Plastic preparation is a fundamental step before injection molding, and its quality directly impacts the stability of the subsequent injection molding process and the final performance of the plastic part. The core of plastic preparation is to uniformly mix the plastic raw materials with various additives in a specific proportion to improve the plastic’s processing properties, mechanical properties, weather resistance, and other characteristics to meet the requirements of different plastic parts. For example, when producing plastic parts that require impact resistance, a toughening agent must be added to the plastic raw materials; when producing plastic parts for outdoor use, a UV inhibitor is added to prevent aging. The plastic preparation process may seem simple, but it actually involves many details, including raw material selection, determining the mixing ratio, and choosing the mixing method. Mistakes in any step can lead to preparation failure, affecting subsequent production.
The first step in plastics formulation is raw material selection and pretreatment. The choice of plastic raw materials must be determined based on the intended use environment, performance requirements, and injection molding process conditions. Commonly used plastic raw materials include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polycarbonate (PC), each with its own unique performance characteristics. For example, polyethylene offers excellent flexibility and chemical resistance, making it suitable for container-type plastics; polycarbonate, with its high strength and light transmittance, is often used in optical components. After raw materials are selected, pretreatment is required, such as removing impurities and breaking up large pieces, to ensure uniform formulation. Highly hygroscopic plastic raw materials, such as polyamide (PA) and polycarbonate (PC), also require drying to prevent moisture from affecting mixing during the formulation process .
The selection and proportioning of additives are crucial steps in plastics formulation. Common additives include plasticizers, stabilizers, fillers, colorants, and lubricants, each with its own specific function. Plasticizers increase the plastic’s flexibility and plasticity. For example, adding phthalate plasticizers to polyvinyl chloride (PVC) can soften it from a hard and brittle state. Stabilizers prevent plastic degradation from heat, light, and other factors during processing and use. For example, adding lead salt stabilizers to PVC improves its thermal stability. Fillers reduce costs and improve the mechanical properties of plastics. For example, adding glass fiber to polypropylene significantly increases its strength and rigidity. Colorants impart the desired color to plastic parts and are categorized as organic pigments, inorganic pigments, and dyes. Lubricants improve the processing fluidity of plastics and reduce friction between the melt and equipment. For example, adding stearic acid and its salts to polyolefin processing can improve the processing fluidity of plastics and reduce friction between the melt and equipment. The appropriate additive ratio must be determined experimentally based on the performance requirements of the plastic part. Excessive proportions may lead to reduced plastic performance, while too low a proportion may not achieve the desired effect.
Mixing and dispersing plastics are crucial steps in ensuring formulation quality. The goal of mixing is to evenly distribute the plastic raw materials and additives, forming a homogeneous mixture. Dispersing, on the other hand, involves fully breaking up the additive particles and evenly dispersing them throughout the plastic matrix, preventing agglomeration that could affect the performance of the plastic part. Commonly used mixing equipment includes high-speed mixers, kneaders, and rubber mixers. High-speed mixers use the centrifugal force generated by the high-speed rotating impeller to cause the materials to churn and collide violently within the mixer, achieving mixing. This allows for rapid mixing and high efficiency, making them suitable for pre-mixing. Kneaders use rotating impellers to knead the materials while heating them, making them suitable for mixing processes that require heating and melting. Rubber mixers use two counter-rotating rollers to compact and mix the materials, making them suitable for mixing high-viscosity materials. During the mixing process, parameters such as mixing time, temperature, and speed must be carefully controlled to ensure uniform mixing. For example, the speed of a high-speed mixer is typically 500-2000 rpm, the mixing time is 5-15 minutes, and the temperature is determined based on the characteristics of the plastic raw material, generally staying within its melting point.
Quality testing of plastics after formulation is crucial to ensuring smooth subsequent injection molding production. Common testing items include mixture homogeneity, moisture content, and melt index. Mixture homogeneity can be tested through microscopic observation or chemical analysis to ensure even distribution of additives within the plastic matrix. Moisture content can be tested using a Karl Fischer titrator; for hygroscopic plastics, the moisture content should be controlled below 0.02%. The melt index reflects the flowability of the plastic mixture and can be measured using a melt indexer. Its value must meet the requirements of the injection molding process. Furthermore, small-batch trial injection molding of the formulated plastic is necessary to verify the formulation by observing the appearance quality of the molded parts and testing their mechanical properties. If any issues are identified, the raw material ratio or mixing parameters should be adjusted promptly until the required results are met. With the development of the plastics industry, plastics formulation technology is also constantly advancing. For example, computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies are being used to optimize formulations and mixing processes, improving formulation efficiency and precision. New additives and mixing equipment are being developed to improve the performance and processability of plastics, providing the injection molding industry with higher-quality raw materials.
