Many engineers are familiar with the need to anneal metals for stress-relieving, but not so many engineers know about annealing plastics. In this blog post, we are going to talk about the subject of annealing polymers.
Why Anneal Plastics?
In manufacturing, when converting resins to finished parts or stock shapes, internal stresses are inherent due to the heating, pressures and cooling that takes place during the conversion process. Some materials develop more internal stresses than others, and need additional annealing to achieve proper physical properties or to maintain specific dimensional tolerances.
Machined-in stresses are sometimes caused by heat from excessive speeds and feeds, the use of dull or worn tools, drilling deep holes, uneven thickness reduction, and removing a significant volume of material (usually from one side of the stock).
These residual stresses can compromise the strength of a polymer part and cause it to experience warping, twisting, and similar dimensional changes. Problems with residual stresses can be compounded when aggressive chemicals are involved. It can lead to premature failure of the part.
The Annealing Process
The general annealing process involves slowly heating a polymer part to a temperature a little bit below its softening point at a controlled rate, holding it at that elevated temperature for a certain amount of time, and then allowing it to slowly cool back to room temperature. The annealing temperature and holding time are both dependent on the type of plastic being annealed. The heating process takes place in a controlled environment such as a specially designed oven.
Annealing of high performance plastic parts is dependent upon whether the part was injection molded or machined from extruded or compression molded stock shapes. In some applications, an injection molded part can be annealed after molding. In the case of parts manufactured from a stock shape, the annealing process would occur on the raw material prior to machining to eliminate internal stresses induced during the extrusion or compression molding process. In materials that have a high coefficient of thermal expansion (CTE), it may be necessary to anneal the part prior to the final machining process.
Benefits of Annealing Plastics
There are several benefits to annealing high performance plastics, such as:
- Ability to achieve tighter tolerances
- Significantly better dimensional stability
- Improved flatness
Clear plastics such as polycarbonate and polysulfone can attain improved chemical resistance because annealing will minimize the occurrence of stress craze marks. Torlon in particular will experience improved wear resistance after annealing.
If you have concerns about residual stresses in your machined part or if a significant volume of material was removed from it, you should consider annealing your plastic machined part. It will be able to conform to higher tolerances and be much more dimensionally stable and, depending on the type of plastic, you may see other improvements also.