PA material has excellent mechanical properties, corrosion resistance, wear resistance, and high-temperature resistance, so it is one of the most widely used engineering plastics, used in electrical and electronic, automotive, construction, office equipment, machinery, aerospace, and other industries.

PA material tensile strength, bending strength, impact strength, hardness, rigidity, modulus, and toughness is better, especially after modified and enhanced PA material or PA alloy material, its strength, modulus, hardness, rigidity, wear resistance are further improved.

Types of PA materials

PA- Polyamide, also known as nylon, is a generic term for a large group of amide-type polymers. The most common ones are PA6, PA66 and PA1010.

Recently, with the development of IT industry, the use of a new type of polyamide – PA46, which is used to replace LCP (liquid crystal polymer) in the production of computer plug-ins, has increased dramatically.

Because PA has good mechanical properties, good toughness, impact resistance, wear resistance, self-lubricating, flame retardant, insulation, etc., so it is widely used in many fields such as automobile, machinery, electronics, instrumentation, chemical industry, such as gears, pulleys, bearings, impellers, bushings, containers, brushes, zippers, etc.

PA6, PA66, PA46 are aliphatic polyamides, which are linear polymers with highly polar amide groups in their molecular structure, so they have a high degree of crystallinity.

The performance of PA injection molded products depends on their crystalline form and crystallinity.

The processing conditions have an influence on the crystalline form and crystallinity, the crystallization of PA products can vary up to 40% depending on the processing conditions, the products cool slowly, have high crystallinity and form larger size crystalline forms. Water absorption also has an effect on its crystallinity.

In addition, PA in the processing process due to flow, shear will produce a certain degree of orientation, resulting in product performance anisotropy, the strength along the orientation direction is better than the direction of non-orientation, orientation is also conducive to the crystallization process, in the mold design to consider this factor.

PA properties and modified PA

PA properties: PA is a kind of polymer containing amide group inside, which can be made by the polymerization of internal acid and ammonia dicyclic, or formed by the condensation of binary ammonia and binary acid. Earlier it was often used as a fiber material, and then gradually expanded to industrial applications.

PA plastic has high mechanical strength, high rigidity, high toughness, excellent wear resistance, good electrical properties, the main varieties include PA6, PA66, widely used in machinery, automotive, home appliances, aviation, metallurgy and other fields.

PA structure contains amide groups, moisture absorption, size instability, so the modification of PA materials is inevitable, the current commonly used PA modification methods are to add glass fiber and add additives two methods.

Adding 30% glass fiber to PA plastic can effectively improve the mechanical properties, heat resistance and aging resistance of PA injection molding products, especially the fatigue resistance can be increased to 2.5 times of that without adding glass fiber.

With the addition of glass fiber in PA, the conditions of the production process also need to be changed, such as to increase the injection pressure and injection speed, to increase the temperature of the barrel and mold, and the barrel temperature should be increased by 10-40 degrees Celsius as much as possible.

Nylon 6 Tm is 220-230℃, Nylon 66 is 260-270℃, Nylon itself has water absorbing base, so it has water absorption, it must be dried before forming, the temperature is too high to dry, then the nylon grain is discolored.

Processing performance of PA injection molding

There are many varieties of PA, such as PA6, PA66, PA610, PA1010, etc. They have some differences in performance, but the basic performance is the same, its main molding processing properties are as follows.

1. PA has strong hygroscopicity, and must be preheated and dried before injection molding, and should prevent rehygroscopicity after preheating and drying, and the moisture content after drying should not exceed 0.3%.

2. PA is a crystalline plastic, high melting point, poor thermal stability when melting, if the material temperature exceeds 300 ℃, the retention time at this temperature exceeds 90min, it is easy to produce decomposition. It is easy to absorb moisture, need to be dried, the water content should not exceed 0.3%. 3.

3. PA melt viscosity is low, excellent fluidity, easy to fill the mold, but also easy to overflow material and “salivation” phenomenon, with the screw injection machine injection should be self-locking nozzle, and should be heated, screw head should be with a non-reverse ring.

4. PA injection mold pouring system form and size is similar to the processing of PS, but increase the size of the runner and inlet cross-section can improve the shrinkage and dent phenomenon.

5. The molding shrinkage range of PA is large and the shrinkage rate is also large and the directionality is obvious, so it is easy to occur shrinkage, dent and deformation, therefore, stable molding conditions are required.

6. PA plastic wall thickness should be as uniform as possible, the release material degree should be slightly larger, especially for thick-walled and deep high type plastic parts should take a large value.

7. The molecular bond orientation of PA is less sensitive to the shear rate during molding, so the molding pressure has less influence on the performance of plastic parts.

8. Because of the large shrinkage of PA, in order to get high precision plastic parts, in addition to strict control of the injection molding process conditions, the best in the mold design on the basis of testing.

9. PA has obvious high and narrow melting point, so PA needs to be molded at a higher temperature.

10. The heat diffusion coefficient of PA is larger than PS, easy to dissipate heat, the wall of the sprue is easy to generate a thicker shell layer, for this reason, the main channel cross-section should be slightly larger than the PS.

11. When injecting PA melt, in order to prevent PA melt from solidifying quickly in the cavity, a higher injection speed should be used.

12. PA melting temperature range is narrow, so the main channel used for molding PA should be thick and short, otherwise it is easy to cause solidification.

13. PA has good flowability and easy to overflow. It is suitable to use self-locking nozzle, and should be heated.

14. The mold temperature is selected according to the wall thickness of the plastic part in the range of 20-90 degrees, the injection pressure is selected according to the type of injection molding machine, the material temperature, the shape and size of the plastic part, the mold pouring system, and the injection molding cycle is selected according to the wall thickness of the plastic part.

When the resin viscosity is small, the injection and cooling time should be long, and white oil should be used as a mold release agent.

PA injection molding conditions

Barrel temperature

The barrel temperature has a great influence on the shrinkage rate of the plastic parts, high temperature will lead to a large shrinkage rate; on the contrary, the shrinkage rate will be small.

The barrel temperature should be determined according to the melting point of the plastic. The barrel temperature of the screw injection molding machine is 10~30℃ higher than the melting point of plastic, while the barrel temperature of the plunger injection molding machine is 30~50℃ higher than the melting point. The nozzle temperature is generally 10~20℃ lower than the front of the barrel.

Mold temperature

80℃ is recommended. Mold temperature will affect the crystallinity, and the crystallinity will affect the physical properties of the product. For thin-walled plastic parts, if a mold temperature lower than 40C is used, the crystallinity of the plastic part will change over time, and annealing is required to maintain the geometric stability of the plastic part.

The mold temperature should be determined according to the thickness of the plastic part and the requirements for performance, generally controlled at 40~100C.

When the mold temperature is high, the hardness of the plastic part is large and the wear resistance is good; when the mold temperature is low, the elongation is large and the transparency and toughness are good, but when the mold temperature is too low, the cooling rate of the plastic part will be uneven to the defects, in general, the most important factor to decide the mold temperature is the wall thickness of the product.

Drying treatment

If the material is sealed before processing, then there is no need to dry. However, if the storage container is opened, then drying in hot air at 85C is recommended. If the humidity is greater than 0.2%, vacuum drying at 105C for 12 hours is also required.

Melting temperature

260~290C. 275~280C for glass additive products. melting temperature should be avoided above 300C.

Injection pressure

Usually at 750~1250bar, depending on the material and product design.

Injection speed

High speed (should be a little lower for reinforced material).

Flow channel and gate

Since the solidification time of PA66 is very short, the location of the gate is very important. The gate aperture should not be less than 0.5*t (where t is the thickness of the plastic part).

If using a hot runner, the gate size should be smaller than with a conventional runner, because the hot runner can help stop the material from solidifying prematurely.

If submerged gates are used, the minimum diameter of the gate should be 0.75 mm. to avoid defects such as air bubbles and scorching.

Screw speed

Medium speed is suitable. Too fast speed will degrade the plastic due to excessive shearing, resulting in discoloration and performance degradation of the product, too slow speed will affect the quality of the melt and also affect the production efficiency due to long melt time.

Back pressure

Under the premise of ensuring the quality of the products, the lower the back pressure the better, high back pressure will make the melt shear too much and overheat degradation.

PA materials advantages and disadvantages

Advantages

1. High mechanical strength, good toughness, with high tensile and compressive strength. Tensile strength is close to yield strength, more than twice as high as ABS.

2. Outstanding fatigue resistance, the parts can still maintain the original mechanical strength after repeated bending. Common escalator handrails, new bicycle plastic wheel rims and other occasions where the cyclic fatigue effect is very obvious are often used PA.

3. Smooth surface, small coefficient of friction, wear-resistant. It is self-lubricating when used as movable mechanical components, low noise, and can be used without lubricant when the friction is not too high.

4. Corrosion resistance, very resistant to alkali and most salt liquids, but also resistant to weak acids, oils, gasoline and other solvents, aromatic compounds are inert, can be used as packaging materials for lubricants, fuels, etc.

Disadvantages

1. Easy to absorb water.Water absorption will affect the size and accuracy of the parts to a certain extent, especially the thickening of thin-walled parts have a greater impact; water absorption will also greatly reduce the mechanical strength of plastic. In the selection of materials, should take into account the use of the environment and other components with the impact of precision.

2. Poor light resistance. In the long-term high temperature environment will be oxidation with the oxygen in the air, the color will turn brown at the beginning, followed by the surface broken and cracked.

3. Injection molding technology requirements are more stringent: the presence of trace moisture will cause great damage to the molding quality; because of the thermal expansion of the product dimensional stability is difficult to control;

The existence of sharp corners in the product will lead to stress concentration and reduce the mechanical strength; wall thickness if uneven will lead to distortion, and deformation of the parts; parts after the processing of equipment require high precision.

4. Will absorb water, alcohol, and swell, not resistant to strong acids and oxidizers, can not be used as acid-resistant materials.

Application fields of PA material

Application: widely used in machinery, instrumentation, automobile parts, electrical and electronics, railroad, home appliances, communication, spinning machines, sports and leisure goods, oil pipes, oil tanks and some precision engineering products.

Electronic appliances: connectors, spools, timers, cover circuit breakers, switch shell holders

Automobile: cooling fan, door handle, fuel tank cover, air intake grille, water tank cover, lamp holder

Industrial parts: chair seat, bicycle frame, skate base, textile shuttle, pedal, slide

Summary

This blog introduces the types, properties and injection molding process of engineering plastics PA, the selection of equipment, product modeling and mold design considerations in actual production, as well as the solution of common defects.

In the actual PA material selection and injection molding product manufacturing, it is recommended to choose an experienced injection mold factory and injection molding product supplier to recommend a suitable PA material in terms of product use and functional requirements, appearance and other aspects to ensure the smooth implementation of the project.