Machining and finishing of high temperature polymer materials can release residual stress. It is recommended that a second annealing is performed on the semi-finished stock prior to the machining process. Annealing can limit dimensional changes, remove stress, and increase levels of crystallininty.
Typical annealing for PEEK polymers:
- Dry component for minimum of three hours at 300ºF
- Determine ramp up and hold at specified temperature for 4 hours
- Determine ramp down to 275ºF
- Turn off oven and allow to cool back to room temperature
Note: If finished component has extreme cross sections further annealing and procedures may be required to achieve optimum tolerances.
We recommend the following to minimize as much stress as possible:
- Machine a skim cut of about .010 to .015 off od (sometimes id as well) just make sure you allow ample material to machine finished part.
- Certain materials may required an additional annealing process.
- Use the recommend tooling, speeds and feeds, but do your own evaluation on machining process as this is only a recommendation.
- Use minimal cutting depth to reduce frictional heat build up.
- Machining to within .010 to .020 of final dimension and let set at ambient temperature for 24 hours, if required.
- Take final cuts to print specifications.
Critical Maching Concerns Engineering Plastics
Engineering plastics can be machined in essentially the same manner as metals. Nevertheless, their physical properties will require closer attention to several factors —
- Limiting Heat Build-up: The softening or melting temperatures of engineering plastics are roughly 1/10th those of metals.
- Melting or Scorching: The thermal conductivity of plastics is low relative to metals. Most of the heat generated by machining will stay at the surface. Temperatures at the surface can rinse unexpectedly high.
- Loss of Tolerance: If the overall temperature of the stock changes during or after machining, expansion or contraction can cause the part to fall out of tolerance. Softening of the stock can allow it to deflect at the surface under the pressure of the cutting tool. When the pressure is removed, the stock will recover and fall out of tolerance. This can frequently be managed by using lubricants and changing tooling or speed.
- Controlling Deflection: Plastics inherently vary in their stiffness (modulus) and are more elastic at higher temperatures. The entire stock can deflect under the pressure of cutting. Proper tooling and support remains important and particular attention should be given to adequately supporting the work.
Tooling should be designed or selected with the following characteristics in mind.
- Positive Rake Angle: Provides the cleanest, smoothest, and most accurate cut.
- Positive Relief Angle: Helps minimize tool wear by allowing space for the plastic to recover from being compressed during cutting.
- Polished Upper Surfaces: Helps minimize heat build-up. Sharp tools are essential for accurately removing material by achieving satisfactory surface finish and limiting heat build-up.
- Tooling Hardness: Hardness affects tool life. Diamond surpasses Carbide and Carbide surpasses High Speed Steel. Diamond tools are strongly recommended for glass- and carbon- fiber filled grades or for high volume operations.
Water or oil coolants should be used to cool the tool tip and to help remove chips or shavings. Parts should be machined symmetrically using shallow cutting depths per pass to minimize machining stresses.