Many plastic parts are injection molded but it is frequently faster and less expensive to produce high precision parts from stock shapes. This guide will help explain the differences between engineering plastics and metal using specific examples and underlying principles.
Factors that will be covered include chip formation, breakdown of the cutting edge, heat generation, and deformation of materials during machining.
There are two basic types of polymers:
Thermoplastic – This polymer is capable of being repeatedly softened by an increase in temperature. Increasing the temperature leads to a physical change.
PEEK is a thermoplastic.
Thermoset - A polymer that changes in to a substantially infusible and insoluble material when cured by application of heat or chemical means.
Torlon is a Thermoset polymer.
1. Stock shapes of extruded plastics have already been processed into their final shape at temperature so parts can be machined to a higher precision than with molding. All injections molded parts have a certain percentage of shrinkage that varies by axial orientation or thickness and inline.
2. The cutting edge should be designed to produce tensile fracture.
3. Continuous chip formation is of critical importance with elastic chip formation. The Positive Rake angle, Relief angles, Polished flanks, and sharp cutting edges are very important.
4. Tensile and flexural strength decreases as temperature increases. As temperature falls the brittleness increases. Properties such as specific heat, coefficient of thermal expansion, glass transition temperature Tg, and thermal conductivity are significanlty different than metals.
Tg glass transition is the temperature which increased molecular mobility results in significant mobility and changes in the properties of polymer including a different thermal expansion.
Machining and finishing of polymer material can release residual stress. It is recommended that a second annealing is performed on the part prior to final finishing if a large amount of machining or finishing is carried out. Annealing can limit dimensional changes, remove stress, and increase levels of crystanllininty.
Typical annealing for PEEK polymers:
Dry component for minimum of three hours at 300ºF
Allow component to heat up 18ºF per hour until 400ºF is reached.
Hold for 4 hours
Ramp down to 275ºF at 18ºF per hour
Turn off oven and allow to cool to room temperature
BENEFITS of STOCK SHAPES VS INJECTION MOLDING
Machining is preferred based on one of the following reasons:
1. Lower Cost at Low Volume – Machining eliminates the need for expensive custom tooling. The tooling must be amortized over a high production volume in order to achieve acceptable costs.
2. Faster Turn-Around Time – Machinging requires little or no custom tooling, the time required producing and testing molds can be eliminated. This is particularly useful when parts are still in development.
3. Ability to Hold Tighter Tolerance – Plastics expand or contract with changes in temperature. Injection molded parts must be designed to allow for shrinkage after the part is removed from the mold. Machined parts experience little temperature changed between when they are shaped and finally used.
4. Lower Internal Stress– Injection molding creates irregular internal stress while the plastic flows through the mold. This can affect part strength, dimensional stability, and chemical resistance.
5. Part Complexity– Molds for complex parts are expensive. Machining is often more economical even at moderately high volume.
These attributes make machining the preferred choice for prototyping, producing custom parts, and for producing parts with extremely high tolerances.
CRITICAL MACHINING CONCERNS ENGINERRING PLASTICS
Engineering plastics can be machined in essentially the same manner as metals. Nevertheless, their physical properties will require closer attention to several factors:
1. Limiting Heat Build-up – The softening or melting temperatures of engineering plastics are roughly 1/10th those of metals.
2. 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.
3. 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.
4. 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:
1. Positive Rake Angle - Provides the cleanest, smoothest, and most accurate cut.
2. Positive Relief Angle - Helps minimize tool wear by allowing space for the plastic to recover from being compressed during cutting.
3. 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.
4. 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.
Chip formation depends greatly on the depth of cut. The cutting force rises greatly as depth of cut increases and the separation of the material friction generated per unit volume.
As the depth of cut decreases in size the heating generated increases and as cutting depth decreases the specific heat per volume increases.
Friction heating between the cutting and rake surface of the cutting tool and heat from material separation are the two primary heat generators. Polished relief surfaces of the tool minimize heat generated. Diamond coated tools are highly recommended for the very abrasive glass and carbon fiber-reinforced grades of PEEK. Diamond tooling does not gum up with melted chips and cutting edges do not breakdown.
THERMAL DETAILS for PEEK
The specific heat per unit volume is less than a metal because he specific heat capacity of PEEK is (0.52 to 0.44) Btu lb-1 ºF-1 compared to steel’s 0.12 Btu/lb-1 ºF-. PEEK’s density is also lower (1.26 – 1.44 g/cm-3) than steel’s (7.85 g/cm-3). PEEK will rise in temperature much faster and higher than the same metal part. If 100 BTU of heat is applied to the same cubic (inch), the temperature of the metal will be 15 ºF compared to 45 ºF for PEEK.
The thermal for reinforced PEEK glass and carbon fiber 0.8 x10-5 Btu/hr-1 ft-2 ºF-1 compared to metals 21 Btu/hr-1 ft-2 ºF-1. This means that most of the friction heat will be conducted to the cutting edge. The thermal conductivity of PEEK is 1.73 (unfilled); 6.38 Btu in hr-10 ft-2 ºF-1 for reinforced grades which compared to metal is 31. This has the most detrimental effect on drilling since greater expansion of the plastic part further worsens increasing friction and heating produced.
It is very important to produce the continuous chip by selecting the proper geometry of the cutting tool, and ensuring that the cutting speed depth of feed is correct. Coolant is very helpful to keep the PEEK parts lubricated and remove heat generated.
Chips are continuous and heat generation is reduced when the correct depth of cut and rotational speed are followed. It is important to grind the outside cutting edges of the drill as sharp as possible with polished flutes. Hand drilling is not recommend on finished parts as the correct feed and dimensional accuracy is greatly increased by using the correct depth of cut and feed.
Clearance angle and flutes of drills must be large enough to eject continuous chips. The correct coolant will reduce breakdown of the cutting edge and remove unwanted heat. As speeds decrease gumming can occur.
Guidelines for Machining and Finishing of Victrex PEEK
Ft/Min-1/ m min-1
390 – 590 ( 120- 180)
in Rev-1/ mm min-1
Top Rake Angle
6 - 12º
Standard or Carbide
Carbide or Diamond Tip
Ft/Min-1/ m min-1
180 – 230(590 – 754)
255 – 360(78– 110)
Ft/Min-1/ m min-1
245 – 395(75 - 120)
in Rev-1/ mm Rev-1
0.002 – 0.008 (0.05 – 0.20)
0.002 – 0.008 (0.05 – 0.20)
100 - 200
Water/ SolubleOilWe use TrimC210
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