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Nylon used in injection molding

Nylon is a crystalline plastic with a wide variety of varieties, reaching more than 130 varieties. Applied to injection molding are nylon 6, nylon 66, nylon 610, nylon 1010 and copolymerized nylon, super tough nylon, glass fiber reinforced nylon, mineral reinforced nylon and the like. In the world's plastics processing market, the most widely used in plastic molding is nylon 66.

The application fields of nylon 66 are generally in the fields of automobiles, electronic appliances, chemical equipment, machinery and the like. From the end use point of view, the nylon 66 consumed by the automotive industry took the first place, and the electronic appliances accounted for the second place. About 88% of nylon 66 is processed into various plastic products by injection molding, and about 12% of nylon 66 is processed into corresponding plastic products by extrusion, blow molding and other molding processes.

Due to its excellent heat resistance, chemical resistance, strength and ease of processing, nylon 66 has been widely used in the automotive industry. Almost all parts of the car, such as engine parts, electrical parts and body parts, are now available. The engine parts include the intake system and the fuel system, such as the engine cylinder head cover, throttle, air filter machine casing, vehicle air horn, vehicle air conditioning hose, cooling fan and its casing, inlet pipe, brake oil tank And filling, and so on. The parts of the car body include automobile fenders, rear view frames, bumpers, instrument panels, luggage racks, door handles, wiper brackets, seat belt buckles, and various decorative parts in the car. In-car electrical appliances, such as electronically controlled doors and windows, connectors, crisper boxes, cable ties, etc.
plastic parts
Nylon injection molding process characteristics
1. Water absorption. Nylon 66 is more hygroscopic and will absorb moisture from the atmosphere if exposed to air for a long time. Volume expansion occurs after water absorption, which affects the dimensional accuracy of plastic products. If the excess moisture is absorbed before injection molding, the appearance and mechanical properties of the plastic part will be impaired.

2. Crystallinity. Nylon 66 is a crystalline high polymer, generally between 20% and 30%. The degree of crystallinity is related to performance. The crystallinity is high, the tensile strength, the abrasion resistance, the hardness, the lubricity and the like are improved, and the thermal expansion coefficient and the water absorption tend to decrease.

3. Thermal stability. At a temperature above the melting point, about 254 ° C, water molecules will chemically react with nylon 66 to hydrolyze or cleave the polymer, discoloring the nylon 66, the molecular weight and toughness of the resin are relatively weakened, and the fluidity is increased, which not only brings difficulty in processing, but also damage to product performance. During injection molding, injection molded part occurs flash. The gas generated by the cracking of the polymer and the moisture absorbed from the air together pinch the injection molded part, the surface might be not smooth, and generate silver, speckle, microspores, bubbles, even the melt expands which lead to the unsuccessful injection molding or the strength is reduced. Finally, the properties of this lysed nylon are completely irreversible and cannot be reused even after re-drying.

If the dried raw materials are exposed in the air, they will quickly absorb moisture in the air and the drying effect will be lost. Even in the covered hopper of the machine, the storage time should not be too long. Generally, the rainy day does not exceed 1h, and the sunny day is limited to 3h.

Although the melting temperature of nylon 66 is high, when it reaches the melting point, its viscosity is much lower than that of a general thermoplastic such as polystyrene, so fluidity during injection molding is not a problem. The rheological property of nylon 66 is that the surface viscosity decreases when the shear rate increases, and the melting temperature range is narrow, between 3-5 degrees Celsius.

4. Liquidity. Nylon 66 melt has low viscosity and high fluidity, and it is easy to molding thin-walled injection molding plastic products. The melt can be quickly solidified, so the injection molding cycle is short.

Injection molding machine used for Nylon injection molding
Although both the plunger type injection molding machine and the screw type injection molding machine can process nylon plastic, it is preferable to use a screw type injection molding machine in view of the plasticizing effect and the residence time of the materials in the barrel. In particular, nylon 66 has a slightly poorer thermal stability.

Nylon drying
In order to successfully molding nylon 66 and ensure nylon plastic parts quality, nylon pellets must be dried before injection molding. The moisture content of the nylon after treatment should be less than 0.1%. The nylon pellets should be prevented from oxidative discoloration when dry, because the amide group is sensitive to oxygen and is prone to oxidative degradation. If the material is sealed before processing, then drying is not necessary. However, if the storage container is turned on, it is recommended to dry it in hot air at 85 degrees Celsius. If the humidity is greater than 0.2%, it is also necessary to carry out vacuum drying at 105 degrees Celsius for 12 hours. The following are drying methods and conditions.

Normal pressure hot air drying method: Drying time 20-24h, drying temperature 80-90 degrees Celsius, material layer thickness 25mm.

Vacuum drying method: Drying time 12-16h, drying temperature 100-110 degrees Celsius, vacuum degree 93Kpa, material layer thickness less than or equal to 50mm. Boiling drying method: Drying time 15-30min, drying temperature 110 degrees Celsius, one feeding 40-80KG.

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