PA6、PA66、PA12、PA1010Introduction of four nylon injection molding processes
TIME：2021-10-7
Nylon 6 injection molding process
Chemical and physical properties
The chemical and physical properties of PA6 are similar to those of PA66. However, its melting point is low and the process temperature range is wide. Its impact resistance and solubility are better than PA66, but its moisture absorption is also stronger. Because many quality characteristics of plastic parts are affected by moisture absorption, this should be fully considered when using PA6 to design products.
In order to improve the mechanical properties of PA6, various modifiers are often added. Glass is the most common additive. Sometimes synthetic rubber, such as EPDM and SBR, is added to improve impact resistance.
For products without additives, the shrinkage of PA6 is between 1% and 1.5%. The addition of glass fiber additives can reduce the shrinkage to 0.3% (but slightly higher in the direction perpendicular to the process). The shrinkage of molding assembly is mainly affected by the crystallinity and moisture absorption of the material. The actual shrinkage is also a function of plastic part design, wall thickness and other process parameters.
Injection molding process conditions
Drying treatment: because PA6 is easy to absorb water, special attention should be paid to the drying before processing. If the material is supplied in waterproof packaging, the container shall be kept closed. If the humidity is greater than 0.2%, it is recommended to dry in hot air above 80 ℃ for 16 hours. If the material has been exposed to air for more than 8 hours, vacuum drying at 105 ℃ for more than 8 hours is recommended.
Melting temperature: 230 ~ 280 ℃, 250 ~ 280 ℃ for reinforced varieties.
Mold temperature: 80 ~ 90 ℃. Mold temperature significantly affects the crystallinity, which in turn affects the mechanical properties of plastic parts. For structural parts, crystallinity is very important, so it is recommended that the mold temperature is 80 ~ 90 ℃.
For thin-walled plastic parts with long process, higher die temperature is also recommended. Increasing the mold temperature can improve the strength and stiffness of plastic parts, but reduce the toughness.
If the wall thickness is greater than 3mm, it is recommended to use a low-temperature mold at 20 ~ 40 ℃. For glass fiber reinforced materials, the mold temperature shall be greater than 80 ℃.
Injection pressure: generally between 750 ~ 1250bar (depending on material and product design).
Injection speed: high speed (slightly lower for reinforced materials).
Runner and gate: because the solidification time of PA6 is very short, the position of gate is very important. The gate aperture shall not be less than 0.5 * t (where t is the thickness of plastic parts).
If hot runner is used, the gate size should be smaller than that of conventional runner, because hot runner can help prevent premature solidification of material. If a submerged gate is used, the minimum diameter of the gate shall be 0.75mm.
Nylon 66 injection molding process
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 nylon are crystalline polymers with high crystallinity. The tensile strength, wear resistance, hardness, lubricity and other properties of the products are improved, and the coefficient of thermal expansion and water absorption tend to decline, but it is unfavorable to transparency and impact resistance. Mold temperature has a great influence on crystallization. Mold temperature is high, crystallinity is high, mold temperature is low and crystallinity is low
Shrinkage: similar to other crystalline plastics, nylon resin has the problem of large shrinkage. Generally, the shrinkage of nylon has the greatest relationship with crystallization. When the crystallinity of the product is large, the shrinkage of the product will also increase. During the molding process, reducing the mold temperature, increasing the injection pressure and reducing the material temperature will reduce the shrinkage, but the internal stress of the product is increased and easy to deform. The shrinkage of PA66 was 1.5-2%.
Molding equipment: during nylon molding, the main attention is to prevent "nozzle casting phenomenon", so the self-locking nozzle is generally selected for nylon material processing.
Products and molds
1. The wall thickness of the product and the flow length ratio of nylon are 150-200. The wall thickness of nylon is not less than 0.8mm, which is generally selected between 1-3.2mm. Moreover, the shrinkage of the product is related to the wall thickness of the product. The thicker the wall thickness, the greater the shrinkage.
2. The overflow value of exhaust nylon resin is about 0.03mm, so the exhaust hole slot should be controlled below 0.025.
3. Mold temperature: if the product wall is thin and difficult to form, or the mold heating control with high crystallinity is required, and the product is required to have certain flexibility, cold water temperature control is generally used.
Forming process of nylon 66
Barrel temperature: because nylon is a crystalline polymer, the melting point is obvious. The barrel temperature selected for nylon resin during injection molding is related to the performance of the resin itself, equipment and product shape factors. Nylon 66 is 260 ℃. Due to the poor thermal stability of nylon, it is not suitable to stay in the barrel at high temperature for a long time, so as not to cause the material to change color and turn yellow. At the same time, due to the good fluidity of nylon, it flows rapidly when the temperature exceeds its melting point.
Injection pressure: Nylon melt has low viscosity and good fluidity, but the condensation speed is fast. It is easy to have insufficient problems on products with complex shape and thin wall thickness, so it still needs higher injection pressure. Generally, if the pressure is too high, the product will overflow; If the pressure is too low, the products will produce defects such as ripples, bubbles, obvious fusion marks or insufficient products. The injection pressure of most nylon varieties does not exceed 120MPa. Generally, it is selected in the range of 60-100mpa to meet the requirements of most products. As long as the products do not have bubbles, dents and other defects, it is generally not expected to adopt high pressure holding pressure to avoid increasing the internal stress of the products.
Injection speed: for nylon, fast injection speed is beneficial, which can prevent ripple and insufficient mold filling caused by too fast cooling speed. The effect of fast injection speed on the properties of products is not prominent.
Mold temperature: mold temperature has a certain impact on crystallinity and molding shrinkage. High mold temperature has high crystallinity, increased wear resistance, hardness and elastic modulus, decreased water absorption and increased molding shrinkage of products; Low mold temperature, low crystallinity, good toughness and high elongation.
Nylon 66 molding process parameters
Barrel temperature ℃
Rear 240-285 middle 260-300 front 260-300
Nozzle temperature: 260-280 ℃, mold temperature: 20 - 90 ℃
Injection pressure MPa 60-200
Use of release agent: the use of a small amount of release agent can sometimes improve and eliminate defects such as bubbles. The release agent of nylon products can be zinc stearate and white oil, or mixed into paste. It must be used in a small and uniform amount to avoid surface defects of products.
Empty the screw during shutdown to prevent breaking the screw during the next production.
PA12 injection molding process conditions
Drying: the humidity shall be below 0.1% before processing. If the material is stored exposed to air, it is recommended to dry in hot air at 85 ℃ for 4 ~ 5 hours. If the material is stored in a closed container, it can be used directly after 3 hours of temperature balance.
Melting temperature: 240 ~ 300 ℃; It shall not exceed 310 ℃ for ordinary materials and 270 ℃ for materials with flame retardant characteristics.
Mold temperature: 30 ~ 40 ℃ for unreinforced materials, 80 ~ 90 ℃ for thin-wall or large-area elements, and 90 ~ 100 ℃ for reinforced materials. Increasing the temperature will increase the crystallinity of the material. Accurate control of mold temperature is very important for PA12.
Injection pressure: up to 1000 bar (low holding pressure and high melting temperature are recommended).
Injection speed: high speed (better for materials with glass additives).
Runner and gate: for materials without additives, the runner diameter should be about 30mm due to the low viscosity of the material. For reinforced materials, a large runner diameter of 5 ~ 8mm is required. The runner shape shall be all circular. The injection port shall be as short as possible. Multiple forms of gates can be used. Do not use small gate for large plastic parts, which is to avoid excessive pressure or excessive shrinkage of plastic parts. The gate thickness should preferably be equal to the thickness of the plastic part. If a submerged gate is used, a minimum diameter of 0.8mm is recommended. Hot runner mold is very effective, but it requires accurate temperature control to prevent material leakage or solidification at the nozzle. If a hot runner is used, the gate size should be smaller than the cold runner.
PA1010 injection molding process conditions
Because nylon 1010 contains hydrophilic amide groups in its molecular structure, it is very easy to absorb moisture, and its equilibrium water absorption is 0.8% ~ 1.0%. Moisture has a significant effect on the physical and mechanical properties of nylon 1010; Therefore, the raw materials must be dried before use to reduce their water content to less than 0.1%. When drying nylon 1010, oxidation discoloration should be prevented because the amide group is sensitive to oxygen and easy to be oxidized and degraded. It is better to use vacuum drying when drying, because this method has high dehydration rate, short drying time and good quality of dried pellets. Generally, the drying conditions are vacuum degree of more than 94.6 kPa, temperature of 90 ~ 100 ℃, and drying time of 8 ~ 12h; The water content decreases to 0.1% ~ 0.3%. If ordinary oven is used for dry operation, the drying temperature shall be controlled at 95 ~ 105 ℃ and the drying time shall be extended, generally 20 ~ 24h. Dry materials shall be preserved to avoid re moisture absorption.
Plasticizing process: before nylon 1010 enters the mold cavity, it shall reach the specified forming temperature and provide sufficient molten material within the specified time. The temperature of molten material at each point shall be uniform. In order to meet the above requirements, the screw injection molding machine is adopted according to the characteristics of nylon 1010, and the screw is abrupt or combined. The barrel temperature increases successively from the hopper feed. Since the barrel temperature is controlled near the melting point, which is conducive to the improvement of impact strength of products, and can avoid material leakage and prevent material decomposition, the barrel temperature is generally 210 ~ 230 ℃. In order to reduce the friction between screw and PA1010 during pre molding, liquid paraffin can be used as lubricant. The dosage is generally 0.5 ~ 2 ml; The mold temperature is generally 40 ~ 80 ℃. The increase of back pressure is conducive to compacting the material in the screw groove, expelling the low molecular gas in the material and improving the plasticizing quality; However, the increase of back pressure will increase the leakage and countercurrent between the screw and the barrel, and reduce the plasticizing capacity of the injection molding machine. Generally, the plasticizing back pressure should not be too high, otherwise the plasticizing efficiency will be greatly reduced, and even excessive shear force and shear heat will be generated, resulting in the decomposition of materials. Therefore, the lower the plasticizing back pressure is, the better, generally 0.5 ~ 1.0MPa under the condition of meeting the application requirements of injection molding.
Filling process: in this process, pay attention 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 change characteristics of mold cavity pressure, the injection molding filling process can be divided into three stages: material introduction, flow filling and cooling setting. The cooling shaping process can be divided into three stages: pressure maintaining and feeding, backflow and cooling after gate freezing.
Certain conditions must be met to realize pressure maintaining and feeding. On the one hand, it shall be ensured that there is enough molten material, that is, there is material to supplement; At the same time, the gating system can not solidify prematurely, so that the molten material can go. This is a necessary condition for feeding; On the other hand, the injection pressure should be high enough and the pressure holding time should be long enough, which is the sufficient condition for the realization of replenishment.
The pressure holding time is usually determined by experiment and can not be too long or too short. If the holding time is too long, the forming cycle will be prolonged; Moreover, the residual pressure in the mold cavity will be too large, resulting in difficult demoulding and even failure to open the mold; It also increases energy consumption. The best holding time is to make the residual pressure in the mold cavity zero when opening the mold. Generally, the molding and pressure holding time of nylon 1010 injection molded parts is 4 ~ 50 s.
Demoulding: Nylon 1010 parts can be demoulded when they are cooled in the mold to have sufficient stiffness. The demoulding temperature should not be too high. It is generally controlled between the thermal deformation temperature of PA1010 and the mold temperature. During demoulding, the residual pressure in the mold cavity should be close to zero, which is determined by the pressure holding time. Generally, the molding time of PA1010 injection molded parts is: injection time 4 ~ 20s, pressure holding time 4 ~ 50s and cooling time 10 ~ 30s.
