Processing Techniques for Nylon Material
1. PA6 Injection Molding Process
1.1 Chemical and Physical Characteristics
The chemical and physical characteristics of PA6 are very similar to PA66.
However, its melting point is lower, and it has a wide processing temperature range.
It exhibits better impact resistance and solvent resistance than PA66, but it has a higher moisture absorption.
Because many quality characteristics of plastic parts are affected by moisture absorption, it is essential to consider this aspect when designing products with PA6.
To enhance the mechanical properties of PA6, various modifiers are often added. Glass is the most common additive, and sometimes synthetic rubbers such as EPDM and SBR are added to improve impact resistance.
For products without additives, the shrinkage rate of PA6 is between 1% and 1.5%.
Adding glass fiber modifiers can reduce the shrinkage rate to 0.3% (slightly higher in the direction perpendicular to the flow). The shrinkage rate of molded assemblies is mainly influenced by material crystallinity and moisture absorption.
The actual shrinkage rate is also a function of part design, wall thickness, and other process parameters.
1.2 Injection Molding Process Conditions
1.2.1 Drying Treatment
Due to the high moisture absorption of PA6, special attention should be given to drying before processing.
If the material is supplied in waterproof packaging, the container should remain airtight. If the humidity exceeds 0.2%, it is recommended to dry in hot air at temperatures above 80°C for 16 hours. If the material has been exposed to the air for more than 8 hours, vacuum drying at 105°C for at least 8 hours is advised.
1.2.2 Melting Temperature: 230-280°C. For reinforced varieties, the melting temperature is 250-280°C.
1.2.3 Mold Temperature: 80~90°C.
Mold temperature significantly affects crystallinity, which, in turn, impacts the mechanical properties of the plastic parts.
- For structural components where crystallinity is crucial, it is recommended to set the mold temperature at 80~90°C.
- For thin-walled, longer-flow plastic parts, such as the nylon cable tie production, a higher mold temperature is also recommended. Increasing the mold temperature can enhance the strength and rigidity of the plastic parts but reduces toughness.
- If the wall thickness is greater than 3mm, it is advisable to use a low-temperature mold ranging from 20~40°C.
- For glass fiber-reinforced materials, the mold temperature should be above 80°C.
1.2.4 Injection Pressure: Generally between 750-1250 bar (depending on the material and product design).
1.2.5 Injection Speed: High-speed (slightly reduced for reinforced materials).
1.2.6 Runners and Gates: Due to the short solidification time of PA6, the gate’s position is crucial. The gate aperture should not be smaller than 0.5*t (where t is the thickness of the plastic part).
If using a hot runner, the gate size should be slightly smaller than with a conventional runner, as the hot runner helps prevent premature solidification of the material. If using a submerged gate, the minimum diameter of the gate should be 0.75mm.
2. PA66 Injection Molding Process
2.1 Drying of Nylon 66
2.1.1 Vacuum Drying: Temperature at 95-105°C, Time in 6-8 hours
2.1.2 Hot Air Drying: Temperature at 90-100°C, Time around 4 hours
2.1.3 Crystallinity
Except for transparent nylon, most nylon is crystalline thermoplastics. High crystallinity improves tensile strength, wear resistance, hardness, lubricity, etc., while reducing the coefficient of thermal expansion and water absorption. However, this is disadvantageous for transparency and impact resistance.
Mold temperature has a significant impact on crystallinity. Higher mold temperature results in higher crystallinity, while lower mold temperature results in lower crystallinity.
2.1.4 Shrinkage
Similar to other crystalline plastics, nylon resin exhibits significant shrinkage. Generally, the shrinkage of nylon is most related to crystallinity. When the product has high crystallinity, the shrinkage increases.
Reducing mold temperature, increasing injection pressure, and lowering material temperature during the molding process can reduce shrinkage, but it may lead to increased internal stress and deformation.
The shrinkage rate of PA66 is 1.5-2%.
2.1.5 Molding Equipment
During nylon molding, special attention is given to preventing the “stringing phenomenon” of the nozzle(https://tekwellmachinery.com/injection-molding-machine-nozzle/). Therefore, for the processing of nylon materials, a self-locking nozzle is generally selected.
2.2 Products and Molds
2.2.1 Product Wall Thickness
The flow length ratio of nylon is between 150-200mm, and the wall thickness of nylon products should not be less than 0.8mm, typically chosen between 1-3.2mm.
Additionally, the shrinkage of the product is related to the wall thickness: the thicker the wall, the greater the shrinkage.
2.2.2 Venting
The flash value of nylon resin is around 0.03mm, so the vent hole groove should be controlled below 0.025.
2.2.3 Mold Temperature
For products with thin walls that are difficult to form or molds requiring high crystallinity, heating control is applied. Products requiring a certain degree of flexibility generally use cold water temperature control.
2.3 PA66 Molding Process
2.3.1 Barrel Temperature
Nylon is a crystalline polymer, so its melting point is distinct. The barrel temperature chosen for molding nylon resin depends on the resin’s properties, equipment, and the shape of the product. For Nylon 66, the temperature is set at 260℃.
Due to the poor heat stability of nylon, it is not advisable for it to stay at high temperatures in the barrel for an extended period to prevent discoloration and yellowing of the material. Additionally, as nylon has good flowability, it flows rapidly once the temperature exceeds its melting point.
2.3.2 Injection Pressure
Nylon melt has low viscosity and good flowability. However, due to its fast solidification speed, there may be insufficiencies, especially in products with complex shapes and thin walls, necessitating higher injection pressure.
Excessive pressure can lead to overflow issues in the product.
Insufficient pressure may result in defects such as ripples, bubbles, obvious weld lines, or inadequate product formation.
The injection pressure for most nylon varieties does not exceed 120 MPa, typically selected within the range of 60-100 MPa to meet the requirements of most products.
Unless defects like bubbles or dents appear, it is generally not advisable to use higher holding pressure to avoid increasing internal stress in the product.
2.3.3 Injection Speed
For nylon, a faster injection speed is beneficial to prevent issues such as ripples and insufficient filling caused by too rapid cooling.
The impact of a fast injection speed on the product’s performance is not particularly pronounced.
2.3.4 Mold Temperature
Mold temperature has a certain influence on crystallinity and molding shrinkage.
High mold temperature results in increased crystallinity, wear resistance, hardness, elastic modulus, decreased water absorption, and increased molding shrinkage of the product.
Low mold temperature leads to lower crystallinity, good toughness, and higher elongation.
2.4 PA66 Molding Process Parameters
2.4.1 Barrel Temperature
Rear: 240-285°C
Middle: 260-300°C
Front: 260-300°C
2.4.2 Nozzle Temperature: 260-280°C
2.4.3 Mold Temperature: 20-90°C
2.4.4 Injection Pressure: 60-200MPA
2.4.5 Use of Mold Release Agent
The use of a small amount of mold release agent sometimes helps improve and eliminate defects such as bubbles. For nylon products, mold release agents such as zinc stearate and white oil can be used, and they can also be mixed into a paste for application. When used, it must be applied sparingly and evenly to avoid surface defects on the product.
2.4.6 Cleaning
During shutdown, the screw should be emptied to prevent screw twisting during the next production.
3. PA12 Injection Molding Process
3.1 Drying Treatment
Ensure humidity is below 0.1% before processing.
If the material is stored exposed to the air, it is recommended to dry it in 85°C 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 balance.
3.2 Melting Temperature
240-300°C
For standard materials, do not exceed 310°C, and for flame-retardant materials, do not exceed 270°C.
3.3 Mold Temperature
For unreinforced materials, it is 30-40°C. For thin-walled or large-area components, it is 80-90°C. For reinforced materials, it is 90-100°C.
Increasing the temperature will increase the crystallinity of the material. Precise control of the mold temperature is crucial for PA12.
3.4 Injection Pressure
Can go up to a maximum of 1000 bar (it is recommended to use low holding pressure and high melting temperature).
3.5 Injection Speed
High speed (preferable for materials with glass additives).
3.6 Runner and Gate
For materials without additives, due to the low viscosity of the material, the runner diameter should be around 30mm.
For reinforced materials, a large runner diameter of 5-8mm is required.
The runner shape should be entirely circular. The gate should be as short as possible. Various forms of gates can be used. Large plastic parts should not use small gates to avoid excessive pressure or excessive shrinkage. The gate thickness is preferably equal to the thickness of the plastic part.
If using a submerged gate, it is recommended to have a minimum diameter of 0.8mm. Hot runner molds are very effective but require precise temperature control to prevent material leakage or solidification at the nozzle.
If using a hot runner, the gate size should be smaller than that of a cold runner.
4. Nylon 1010 Injection Molding Process
Due to the presence of hydrophilic amide groups in the molecular structure of Nylon 1010, it is highly hygroscopic, with a balanced water absorption rate of 0.8% to 1.0%.
Moisture significantly affects the physical and mechanical properties of Nylon 1010. Therefore, the raw material must be dried before use to reduce its moisture content to below 0.1%.
During the drying of Nylon 1010, precautions should be taken to prevent oxidation discoloration, as amide groups are sensitive to oxygen and prone to oxidation degradation. Vacuum drying is preferred as it provides a high dehydration rate, short drying time, and good pellet quality.
Typical drying conditions include a vacuum degree of 94.6 kPa or above, a temperature of 90-100 ℃, and a drying time of 8-12 hours, reducing the moisture content to 0.1%-0.3%.
If using a conventional oven, the drying temperature should be controlled between 95-105℃, and the drying time extended to approximately 20-24 hours.
Dried materials should be stored with care to prevent reabsorption of moisture.
4.1 Plasticization Process
Before Nylon 1010 enters the mold cavity, it should reach the specified molding temperature, providing a sufficient amount of molten material uniformly at all points.
- To meet these requirements, a screw-type injection molding machine is used based on the characteristics of Nylon 1010, with a screw that is either of the toggle type or combination type.
- The barrel temperature increases sequentially from the material hopper towards the feed inlet. Controlling the barrel temperature near the melting point is advantageous for improving the impact strength of the product and preventing material backflow and decomposition. The barrel temperature is generally set at 210-230 ℃.
- To reduce friction between the screw and PA1010 during pre-plasticization, liquid paraffin can be used as a lubricant, with a typical dosage of 0.5-2 mL/kg.
- The mold temperature is generally set between 40-80℃.
- Increasing back pressure helps compact the material in the screw groove, expelling low-molecular-weight gases from the material, and improving plasticization quality.
However, raising the back pressure increases leakage and reverse flow between the screw and barrel, reducing the plasticizing capacity of the injection molding machine.
Excessive back pressure is not advisable as it significantly decreases plasticization efficiency and may lead to excessive shear force and shear heat, causing material decomposition.
Therefore, it is preferable to keep the plasticization back pressure as low as possible, usually within the range of 0.5-1.0 MPa.
4.2 Mold Filling Process
During this process, attention should be paid to the injection pressure and injection speed of Nylon 1010 injection molding.
- The recommended injection pressure is generally 2-5 MPa, and a slow injection speed is preferred.
Excessive injection pressure and rapid injection speed can lead to turbulent mold filling, which is not conducive to eliminating bubbles in the product.
- According to the characteristics of mold cavity pressure changes, the injection molding filling process can be divided into the feeding into the mold, flowing filling, and cooling and setting stages.
The cooling and setting process can be further divided into the pressure-holding replenishment, backflow, and cooling after gate freezing stages.
- Achieving pressure-holding replenishment requires certain conditions. On the one hand, there should be enough molten material available for replenishment. At the same time, the casting system should not solidify too early, allowing a path for molten material. This is a necessary condition for replenishment.
On the other hand, the injection pressure should be sufficiently high, and the pressure-holding time should be long enough, which are sufficient conditions for replenishment to occur.
- The pressure-holding time is usually determined through experiments and should not be too long or too short.
An excessively long pressure-holding time not only extends the molding cycle but also leads to excessive residual pressure in the mold cavity, making demolding difficult and, in some cases, preventing the mold from opening.
Additionally, it increases energy consumption. The optimal pressure-holding time should be such that the residual pressure in the mold cavity is zero at the time of mold opening.
Generally, the molding pressure-holding time for Nylon 1010 injection-molded parts is 4-50 seconds.
4.3 Mold Release
Nylon 1010 parts can be demolded when they have cooled inside the mold to a sufficient stiffness. The demolding temperature should not be too high, generally controlled between the heat distortion temperature of PA1010 and the mold temperature.
During demolding, the residual pressure in the mold cavity should be close to zero, which is determined by the pressure-holding time.
