced at the bottom of the cut in order to prevent the drill or flat boring tool from pulling through at the exit-side, causing chipping or breaking out. It is not recommended to hand feed the drill because the drill may "grab" and stress the material.
 

Sawing

 
Band saws, circular saws or reciprocating saws that have widely spaced teeth in order to assure good chip removal can be used. They should also have enough set to minimize the friction between the saw and the work and also to avoid close-in behind the cutting edge, causing excessive heat build-up and even blocking of the saw. Proper clamping of shapes on the worktable is required to avoid vibrations and consequent rough cutting or even rupture.