This paper mainly introduces the performance of engineering plastics
PA and the injection molding process, the selection of equipment in the actual production, product modeling and mold design considerations.
PA-polyamide, also known as nylon, is a general term for a large class of amide-type polymers. The most common are PA6, PA66, and PA1010. Recently, with the development of the IT industry, the use of a new type of polyamide-PA46 has increased dramatically, and it has been used to replace the LCP (liquid crystal polymer) to produce plug-ins on computers. Because PA has good mechanical properties, good toughness, impact resistance, wear resistance, self-lubrication, flame retardant, insulation, etc., it is widely used in many fields such as automobile, machinery, electronics, instrumentation, chemical industry, such as gears and pulleys. , bearings, impellers, bushings, containers, brushes, zippers, etc.
PA6, PA66, and PA46 are all aliphatic polyamides. They are linear polymers with a very polar amide group in their molecular structure, so they have a high degree of crystallization. The properties of PA products depend on their crystalline morphology and crystallinity. The processing conditions have an effect on the crystal form and crystallinity, and the processing conditions are different. The crystal change of the PA product can reach 40%, the product cools slowly, the crystallinity is high, and a large-sized crystal form is formed, and the water absorption has crystallinity. influences. In addition, PA will have a certain degree of orientation during processing due to flow and shear, resulting in anisotropic properties of the product, and the strength in the orientation direction is better than the non-oriented direction. The orientation is also beneficial to the crystallization process. This factor should be considered when designing.
PA performance
• Physical properties
No ※, tasteless, not mildew, appearance is translucent or transparent, milky white or light yellow, density 1.04-1.36, emits a special protein flavor when burning, the flame is blue at the upper end.
•Mechanical behavior
Good toughness, resistance to repeated shock and vibration, use temperature of –40-100 °C, friction resistance, wear resistance, self-lubrication, but poor creep resistance, poor dimensional stability, can be enhanced by adding glass fiber or other materials Blending to overcome this disadvantage.
• Chemical resistance and weather resistance
PA has few organic solvents and is chemically good. The organic solvent of PA is formic acid and phenolic compounds. Different concentrations of inorganic acids, bases, and salts can cause PA to swell, dissolve, or hydrolyze. Under the condition of being exposed to sunlight, its aging resistance is good, but it is aging under the action of heat, light and radiation, the product is discolored, and the performance is degraded.
•Processing performance
It is easy to absorb moisture and must be dried before molding.
The melting point is high, the melting limit is narrow, and the melting points are: PA6: 215 ° C, PA 66: 255 ° C, PA 46: 290 ° C.
PA melt viscosity has high temperature sensitivity and shear sensitivity, low melt viscosity and good fluidity. The thermal stability of the PA molten state is poor, and it is easily decomposed at high temperatures.
The cooling rate of the melt has a significant effect on the structure and properties of the article.
The molding process is accompanied by crystallization and the shrinkage rate is large.
Molding equipment and mold
Injection molding machine selection
Screw and barrel
Because PA has the characteristics of toughness, self-lubricity, difficulty in feeding, large heat of shearing, high melting point, fast melting speed and easy decomposition. The screw is required to have a concentrated exhaust gas concentration, strong feeding ability, large driving force and good wear resistance. Therefore, the plasticizing system of PA injection molding process is: the feeding section of the screw is long, the screw groove of the feeding section is deep, the compression section and the homogenization section are short, and the groove is added at the feeding section of the barrel to increase the power of the oil motor. The screw has an efficient check ring.
•The nozzle
Since the viscosity of the PA is low in the molten state, the fluidity is good, and some residual pressure is inevitably left in the barrel. If an open nozzle is used, the melt will flow out from the nozzle when the product is taken out by the mold (ie, the flow phenomenon). It not only wastes materials but also affects normal production. Therefore, it is necessary to use a self-locking nozzle, and a common spring needle valve type nozzle.
• Product creation and mold design
Under the premise of ensuring the performance of the product, the wall thickness should be as small as possible. The thicker the product, the larger the shrinkage and the insufficient strength, and the reinforcing ribs can be increased. The product has a large shrinkage rate and is easy to demould, and the draft angle is 40'-1 ゚40'.
• runners and gates
The diameter of the runner is -63-6mm, and the diameter of the gate is 2/3-3/4 of the wall thickness. But not less than 0.8mm. If the gate is too small, the melt will be sheared and overheated to degrade, which will affect the performance of the product.
•Inlay
The coefficient of thermal expansion of nylon is 9-10 times larger than that of steel and 4-5 times larger than that of aluminum. Metal inserts hinder the shrinkage of nylon and generate large stress, which may cause cracking. The thickness around the insert is required to be not less than the metal of the insert. Diameter size.
•exhaust
The viscosity of PA is small. Under high pressure injection, the filling is fast. If the gas cannot be discharged in time, the product is prone to bubbles, burns and other defects. The mold must have a vent or a venting groove, which is generally open on the opposite side of the gate. The diameter is Ф1.5-1mm and the vent depth is less than 0.03mm.
•Molding
1. Preparation of raw materials
Because PA is easy to absorb moisture, it has an effect on the processing after moisture absorption. For example, the viscosity of the melt decreases, and the surface of the product has bubbles and silver streaks. Moreover, the mechanical properties of the product are also significantly reduced. Therefore, it is necessary to carry out drying treatment before molding. Further, PA is easily oxidized to discolor and degrade at a high temperature. Therefore, it is preferred to use vacuum drying, but it can be dried by atmospheric hot air without vacuum drying. The vacuum drying temperature is 85-95 ° C, the time is 4-6H, the hot air drying is: temperature: 90-100 ° C, time 8-10H, and the dried PA material should not be placed in the air for a long time (not exceeding 1-3H).
2, molding temperature
The choice of barrel temperature is based on the melting point of PA, and is related to the type of injection molding machine, the shape and size of the product. Generally, it is 220-320 ° C, PA6: 220-300 ° C; PA66: 260-320 ° C, because the processing temperature of PA is narrow, the barrel temperature must be strictly controlled to avoid the degradation of the melt and the deterioration of the product. The temperature setting of the barrel has a great influence on the plasticization of the plastic and the speed of the melt. The middle temperature of the barrel is higher than the melting point of 20--40 ° C, lower than the decomposition temperature of 20-30 ° C, and the front section temperature is lower than the middle temperature. -10 °C, the temperature of the rear section (feeding section) is 20-50 °C lower than the temperature of the middle section; the cooling at the feeding port must be effective; if the temperature of the middle section is too low, the screw speed is too fast, the card phenomenon may occur, and the temperature in the rear section is too high. Will affect the conveying capacity, and the screw feeds slowly, which affects the production efficiency.
3, injection pressure
Injection pressure has little effect on the mechanical properties of PA. The choice of injection pressure depends mainly on the type of injection molding machine, barrel temperature, product shape, size, mold structure, and also depends on factors such as injection speed, injection time, and holding time.
4, injection speed
The selection of injection speed is related to the wall thickness of the product, the temperature of the melt, the size of the gate, etc. For thin-walled products, the injection speed can be faster, while for thick-walled products, the injection speed can be slower and the melt temperature is high. The injection speed is slower, the gate size is small, and the injection speed cannot be too fast. Otherwise, the melt temperature will be too high due to excessive shearing, which will lead to discoloration and mechanical properties. If the injection speed is too fast, the product will also have defects such as bubbles and charring.
• Screw speed
It is suitable to use medium speed. If the rotation speed is too fast, the plastic will degrade due to excessive shearing, resulting in discoloration and performance degradation of the product. The rotation speed is too slow, which will affect the quality of the melt rubber, and will also affect the production efficiency due to the long glue time.
• Back pressure
Under the premise of ensuring the quality of the product, the lower the back pressure, the better, and the high back pressure will cause the melt to shear excessively and thermally degrade.
• mold temperature
The mold temperature is high, and the hardness, density, tensile strength, and elastic modulus of the product are improved. The mold temperature is related to the performance requirements of the product. For thin products requiring good elongation and transparency, the mold temperature is lower; while high strength, good wear resistance and low deformation are required, the mold temperature is high. Something is good.
details as follows:
Product thickness Mold temperature: less than 3mm20-40°C, 3-6mm40-60°C, 6-10mm60-90°C, more than 10mm100°C mold temperature has a great influence on the shrinkage rate of the product, the higher the mold temperature, the greater the shrinkage rate, and vice versa. The shrinkage rate is small.
• Molding cycle
Mainly depends on the wall thickness of the product, for thin-walled products, injection time, dwell time, cooling time can be shorter, and thick-walled products, in order to prevent shrinkage deformation, dents, bubbles, etc., injection time, dwell time Correspondingly longer, using a high mold temperature, the cooling time should be longer.
• Product post processing
The molecular orientation of the PA during the molding process and the crystallization during the cooling process cause the product to have a certain internal stress, and the size and shape of the product may change during the subsequent storage or use. Therefore, annealing and humidity conditioning treatment are required.
•annealing
The product is used at a temperature higher than 80 ° C or with high precision. After the product is taken out of the mold, it is annealed in oil or paraffin. The annealing temperature is higher than the use temperature of 10-20 ° C for 10 to 60 minutes. (depending on the thickness of the product)
• humidity control
For products that have been used for a long time in damp or aqueous solution, in the molding process, after the product is taken out, it is placed in boiling water or potassium acetate aqueous solution for 1-2 days.
• barrel retention time
In the production process, if the temperature of the glue is above 300 °C, it is necessary to avoid the residence time of the melt in the barrel for too long (20 minutes), otherwise it will overheat and decompose, making the product discolored or brittle. If you need to temporarily stop for more than 20 minutes, you can reduce the barrel temperature to 200 °C. For long periods of downtime, the barrel must be cleaned with a higher viscosity polymer and can be cleaned with HDPE or PP.
