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Detailed description of injection moulding process of five general plastics

2019/8/26 9:32:27   点击:2101

 "Plastic properties are the foundation of injection technology". To master the technological properties and characteristics of all kinds of plastics is the basic professional knowledge that every injection worker must know. The performance of plastics is the basis for setting up the "injection process conditions". It is also the reason that must be considered when analyzing the quality problems and abnormal phenomena in the injection process. One of the elements.



1. Injection Molding Processing of Polypropylene (PP)





PP is commonly known as polypropylene because of its good fracture resistance, also known as "100 fold gum". PP is a translucent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high thermal deformation temperature, low density and high crystallinity. Modified fillers usually include glass fibers, mineral fillers, thermoplastic rubber, etc.





The fluidity of PP for different uses varies greatly, and the flow rate of PP used in general is between ABS and PC.





Pure PP is translucent ivory white and can be dyed in various colors. The dyeing of PP can only be done with masterbatch on general injection moulding machines. In some machines, there are independent plasticizers to enhance the mixing effect. They can also be dyed with dyes. Outdoor products are usually filled with UV stabilizers and carbon black. The proportion of recycled materials should not exceed 15%. Otherwise, it will cause strength reduction and decomposition discoloration. PP does not need special drying treatment before injection moulding.





There is no special requirement for the selection of injection moulding machine. Because PP has high crystallinity. Computer injection moulding machine with high injection pressure and multi-stage control is needed. The clamping force is generally determined by 3800t/m2 and the injection volume is 20%-85%.





Mould temperature is 50 - 90 C, and high die temperature is required for high dimension. Core temperature is lower than cavity temperature by more than 5 C, runner diameter is 4-7 mm, needle gate length is 1-1.5 mm, diameter can be smaller than 0.7 mm. The shorter the length of the edge gate, the better. It is about 0.7mm. The depth is half of the wall thickness and the width is twice the wall thickness. The die must have good exhaust performance. The exhaust hole depth is 0.025mm-0.038mm and the thickness is 1.5mm. To avoid shrinkage marks, large and round nozzles and circular runners should be used, and the thickness of reinforcing ribs should be small (for example, 50-60% of wall thickness). Homopolymer PP products should not be more than 3 mm thick, otherwise there will be bubbles (thick-walled products can only use copolymer PP).





The melting point and decomposition temperature of PP are 160-175 (?) and 350 (?) respectively, but the setting temperature of PP during injection processing should not exceed 275 (?) and the melting temperature of PP should be 240 (?)





In order to reduce the internal stress and deformation, high-speed injection should be selected, but some grades of P P and die are not suitable (bubble, air striation). If there are light and dark interfacial stripes on the surface carved with pattern diffused by gate, low-speed injection and higher die temperature should be used.





The back pressure of 5bar melt glue can be used, and the back pressure of color powder can be adjusted appropriately.





High injection pressure (1500-1800 bar) and holding pressure (about 80% of injection pressure) were used. About 95% of the journey to the pressure, with a longer holding time.





In order to prevent shrinkage and deformation caused by post-crystallization, products generally need to be treated by hot water immersion.

"2. Injection Molding Technology of Polyethylene (PE)






PE is a crystalline material with very little moisture absorption, not exceeding 0.01%. Therefore, drying treatment is not necessary before processing. PE molecule chain has good flexibility, low interbond force, low melt viscosity and excellent fluidity, so it can form thin-walled long process products without too high pressure. The shrinkage range of PE is large, the shrinkage value is large and the orientation is obvious. The shrinkage rate of LDPE is about 1.22%, and that of HDPE is about 1.5%. Therefore, it is easy to deform and warp, and the cooling condition of the die has a great influence on the shrinkage rate. Therefore, the temperature of the die should be controlled to keep the cooling uniform and stable.





PE has high crystallization ability, and the temperature of the mould has a great influence on the crystallization of the plastic parts. High mould temperature, slow melt cooling, high crystallinity and high strength of plastic parts.





The melting point of PE is not high, but its specific heat capacity is large, so it still needs to consume more heat when plasticizing. Therefore, the plasticizing device is required to have larger heating power in order to improve production efficiency. The softening temperature range of PE is small and the melt is easy to oxidize. Therefore, the contact between melt and oxygen should be avoided as far as possible in the forming process, so as not to reduce the quality of plastic parts.





PE parts are soft and easy to demould, so when plastic parts have shallow grooves, they can be demoulded forcefully. The non-Newtonian property of PE melt is not obvious, the change of shear rate has little effect on the viscosity, and the influence of temperature on the viscosity of PE melt is also small. The cooling rate of PE melt is slow, so it must be fully cooled. The mould should have a better cooling system.





If the PE melt is fed directly at the inlet and outlet during injection, the stress should be increased, and the uneven shrinkage and obvious increase of deformation in direction should be produced. Therefore, attention should be paid to the selection of the inlet and outlet parameters. The forming temperature of PE is wide, and a little fluctuation of temperature has no effect on injection moulding under flowing condition. PE has good thermal stability, generally no obvious decomposition phenomenon below 300 degrees, and has no effect on the quality.







Main Forming Conditions of PE






Barrel temperature: Barrel temperature is mainly related to the density of PE and the melt flow rate. It is also related to the type and performance of injection moulding machine and the shape of the first grade plastic parts. Because PE is a crystalline polymer, the grain absorbs a certain amount of heat during melting, so the barrel temperature should be higher than its melting point of 10 degrees. For LDPE, the barrel temperature is controlled at 140-200 C, the barrel temperature of HDPE is controlled at 220 C, the barrel rear is the minimum and the front is the maximum.





Mold temperature: Mold temperature has a great influence on the crystallization of plastic parts. Mold temperature is high, melt crystallinity is high, strength is high, but shrinkage rate will also increase. Usually the temperature of LDPE die is controlled at 30 - 45 C, while the temperature of HDPE is 10 - 20 C higher correspondingly.





Injection pressure: Increasing injection pressure is beneficial to the filling of melt. Because PE has good fluidity, besides thin-walled and slender products, lower injection pressure should be selected carefully. Generally, the injection pressure is 50-100 MPa. The shape is simple. For larger plastic parts behind the wall, injection pressure can be lower and vice versa.







3. Injection Molding Technology of Polyvinyl Chloride (PVC)




Typical applications: water supply pipeline, household pipeline, house wallboard, commercial machine shell, electronic product packaging, medical equipment, food packaging, etc.





Chemical and Physical Properties: PVC is a non-crystalline material. In the practical use of PVC materials, stabilizers, lubricants, auxiliary processing agents, pigments, impact resistance agents and other additives are often added. PVC material has non-flammability, high strength, weather resistance and excellent geometric stability.





PVC has strong resistance to oxidants, reducing agents and strong acids. However, it can be corroded by concentrated oxidizing acids such as concentrated sulfuric acid and concentrated nitric acid, and it is not suitable for contacting aromatic hydrocarbons and chlorinated hydrocarbons.





The melting temperature of PVC is a very important process parameter in processing. If this parameter is not appropriate, it will lead to material decomposition. The flow characteristics of PVC are rather poor, and its technological range is very narrow.





In particular, large molecular weight PVC materials are more difficult to process (this material usually adds lubricants to improve the flow characteristics), so they are usually used in small molecular weight PVC materials. The shrinkage of PVC is quite low, generally 0.2-0.6%.







Injection moulding process conditions:




Drying treatment: Drying treatment is usually not required.



Melting temperature: 185 ~ 205 Mold temperature: 20 ~ 50 C.



Injection pressure: up to 1500bar.



Pressure holding pressure: up to 1000 bar.



Injection speed: In order to avoid material degradation, a considerable injection speed is usually used.



Runner and gate: All conventional gate can be used. If smaller parts are processed, it is better to use pinpoint gate or submerged gate; for thicker parts, it is better to use sector gate. The minimum diameter of pinpoint gate or submerged gate shall be 1 mm, and the thickness of sector gate shall not be less than 1 mm.



Chemical and Physical Properties: Rigid PVC is one of the most widely used plastic materials.
4. Polystyrene (PS) Injection Molding Process




Typical applications: product packaging, household goods (tableware, tray, etc.), electrical (transparent container, light scatterer, insulating film, etc.).





Chemical and Physical Properties: Most commercial PS is transparent and amorphous. PS has excellent geometric stability, thermal stability, optical transmission, electrical insulation and very small moisture absorption tendency. It can resist water and diluted inorganic acids, but can be corroded by strong oxidized acids such as concentrated sulfuric acid, and can expand and deform in some organic solvents. Typical shrinkage rates range from 0.4% to 0.7%.







Injection moulding process conditions:




Drying treatment: Unless improperly stored, drying treatment is usually not required. If drying is needed, it is suggested that the drying conditions should be 80 C for 2 - 3 hours.



Melting temperature: 180 ~ 280 C. For flame retardant materials, the upper limit is 250 C.



Temperature of die: 40 ~ 50 C.



Injection pressure: 200-600 bar.



Injection speed: Fast injection speed is recommended.



Runner and gate: All conventional types of gate can be used.







5. ABS Injection Molding Process




Typical applications: automobiles (dashboards, tool hatches, wheel covers, reflector boxes, etc.), refrigerators, high-strength tools (hair dryers, mixers, food processors, lawn mowers, etc.), telephone shells, typewriter keyboards, entertainment vehicles such as golf carts and jet sledgers.





Chemical and physical properties: ABS is synthesized from acrylonitrile, butadiene and styrene. Each monomer has different characteristics: acrylonitrile has high strength, thermal stability and chemical stability; butadiene has toughness and impact resistance; styrene has easy processing, high finish and high strength. From the morphological point of view, ABS is a non-crystalline material.





The copolymerization of the three monomers produces a terpolymer with two phases: a continuous phase of styrene-acrylonitrile and a dispersed phase of polybutadiene rubber. The properties of ABS depend mainly on the ratio of three monomers and the molecular structure in the two phases. This allows for great flexibility in product design, resulting in hundreds of ABS materials of different qualities on the market. These materials of different qualities provide different properties, such as medium to high impact resistance, low to high finish and high temperature distortion.





ABS material has super machinability, appearance characteristics, low creep, excellent dimensional stability and high impact strength.







Injection moulding process conditions:






Drying treatment: ABS material has hygroscopicity, requiring drying treatment before processing. It is suggested that the drying conditions should be at least 2 hours at 80-90 C. The material temperature should be guaranteed to be less than 0.1%.



Melting temperature: 210 ~ 280 and recommended temperature: 245 C.



Temperature of die: 25 - 70 C. (Mold temperature will affect the smoothness of plastic parts, lower temperature will lead to lower smoothness).



Injection pressure: 500-1000bar.



Injection speed: medium and high speed.



The origin of this article is reproduced in the injection moulding band.

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