Insufficient transparency of injection molded parts and solutions
The transparency of injection-molded parts is a core quality requirement for many high-end products, such as medical infusion sets, food packaging containers, and optical lenses. Insufficient transparency can directly impact product functionality and market competitiveness. Insufficient transparency in injection-molded parts manifests primarily as high surface haze, low gloss, and internal cloud-like streaks or bubbles. These defects stem from uneven molecular alignment, impurity contamination, or excessive crystallinity in the plastic melt during the molding process. For example, insufficient transparency in lenses made of PC material can reduce light transmittance and affect optical performance. Poor transparency in PET preforms can make the packaging appear cloudy, reducing consumer acceptance. In production, plastic parts with insufficient transparency often require rework or scrapping. According to statistics, the scrap rate due to such defects can reach 10%-15%. Therefore, identifying the causes and implementing targeted solutions is crucial.
Improper raw material purity and pretreatment are common causes of insufficient transparency in injection molded parts. Impurities in plastic raw materials, such as dust, metal particles, and unmelted particles, can create light scattering points in the melt, reducing transparency. For example, if PMMA raw materials contain impurity particles larger than 0.1mm, visible white spots will form on the surface of the molded part after molding. Furthermore, moisture in the raw materials is a key factor affecting transparency. Hygroscopic plastics such as PA and PC undergo hydrolysis at high temperatures, breaking the molecular chains and forming tiny bubbles. These bubbles reflect light within the molded part, reducing transparency. Experiments have shown that when the moisture content of PC raw materials exceeds 0.02%, the haze of the molded part increases by more than 5%. Therefore, raw materials must be rigorously screened and dried before use. Screening can be performed using a 100-200 mesh filter. Drying parameters should be set according to the material’s characteristics. For example, PC should be dried at 120°C for 4-6 hours to ensure a moisture content below 0.01%.
Improperly set injection molding process parameters can seriously affect the transparency of plastic parts. When the injection temperature is too low, the plastic melt has poor fluidity, preventing the molecular chains from fully expanding. This leads to stress concentration within the part, resulting in subtle optical distortion and reduced transparency. For example, when processing PS plastic, if the barrel temperature is below 180°C, the melt will not fully plasticize, resulting in a noticeable haze on the part. Excessively high temperatures can cause thermal degradation of the plastic, producing small volatile molecules. These volatiles can form bubbles or silver streaks within the part, affecting transparency. For example, PMMA is prone to degradation above 260°C, resulting in yellowing and reduced transparency. The injection speed and pressure also require precise control. Excessive speeds can cause severe shearing of the melt within the mold cavity, leading to uneven molecular orientation and the formation of flow marks. Excessively slow speeds prolong the melt’s cooling time in the mold cavity, increasing the crystallinity of crystalline plastics. For example, slow injection of PE increases crystallinity and reduces transparency.
Mold design and surface quality have a direct impact on the transparency of injection molded parts. High surface roughness in the mold cavity can lead to uneven surfaces, diffusely reflecting light, and reducing gloss and transparency. For example, if the cavity surface roughness Ra exceeds 0.2μm, the light transmittance of PC parts will decrease by 3%-5%. Therefore, the mold cavity must be precisely polished to achieve a mirror finish, typically requiring an Ra of 0.02μm or less. Improper gate design can also contribute to insufficient transparency. If the gate is too small or improperly positioned, excessive shear forces will be generated when the melt enters the cavity, causing molecular chain breakage or alignment, resulting in visible flow marks. For large transparent parts, fan gates or film gates should be used to reduce melt flow resistance. For small parts, point gates can be used to ensure rapid and even melt filling. Furthermore, the mold’s venting system must be unobstructed. Otherwise, gases trapped in the cavity cannot escape, forming bubbles within the part and affecting transparency. The venting groove should be controlled within a depth of 0.01-0.03mm and a width of 5-8mm.
Targeted solutions are required for different types of transparency defects. For plastic parts with high surface haze caused by low mold temperature, the mold temperature can be increased by 10-20°C. For example, for PC, the mold temperature can be raised from 70°C to 90°C to promote uniform molecular chain alignment. If the cause is raw material impurities, raw material screening and barrel cleaning should be strengthened, and filters should be regularly checked for clogs. Parts with internal hazy streaks are often caused by uneven melt plasticization. This can be improved by increasing barrel temperature, extending plasticization time, or increasing back pressure (typically to 10-15 MPa). For insufficient transparency due to excessive crystallinity (such as in PP parts), the mold temperature can be lowered or a nucleating agent can be added to inhibit grain growth and achieve finer and more uniform crystals. For transparency issues caused by bubbles, raw material drying and optimized venting should be implemented to ensure the moisture content meets the standard. Venting slots should also be added at the final melt filling point. By combining these measures, the light transmittance of injection molded parts can be increased to over 90%, meeting the transparency requirements of high-end products.
