Annealing is a heat treatment for plastic molded products to release residual stress and promote crystallization to prevent defects over time and to suppress deformation.
Injection molding is one of the processes used in the mass production of plastic molded products.
Resin pellets are forced to plasticize by heat and a screw, and then filled into a narrow mold by applying pressure through a narrow injection port. Naturally, it is not surprising that some residual stress (here for ease of image, I will use the term "stress") remains in the resin.
If the molded product is used while stress remains, deformation may occur over time and unpredictable fractures may occur.
If this were to occur at the point of delivery or on the market, it could result in complaints or accidents.
Annealing treatment releases internal stress by treating resin molded products at a temperature 10°C to 15°C lower than the load deflection temperature. Poor treatment conditions can result in dimensional changes that may cause the product to deviate from target dimensional tolerances.
Therefore, the product can finally be delivered after measurement and inspection with appropriate annealing conditions and mold modification in anticipation of dimensional changes.
As mentioned above, annealing is a separate process after molding, which increases lead time and manufacturing costs. Recently, annealing-less processes are being selected in an increasing number of cases due to material improvement and review of the operating environment.
One approach that we can take is to reduce the stress on the resin during the filling process by proactively installing a gas venting port in the mold.
Annealing is considered on a case-by-case basis, depending on the operating environment, resin used, and required dimensional change over time.
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