7. Which CNC machining plastics typically require annealing?
We have discussed the situations that need plastic anneal. Some plastics for CNC machining may need to anneal more frequently than others.
7.1 Annealing POM/Delrin plastic
Most manufacturers of stock shapes anneal their products to alleviate stress. However, additional annealing may be necessary during the machining of parts with tight tolerances to relieve machined-in stresses, particularly after significant machining cuts. Typically, annealing of machined parts occurs before the final light finishing or polishing cuts.
Air Annealing:
For DELRIN®, air annealing is best carried out in ovens with circulating air, capable of maintaining a uniform air temperature controllable to within +2°C. In air, achieving the same degree of annealing takes one hour at 160°C, as compared to 30 minutes in oil at the same temperature. This is because heat transfer occurs more slowly in air than in oil. The recommended annealing time is 30 minutes for part heat-up to 160 + 2°C, and then an additional 5 minutes for each 1 mm of wall thickness.
Oil Annealing:
Recommended oils include “Primol” 342 and “Ondina” 33 or other refined annealing oils. Parts can be annealed at a “part” temperature of 160° + 2°C. The annealing time at 160°C is 5 minutes per 1 mm of wall thickness after the parts reach the annealing bath temperature (15-20 min). Adequate agitation is essential to ensure a uniform bath temperature and prevent localized overheating of the oil, which could lead to deformation or melting of the parts.
During annealed parts’ removal from the annealing chamber, they should be gradually cooled to room temperature without disturbance. Stacking or piling, which might deform the still-hot parts, should be postponed until the parts are cool to the touch.
Below are steps to anneal or stress-relieve Delrin with air annealing.
1 Place parts in an air-circulating oven to ensure proper air circulation around them.
2 Heat the oven to 300°F with a maximum heating rate of 20°F per hour.
3 Maintain the oven temperature at 300°F for 30 minutes plus an additional 15 minutes for every 1/8” of cross-section.
4 Cool the oven to 150°F over 15 hours, with a maximum cooling rate of 10°F per hour.
5 Turn off the oven and allow it to cool to room temperature before removing parts.
7.2 Annealing Peek plastic
PEEK plastic machining poses challenges due to the necessary preparations the material must undergo before the machining process can take place. Given the specific properties of PEEK plastic, it requires annealing before it can be shaped and sized according to requirements. Annealing involves the application of heat to the plastic to soften it and relieve internal stresses. This crucial step enhances PEEK machinability, preventing the development of surface cracks, imperfections, and weak points.
The challenge lies in achieving the right temperature to soften the PEEK without reaching its melting point (649.4°F). Annealing in PEEK machining is not a one-time procedure; for longer processes, multiple annealing instances may be required to ensure the absence of cracks and stress damage. Below are steps to anneal or stress-relieve PEEK.
1 Place parts in an air-circulating oven to allow air circulation around them.
2 Heat the oven to 300°F with a maximum heating rate of 20°F per hour.
3 Hold the oven temperature at 300°F for 120 minutes, adding 30 minutes for every 1/8″ of cross-section.
4 Heat the oven to 390°F with a maximum heating rate of 20°F per hour.
5 Hold the oven temperature at 390°F for 2 hours, adding 3 minutes for every 1/8″ of cross-section.
6 Cool the oven to 150°F over 25 hours with a maximum cooling rate of 10 per hour.
7 Turn off the oven and allow it to cool to room temperature before removing parts.
7.3 Annealing Polycarbonate plastic
Annealing polycarbonate parts involves heating the part to just below its glass transition (Tg) temperature to alleviate internal stress generated during CNC machining. Below are steps to anneal or stress relieve Polycarbonate.
1 Place parts in an air-circulating oven to ensure proper air circulation around them.
2 Heat the oven to 250°F with a maximum heating rate of 20°F per hour.
3 Maintain the oven temperature at 250°F for 30 minutes plus an additional 15 minutes for every 1/8” of cross-section.
4 Cool the oven to 150°F over 10 hours, with a maximum cooling rate of 10°F per hour.
5 Turn off the oven and allow it to cool to room temperature before removing parts.
7.4 Annealing acrylic plastic
Annealing acrylic is a stress-relieving process involving heating followed by slow cooling. Here are the steps:
1 Anneal sheets and fabricated parts in a forced air-circulating oven to ensure uniform heating and proper ventilation. Maintain an air velocity of approximately 1 m/s. Temperature control should be within ±6°C.
2 Preheat the oven to 80°C, just below the deflection temperature of ACRYLITE®. Avoid higher temperatures to reduce the risk of warpage.
3 Place the part in the oven, considering the use of jigs to support thermoformed parts in case of relaxation.
4 Maintain the annealing temperature for 2 hours for parts 6.0 mm and thinner. For thicknesses above 6.0 mm, divide the millimeter thickness by 3 (refer to the annealing chart for details).
5 Turn off the oven or run a ramped cooling cycle down to at least 60°C. Avoid removing the part too soon, as it can offset the positive effects of annealing. Cool for at least 2 hours. For thicknesses above 8.0 mm, divide the millimeter thickness by 4.
7.5 Annealing Nylon plastic
Annealing/Stress Relieving Procedures for Nylon:
1 Place parts in an air-circulating oven to ensure proper air circulation around them.
2 Heat the oven to 320°F with a maximum heating rate of 20°F per hour.
3 Maintain the oven temperature at 320°F for 30 minutes plus an additional 15 minutes for every 1/8 of the cross-section.
4 Cool the oven to 150°F over 15 hours, with a maximum cooling rate of 10°F per hour.
5 Turn off the oven and allow it to cool to room temperature before removing parts.
7.6 Annealing HDPE/UHMWPE
Thermoplastics are susceptible to stress cracking and premature part failure when subjected to high heat and tensile load. Hence, annealing is a crucial step to achieve a quality, precision-machined part from the stock shape. This process significantly minimizes the risk of stress-related issues caused by the heat generated during machining HDPE/UHMWPE and other polymers. Our machinists utilize computer-controlled annealing ovens to ensure the highest quality precision machining.
In general, HDPE/UHMWPE should undergo the following annealing process:
1 Heat the material between 135°C to 138°C in an oven or a liquid bath of silicone oil or glycerine.
2 Cool it down at a rate of 5°C per hour to at least 65°C.
3 Wrap it in an insulating blanket for 24 hours to bring it to room temperature.
This annealing procedure is essential for achieving optimal mechanical properties and preventing stress-related issues in the final precision-machined UHMWPE parts.
There are many methods to anneal plastics, but the process has some similar characteristics. Below is a brief description of the three stages about how to anneal plastic.
