Paper on Matching of Molding Materials

Injection molding technology of polyamide (nylon)

I. Classification and characteristics of nylon

Classification:

1 According to the number of carbon atoms of diamines and diacids, the nylons synthesized from two monomers are:

46、66、6 10、6 12、6 13、 10 10、 13 13

2. According to the number of carbon atoms contained in the monomer, they are named as follows:

Nylon 4,5,6,7,8,9, 1 1, 12, 13.

characteristic

1, nylon has excellent toughness, self-lubrication, wear resistance, chemical resistance, air permeability, oil resistance, non-toxicity and easy coloring, so nylon is widely used in industry.

Second, the technological characteristics of nylon

Rheological characteristics of nylon

Nylon is mainly crystalline resin. When the temperature exceeds its melting point, its melt viscosity is small and its melt fluidity is excellent. Overflow should be prevented. At the same time, due to the fast coagulation speed of the solution, it is necessary to prevent the phenomenon of insufficient products caused by the blockage of nozzles, runners and gates by materials. The overflow value of die is 0.03, and the melt viscosity is more sensitive to temperature and shear force, but more sensitive to temperature.

First, the melt viscosity is reduced from the barrel temperature.

Nylon has higher water absorption than dry nylon. During the molding process of wet nylon, the viscosity drops sharply, silver wires appear on the surface of products mixed with bubbles, and the mechanical strength of products decreases. The materials must be dried before processing.

Allowable water content for partial nylon injection:

Resin name nylon 6,66 nylon 1 1 nylon 6 10

Allowable water content% 0.1.150.1-0.15.

Drying of nylon PA66

Vacuum drying and hot air drying

Temperature 95- 105

The time is about h 6-8 4

Crystallinity:

Except transparent nylon, nylon is mostly a crystalline polymer with high crystallinity. The tensile strength, wear resistance, hardness and lubricity of the products are improved, and the coefficient of thermal expansion and water absorption tend to decrease, but it is not good for transparency and impact resistance. Mold temperature has a great influence on crystallization, with high mold temperature and low mold temperature.

Shrinkage rate:

Similar to other crystalline plastics, nylon resin also has the problem of large shrinkage. Generally speaking, the shrinkage of nylon has the greatest relationship with crystallization. When the crystallinity of the product is high, the shrinkage rate of the product will also increase. In the molding process, lowering the mold temperature \ increasing the injection pressure \ lowering the material temperature will reduce the shrinkage rate, but the internal stress of the product will increase and the product will be easily deformed. PA66 molding equipment with shrinkage of 1.5-2%.

When nylon is molded, attention should be paid to prevent "casting phenomenon of nozzle", so self-locking nozzle is generally used for processing nylon materials.

Products and molds

1, the flow length ratio of nylon is between 150-200, and the wall thickness of nylon products is generally between1-3.2 mm. The shrinkage of products is related to the wall thickness of products. The thicker the wall thickness, the greater the shrinkage.

2. The overflow value of exhaust nylon resin is about 0.03mm, and the exhaust slot should be controlled below 0.025.

3. It is difficult to mold the products with temperature, or the products with high crystallinity need mold heating control, and the products with certain flexibility generally use cold water control. Third, the molding process of nylon

Barrel temperature Because nylon is a crystalline polymer, its melting point is obvious. The barrel temperature of nylon resin in injection molding is related to the performance of the resin itself, the equipment and the shape of the product. Generally, the lowest dissolution temperature of nylon 6 is 225℃, and that of nylon 66 is 260℃. * Due to the poor thermal stability of nylon, it is not suitable to stay in the barrel for a long time at high temperature, so as not to cause discoloration and yellowing of the material. At the same time, because nylon has good fluidity, it flows quickly when the temperature exceeds its melting point.

Injection pressure nylon solution has low viscosity and good fluidity, but its coagulation speed is fast, which is easy to cause insufficient problems on products with complex shape and thin wall thickness, so it still needs high injection pressure. Usually, if the pressure is too high, the product will overflow; If the pressure is too low, the product will have defects such as ripples, bubbles, obvious weld marks or insufficient products. The injection pressure of most nylon varieties is less than 120MPA, and it is generally selected within the range of 60- 100MPA to meet the requirements of most products. As long as there are no defects such as bubbles and dents in the product, it is generally not advisable to adopt higher holding pressure to avoid increasing the internal stress of the product.

Injection speed for nylon, the injection speed is favorable, which can prevent ripple and insufficient mold filling caused by too fast cooling speed. The effect of high injection speed on the properties of products is not significant.

Mold temperature mold temperature has certain influence on crystallinity and molding shrinkage. The mold temperature is high, the crystallinity is high, the wear resistance, hardness and elastic modulus increase, the water absorption decreases, and the molding shrinkage of the product increases. Low mold temperature, low crystallinity, good toughness and high elongation.

Four, nylon molding process parameter table

Project nylon 66 glass fiber reinforced nylon 66

The temperature of the back barrel is 240-285, 290-300℃.

Central 260-300 285-320

Front 260-300 285-320

Nozzle temperature℃ 260-280 280-285

Mold temperature℃ 20- 90 80-85

Injection pressure MPA 60-200 60-200

Screw speed r/min 50-120 50-120 V. Precautions for nylon molding

1. It is best to use recycled materials no more than three times, so as not to cause discoloration of products or sharp decline of mechanical and physical properties. The application amount should be controlled below 25%, too much will cause fluctuations in process conditions, and recycled materials must be dried when mixed with new materials.

2. Safety precautions: When starting nylon resin, the nozzle temperature should be turned on first, and then the feed cylinder should be preheated. When the nozzle is blocked, avoid facing the nozzle hole to prevent the solution in the cylinder from being suddenly released due to pressure accumulation, causing danger.

3, the use of release agent The use of a small amount of release agent can sometimes improve and eliminate defects such as bubbles. The release agent of nylon products can be zinc stearate, white oil, etc. It can also be made into paste, and the dosage must be small and even when used, so as not to cause surface defects of the product.

4. Empty the screw when stopping to prevent the screw from breaking in the next production. 6. Post-treatment of nylon products.

Post-treatment of products: the post-treatment of nylon products is to prevent and eliminate the residual stress or dimensional change caused by moisture absorption in the products. There are two post-treatment methods: heat treatment and wet conditioning. 1. Common heat treatment methods In high-boiling liquids such as mineral oil, glycerin and liquid paraffin, the heat treatment temperature should be higher than the use temperature 10-20℃, and the treatment time depends on the wall thickness of the product. 10-15min thickness is below 3mm,15min thickness is 3-6mm. 2. Humidification treatment Humidification treatment is mainly carried out for products with high environmental humidity. There are two methods: one is to humidify with boiling water, and the other is to humidify with potassium acetate aqueous solution (the ratio of potassium acetate to water is 1.25: 1, and the boiling point is 12 1℃). The method of boiling water humidification is simple, as long as the product is placed in an environment with humidity of 65438 0℃. The treatment temperature of potassium acetate aqueous solution conditioning method is 80- 100, and the treatment time mainly depends on the wall thickness of the product. When the wall thickness is 1.5mm, it is about 2 hours, 3mm is 8 hours, and 6mm is 16- 18 hours. Seven. Common defects of nylon products and their treatment.

* Note-The injection pressure is unstable.

This situation is generally related to the nozzle hole of injection molding machine is too small, because the contact time between the nozzle and the mold is long, the mold temperature is very low 20-90℃, and the nozzle temperature is 240-280℃, so there is a temperature difference between them, and heat exchange will inevitably occur. When the temperature of the nozzle drops below the melting point of nylon, the nozzle hole will be frozen, and it will be pushed away by huge pressure in the next injection, resulting in pressure loss and injection. However, when the injection pressure increases at this time, from the phenomenon, the injection molding machine is unstable. In fact, if the nozzle hole is too small, the phenomenon that the nozzle hole becomes larger will disappear.

Papoma ABS, these materials will cause this problem, wavy flow marks.

The mechanism is that the flow rate of rubber in the mold cavity is too slow, and the solidified rubber can not stick to the mold.

Solution:

1, improve the injection speed.

2, improve the mold temperature

3, improve the barrel temperature

4. Appropriately increase the nozzle aperture or gate.

Silver/gray hair

The mechanism is that there is gas in the plasticizing material, and the gas is extruded on the surface of the mold during injection, and white silk threads appear on the surface of the product.

Solution:

1, check whether the raw materials are wet or mixed with other raw materials.

2, check whether the raw materials in the barrel decomposition (barrel temperature

Too high, screw speed is too fast)

3. Check whether the nozzle hole is too small.

4. Check whether the mold temperature is too low.

5, mold exhaust is not good

6. Is the size of the door too small?

7. The back pressure is too low, and recycled materials are used too much.

Melting mark

The mechanism is that at the end of the flow, the temperature of the rubber compound is very low, the adhesion is poor, and the pressure transmission is weak, so that the two flows are not closely combined.

Solution:

1, improve the injection pressure and speed.

2, improve the mold temperature

3, improve the material temperature

4, improve the mold

unleash

shrinkage cavity

The mechanism is that the product is injected or contracted.

If it is * note, use * note method to solve it.

If it is contraction, use contraction to solve the focal spot.

The mechanism is that rubber occupies the mold cavity at high speed during injection. When the gas in the mold cavity is too late to be removed, this part of the gas is compressed, and the compressed gas is heated to make the product burn.

Solution:

1, reduce the injection speed or pressure.

2. Reduce the melt temperature

3, improve the mold exhaust

4. Reduce the clamping force

5, increase the nozzle aperture

Poor demoulding

1, improper mold temperature control, uneven shrinkage of each part, resulting in uneven mold coverage.

2. The injection residual stress in the product is large and it is difficult to demould.

How to solve this problem:

1, reducing injection and holding pressure;

2. Reduce injection and pressure holding time.