Nylon's special properties make it widely used in automobile, electronic and electrical equipment, mechanical structure, sports equipment, textile and so on. However, with the miniaturization of automobiles, the high performance of electronic and electrical equipment, and the process of lightweight mechanical equipment, the demand for nylon and performance requirements have gradually increased. Therefore, the modification of nylon is particularly important.

Nylon modification precautions

01 Cylinder temperature setting

(1) Because nylon is a crystalline polymer, the melting point is obvious, and the barrel temperature selected by nylon resin during injection molding is related to the performance of the resin itself, the equipment and the shape factor of the product.

(2) Too high material temperature is easy to make the rubber parts appear color change, brittle quality and silver wire, and too low material temperature makes the material very hard and may damage the mold and screw.

(3) Generally, the melt temperature of PA6 is lower than 220℃, and that of PA66 is 250℃. Due to the poor thermal stability of nylon, it is not appropriate to stay in the barrel at high temperature for a long time, so as not to cause the material to discolor and yellow, and because of the good fluidity of nylon, the temperature exceeds its melting point and flows rapidly.

02 Mold temperature setting

(1) Mold temperature has a certain effect on crystallinity and molding shrinkage, and the mold temperature range is 80 ~ 120℃.

Select high mold temperature, high crystallinity, wear resistance, hardness, elastic modulus increase, water absorption decrease, product shrinkage increase, suitable for thick products; Select low mold temperature, low crystallinity, good toughness, high elongation, reduced shrinkage, suitable for thin products with good transparency.

(2) If the wall thickness is greater than 3mm, it is recommended to use a low temperature mold of 20~40℃. For glass reinforced material mold temperature should be greater than 80℃.

(3) The product wall is thin and difficult to form or requires high crystallinity mold temperature control, requiring products to have a certain degree of flexibility generally use cold water temperature control.

03 Wall thickness of the articles

The flow length ratio of nylon is 150 ~ 200, the wall thickness of the product is not less than 0.8mm, generally in the selection of 1 ~ 3.2mm, and the contraction of the product is related to the wall thickness of the product, the thicker the wall thickness, the greater the contraction.

04 Exhaust

The overflow edge value of nylon resin is about 0.03mm, so the vent slot should be controlled below 0.025.

05 Runner and gate

The gate aperture should not be less than 0.5t (t is the thickness of the plastic part). With a submerged gate, the minimum diameter of the gate should be 0.75mm.

06 Glass fiber filling range

In the molding process of nylon, reducing the mold temperature, increasing the injection pressure, and reducing the material temperature will reduce the shrinkage rate of nylon to a certain extent, so that the internal stress of the product is easy to deform. For example, the shrinkage rate of PA66 is 1.5% to 2%, the shrinkage rate of PA6 is 1% to 1.5%, and the shrinkage rate can be reduced to about 0.3% after the addition of glass fiber additives.

Practical experience tells us that the more glass fiber is added, the smaller the shrinkage of nylon resin. However, adding more glass fiber will cause surface floating fiber, poor compatibility and other consequences, generally adding 30% effect is relatively good.

07 Use of recycled materials

It is better not to exceed three times, so as not to cause discoloration or a sharp decline in physical and mechanical properties of the product, the application should be controlled below 25%, too much will cause fluctuations in process conditions, and the mix of recycled materials and new materials must be dried.

08 Safety Notice

Nylon resin should first open the nozzle temperature, and then in the feed cylinder heating, when the nozzle blocked, do not face the spray hole, to prevent the melt in the cylinder due to pressure accumulation and sudden release, dangerous.

09 Use of release agent

The use of a small amount of release agent sometimes has the effect of improving and eliminating bubbles and other defects. The release agent of nylon products can be selected zinc stearate and white oil, etc., and can also be mixed into a paste, which must be used in a small and uniform amount to avoid surface defects of the product. In the shutdown to empty the screw, to prevent the next production, to break the screw.

10 Post-Processing

(1) The product should be heat treated after molding

In the high boiling point liquid such as mineral oil, glycerin, liquid paraffin, the heat treatment temperature should be higher than the use temperature of 10 ~ 20℃, the treatment time varies depending on the wall thickness of the product, the thickness is 10 ~ 15min below 3mm, the thickness is 3 ~ 6mm, the time is 15 ~ 30min, the heat-treated product should pay attention to slow cooling to room temperature. To prevent sudden cooling caused by the stress in the product.

(2) The product should be moistened after molding

Humidification treatment is mainly used for products with high ambient humidity, and there are two methods: one is boiling water humidification method; The second is the potassium acetate aqueous solution humidification method (the ratio of potassium acetate to water is 1.25:1, the boiling point is 121℃).

The boiling water humidification method is simple, as long as the product is placed in the humidity of 65% environment, so that it can reach the equilibrium moisture absorption, but the time is longer, and the potassium acetate water solution humidification method has a treatment temperature of 80 ~ 100℃, and the processing time mainly depends on the wall thickness of the product, when the wall thickness is 1.5mm, about 2h, about 8h, when the wall thickness is 3mm. 16 ~ 18h at 6mm.

Case analysis

01 Glass fiber reinforced nylon

When 30% glass fiber is added to nylon, the mechanical properties, dimensional stability, heat resistance and aging resistance of nylon are significantly improved, and the fatigue strength is 2.5 times that of unenhanced nylon. The molding process of glass fiber reinforced PA is roughly the same as that without reinforcement, but because the flow is worse than that before reinforcement, the injection pressure and injection speed should be appropriately increased, and the barrel temperature should be increased by 10 ~ 40℃.

Since the glass fiber will be oriented along the flow direction during the injection molding process, causing the mechanical properties and shrinkage rate to increase in the orientation direction, resulting in product deformation and warping, therefore, the mold design, the position and shape of the gate should be reasonable, the process can improve the temperature of the mold, and the product is put into hot water to cool slowly. In addition, the greater the proportion of glass fiber added, the greater the wear on the plasticizing components of the injection molding machine, and the better is to use bimetal screw and barrel.

02 Flame retardant nylon

Due to the addition of flame retardants in nylon, most of the flame retardants are easy to decompose at high temperatures, release acidic substances, and have a corrosive effect on the metal, therefore, plasticizing components (screws, rubber heads, rubber rings, rubber gaskets, flanges, etc.) need to be plated with hard chromium. In terms of process, try to control the temperature of the barrel can not be too high, the injection speed can not be too fast, in order to avoid the discoloration and mechanical properties of the product caused by the decomposition of the rubber temperature is too high.

03 Transparent nylon

It has good tensile strength, impact strength, rigidity, wear resistance, chemical resistance, surface hardness and other properties, high light transmission, similar to optical glass, processing temperature is 300 ~ 315℃, molding processing, it is necessary to strictly control the temperature of the barrel, melt temperature is too high due to degradation and product discolation, temperature is too low due to plasticization and poor affect the transparency of products. The mold temperature should be as low as possible, and the high mold temperature will reduce the transparency of the product due to crystallization.

04 Weather resistant nylon

Ultraviolet absorbing additives such as carbon black are added to nylon, which greatly enhance the self-lubricating property of nylon and the wear of metal, and affect the material and wear parts during molding processing. Therefore, it is necessary to use a combination of screw, barrel, rubber head, rubber ring and rubber washer with strong feeding capacity and high wear resistance.