Injection molded products and plastics used
Injection molded products are various plastic parts produced through the injection molding process. They are widely used in the automotive, electronics, home appliances, medical device and other fields. Their structures are diverse, ranging from micro electronic connectors to car bumpers, and their performance requirements vary significantly depending on the application scenario. For example, automotive interior parts must have temperature resistance (-40-80°C), low VOC (volatile organic compounds), and a good feel; electronic housings must have flame retardancy (UL94 V0 grade), impact resistance, and electromagnetic shielding properties; and medical device components must comply with biocompatibility (ISO 10993) and sterilization resistance. These performance requirements directly determine the type of plastic used and the modification method, so there is a close matching relationship between injection molded products and plastic materials.
Plastics used in injection molding can be categorized into three main categories: general-purpose plastics, engineering plastics, and specialty plastics, each with unique properties and applications. General-purpose plastics, including PE, PP, PVC, and PS, are inexpensive and offer excellent moldability, making them suitable for low-performance products such as packaging and daily necessities. For example, PP, due to its chemical resistance and toughness, is often used in food containers and toys. PVC, due to its low cost and ability to be manufactured in both soft and hard forms, is widely used in pipes and profiles. However, caution is advised regarding the potential release of HCl gas during processing, requiring specialized screws and molds. Engineering plastics, including ABS, PC, PA, and POM, offer high strength, heat resistance, and corrosion resistance, making them suitable for structural and functional parts. For example, ABS, due to its excellent overall properties, is often used in appliance housings. PC, due to its high light transmittance and strong impact resistance, is used in eyeglass lenses and lampshades. Specialty plastics, such as PEEK and PI, offer extreme properties such as high-temperature resistance (>250°C) and radiation resistance, making them suitable for high-end applications such as aerospace and medical devices. However, these plastics are expensive and difficult to mold.
Plastic performance parameters are key considerations for selecting materials for injection molding products, primarily including mechanical, thermal, chemical, and molding properties. Mechanical properties, such as tensile strength and impact strength, determine the product’s load-bearing capacity and impact resistance. For example, PA66 has a tensile strength of up to 80 MPa, making it suitable for load-bearing components; PC has an impact strength (notched) of up to 60 kJ/m², making it suitable for products requiring high impact resistance. Thermal properties, such as heat deflection temperature (HDT) and glass transition temperature (Tg), determine the product’s operating temperature range. For example, POM has an HDT of 110°C, allowing for use in higher temperature environments; PS, with an HDT of only 70°C, is not suitable for high-temperature applications. Chemical properties, such as solvent resistance and corrosion resistance, must be tailored to the intended use environment. For example, PP is resistant to most acids and alkalis and can be used in chemical pipelines; PC, on the other hand, is not resistant to organic solvents and should avoid contact with reagents such as alcohol. The melt flow rate (MFR) in the molding performance affects the injection molding process. Plastics with high MFR (such as PP) have good fluidity and are suitable for thin-walled products; plastics with low MFR (such as PC) have poor fluidity and require higher injection pressure.
The structural design of injection molded products must be tailored to the properties of the plastic used to maximize material performance. For brittle plastics (such as PS and PMMA), avoid sharp corners and sudden changes in wall thickness. Corners should be rounded (radius ≥ 1mm) to reduce stress concentration and cracking. For tough plastics (such as PE and PP), more complex designs are possible, but wall thickness must be uniform (deviation ≤ 10%) to avoid sink marks. When high strength is required, glass fiber-reinforced plastics (such as 30% glass fiber-reinforced PA66) can be used. However, be aware that reinforced plastics have poor flowability, requiring larger gate and runner dimensions. They are also susceptible to mold wear, so core and cavity materials should be constructed of wear-resistant materials (such as Cr12MoV). For transparent products, use plastics with high light transmittance (such as PC and PMMA), and ensure the mold cavity has a smooth surface (Ra ≤ 0.02μm) to avoid scratches that could affect light transmission.
The production process for injection molded products must be optimized based on the characteristics of the plastics used to ensure product quality. Hygroscopic plastics (such as PA and PC) must be thoroughly dried, otherwise silver streaks and bubbles will appear in the finished product. Heat-sensitive plastics (such as PVC and POM) require strict barrel temperature control to prevent overheating and degradation. For example, the barrel temperature for PVC should be kept between 160-190°C; temperatures exceeding 200°C will cause decomposition. For crystalline plastics (such as PP and PA), mold temperature must be controlled to adjust the degree of crystallinity. For example, increasing the mold temperature can increase the crystallinity of PA and improve product strength. For amorphous plastics (such as PC and ABS), mold temperature primarily affects the cooling rate. Excessively low temperatures can increase internal stress, necessitating an appropriate increase in mold temperature (50-80°C). Furthermore, the shrinkage rate of different plastics varies significantly, so mold dimensions must be calculated based on this shrinkage rate during mold design. For example, the shrinkage rate of PP is 1.5-2.5%, so the mold dimensions must be increased accordingly to ensure product dimensional accuracy. By properly matching product structure, plastic properties, and process parameters, high-quality injection molded products can be produced to meet the needs of various applications.
