Introduction:

Welcome to our comprehensive guide on improving the quality of nylon injection molded parts. If you're involved in the manufacturing industry, specifically working with polyamide resin such as Nylon 6 or Nylon 66, then this blog post is tailor-made for you. We understand that achieving top-notch quality in nylon injection molded parts is crucial for the success and reputation of any company. That's why we have curated a range of valuable insights, tips, and techniques to help you elevate your manufacturing process and produce impeccable nylon injection molded parts. Join us as we dive deep into the world of polyamide resin and uncover the secrets to optimizing quality in your production line. 

1.Ensuring drying effectiveness:

Nylon is prone to moisture absorption, and if exposed to air for a long time, it will absorb moisture from the atmosphere. Excessive moisture absorption before injection molding can damage the appearance and mechanical performance of the molded parts. At temperatures above the melting point (around 254°C), water molecules can chemically react with nylon, resulting in hydrolysis or cracking. This chemical reaction causes nylon to oxidize, discolor, reduce resin molecular weight and toughness, and increase flowability. It not only brings processing difficulties but also damages the properties of the molded parts. During injection molding, there may be drooling from the nozzle and excessive flash. Moisture absorbed by the plastic, as well as gases released from cracking, can lead to surface defects such as roughness, silver streaks, spots, micropores, and bubbles. In severe cases, melt expansion and significant decrease in mechanical strength can occur. Nylon that has undergone such hydrolysis and cracking cannot be restored to its original properties, even if it is re-dried.

Therefore, drying nylon materials before injection molding is crucial. The degree of drying depends on the requirements of the finished product, usually below 0.25%, preferably not exceeding 0.1%. If the raw material is well dried, injection molding becomes easier and quality issues can be avoided.

Vacuum drying is preferred for nylon to minimize the risk of oxidation caused by contact with oxygen in the air at high temperatures during regular drying. Excessive oxidation can make the parts brittle.

In the absence of vacuum drying equipment, regular drying is the only option, although the effectiveness is lower. There are different suggestions for regular drying conditions. Here are a few examples: first, drying at 60°C to 70°C with a material layer thickness of 20mm for 24 to 30 hours; second, drying below 90°C for no more than 10 hours; third, drying at 93°C or below for 2 to 3 hours, and if the air temperature exceeds 93°C continuously for more than 3 hours, the temperature should be reduced to 79°C; fourth, if the nylon has been exposed to the air for too long or the drying equipment is not performing well, the temperature can be raised to above 100°C, or even 150°C; fifth, using hot air in the injection machine's hopper dryer, increasing the temperature of the hot air to no less than 100°C to evaporate the moisture inside the plastic, and then removing the moist air along the top of the hopper.

The specific drying conditions should be determined based on the drying effectiveness. Too low temperature and long drying time lead to low efficiency, while too high temperature can cause polymer aggregation or cross-linking, resulting in increased melt viscosity during processing. The vacuum desiccant dryer is a closed structure. It can heat the material internally and extract the water vapor released by the plastic, so there is no need to raise the drying temperature excessively. This can significantly reduce the drying time and achieve good desiccation without causing color change or decrease in mechanical properties.

Even properly dried plastic will quickly absorb moisture from the air if left exposed. Even in a covered machine hopper, the storage time should not be too long. Generally, it should not exceed 1 hour on rainy days and should be limited to within 3 hours on sunny days.


2. Controlling the temperature of the barrel

Although the melting temperature of nylon is high, once it reaches its melting point, its viscosity is much lower than that of general thermoplastic materials such as polystyrene, so flowability is not a problem during molding. In addition, due to the rheological characteristics of nylon, its apparent viscosity does not decrease significantly when the shear rate increases. Furthermore, the melting temperature range of nylon is relatively narrow, between 3°C and 5°C. Therefore, maintaining a high temperature of the material is crucial for successful molding.

However, nylon has poor thermal stability in its molten state, and excessively high material temperature and prolonged heating time during processing can cause degradation of the polymer, resulting in bubbles and reduced strength in the molded parts. Therefore, the temperature of each section of the barrel must be strictly controlled, ensuring that the material pellets are heated at a high melting temperature as reasonably and evenly as possible, avoiding poor melting and localized overheating. In terms of the entire molding process, the temperature of the barrel should not exceed 300°C, and the heating time of the material pellets inside the barrel should not exceed 30 minutes.

3.Improving equipment components

First, regarding the situation inside the barrel, during injection, although there is a large amount of material pushing forward, the backflow of the molten material in the screw groove and the leakage between the screw end face and the inclined barrel wall also increase due to the high flowability. As a result, not only does it reduce the effective injection pressure and feeding amount, but sometimes it also hinders smooth feeding, causing the screw to slip and cannot retract. Therefore, a check valve must be installed at the front end of the barrel to prevent reverse flow. However, after installing the check valve, the material temperature needs to be increased by 10°C to 20°C to compensate for the pressure loss.

Secondly, regarding the situation of the nozzle, after the injection action is completed and the screw retracts, the molten material in the front barrel may flow out from the nozzle under residual pressure, which is called "drooling phenomenon". If the drooling material enters the mold cavity, it will cause cold spots or difficulty in filling the parts. If it is removed before the nozzle contacts the mold, it greatly increases the operational difficulties and is not cost-effective. Installing a separately adjustable heating coil on the nozzle is an effective method for controlling the nozzle temperature, but the fundamental solution is to use a nozzle with a spring-loaded valve or similar flow resistance structure. Of course, the spring material used in such nozzles must be heat-resistant, otherwise it will lose its elasticity due to repeated compression annealing at high temperatures.



4.Ensuring Mold Venting and Controlling Mold Temperature

Due to nylon's high melting point, its solidification point is also high. The molten material entering the cold mold can solidify below its melting point at any time, hindering the completion of the filling process. Therefore, high-speed injection must be used, especially for thin-walled or long-flow parts. In addition, high-speed filling poses a venting problem for nylon molds, requiring sufficient venting measures.

Nylon requires a much higher mold temperature than general thermoplastics. In general, a higher mold temperature is beneficial for flow. This is particularly important for complex parts. The problem is that the cooling rate of the molten material after filling the mold cavity has a significant impact on the structure and performance of nylon parts. This is mainly due to its crystallinity. When it enters the mold cavity in an amorphous state at high temperature, crystallization begins. The degree of crystallization depends on the mold temperature and the rate of heat transfer. When high elongation, transparency, and toughness are required for thin parts, the mold temperature should be relatively low to reduce the degree of crystallization. When high hardness, abrasion resistance, and minimal deformation during use are required for thick-walled parts, the mold temperature should be relatively high to increase the degree of crystallization. Nylon has higher requirements for mold temperature because it has a large molding shrinkage, and when it transitions from a molten state to a solid state, its volume shrinks significantly, especially for thick-walled products. A mold temperature that is too low can cause internal voids. Only when the mold temperature is well-controlled can the dimensions of the components be relatively stable.

The temperature control range for nylon molds is 20°C to 90°C, and it is best to have both cooling (such as tap water) and heating (such as insert-type heating rods) devices.

For parts that are used at temperatures above 80°C or have strict accuracy requirements, they should be annealed in oil or paraffin after molding. The annealing temperature should be 10°C to 20°C higher than the operating temperature, and the time required varies depending on the thickness, ranging from about 10 minutes to 60 minutes. After annealing, slow cooling should be applied. Components that have been annealed can obtain larger nylon crystals and increased rigidity. Fully crystallized components have minimal density changes and are resistant to deformation and cracking. Components fixed using quenching have low crystallinity, smaller crystals, higher toughness, and transparency.

Nucleating agents can be added to nylon to generate high-crystallinity crystals during injection molding, which can shorten the molding cycle and improve the transparency and rigidity of the parts.

Changes in environmental humidity can affect the dimensions of nylon parts. Nylon itself has a high shrinkage rate. To maintain relative stability, the parts can be subjected to moisture adjustment treatment using water or water solutions. The method involves immersing the parts in boiling water or a potassium acetate solution (with a ratio of 1.25:100, boiling point 121°C). The soaking time depends on the maximum wall thickness of the parts: 2 hours for 1.5 mm thickness, 8 hours for 3 mm thickness, and 16 hours for 6 mm thickness. Moisture adjustment treatment also has benefits for improving the crystal structure inside the plastic, enhancing part toughness, and improving internal stress distribution, with better effects compared to annealing treatment.
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