NylonPolyamide (PA for short) is a plastic composed of polyamide resins. Such resins can be made from diamines and dibasic acids through condensation, or from lactams formed by dehydration of amino acids through ring-opening polymerization.
There are many varieties of PA, mainly PA6, PA66, PA610, PA11, PA12, PA1010, PA612, PA46, PA6T, PA9T, MXD-6 aromatic amides, etc. PA6, PA66, PA12, and PA1010 are the most commonly used. This article will discuss the characteristics of the four nylon injection molding processes, PA6, PA66, PA12, and PA1010, in more depth.
Nylon 6 injection molding process
Chemical and physical properties
The chemical and physical properties of PA6 are very similar to PA66, however, it has a lower melting point and a wide range of process temperatures. Its impact and dissolution resistance is better than PA66, but it is also more hygroscopic.
Because many of the quality characteristics of plastic parts are affected by moisture absorption, it is important to take this into account when designing products with PA6.
To improve the mechanical properties of PA6, a variety of modifiers are often added. Glass is the most common additive, and sometimes synthetic rubber, such as EPDM and SBR, is added to improve impact resistance.
For products without additives, the shrinkage of PA6 ranges from 1% to 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 the molded assembly is mainly influenced by the crystallinity and moisture absorption of the material. The actual shrinkage is also a function of the part design, wall thickness, and other process parameters.
Injection Molding Process Conditions
Drying: Since PA6 readily absorbs moisture, special attention must be paid to drying before processing. If the material is supplied in a waterproof material package, the container should be kept airtight.
If the humidity is greater than 0.2%, drying in hot air above 80°C for 16 hours is recommended. If the material has been exposed to air for more than 8 hours, vacuum drying at 105°C for more than 8 hours is recommended.
Melting temperature: 230~280℃, for reinforced varieties 250~280℃.
Mold temperature: 80~90℃. Mold temperature significantly affects the crystallinity, which in turn affects the mechanical properties of the injection molded part. For structural parts crystallinity is important, so the recommended mold temperature is 80~90℃.
Higher mold temperatures are also recommended for thin-walled, long-flow plastic parts. Increasing the mold temperature increases the strength and stiffness of the injection molded part, but decreases the toughness.
If the wall thickness is greater than 3mm, it is recommended to use a low-temperature mold of 20~40℃. For glass fiber reinforced material mold temperature should be greater than 80℃.
Injection pressure: generally between 750~1250bar (depending on the material and product design).
Injection speed: High speed (to be reduced slightly for reinforced materials).
Runners and gates: Due to the short solidification time of PA6, the location of the gates is very important. The gate aperture should not be smaller than 0.5*t (where t is the thickness of the molded part).
If using a hot runner, the gate size should be smaller than with a conventional runner, because the hot runner helps to stop the material from solidifying prematurely. If a submerged gate is used, the minimum diameter of the gate should be 0.75mm.
The 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: In addition to transparent nylon, nylon is a mostly crystalline polymer, with high crystallinity, the product tensile strength, abrasion resistance, hardness, lubricity, and other properties have improved, and the coefficient of thermal expansion and water absorption tends to decline, but the transparency and impact resistance is not conducive to performance.
Mold temperature has a great influence on crystallization, with high crystallinity at high mold temperature and low crystallinity at low mold temperature.
Shrinkage: Similar to other crystalline plastics, nylon resin has the problem of shrinkage, generally, the shrinkage of nylon has the greatest relationship with crystallization, when the product crystallization is large, the product shrinkage will also increase.
In the molding process reducing the mold temperature and increasing the injection pressure and reducing the material temperature will reduce the shrinkage, but the product’s internal stress increases easily deformed. 1.5-2% shrinkage of PA66.
Molding equipment: nylon injection molding, the main attention to prevent the “nozzle of the flow phenomenon”, so the processing of nylon material generally uses a self-locking nozzle.
Product and mold
1. Wall thickness of the product The flow length ratio of nylon is between 150-200, the wall thickness of nylon products is not less than 0.8mm, generally between 1-3.2mm, and the shrinkage of the product is related to the wall thickness of the product, the thicker the wall thickness, the greater the shrinkage.
2. Exhaust The overflow value of nylon resin is about 0.03mm, so the exhaust hole slot should be controlled at 0.025 or less.
3. Mold temperature The thin wall of the product is difficult to form or requires high crystallinity mold temperature control, the product requires a certain degree of flexibility and is generally used to control the temperature of cold water.
The molding process of nylon 66
Barrel temperature: nylon is a crystalline polymer, so the melting point is obvious, nylon resin in injection molding barrel temperature selected with the performance of the resin itself, equipment, and the shape of the product factors.
Nylon 66 is 260℃. Because of the poor thermal stability of nylon, it is not suitable to stay in the barrel for a long time at a high temperature, so as not to cause material discoloration and yellowing, and because of the good fluidity of nylon, the temperature exceeds its melting point and then flows rapidly.
Injection pressure: nylon melt viscosity is low and has good fluidity, but the condensation rate is faster, in the shape of the complex and thin wall thickness of the product prone to deficiencies, so still needs a higher injection pressure.
Usually, if the pressure is too high, the product will appear to overflow edge problems; if pressure is too low, the product will produce ripples, bubbles, obvious fusion marks or product shortage, and other defects, most nylon varieties of injection pressure do not exceed 120MPA.
Generally selected in the range of 60-100MPA is to meet the requirements of most products, as long as the product does not appear bubbles, dents, and other defects, generally do not want to use a higher holding pressure, to avoid the increase in internal stress. To avoid the increase of stress on the product.
Injection speed: For nylon, the injection speed is faster to prevent ripples caused by too fast cooling speed and insufficient mold filling. Fast injection speed does not have a significant impact on the performance of the product.
Mold temperature: mold temperature has a certain influence on the crystallinity and injection molding shrinkage, high mold temperature crystallinity, high abrasion resistance, hardness, elastic modulus increase, water absorption decrease, molding shrinkage of products increase; low mold temperature crystallinity low, good toughness, elongation higher.
Nylon 66 molding process parameters:
1）Barrel temperature ℃
2）Rear 240-285 Middle 260-300 Front 260-300
3）Nozzle temperature ℃ 260-280 Mold temperature ℃ 20- 90
4）Injection pressure MPA 60-200
The use of mold release agent: The use of a small amount of mold release agent sometimes has the effect of improving and eliminating defects such as air bubbles.
The release agent for nylon products can be zinc stearate and white oil, etc. It can also be mixed into a paste, which must be used in small amounts and evenly to avoid surface defects of the products.
When stopping the machine, empty the screw to prevent the next production, and twist the screw.
PA12 injection molding process conditions
Drying treatment: The humidity should be ensured to be below 0.1% before processing. If the material is exposed to air for storage, it is recommended to be dried in hot air at 85℃ for 4~5 hours. If the material is stored in an airtight container, then it can be used directly after 3 hours of temperature equilibration.
Melting temperature: 240~300℃; do not exceed 310℃ for common characteristic materials, and do not exceed 270℃ for materials with flame retardant characteristics.
Mold temperature: 30~40℃ for unreinforced materials, 80~90℃ for thin wall or large area components, 90~100℃ for reinforced materials. Increasing the temperature will increase the crystallinity of the material. Precise control of mold temperature is 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).
Runners and gates: For materials without additives, the runner diameter should be around 30mm due to the low viscosity of the material. For reinforced materials, a large runner diameter of 5~8mm is required.
The runner shape should be all round. The injection port should be as short as possible. Various types of gates can be used. Do not use small gates for large parts to avoid excessive pressure or shrinkage on the part.
It is better to have a gate thickness equal to the thickness of the part. If a submerged gate is used, a minimum diameter of 0.8 mm is recommended. hot runner injection molds are effective, but require very 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 a cold runner.
PA1010 injection molding process conditions
Because the molecular structure of nylon 1010 contains hydrophilic amide groups, it is very easy to absorb moisture, and its equilibrium water absorption rate is 0.8%~1.0%.
Moisture has a significant impact on the physical and mechanical properties of nylon 1010; therefore, the raw material must be dried before use, so that its water content is reduced to less than 0.1%.
When drying nylon 1010 should prevent oxidation discoloration, because the amide group is sensitive to oxygen and easy to oxidative degradation.
It is better to use vacuum drying when drying because this method has a high dehydration rate, short drying time, and good quality of the dried granules.
The drying conditions are generally vacuumed degrees above 94.6 kPa, temperature 90~100 ℃, drying time 8~12h; water content is reduced to 0.1%~0.3%.
If the ordinary oven dry operation is used, the drying temperature should be controlled at 95~105 ℃, and the drying time should be extended, generally needing 20~24h. The dried material should be preserved with care to avoid moisture absorption again.
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 molten material should be uniform at all points.
To achieve the above requirements, a screw-type injection molding machine is used according to the characteristics of nylon 1010, and the screw is mutation type or combination type.
The barrel temperature increases from the hopper inlet to the front in sequence. As the barrel temperature is controlled near the melting point, it is beneficial to the improvement of the impact strength of the product and can avoid the leakage of material and prevent the decomposition of the material, the barrel temperature is generally 210~230 ℃.
To reduce the friction between the screw and PA1010 during pre-molding. Liquid paraffin can be used as a lubricant. The dosage is generally 0.5~2 mL/kg; the mold temperature is generally 40~80℃. The increase in back pressure is good for compacting the material in the screw groove.
Expel the low molecular gas in the material. Improve the quality of plasticization, but the increase in back pressure will increase the leakage and backflow between the screw and the barrel so that the plasticizing ability of the injection molding machine is reduced.
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 so that the material decomposes.
Therefore, under the condition that the requirements of injection molding can be met. The lower the plasticizing back pressure, the better, generally taking 0.5 ~ 1.0MPa.
Mold filling process: In this process, we should 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. The injection speed is too fast. Easy to form a turbulent flow of mold filling, not conducive to the elimination of air bubbles in the product.
According to the characteristics of the cavity pressure changes, the injection molding process can be divided into the introduction of material into the mold, the flow of mold fil, ling, and the cooling and shaping stages.
The cooling and shaping process can be divided into three stages: pressure-holding and replenishment, backflow and cooling after the gate freezes.
To achieve the pressure-holding material must have certain conditions: on the one hand, there should be enough molten material, that is, there is material to fill; at the same time, the pouring system cannot be solidified too early, so that the molten material has a way to go.
On the other hand, the injection pressure should be high enough and the holding time should be long enough, which is a sufficient condition for the material to be filled. The holding time is usually determined by the experiment, and should not be too long or too short.
If the holding time is too long, it will not only prolong the molding cycle, but also make the residual pressure in the mold cavity too large, which will make it difficult to release the mold, and even not open the mold.
The best pressure-holding time should be zero when the mold cavity residual pressure is opened. The general holding time of nylon 1010 injection molding parts is 4~50 s.
Demolding: Nylon 1010 parts can be demolded when they are cooled in the mold until they have sufficient rigidity.
The demolding temperature should not be too high. Generally, it is controlled between the heat deflection temperature of PA1010 and the mold temperature.
The residual pressure in the mold cavity should be close to zero when demolded, which is determined by the holding time. Generally, the injection molding time of PA1010 injection parts is: injection time 4~20 s, holding time 4~50 s, and cooling time 10~30 s.
Conclusion
Injection molding is a process that allows the manufacture of high-quality parts with complex geometries. The four nylon injection molding processes we have described should give you a good starting point for understanding the different types of nylon and how to produce them. If you have any questions about the process or would like to learn more, please don’t hesitate to contact our team and we’ll be happy to help!
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