Detailed explanation of injection molding process

In the field of plastic processing, injection molding is the most common processing method for PA6, PA66, nylon 66, PA12 and nylon 1010. Different plastic materials have different characteristics and processing requirements. This article will explain in detail the injection molding process of several materials such as PA6, PA66, nylon 66, PA12 and nylon 1010.

1. PA6 injection molding process

Chemical and physical properties

The chemical and physical properties of PA6 are similar to those of PA66, but it has a lower melting point and a wider process temperature range. Its impact resistance and solvent resistance are better than those of PA66, but its hygroscopicity is stronger. Hygroscopicity is an important factor affecting the quality characteristics of PA6 products, so hygroscopicity should be fully considered when designing products.

Injection molding process conditions

Drying treatment:PA6 is easy to absorb moisture, so drying before processing is particularly important. If the material is supplied in waterproof packaging, the container should be kept airtight. If the humidity is greater than 0.2%, it is recommended to dry it in hot air above 80℃ for 16 hours. If the material has been exposed to air for more than 8 hours, it is recommended to vacuum dry it at 105℃ for more than 8 hours.

Melting Temperature:230~280℃, for reinforced varieties it is 250~280℃.

Mold temperature:80~90℃. Mold temperature has a significant impact on crystallinity, which in turn affects the mechanical properties of plastic parts. Structural components should choose a higher mold temperature, such as 80~90℃. For plastic parts with thin walls and long processes, it is also recommended to use higher mold temperatures. Increasing mold temperature can improve the strength and stiffness of plastic parts, but reduces the toughness. If the wall thickness is greater than 3mm, it is recommended to use a low-temperature mold at 20~40℃. The mold temperature of glass fiber reinforced materials should be greater than 80℃.

Injection pressure:Generally between 750~1250bar (depending on materials and product design).

Injection speed:High speed (slightly reduced for reinforced materials).

Runners and gates:Since PA6 has a very short solidification time, the location of the gate is critical. The gate aperture should not be less than 0.5*t (where t is the thickness of the plastic part). If a hot runner is used, the gate size should be smaller than that of a conventional runner, because the hot runner helps prevent the material from solidifying prematurely. If a submerged gate is used, the minimum gate diameter should be 0.75mm.

2. Injection molding process of PA66

Drying of Nylon 66

Vacuum drying:Temperature 95~105℃, time 6-8 hours

Hot air drying: Temperature 90~100℃, time about 4 hours

Crystallinity:Except for transparent nylon, most nylons are crystalline polymers with high crystallinity. The tensile strength, wear resistance, hardness, lubricity and other properties of the products are improved, the thermal expansion coefficient and water absorption tend to decrease, but the transparency and impact resistance are unfavorable. The mold temperature has a great influence on crystallization. The higher the mold temperature, the higher the crystallinity, and the lower the mold temperature, the lower the crystallinity.

Shrinkage:Similar to other crystalline plastics, nylon resin has a large shrinkage problem. Generally, the shrinkage of nylon is most closely related to crystallization. When the product has a high degree of crystallinity, the shrinkage of the product will also increase. During the molding process, lowering the mold temperature, increasing the injection pressure, and lowering the material temperature will reduce the shrinkage, but the internal stress of the product will increase and it will be easy to deform. The shrinkage of PA66 is 1.5~2%.

Molding equipment:When molding nylon, the main attention should be paid to preventing the "nozzle casting phenomenon", so self-locking nozzles are generally used for the processing of nylon materials.

3. Injection molding process of PA12

Drying treatment:Before processing, the humidity should be kept below 0.1%. If the material is stored exposed to the air, it is recommended to dry it in 85℃ hot air for 4~5 hours. If the material is stored in a sealed container, it can be used directly after 3 hours of temperature equilibrium.

Melting Temperature:240~300℃; for materials with common properties, do not exceed 310℃; for materials with flame retardant properties, do not exceed 270℃.

Mold temperature:For unreinforced materials, it is 30~40℃, for thin-walled or large-area components, it is 80~90℃, and for reinforced materials, it is 90~100℃. Increasing the temperature will increase the crystallinity of the material. Accurately controlling the mold temperature is very important for PA12.

Injection pressure:The maximum value is 1000 bar (low holding pressure and high melting temperature are recommended).

Injection speed:High speed (better for materials with glass additives).

Runners and gates:For materials without additives, the flow channel diameter should be around 30mm due to the low viscosity of the material. For reinforced materials, a large flow channel diameter of 5~8mm is required. The flow channel shape should be all circular. The injection port should be as short as possible. Various forms of gates can be used. Do not use small gates for large plastic parts to avoid excessive pressure or excessive shrinkage on the plastic parts. The gate thickness should be equal to the thickness of the plastic part. If a submerged gate is used, the recommended minimum diameter is 0.8mm. Hot runner molds are very effective, but require precise temperature control to prevent material leakage or solidification at the nozzle. If a hot runner is used, the gate size should be smaller than that of a cold runner.

4. Injection molding process of nylon 1010

Nylon 1010 contains hydrophilic amide groups in its molecular structure and is highly hygroscopic. Its equilibrium water absorption rate is 0.8% to 1.0%. Moisture has a significant impact on the physical and mechanical properties of nylon 1010; therefore, the raw materials must be dried before use to reduce the moisture content to less than 0.1%. When drying nylon 1010, oxidative discoloration should be prevented because the amide group is sensitive to oxygen and prone to oxidative degradation. It is best to use vacuum drying when drying, because this method has a high dehydration rate, short drying time, and good quality of dried pellets. Drying conditions are generally vacuum degree above 94.6 kPa, temperature 90~100°C, drying time 812h; moisture content is reduced to 0.1%~0.3%. If ordinary oven drying is used, the drying temperature should be controlled at 95~105°C and the drying time should be extended, which generally takes 20~24 hours. Dry materials should be stored carefully to avoid moisture absorption.

Plasticization process:Before nylon 1010 enters the mold cavity, it should reach the specified molding temperature, and be able to provide a sufficient amount of molten material within the specified time, and the temperature of each point of the molten material should be uniform. In order to meet the above requirements, a screw injection molding machine is used according to the characteristics of nylon 1010, and the screw is a sudden change or a combination type. The barrel temperature increases from the hopper feed forward. Since the barrel temperature is controlled near the melting point, it is beneficial to improve the impact strength of the product, and it can avoid leakage and prevent material decomposition. The barrel temperature is generally 210~230 ℃. In order to reduce the friction between the screw and PA1010 during pre-plasticization. Liquid paraffin can be used as a lubricant. The dosage is generally 0.52 mL/kg; the mold temperature is generally 40~80℃. The increase in back pressure is conducive to compacting the material in the screw groove, expelling low-molecular gases in the material, and improving the plasticization quality; however, the increase in back pressure will increase the leakage and backflow between the screw and the barrel, reducing the plasticization capacity of the injection molding machine. The plasticizing back pressure should not be too high, otherwise it will greatly reduce the plasticizing efficiency, and even produce excessive shear force and shear heat, causing the material to decompose. Therefore, under the condition that it can meet the requirements of injection molding, the lower the plasticizing back pressure, the better, generally 0.5-1.0MPa.

Filling process:In this process, attention should be paid to the injection pressure and injection speed of nylon 1010 injection molding. Generally, the injection pressure should be 2~5MPa, and the injection speed should be slow. If the injection pressure is too high and the injection speed is too fast, it is easy to form mold filling turbulence, which is not conducive to eliminating bubbles in the product. According to the characteristics of the change of cavity pressure, the injection molding process can be divided into the stages of leading the material into the mold, flowing the mold and cooling and shaping. The cooling and shaping process can be divided into three stages: pressure maintenance and material filling, backflow and cooling after gate freezing.

Certain conditions must be met to achieve pressure-holding and material replenishment. On the one hand, there should be enough molten material, that is, there should be material to replenish; at the same time, the pouring system should not solidify too early, so that the molten material has a way to go. This is a necessary condition for material replenishment; on the other hand, the injection pressure should be high enough and the pressure holding time should be long enough, which are sufficient conditions for material replenishment.

The holding time is usually determined by experiments and should not be too long or too short. If the holding time is too long, it will not only extend the molding cycle, but also cause excessive residual pressure in the mold cavity, making demolding difficult, and even causing deformation or rupture of the plastic part. When the pouring system solidifies or the pressure-holding filling is completed, the injection pressure can be reduced to solidify and seal the gate, and then the molten material in the mold cavity and the pouring system are balanced, and the mold cavity pressure rises. After that, the molten material in the mold cavity cools faster than the gate and the pouring system, resulting in a pressure difference between the gate and the mold cavity, and the plastic melt flows back to the pouring system under high pressure, which forms a backflow.

During the cooling process of the melt in the mold cavity, when the mold cavity pressure gradually decreases to a level lower than the pressure at the gate freezing point, the melt cools and shrinks, generating a clamping force that keeps the melt in the gating system. Then it continues to cool until the melt at the gate freezes. Therefore, cooling after the gate freezes is actually the final stage of the cooling and shaping process.

In the process of injection molding nylon 1010, since moisture has a great influence on the mechanical properties of nylon 1010, the moisture content should be strictly controlled during the production process to ensure the quality of the product.

The above is a detailed description of the injection molding process of PA6, PA66, nylon 66, PA12 and nylon 1010. Different materials have different characteristics and processing requirements, so it is necessary to select appropriate injection molding process conditions according to the characteristics of the materials to ensure the quality and performance of the product. In the actual production process, it is also necessary to continuously optimize the process conditions to obtain the best processing effect.