nylon injection molding

PA injection molding

Pros:

  • High mechanical strength

The tensile strength is higher than that of metal, and the compression strength is equal to that of metal, but its rigidity is lower than that of metal.

Tensile strength is close to yield strength, which is more than twice as high as ABS.

It has strong absorption ability to shock and stress vibration, and its impact strength is much higher than that of the other plastics, and superior to acetyl.

  • The fatigue resistance

and the parts can still maintain the original mechanical strength after repeated bending.

PA is often used in escalator handrails, bicycle plastic cycles and other circumstance where periodic fatigue effect is very obvious.

  • High softening point and heat resistance

nylon 46  injection molding has a high thermal deformation temperature and can be used for a long time at 150 degrees.

After being reinforced with glass fiber, PA 66 injection molded part has a thermal deformation temperature above 250 degrees.

  • Smooth surface, small friction coefficient and wear resistance

Nylon injection molding has self-lubricating property and low noise when used as a movable mechanical component, and it can be used without lubricant when the friction effect is not too high.

If lubricant is needed to reduce friction or help heat dissipation, water, oil, grease, etc. can be selected. Therefore, as a transmission component, its service life is long.

  • Excellent electrical performance

Nylon has good electrical insulation, high volume resistance and high breakdown voltage resistance.

It can be used as power frequency insulation material in dry environment and it has good electrical insulation even in high humidity environment.

The product is light in weight and easy to dye. Due to its low melt viscosity, it can flow rapidly.

It can be quickly injection molded, so the molding cycle is short and the production efficiency is high.

How to process Nylon injection molding:

  • Drying 

Vacuum drying: 95-105℃ for 6-8 hours

Hot air drying: temperature 90-100℃ for 4 hours.

Crystallization: Except transparent nylon, nylon is mostly crystalline polymer, with high crystallization.

The tensile strength, wear resistance, hardness, lubricity and other properties of the product are improved, while the coefficient of thermal expansion and water absorption tend to decrease, but it is unfavorable to its transparency and impact resistance.

Mold temperature has a great influence on crystallization.

High mold temperature results in high crystallization, while low mold temperature results in low crystallization.

Shrinkage:Similar to other crystalline plastics, nylon resin has a larger shrinkage rate.

Generally, the shrinkage of nylon will greatly affect crystallization, and the shrinkage of products with large crystallization will also increase.

In the molding process, the shrinkage will decrease if the mold temperature is lowered, the injection pressure is increased, and the material temperature is lowered.

However, the internal stress of the products is increased and easy to deform.

The shrinkage rate of PA 66 is 1.5-2%。

Nozzle: When nylon is injected, the “drooling of nozzles” shall be prevented. Therefore, self-locking nozzles are generally selected for processing nylon injection molding.

  • Products and moulds

The flow length ratio  is between 150 and 200. The wall thickness of nylon injection molded part is not less than 0.8 mm and generally between 1 and 3.2 mm.

The shrinkage of nylon injection is related to the wall thickness of nylon part.

The greater the shrinkage will be,the thicker the wall thickness is .

venting: nylon material venting slot height is about 0.03 mm, so the slot height should be controlled below 0.025.

Mold temperature: For the thin wall product is hard to fill , the mold is required to be heated and controlled.

Cold water is generally used to control the temperature if the product need certain flexibility.

  • Injection:

Barrel temperature

Nylon is a crystalline polymer, so its melting point is fixed.

The barrel temperature  is related to the performance of the resin itself, equipment and the shape of the product. Barrel for nylon injection molding  is 260℃.

Due to the poor thermal stability , it is not suitable to stay in the  barrel at high temperature for a long time, so as not to cause discoloration and yellowing of materials.

At the same time, due to the good fluidity , it will flow rapidly after the temperature exceeds its melting point.

  • Injection pressure

Nylon melt has low viscosity and good fluidity, but its condensation speed is fast.

It is easy to be short shot in products with complex shape and thin wall thickness.

So nylon injection molding needs higher injection pressure.

Usually,if the PA injection pressure is too high, the product will get flash.

If the pressure is too low, the products will have defects such as prints, bubbles, weld marks or short-shot .

The injection pressure of most nylon varieties is not more than 120MPA, and is generally selected within the range of 60-100 MPA.

As long as the products do not have defects such as bubbles and dents, it is generally no need to adopt higher maintaining pressure to avoid increasing internal stress of the products.

  • Injection speed

For nylon injection molding, a faster injection speed can prevent ripples and insufficient mold filling caused by too fast cooling speed.

The effect of fast injection speed on product performance is not obvious.

  • Mold temperature

Mold temperature has certain influence on crystallization and shrinkage.

 When the mold temperature is high, the crystallization, abrasion resistance and hardness of nylon products will increase, and molding shrinkage of products will increase.

Low mold temperature injection molding will lead to low crystallization, good toughness and high elongation.

The injection temperature and drying temperature must be high; The injection pressure and holding pressure should not be too high or too long, and it should be injected at high speed.Venting channels should be cut to avoid burning ; Back pressure 50 ~ 150 kg/cm2

Nylon should never be injected until they are dry completely. Because of the high moisture content, the material is easy to get stuck in the screw groove of the feeding section, which makes it difficult to feed

During injection, the nozzle is the easiest section to get cool. If the injection pressure is increased during cooling, the non-return valve will break easily, so the temperature control at the nozzle must be appropriate. In order to prevent the nylon from flowing into the mold , a controlled nozzle should be used.

When using other plastics material instead of nylon, it should be noted that the heating temperature of the nylon is above 270℃, while the melt temperature of the common plastic material is only about 200℃, so it is necessary to heat the cylinder to the melt temperature of nylon before operating, otherwise the non-return valve of the screw is easily broken.

To prevent the NG products, precision plastic mold must be adopted.

The design of products should avoid surface sink marks and consider dimensional stability

Nylon injection molding shrinkage is about 1.5~2.5%

The heating temperature of PA6 is 220~300℃, mold temperature is 100~160℃, barrel temperature is 200~260℃, ejection temperature is 220~300℃, ejection pressure is 700 ~ 2000 kg/cm, lowest nozzle injection temperature is 225℃, melting point temperature is 215℃.

Barrel temperature:

  • section 1 :230℃
  • section 2 :230℃
  • section 3 :220℃
  • section 4 :210℃

The second stage is 230℃; The third stage is 220℃; The fourth stage is 210℃; The plastic temperature is 25~70℃. Linear expansion coefficient 0.000083/℃, molding shrinkage 0.6~2.1%, specific gravity 1.1~1.4, thermal deformation temperature 149~185℃.

The temperature of PA66 injection molding: 250~380℃, mold temperature : 30~100℃, barrel temperature : 240~300℃, injection temperature : 250~310℃, ejection pressure : 600 ~ 2100 kg/cm2, lowest injection temperature : 260℃, melting  temperature : 215℃. The injection molding shrinkage is 1.3~2.4%, the specific gravity is 1.1~1.4, and the thermal deformation temperature is 66~86℃.

Barrel temperature

  • nozzle 235~265℃,
  • front section 235~245℃,
  • middle section 210~240℃,
  • rear section 195 ~ 230℃;

The screw speed is 100-105 rpm, and the back pressure is 5-10 kg/cm2