PC, a high-performance engineering plastic, is widely used in injection molding. However, problems can sometimes arise with molded parts becoming brittle and producing a white haze, which not only affects the product’s appearance but also reduces its mechanical properties. From a raw material perspective, the molecular weight distribution, purity, and presence of impurities in the PC material can all be the root causes of these problems. If the PC material is contaminated during the production process by the incorporation of other low-molecular-weight substances or incompatible plastic components, the integrity of its molecular chain structure can be compromised. During injection molding, these impurities can act as stress concentration points, causing the molded part to fracture easily when subjected to external forces, resulting in brittleness. Furthermore, the presence of impurities can affect the crystallization of the PC material, resulting in an uneven internal structure and the formation of white haze-like defects on the surface.
Improper injection molding process parameters are also a major cause of brittleness and white haze in PC molded parts. First, the melt temperature. Excessively high melt temperatures can cause thermal degradation of the PC material, breaking molecular chains and reducing molecular weight. This decreases the material’s toughness, leading to brittle parts. Furthermore, excessively high temperatures can cause some components in the PC material to decompose and produce gases. These gases form tiny bubbles within or on the surface of the molded part, creating a white haze. Conversely, if the melt temperature is too low, the PC material fails to fully melt, resulting in poor melt flow and difficulty filling the mold cavity during the molding process, leading to significant internal stress. Areas of concentrated internal stress are prone to cracking, leading to brittle parts. Incompletely melted particles can also cause a white haze on the surface.
The rationality of mold design and manufacturing has a direct impact on the quality of PC injection molded parts. Improper mold gate placement, undersize, or poor runner design can subject the melt to excessive shear during flow. PC materials are sensitive to shear forces, and excessive shear can disrupt their molecular chain structure, causing material degradation and brittleness in the molded part. Furthermore, excessive shear can cause localized increases in the melt temperature, triggering thermal degradation and leading to the formation of white haze on the surface. Furthermore, an improperly designed mold cooling system and uneven cooling rates can lead to significant internal stress within the molded part. Internal stress can degrade the material’s mechanical properties, resulting in brittleness and potentially causing surface defects such as white haze.
During the injection molding process, improper operation can also cause quality issues with PC molded parts. For example, unstable nozzle temperature control on the injection molding machine, with fluctuations in temperature, can affect the melt state of the PC material. When the nozzle temperature is too high, the PC material flowing through it will thermally degrade; when the temperature is too low, the PC material will not fully melt. Both of these factors can cause the molded part to become brittle and produce white fog. Furthermore, improper holding pressure and holding time settings. If the holding pressure is insufficient or the holding time is too short, the molded part will not receive adequate shrinkage during cooling and shrinkage, resulting in internal vacuum bubbles or shrinkage cavities, which not only make the part brittle but also produce white fog on the surface. Conversely, excessive holding pressure or holding time can generate excessive internal stress within the molded part, similarly leading to brittleness and white fog.
In addition to the aforementioned factors, the storage and pretreatment of PC materials can also affect the quality of injection molded parts. PC materials are hygroscopic. High humidity during storage can cause the material to absorb significant amounts of water. If PC materials are not adequately dried before injection molding, the moisture can vaporize at high temperatures, forming bubbles. These bubbles can accumulate within or on the surface of the molded part, causing white haze and disrupting the material structure, making the part brittle. Furthermore, if PC materials are exposed to direct sunlight or high temperatures during storage, they can undergo oxidative degradation, breaking molecular chains and degrading material properties. This can also lead to brittleness and white haze after injection molding. Therefore, proper storage and pretreatment of PC materials are crucial for ensuring the quality of injection molded parts.
