Annealing of Plastic Stock Shapes

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.