Post-processing of Plastic Parts

Why post-processing of plastic parts?

The deformation and flow behavior of the plastic parts as it melts under the influence of temperature and pressure during the molding process is quite complex. Uneven crystallization, orientation, and shrinkage cause corresponding crystallization, orientation, and shrinkage stress in the plastic portion, in addition to the uneven plasticization before flow, unevenness, and various cooling rates after filling. After demolding, it not only results in aging deformation but also deteriorates the mechanical, optical, and apparent quality of the plastic components, and in extreme situations, breaks them. We can properly post-process the plastic components to address these issues.

Annealing of plastic parts

Plastic components are heated to a specific temperature and then kept warm for a specific amount of time during the heat treatment process known as annealing. The idea is to employ heat during annealing to hasten the relaxation of plastics’ macromolecules and eliminate or drastically reduce the residual tension on objects made of plastic after molding. Its functions include:

• Standards for annealing of plastics:  Due to the uneven plasticization or crystallization of polymers in the cavity, uneven cooling, or due to the influence of metal inserts and incorrect secondary processing of injection molded components, which causes inconsistent shrinkage of different regions of the parts. Internal stress in the injection molded pieces is unavoidable for various reasons. Injection-molded parts frequently need to undergo an annealing process to remove residual stress since the presence of internal stress frequently causes deformation or breaking of injection-molded parts during usage.

• Annealing method: The injection-molded components should either be heated in an oven to a specific temperature or placed for some time in a liquid medium (such as water, mineral oil, glycerin, ethylene glycol, liquid paraffin, etc.) before being allowed to gradually cool to room temperature. Utilize the heat produced by annealing to hasten the relaxation of plastics’ macromolecules, which will eliminate or significantly reduce any residual tension left over after the molding of plastic components.

• Annealing temperature: generally controlled at 10-200C higher than the service temperature of the plastic part or 10-200C lower than the thermal deformation temperature of the plastic. The temperature should not be too high, otherwise, the plastic parts will warp, and the temperature should not be too low, otherwise, the purpose of post-processing will not be achieved.

• Annealing time: The length of time needed for annealing is determined by the type of plastic being used, the temperature of the heating medium, the shape and wall thickness of the plastic component, the level of precision required for the plastic component, and other considerations. If there are no data, it can alternatively be calculated using the rule of thumb of around 0.5h/mm of thickness.

• Annealing heat source or heat preservation medium: infrared lamp, blast oven, hot water, hot oil, hot air and liquid paraffin, etc.
• Note: During annealing and cooling, the cooling rate should not be too fast, otherwise temperature stress may be regenerated.

Humidity treatment of plastic parts

To speed up the moisture absorption balance and maintain its size, the newly released injection-molded components (polyamide) are placed in hot water to isolate the air, prevent oxidation, and remove internal stress. This process is known as humidity control treatment. For instance, when polyamide injection molded components are taken out of the mold, they are prone to swelling and water absorption when used or kept in the air, as well as to oxidation and discoloration when exposed to air at high temperatures. Injection molded parts immediately reach a condition of moisture absorption equilibrium after humidity control treatment, stabilizing the size of the plastic component.

A post-treatment procedure called humidity conditioning is used to modify the water content of plastic components. It is mostly utilized for polyamide plastic components that are highly hygroscopic and easily oxidized. It can help plastic parts establish moisture absorption equilibrium in a heating medium to prevent their dimensions from changing while being used, in addition to removing residual stress under heating and heat preservation settings.

Boiling water or potassium acetate solution (which has a boiling point of 121°C) is the heating medium used in the humidity control treatment. The heating temperature ranges from 100 to 120°C; the upper limit is used when the heat distortion temperature is high and the lower limit is used in all other situations.

The length of the humidity conditioning treatment, which is typically 2–9 hours, is determined by the type of plastic, the shape of the injection-molded part, the wall thickness, and the degree of crystallinity. The room’s temperature is gradually lowered when the humidity-conditioning treatment period has ended.

But not all plastic parts need post-processing. Usually, only nylon plastic parts, metal inserts, large temperature range changes, high dimensional accuracy requirements, and large thick-walled plastic parts are necessary.