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General Guidelines for Annealling (Stress-Relieving) of Plastics

General Guidelines for Annealing (Stress-Relieving) of Plastics

The following guidelines are presented for those machinists not familiar with post-machining annealing (stress-relief) for high-performance plastics. They are intended as guidelines only, and may not represent the most optimum conditions for all parts.

Most quality stock shape materials are stress-relieved by the manufacturer after molding to ensure the highest degree of machinability and dimensional stability. However, some components may benefit from post-machining annealing or stress-relieving using the instructions below.


Post-Machining Annealing (Stress-Relief)

When should parts be annealed after machining to ensure optimum part performance?
Experience has shown us that very few machined plastic parts require annealing after machining to meet dimensional or performance requirements.

Generally speaking, most stock shapes are annealed after molding using a proprietary stress relieving cycle to minimize any internal stresses that may result from the manufacturing process. This assures you that the material will remain dimensionally stable during and after machining.

Machined-in stress can reduce part performance and lead to premature part failure. To prevent machined-in stress, it is important to identify the causes. Machined-in stress is created by:

  • Using dull or improperly designed tooling
  • Excessive heat –– generated from inappropriate speeds and feed rates
  • Machining away large volumes of material –– usually from one side of the stock shape

To reduce the potential for machined-in stress, review the fabrication guidelines for the specific material. Recognize that guidelines change as the material type changes.

 

BENEFITS OF POST-MACHINING ANNEALING

  • Improved Chemical Resistance Polycarbonate, polysulfone, and Ultem® PEI, like many amorphous (transparent) plastics may be annealed to minimize stress crazing. Torlon® PAI also benefits from post machining annealing. Annealing finished parts becomes more important as machining volume increases. Annealing after machining reduces "machined-in" stresses that can contribute to premature failure.
  • Better Flatness & Tighter Tolerance Capability Extremely close-tolerance parts requiring precision flatness and non-symmetrical contour sometimes require intermediate annealing between machining operation. Improved flatness can be attained by rough machining, annealing and finish machining with a very light cut. Balanced machining on both sides of the shape centerline can also help prevent warpage.
  • Improved Wear Resistance Extruded or injection molded Torlon® PAI parts that require high PV's or the lowest possible wear factor benefit from an additional cure after machining. This curing process optimizes the wear properties. Only PAI benefits from such a cycle.



POST MACHINING AIR ANNEALING GUIDELINES

Material

Heat Up

Hold

Cool Down

Environment

ABS

50°F per hour to 200°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Acrylic

2 hours to 180°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Acetal copolymer

4 hours to 310°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen
or Air

Delrin® acetal homopolymer

4 hours to 320°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen
or Air

Ardel® polyarylate

50°F per hour to 330°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Ertalyte® PET-P

4 hours to 350°F

30 minutes per 1/4” thickness

50°F per hour

Oil or Nitrogen

Halar® ECTFE

50°F per hour to 225°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Hydex® 4101 PBT-P

4 hours to 300°F

60 minutes per 1/4” thickness

50°F per hour

Nitrogen
or Air

Kynar® PVDF

2 hours to 275°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Noryl® PPO

50°F per hour to 250°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Noryl® PPO
30% glass filled

50°F per hour to 260°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Nylon - type 6

4 hours to 300°F

30 minutes per 1/4” thickness

50°F per hour

Oil or Nitrogen

Nylon - type 6/6

4 hours to 350°F

30 minutes per 1/4” thickness

50°F per hour

Oil or Nitrogen

Nylon - glass-filled

4 hours to 375°F

30 minutes per 1/4” thickness

50°F per hour

Oil or Nitrogen

PCTFE (formerly Kel-F®)

3 hours to 225°F

60 minutes per 1/4” thickness

50°F per hour

Air

PEEK polyetheretherketone

2 hours to 300°F
then
2 hours to 375°F

60 minutes per 1/4” thickness

60 minutes per 1/4” thickness

50°F per hour

Air

Polycarbonate (unfilled)

4 hours to 275°F

30 minutes per 1/4” thickness

50°F per hour

Air

Polycarbonate (glass-filled)

4 hours to 290°F

30 minutes per 1/4” thickness

50°F per hour

Air

Polyethylene (UHMW)

2 hours to 220°F

30 minutes per 1/4” thickness

10°F per hour

Nitrogen

Polypropylene

2 hours to 185°F

30 minutes per 1/4” thickness

50°F per hour

Air

Polystyrene

50°F per hour to 170°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Radel® R polyethersulfone

4 hours to 390°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen
or Air

Ryton® PPS

4 hours to 350°F

30 minutes per 1/4” thickness

50°F per hour

Air

Techtron® PPS

4 hours to 350°F

30 minutes per 1/4” thickness

50°F per hour

Air

Torlon® PAI

4 hours to 300°F
then
4 hours to 420°F
then
4 hours to 470°F
then
4 hours to 500°F


1 day

1 day

1 day

3 to 10 days

50°F per hour

Air

TPX® polymethylpentene

50°F per hour to 200°F

30 minutes per 1/4” thickness

50°F per hour

Nitrogen

Udel® polysulfone

4 hours to 330°F

30 minutes per 1/4” thickness

50°F per hour

Air

Ultem® PEI (unfilled)

4 hours to 390°F

30 minutes per 1/4” thickness

50°F per hour

Air

Ultem® PEI (20%, 30% glass filled)

4 hours to 400°F

30 minutes per 1/4” thickness

50°F per hour

Air

Finish machining of critical dimensions should be performed after annealing.

Important: Annealing cycles have been generalized to apply to a majority of machined parts. Changes in heat up and hold time may be possible if cross sections are thin. Parts should be fixtured during annealing to prevent distortion.

 

Trademark Acknowledgments:

RADEL and TORLON are registered trademarks of Solvay Engineering Polymers.
RYTON is a registered trademark of Chevron Phillips Chemical Company.
DELRIN is a registered trademark of DuPont.
ULTEM is a registered trademark of General Electric Company.
HYDEX is a registered trademark of A.L. Hyde Company.
ERTALYTE and TECHTRON are registered trademarks of Quadrant Engineering Plastic Products.
PEEK is a trademark of Victrex plc.

The information on this page originally provided by Quadrant Engineering Plastics & Westlake Plastics Company.
Other stock shape manufacturers have also provided relevant information.

 

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