Detailed Explanation of the Injection Molding Process for the 5 Major General-Purpose Plastics

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1. Injection molding process for polypropylene (PP)
PP is commonly known as polypropylene; due to its excellent resistance to breakage, it is also referred to as “unbreakable plastic”. PP is a translucent, semicrystalline thermoplastic with high strength, good insulation properties, low water absorption, a high heat distortion temperature, low density, and high crystallinity. Modified fillers typically include glass fibers, mineral fillers, thermoplastic rubbers, etc. PP used for different purposes exhibits significant differences in flowability; generally, the flow rate of commonly used PP lies between that of ABS and PC. Pure PP is translucent ivory white and can be dyed in various colors. For coloring PP, color masterbatches can only be used on conventional injection molding machines. Some machines are equipped with separate plasticizing elements that enhance mixing, and they can also be colored using pigment powders. Products intended for outdoor use generally contain UV stabilizers and carbon black filler. The proportion of recycled material used should not exceed 15%, otherwise it will cause a decrease in strength and discoloration due to decomposition. PP generally does not require special drying treatment before injection molding. There are no special requirements for selecting the injection molding machine. Due to the high crystallinity of PP. A computerized injection molding machine with high injection pressure and multi-stage control is required. The mode-locking force is generally set at 3800 t/m2, while the injection volume of 20%-85% is sufficient. The mold temperature should be 50–90°C; a higher temperature is used for applications with strict size requirements. The core temperature is at least 5°C lower than the cavity temperature; the runner diameter ranges from 4 to 7 mm, the length of the pin gate is 1 to 1.5 mm, and its diameter can be as small as 0.7 mm. The length of the polygonal gate should be as short as possible—approximately 0.7 mm. Its depth should be half of the wall thickness, and its width should be twice the wall thickness. Additionally, it gradually increases along with the length of the molten flow within the mold cavity. The mold must have good venting capabilities; the vent holes should be 0.025mm to 0.038mm in depth and 1.5mm thick. To avoid shrinkage marks, large, round gates and circular runners should be used, and the thickness of the ribs should be low (for example, 50-60% of the wall thickness). Products made from homopolymer PP cannot have a thickness exceeding 3 mm, otherwise bubbles will form (thick-walled products can only be made from copolymer PP). The melting point of PP is 160–175°C, and its decomposition temperature is 350°C; however, during injection molding the temperature should not exceed 275°C, with the temperature during the melting stage being ideally around 240°C. To reduce internal stress and deformation, high-speed injection should be used, but it is not suitable for certain grades of PP and molds (causing bubbles and air marks). If alternating light and dark stripes caused by gate diffusion appear on the patterned surface, low injection speed and a higher mold temperature should be used. A melt backpressure of 5 bar can be used; the backpressure for pigment materials can be appropriately increased. A higher injection pressure (1500–1800 bar) and holding pressure (about 80% of the injection pressure) are used. Presure holding is likely to be switched on at about 95% of the total process time, with a longer pressure holding period. To prevent shrinkage deformation caused by post-crystallization, the products generally need to be treated with hot water immersion. 2. Polyethylene (PE) injection molding process: PE is a crystalline material with extremely low moisture absorption, not exceeding 0.01%, so it does not require drying prior to processing. PE molecules have good chain flexibility, weak intermolecular forces, low melt viscosity, and excellent flow properties; as a result, it is possible to produce thin-walled, long-length products during molding without the need for high pressure. PE has a wide range of shrinkage rates, high shrinkage values, and a distinct directionality; the shrinkage rate of LDPE is around 1.22%, while that of HDPE is around 1.5%. As a result, it is prone to deformation and warping. The cooling conditions of the mold have a significant impact on the shrinkage rate; therefore, the mold temperature must be well controlled to ensure uniform and stable cooling. PE has a high crystallization ability, and the temperature of the mold has a significant impact on the crystallization behavior of the plastic parts. A high mold temperature results in slow melt cooling, leading to a high degree of crystallization in the plastic part, and thus higher strength. PE has a low melting point, but a high specific heat capacity; therefore, a significant amount of heat is still required during plasticization. As a result, the plasticization equipment needs to have a high heating power in order to improve production efficiency. PE has a narrow softening temperature range, and its melt is prone to oxidation; therefore, in molding processes, contact between the melt and oxygen should be avoided as much as possible to prevent a decline in the quality of the plastic parts. PE parts are relatively soft and easy to demold; therefore, when the plastic part has shallow side grooves, it can be demolded with ease. The non-Newtonian behavior of the PE melt is not significant; changes in shear rate have little effect on its viscosity, and temperature also has a minor impact on the viscosity of the PE melt. The cooling rate of the PE melt is slow, so it must be cooled thoroughly. The mold should have a good cooling system. If the PE melt is fed through a direct feed port during injection, stress will increase, leading to uneven contraction and significant directional deformation; therefore, care should be taken in selecting the feed port parameters. PE has a wide range of molding temperatures; in a flowing state, slight fluctuations in temperature have no effect on injection molding. PE has good thermal stability; generally, no significant decomposition occurs below 300 degrees, which has no impact on its quality. The main molding conditions for PE: Barrel temperature: The barrel temperature is primarily related to the density of PE and the melt flow rate; it is also influenced by the type and performance of the injection molding machine, as well as the shape of the final plastic part. Since PE is a crystalline polymer, the crystals absorb a certain amount of heat during melting; therefore, the barrel temperature should be 10 degrees higher than its melting point. For LDPE, the barrel temperature should be maintained between 140–200°C, while for HDPE it should be 220°C; the lowest temperature is recorded at the back of the barrel and the highest temperature at its front end. Mold temperature: The mold temperature has a significant impact on the crystallization behavior of the plastic parts. A higher mold temperature leads to greater crystallinity and higher strength in the melt, but it also increases the shrinkage rate. Typically, the mold temperature for LDPE is controlled at 30°C–45°C, while the temperature for HDPE is 10–20°C higher. Injection pressure: Increasing the injection pressure facilitates the filling of the mold with molten material. Since PE has good flow properties, except for thin-walled and elongated products, a lower injection pressure should be chosen; generally, the injection pressure ranges from 50 to 100 MPa. The shape is simple. For larger plastic parts behind a wall, the injection pressure can be lower; conversely, it should be higher. 3. Polyvinyl chloride (PVC) injection molding process. Typical application areas: water supply pipes, household piping, wall panels for buildings, enclosures for commercial machinery, packaging for electronic products, medical devices, food packaging, etc. Chemical and physical properties: PVC is a amorphous material. In practical use, PVC materials often contain stabilizers, lubricants, processing aids, colorants, impact modifiers, and other additives. PVC material features non-flammability, high strength, resistance to weathering, and excellent geometric stability. PVC has strong resistance to oxidizing agents, reducing agents, and strong acids. However, it can be corroded by strong oxidizing acids such as concentrated sulfuric acid and concentrated nitric acid, and it is also not suitable for use in situations involving contact with aromatic hydrocarbons and chlorinated hydrocarbons. The melting temperature of PVC during processing is a very important process parameter; if this parameter is not properly set, it can lead to material decomposition. PVC has quite poor flow properties, and its process window is very narrow. In particular, PVC materials with high molecular weights are more difficult to process (lubricants are usually added to such materials to improve their flow properties); therefore, PVC materials with low molecular weights are generally used. The shrinkage rate of PVC is quite low, generally ranging from 0.2% to 0.6%. Injection molding process conditions: Drying treatment: Usually, no drying treatment is required. Melting temperature: 185~205°C. Mold temperature: 20~50°C. Injection pressure: can reach up to 1500 bar. Holding pressure: can reach up to 1000 bar. Injection speed: To avoid material degradation, a suitable injection speed should generally be used. Flow channels and gates: All conventional gates can be used. When machining smaller parts, it is best to use a needle-point gate or a submersion gate ； For thicker components, it is best to use a fan gate. The minimum diameter of a needle-point gate or submersion gate should be 1 mm ； The thickness of the fan-shaped gate must not be less than 1 mm. Chemical and physical properties: Rigid PVC is one of the most widely used plastic materials. 4. Polystyrene (PS) injection molding process. Typical applications: product packaging, household items (tableware, trays, etc.), electrical applications (transparent containers, light source diffusers, insulating films, etc.). Chemical and physical properties: Most commercially available PS is a transparent, amorphous material. PS possesses very good geometric stability, thermal stability, optical transmittance, electrical insulation properties, and a very low tendency to absorb moisture. It can resist water and diluted inorganic acids, but it can be corroded by strong oxidizing acids such as concentrated sulfuric acid, and it can swell and deform in some organic solvents. The typical shrinkage rate is between 0.4% and 0.7%. Injection molding process conditions: Drying treatment: Unless stored improperly, drying treatment is generally not required. If drying is required, it is recommended to dry at 80°C for 2–3 hours. Melting temperature: 180~280°C. For flame-retardant materials, the upper limit is 250°C. Mold temperature: 40~50°C. Injection pressure: 200~600 bar. Injection speed: A fast injection speed is recommended. Flow channels and gates: All conventional types of gates can be used. 5. ABS injection molding process – Typical applications: automobiles (dashboards, trunk lids, wheel covers, mirror housings, etc.), refrigerators, high-strength tools (hair dryers, mixers, food processors, lawnmowers, etc.), telephone casings, typewriter keyboards, as well as recreational vehicles such as golf carts and jet skis. Chemical and physical properties: ABS is synthesized from three chemical monomers: acrylonitrile, butadiene, and styrene. Each monomer possesses different properties: acrylonitrile has high strength, thermal stability, and chemical stability ； Butadiene possesses toughness and impact resistance ； Styrene features easy processing, high surface finish, and high strength. Morphologically, ABS is an amorphous material. The polymerization of the three monomers yielded a ternary copolymer with two phases: one being a continuous phase of styrene-acrylonitrile, and the other being a dispersed phase of polybutadiene rubber. The properties of ABS mainly depend on the ratio of the three monomers and the molecular structure in the two phases. This allows for great flexibility in product design, and as a result, there are hundreds of different quality grades of ABS materials available on the market. These materials of different qualities offer various properties, such as impact resistance ranging from medium to high, surface finish from low to high, and high-temperature warping resistance. ABS material boasts excellent processability, favorable aesthetic properties, low creep, superior dimensional stability, and high impact strength. Injection molding process conditions: Drying treatment: ABS material is hygroscopic, and it is necessary to carry out drying treatment before processing. It is recommended to dry at 80–90°C for at least 2 hours. The material temperature should be kept below 0.1%. Melting temperature: 210~280℃ ； Recommended temperature: 245°C. Mold temperature: 25~70°C. (The mold temperature affects the finish of the plastic part; lower temperatures result in a poorer finish.) Injection pressure: 500~1000 bar. Injection speed: medium to high speed.