Dr. Dmitri Kopeliovich
Annealing of plastics is a heating of a polymeric part to below it’s glass transition temperature in order to relieve the internal stresses introduced into the part during its fabrication (molding, cooling after molding, machining, welding)/
In order to mold a plastic it is soften by heating and then forced into a cavity where it cools down and shrinks. Non-uniform flowing of the soft plastic and non-uniform and relatively fast cooling result in a formation of internal stresses, which may cause dimensional distortion of the part and even cracking.
Machining of the molded parts introduces additional internal stresses particularly if an improperly designed tool is used or if the plastic part is locally overheated due to excessive cutting and feed speeds.
Drilling deep holes, uneven thickness reduction and screw thread cutting are the machining operations producing internal stresses, which should be reduced by annealing.
Some polymers (e.g. polycarbonate, polysolfone) are more susceptible to internal stress formation.
General annealing process includes the following stages:
There are three technique for annealing of plastics:
|Plastic material||Annealing temperature|
|Polyethylene Terephtalate (PET)||350||177|
|High Density Polyethylene (HDPE)||250||121|
|Polyvinylidene fluoride (PVDF)||275||135|
|Low Density Polyethylene (LDPE)||175||79|