Nylon products are strong and wear-resistant. They will fade if left outside for a long time. When ignited with an open flame, the flame is blue with a yellow tip. It burns slowly and drips like a candle. It emits a pungent odor when burning.
In order to meet the requirements of the product, it is usually not used directly, but “nylon” is modified to make the comprehensive mechanical and material properties of the material meet the requirements of the product. In our factory, PA6, PA66, PA12, glass fiber reinforced nylon, etc. are the main processing materials used.
“Nylon” has excellent fluidity and low viscosity in the molten state.
It has many similarities with polypropylene. When the injection melt enters the cavity, due to the action of inertia and back pressure, the melt will continue to flow forward, which will cause the problem of “drooling”. If it is not handled, it will cause “material encapsulation” when the next product is injected.
The screw of the injection molding machine should preferably be a mutant screw with a length-to-diameter ratio of 18~20 and a compression ratio of 3~3.5 to prevent the material from carbonizing and decomposing too quickly. The screw head is equipped with a check ring, and the nozzle is preferably a self-locking spring nozzle. When the check ring and self-locking nozzle are configured, the injection pressure and material temperature must be increased on the original basis to compensate for the pressure loss.
“Nylon” solidifies quickly in the mold cavity and is a crystalline material.
The condensation and setting time of the product is very short. If the temperature is set low, the cold head produced by condensation and solidification is easy to block the main channel, gate, and bone position. In order to prevent the blockage during injection molding and the inability to mold the next product, the nozzle needs to be equipped with a heating ring with a separate temperature control to solve the problem of too fast melt temperature solidification.
The mold temperature setting of “nylon” products is higher than that of most materials.
If the mold temperature is low, cold material spots are likely to be produced. The dynamic and static molds of the mold are connected to the temperature regulator, and even some of them use the temperature regulator, which cannot reach the temperature required by the product. The heating system (heating rod) is added to the mold because the “nylon” material condenses and solidifies quickly, so high pressure and high speed must be used to fill the mold quickly during molding to fill the product.
The use of high pressure and high speed for filling indirectly leads to the need for a sufficient exhaust system for the mold. The first-level exhaust is 0.03mm, and the second-level exhaust is 0.30mm. The mold core can be removed where the inserts can be removed without affecting the appearance requirements, and exhaust needles can be added to facilitate the exhaust effect when the melt is quickly filled.
The melting temperature of “nylon” is higher than that of most materials, and it is a heat-sensitive material.
The processing temperature range of melting temperature-degradation temperature is narrow, and the thermal stability is poor. If the material stays in the barrel for too long, it is easy to degrade. During a short shutdown, the material in the barrel needs to be replaced before the machine can be started. During the shutdown, the screw should be cleaned with a high-viscosity material to avoid carbonization of the material.
“Nylon” has a large dimensional deviation from the molten state to the solidified state, with a shrinkage rate of 0.3~3.0%.
Too low mold temperature is not conducive to product size adjustment, mainly because the melt is not completely squeezed into the cavity, and there are pores on the surface of the cavity. Only mold temperature adjustment (high mold temperature) is conducive to size control. Sometimes, in order to maintain dimensional stability, hot water or chemical agents are used to “humidify” the product to balance the moisture absorption of the material, improve the toughness of the material, and improve the internal stress and comprehensive mechanical properties of the product.
“Nylon” needs to be dried before molding and is a moisture-sensitive material.
If it is exposed to the air for a long time, it will absorb moisture in the air. Of course, the drying time for each brand is different, but the moisture content of the material must be around 0.1%, and no more than 0.2% at most before injection molding.
Of course, no matter what kind of material with high water absorption rate, low moisture content is conducive to injection molding. Otherwise, after “nylon” is heated in the screw, it will react chemically with water molecules, the material will degrade, and the product will emit odor, oxidize and discolor, and the surface of the product will produce water splashes, bubbles, burnt products, incomplete filling and other molding defects. In severe cases, it may also lead to the inability to punch the product, and the product cannot achieve the required comprehensive mechanical properties, destroying the flexibility of the material.
Compared with other materials, it should be noted that the drying time and drying temperature need to be set according to the material requirements or factory processing. If the drying temperature is set low, the drying cycle is long, and the production capacity is wasted; if the drying temperature is too high, the color of the material changes, the viscosity increases, and the mechanical properties decrease.
After the formed product is scrapped due to water decomposition, the comprehensive performance of the material will decrease. It will be crushed (solid and dense, low crushing efficiency) and reprocessed, and then dried to make products with low requirements.
