Annealing
Due to uneven plasticization or uneven crystallization, orientation, and cooling of the plastic in the mold cavity, inconsistent shrinkage occurs in different parts of the plastic part. Additionally, the influence of metal inserts and improper secondary processing of the plastic part inevitably lead to internal stress. The presence of internal stress often causes deformation or cracking during use, therefore, annealing is frequently required to eliminate residual stress.
The annealing method involves placing the plastic part in an oven at a specific temperature or in a liquid medium (such as water, hot mineral oil, glycerin, ethylene glycol, and liquid paraffin) for a period of time, followed by slow cooling to room temperature. The heat generated during annealing accelerates the relaxation of macromolecules in the plastic, thereby eliminating or reducing residual stress after molding.
Annealing temperature is generally controlled at 10-20℃ above the service temperature of the plastic part or 10-20℃ below the heat distortion temperature of the plastic.
The temperature should not be too high, otherwise the plastic part will warp; the temperature should not be too low, otherwise the purpose of post-treatment will not be achieved.
The annealing time depends on factors such as the type of plastic, the temperature of the heating medium, the shape and wall thickness of the plastic part, and the precision requirements of the plastic part. Table 2-6 shows the heat treatment conditions for commonly used thermoplastics.
Table 2-6 Common Hot Runner Molding Processing Conditions for Thermoplastics
| Material | Processing Temperature (°C) | Time (h) | Heating Method |
|---|---|---|---|
| ABS | 70 | 4 | Oven |
| Polyoxymethylene (POM) | 110 ~ 135 | 4 ~ 8 | Oven |
| 100 ~ 110 | 8 ~ 12 | Far-infrared lamp, Oven | |
| Polyamide (PA/Nylon) | 140 ~ 145 | 4 | Far-infrared heating, Oven |
| Polyurethane (TPU) | 100 ~ 110 | 4 | Salt bath |
| Polymethyl methacrylate (PMMA) | 70 | 4 | Far-infrared heating, Oven |
| Polycarbonate (PC) | 110 ~ 130 | 4 ~ 8 | Far-infrared heating, Oven, Oil bath |
| Poly(ethylene terephthalate) (PET) | 120 | 1 ~ 2 | Oven |
Humidity conditioning
Conditioning involves placing freshly demolded plastic parts in hot water to isolate them from air, prevent oxidation, eliminate internal stress, and accelerate the reaching of moisture equilibrium, thus stabilizing their dimensions. For example, polyamide plastic parts are prone to oxidation and discoloration when exposed to air at high temperatures after demolding, and easily absorb moisture and expand when used or stored in air. Conditioning isolates them from air and allows them to quickly reach moisture equilibrium, stabilizing their dimensions.
Conditioning can also improve the toughness of plastic parts, increasing impact toughness and tensile strength. The conditioning temperature is generally 100–120°C; for plastics with high heat distortion temperatures, the upper limit is used; conversely, the lower limit is used.
The conditioning time depends on the type of plastic, the shape of the part, its wall thickness, and its crystallinity. After the conditioning time is reached, the parts are slowly cooled to room temperature.