In traffic light shell injection molding of plastic products, temperature control is crucial to preventing scorching. Plastics are extremely sensitive to temperature; excessively high barrel, nozzle, or mold temperatures, or prolonged residence time of the material in high-temperature areas, can lead to plastic decomposition and scorching. Scorching not only causes black spots or streaks on the shell surface, affecting appearance quality, but also reduces the material's mechanical properties and may even release irritating gases, polluting the mold and production environment. Therefore, a comprehensive approach is needed, encompassing barrel temperature management, nozzle temperature coordination, mold temperature optimization, material residence time control, process parameter adjustment, mold venting design, and equipment maintenance.
Barrel temperature is a key parameter in plasticization, directly affecting melt flowability and thermal stability. Excessively high barrel temperatures can cause plastic molecular chains to break, generating volatile gases and leading to scorching; excessively low temperatures increase melt viscosity and flow resistance, forcing higher injection pressure and causing localized overheating due to frictional heat. Therefore, a reasonable barrel temperature gradient needs to be set according to the type of plastic (such as modified PC, ABS, etc.), typically with a gradual increase in temperature from the feeding section to the metering section, ensuring sufficient plasticization without overheating. Simultaneously, the heating device and temperature control system must be checked regularly to prevent temperature runaway due to equipment malfunction.
The nozzle, as the channel connecting the barrel and the mold, requires temperature control in traffic light shell injection molding to prevent "drooling" and premature solidification. If the nozzle temperature is too high, the melt may drip at the nozzle, forming "drool," affecting product quality; if the temperature is too low, the melt may solidify prematurely, clogging the nozzle or causing surface defects in the product. Therefore, the nozzle temperature is usually slightly lower than the barrel end temperature to balance flowability and controllability. Furthermore, it is necessary to ensure a good fit between the nozzle and the main runner of the mold to prevent melt stagnation and decomposition at the interface.
Mold temperature has a decisive influence on the filling and cooling process of plastic. If the mold temperature is too high, the melt fluidity increases, but the cooling time is prolonged, which may lead to deformation of the product upon ejection. If the temperature is too low, the melt may become dull due to rapid cooling, or even crack due to internal stress. For traffic light housings, the mold temperature needs to be adjusted according to the crystallinity of the plastic and the product structure, usually controlled by a mold temperature controller to ensure uniform mold surface temperature and avoid localized overheating and scorching. Simultaneously, the cooling water circuit design needs to be optimized to ensure rapid heat dissipation and shorten the molding cycle.
Excessive residence time of material in the barrel is a common cause of scorching. Even within a reasonable temperature range, prolonged heating can lead to plastic degradation. Therefore, the injection speed and cycle time need to be adjusted appropriately based on the injection molding machine's production capacity and product requirements to avoid excessive material residence in the barrel. Furthermore, screw speed and back pressure have a significant impact on the plasticizing and conveying process; appropriate parameters need to be selected based on the plastic's characteristics to avoid localized overheating due to excessive shear force.
Mold venting design is a crucial factor in preventing scorching. If the mold has poor venting, the air inside the cavity of the traffic light shell injection molding die will be compressed by the melt, generating high-temperature gas that causes the plastic to scorch. Therefore, it is necessary to install appropriate venting channels or venting holes at the parting surface of the mold and at the mating point between the core and the cavity to ensure smooth air discharge. At the same time, the venting channels must be cleaned regularly to prevent venting failure due to blockage by plastic powder or residue.
Equipment maintenance and cleaning are fundamental to preventing scorching. The hopper, barrel, screw, and nozzles must be cleaned regularly to prevent foreign matter from entering or resin from remaining and decomposing. In addition, the accuracy of the heating device and temperature control system must be checked to ensure that the temperature display matches the actual value, preventing temperature runaway due to equipment malfunction.
