“Plastic performance is the foundation of injection molding technologyâ€. Mastering the process performance and characteristics of various plastics is the basic professional knowledge that every injection molding worker must understand. The performance of plastics is the basis for setting “injection molding process conditionsâ€. One of the factors that must be considered when analyzing the quality problems and anomalies that occur during the injection molding process.
Polypropylene (PP) injection molding process
PP is called polypropylene, because it has good anti-breaking performance, also known as "100 fold glue." PP is a translucent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high heat distortion temperature, low density and high crystallinity. The modified filler usually has glass fibers, mineral fillers, thermoplastic rubber, and the like.
The PP of different uses has a large difference in fluidity, and the PP flow rate generally used is between ABS and PC.
Pure PP is a translucent ivory white that can be dyed in a variety of colors. The dyeing of PP can only be used with color masterbatch on general injection molding machines. On some machines, there are separate plasticizing elements that enhance the mixing action, and can also be dyed with toner. Products for outdoor use are typically filled with UV stabilizers and carbon black. The proportion of recycled materials should not exceed 15%, otherwise it will cause strength drop and discoloration. PP does not require special drying before injection molding.
There are no special requirements for the selection of injection molding machines. Since PP has high crystallinity. A computer injection molding machine with high injection pressure and multi-stage control is required. The clamping force is generally determined by 3800t/m2, and the injection volume is 20%-85%.
The mold temperature is 50-90 ° C, and the high mold temperature is required for the size. The core temperature is 5 °C lower than the cavity temperature, the runner diameter is 4-7 mm, the needle gate length is 1-1.5 mm, and the diameter can be as small as 0.7 mm. The shorter the length of the edge gate, the better, about 0.7 mm, the depth is half of the wall thickness, the width is twice the wall thickness, and the thickness of the melt flow in the cavity increases. The mold must have good exhaustibility. The vent hole depth is 0.025mm-0.038mm and the thickness is 1.5mm. To avoid shrinkage marks, it is necessary to use large and round nozzles and circular flow passages. The thickness of the ribs should be small ( For example, 50-60% of the wall thickness). Products made of homopolymerized PP should not exceed 3mm in thickness, otherwise there will be bubbles (thick-walled products can only be copolymerized with PP).
The melting point of PP is 160-175 ° C, and the decomposition temperature is 350 ° C, but the temperature setting during injection processing cannot exceed 275 ° C, and the temperature of the melting section is preferably 240 ° C.
In order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and mold are not suitable (bubbles, gas lines appear). If the surface of the pattern has a light and dark streak spread by the gate, a low speed injection and a higher mold temperature are used.
The back pressure of the 5 bar melt can be used, and the back pressure of the toner can be appropriately adjusted.
Use a higher injection pressure (1500-1800 bar) and a holding pressure (about 80% of the injection pressure). It is about to maintain pressure at 95% of the full stroke, with a longer holding time.
In order to prevent shrinkage deformation caused by post-crystallization, the product generally needs to be immersed in hot water.
Polyethylene (PE) injection molding process
PE is a crystalline raw material and has extremely low hygroscopicity of not more than 0.01%, so that it is not required to be dried before processing. The PE molecular chain is flexible, the force between the keys is small, the melt viscosity is low, and the fluidity is excellent. Therefore, the thin wall long process product can be formed without too high pressure during molding. PE has a large shrinkage range, large shrinkage value, and obvious directionality. The LDPE shrinkage is about 1.22%, and the HDPE shrinkage is about 1.5%. Therefore, it is easy to deform and warp, and the mold cooling condition has a great influence on the shrinkage rate, so the mold temperature should be controlled to keep the cooling uniform and stable.
The crystallization ability of PE is high, and the temperature of the mold has a great influence on the crystallization condition of the plastic part. The mold temperature is high, the melt cooling is slow, the plastic part has high crystallinity and the strength is high.
The melting point of PE is not high, but the specific heat capacity is large, so it still needs to consume more heat during plasticization. Therefore, the plasticizing device is required to have a large heating power in order to improve production efficiency. The softening temperature range of PE is small, and the melt is easy to be oxidized. Therefore, the contact between the melt and oxygen should be avoided as much as possible in the molding process, so as to avoid the quality of the plastic part.
PE parts are soft in texture and easy to release, so when the plastic parts have shallow side grooves, they can be strongly demolded. The non-Newtonian properties of the PE melt are not obvious, the change of the shear rate has little effect on the viscosity, and the viscosity of the PE melt is less affected by the temperature. The PE melt has a slower cooling rate and must therefore be sufficiently cooled. The mold should have a better cooling system.
If the PE melt is fed with a direct feed port during injection, the stress should be increased and the shrinkage unevenness and the directionality should be increased. Therefore, the feed port parameters should be selected. PE has a relatively high molding temperature, and in the flow state, a slight fluctuation in temperature has no effect on injection molding. PE has good thermal stability, and generally has no obvious decomposition phenomenon below 300 degrees, and has no effect on quality.
Main molding conditions of PE
Barrel temperature: The temperature of the barrel is mainly related to the density of the PE and the flow rate of the melt. It is also related to the type and performance of the injection molding machine and the shape of the first-class plastic part. Since PE is a crystalline polymer, the crystal grains absorb a certain amount of heat during melting, so the cylinder temperature should be 10 degrees higher than its melting point. For LDPE, the barrel temperature is controlled at 140-200 ° C, the barrel temperature of HDPE is controlled at 220 ° C, the minimum value is taken at the rear of the barrel, and the front end takes the maximum value.
Mold temperature: The mold temperature has a great influence on the crystallization condition of the plastic part. The mold temperature is high, the melt crystallinity is high, and the strength is high, but the shrinkage rate also increases. Generally, the mold temperature of LDPE is controlled at 30 ° C - 45 ° C, and the temperature of HDPE is correspondingly higher by 10-20 ° C.
Injection pressure: increasing the injection pressure is conducive to the filling of the melt. Since the fluidity of PE is very good, in addition to the thin-walled slender products, the lower injection pressure should be selected well, and the general injection pressure is 50-100 MPa. The shape is simple. The larger the plastic part behind the wall, the injection pressure can be lower, and vice versa.
Polyvinyl chloride (PVC) injection molding process
Typical applications: water supply pipes, household pipes, house wall panels, commercial machine casings, electronic product packaging, medical equipment, food packaging, etc.
Chemical and physical properties: PVC material is a non-crystalline material. PVC materials are often added with stabilizers, lubricants, auxiliary processing agents, colorants, impact agents and other additives in actual use. PVC materials are non-flammable, high strength, weather resistant and excellent geometric stability.
PVC is highly resistant to oxidizing agents, reducing agents and strong acids. However, it can be corroded by concentrated oxidizing acids such as concentrated sulfuric acid, concentrated nitric acid, and is also not suitable for contact with aromatic hydrocarbons or chlorinated hydrocarbons.
The melting temperature of PVC during processing is a very important process parameter. If this parameter is improper, it will cause the problem of material decomposition. The flow characteristics of PVC are rather poor and the process range is narrow.
In particular, large molecular weight PVC materials are more difficult to process (this material usually incorporates a lubricant to improve flow characteristics), so small molecular weight PVC materials are commonly used. The shrinkage rate of PVC is quite low, generally 0.2 to 0.6%.
Injection molding process conditions:
Drying treatment: Drying is usually not required.
Melting temperature: 185~205°C Mold temperature: 20~50°C.
Injection pressure: up to 1500 bar.
Holding pressure: up to 1000 bar.
Injection speed: In order to avoid material degradation, a considerable injection speed is generally used.
Runners and gates: All conventional gates can be used. If you are working with smaller parts, it is best to use tip-type gates or submerged gates; for thicker parts, it is best to use fan-shaped gates. The minimum diameter of the tip or submerged gate shall be 1 mm; the thickness of the fan gate shall not be less than 1 mm.
Chemical and physical properties: Rigid PVC is one of the most widely used plastic materials.
Polystyrene (PS) injection molding process
Typical applications: product packaging, household goods (tableware, trays, etc.), electrical (transparent containers, light source diffusers, insulating films, etc.).
Chemical and physical properties: Most commercial PSs are transparent, amorphous materials. PS has very good geometric stability, thermal stability, optical transmission characteristics, electrical insulation properties and a very small tendency to absorb moisture. It is resistant to water, diluted inorganic acids, but can be corroded by strong oxidizing acids such as concentrated sulfuric acid and can swell and deform in some organic solvents. Typical shrinkage is between 0.4 and 0.7%.
Injection molding process conditions:
Drying treatment: Drying is usually not required unless stored improperly. If drying is required, it is recommended to dry at 80 ° C for 2-3 hours.
Melting temperature: 180~280 °C. The upper limit for the flame retardant material is 250 °C.
Mold temperature: 40~50 °C.
Injection pressure: 200~600bar.
Injection speed: A fast injection speed is recommended.
Runners and Gates:: All conventional types of gates can be used.
ABS injection molding process
Typical applications: automotive (dashboard, tool door, wheel cover, mirror box, etc.), refrigerator, high-strength tools (hair dryer, blender, food processor, lawn mower, etc.), telephone housing Body, typewriter keyboard, recreational vehicles such as golf carts and jet skis.
Chemical and physical properties: ABS is synthesized from three chemical monomers of 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 a morphological point of view, ABS is an amorphous material.
The polymerization of the three monomers produces a terpolymer having two phases, one being a continuous phase of styrene-acrylonitrile and the other being a dispersed phase of polybutadiene rubber. The properties of ABS are mainly determined by the ratio of the three monomers and the molecular structure in the two phases. This allows for great flexibility in product design and results in hundreds of different quality ABS materials on the market. These different quality materials offer different properties such as medium to high impact resistance, low to high finish and high temperature distortion.
ABS materials have superior processability, appearance characteristics, low creep and excellent dimensional stability and high impact strength.
Injection molding process conditions:
Drying treatment: ABS materials are hygroscopic and require drying treatment before processing. It is recommended to dry for at least 2 hours at 80~90 °C. The material temperature should be guaranteed to be less than 0.1%.
Melting temperature: 210~280°C; recommended temperature: 245°C.
Mold temperature: 25~70 °C. (The mold temperature will affect the finish of the plastic part, and the lower temperature will result in a lower finish).
Injection pressure: 500~1000bar.
Injection speed: medium to high speed.
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