The performance of plastics is the basis of injection molding technology, and mastering the process performance and characteristics of various plastics is a basic professional knowledge that every injection molding worker must know.
It is also one of the factors that must be considered when analyzing the quality problems and abnormalities that occur during the injection molding process.
I. Polypropylene (PP) injection molding process
PP is a translucent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high heat deflection temperature, low density, and high crystallinity. Modified fillers usually include glass fiber, mineral filler, thermoplastic rubber, etc.
The fluidity of PP varies greatly from application to application, and the general use of PP flow rate is between ABS and PC.
Pure PP is semi-transparent ivory white and can be dyed in various colors; PP can only be dyed with masterbatch on general injection molding machines.
In some machines, there are separate plasticizing elements that enhance the mixing effect, and can also be dyed with color powder.
For products used outdoors, UV stabilizers and carbon black filling is generally used. The proportion of recycled material used should not exceed 15%, otherwise, it will cause strength loss and decomposition, and discoloration. PP generally does not need special drying treatment before injection molding processing.
There is no special requirement for the selection of an injection molding machine. Because PP has high crystallinity.
It is necessary to use a computerized injection molding machine with high injection pressure and multi-stage control. The clamping force is generally determined by 3800t/m2, and the injection volume is 20%-85%.
The mold temperature is 50-90℃, with high mold temperature for high dimensional requirements. The core temperature is more than 5℃ lower than the cavity temperature, the runner diameter is 4-7mm, the needle gate length is 1-1.5mm, and the diameter can be as small as 0.7mm.
The shorter the edge gate length is, the better, about 0.7mm, the depth is half of the wall thickness, the width is twice the wall thickness, and the length of the melt flow in the cavity increases with the mold.
The mold must have good exhaust, the exhaust hole is 0.025mm-0.038mm deep and 1.5mm thick, to avoid shrinkage marks, it is necessary to use a large and round injection mouth and round flow channel, the thickness of the reinforcement should be small (for example, it is 50-60% of the wall thickness).
The thickness of products made of homopolymer PP cannot exceed 3mm, otherwise, there will be bubbles (thick-walled products can only be made of copolymer PP).
The melting point of PP is 160-175℃, and the decomposition temperature is 350℃, but the temperature setting cannot exceed 275℃ during injection processing, and the temperature of the melt section is preferably at 240℃.
To reduce internal stress and deformation high-speed injection should be chosen, but some grades of PP and molds are not applicable (bubbles and air patterns appear).
If the patterned surface light and dark streaks spread by the gate, low-speed injection and higher mold temperature should be used.
The melt back pressure can be 5 bar, and the back pressure of color powder can be adjusted higher.
Use higher injection pressure (1500-1800 bar) and holding pressure (about 80% of injection pressure). Turn the holding pressure at approximately 95% of the full stroke and use a longer holding time.
To prevent shrinkage and deformation caused by post-crystallization, the product is usually treated with hot water soaking.
2. Polyethylene (PE) injection molding process
PE is crystalline raw materials, moisture absorption is very small, not more than 0.01%, so there is no need to dry before processing.
PE molecular chain flexibility, small inter-bond forces, the low viscosity of the melt, and excellent fluidity, so the molding without too high pressure can be molded out of thin-walled long-flow products.
PE shrinkage rate range, shrinkage value, directional obvious, LDPE shrinkage rate of about 1.22%, HDPE shrinkage rate of about 1.5%. HDPE shrinkage rate is around 1.5%.
Therefore, it is easy to deform and warpage and the cooling condition of the mold has a great influence on the shrinkage rate, so the temperature of the injection mold should be controlled well 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 parts.
The mold temperature is high, the melt cools slowly, the crystallization of melted plastic parts is high, and the strength is high.
PE’s melting point is not high, but the specific heat capacity is larger, so plasticizing still needs to consume more heat, so the plasticizing device is required to have a larger heating power to improve production efficiency.
PE’s softening temperature range is small, and the melt is easy to oxidation, so the molding process should be as far as possible to avoid contact between the melt and oxygen, so as not to reduce the quality of plastic parts.
PE parts are soft and easy to demould, so when the plastic parts have shallow side grooves, they can be strongly demoulded.
The non-Newtonian nature of PE melt is not obvious, the change of shear rate has less effect on viscosity, and the viscosity of PE melt is less affected by temperature. the cooling rate of PE melt is slow, so it must be fully cooled. The mold should have a better cooling system.
If PE melt is fed by direct inlet during injection, it should increase the stress and produce uneven shrinkage and directional increase in deformation obviously, so attention should be paid to the selection of inlet parameters.
The molding temperature of PE is wide, in the flow state, a little fluctuation of temperature does not affect injection molding. The thermal stability of PE is good, generally below 300 degrees without obvious decomposition phenomenon, which has little effect on the quality.
The main molding conditions of PE
Barrel temperature: The barrel temperature is mainly related to the density of PE and the size of the melt flow rate, in addition to the type and performance of the injection molding machine and the desired shape of the first-level plastic parts.
As PE is a crystalline polymer, the grains should absorb certain heat when melting, so the barrel temperature should be 10 degrees higher than its melting point.
For LDPE, the barrel temperature is controlled at 140-200℃, and for HDPE, the barrel temperature is controlled at 220℃, with the rear of the barrel taking the minimum value and the front taking the maximum value.
Mold temperature: Mold temperature has a greater impact on the crystallization of plastic parts, high mold temperature, high crystallinity of the melt, and high strength, but shrinkage will also increase.
Usually, the mold temperature of LDPE is controlled at 30℃-45℃, while the temperature of HDPE is 10-20℃ higher accordingly.
Injection pressure: Improving the injection pressure is conducive to the melt-filling mold, due to the good fluidity of PE, so in addition to thin-walled slender products, should be fine to choose a lower injection pressure, the general injection pressure of 50-100MPa. simple shape. After the wall of larger plastic parts, injection pressure can be lower, and vice versa is high.
3. Polyvinyl chloride (PVC) injection molding process
Typical applications: water supply pipes, household pipes, house wall panels, commercial machine housings, electronic product packaging, medical devices, food packaging, etc.
Chemical and physical properties: PVC material is a non-crystalline material. PVC material often adds stabilizers, lubricants, auxiliary process agents, colorants, impact agents, and other additives in actual use.
PVC material has non-flammability, high strength, chemical resistance to atmospheric changes, 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 and nitric acid and is not suitable for contact with aromatic and chlorinated hydrocarbons.
The melting temperature of PVC is a very important parameter in the processing and if this parameter is not appropriate it will lead to the decomposition of the plastic material.
The shrinkage rate of PVC is quite low, generally 0.2~0.6%.
Injection moulding process conditions.
Drying treatment: Usually, no drying treatment is required.
Melting temperature: 185~205℃ Mold temperature: 20~50℃.
Injection pressure: can be as large as 1500bar.
Holding pressure: can be as large as 1000bar.
Injection speed: To avoid excess material degradation, a fairly high injection speed is usually used.
Runners and gates: All conventional gates can be used. For small injection molded parts it is better to use a needle tip gate or a submerged gate; for thicker parts, it is better to use a fan gate.
The minimum diameter of the needle-tip gate or submerged gate should be 1mm; the thickness of the fan gate should not be less than 1mm.
Chemical and physical properties: Rigid PVC is one of the most widely used molten plastic materials.
4. Polystyrene (PS) injection molding process
Typical application areas: product packaging, household products (tableware, trays, etc.), electrical (transparent containers, light scatterers, insulating films, etc.).
Chemical and physical properties: Most commercial PS is a transparent, non-crystalline material. ps has very good geometric stability, thermal stability, optical transmission properties, electrical insulation properties, and a very slight tendency to absorb moisture.
It is resistant to water, and dilutes 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 rates range from 0.4 to 0.7%.
Injection molding process conditions.
Drying: Drying is not normally required unless improperly stored. If drying is required, the recommended drying conditions are 80°C and 2 to 3 hours.
Melting temperature: 180~280℃. For flame retardant materials the upper limit is 250℃.
Mold temperature: 40~50℃.
Injection pressure: 200~600bar.
Injection speed: It is recommended to use fast injection speed.
Runners and gates: All conventional types of gates can be used.
5. ABS injection molding process
Typical application areas: automobiles (instrument panels, tool hatches, wheel covers, reflector boxes, etc.), refrigerators, large strength tools (hair dryers, blenders, food processors, lawn mowers, etc.), telephone housings, typewriter keyboards, recreational vehicles such as golf trolleys and jet sleds, etc.
Chemical and physical properties: ABS is synthesized from three chemical monomers: acrylonitrile, butadiene, and styrene.
Each monomer has different properties: acrylonitrile has high strength and thermal and chemical stability; butadiene has toughness and impact resistance; and styrene has easy processing, a high finish, and high strength. Morphologically, ABS is a non-crystalline material.
The polymerization 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 largely on the ratio of the three monomers and the molecular structure in the two phases.
This allows for a great deal of flexibility in product mold design and has resulted in a hundred different qualities of ABS materials on the market.
These different qualities offer different properties, such as medium to high impact resistance, low to a high gloss, and high-temperature distortion properties.
ABS materials offer superb processability, appearance, low creep and excellent dimensional stability, and high impact strength.
Injection molding manufacturing process conditions.
Drying treatment: ABS material is hygroscopic and requires drying treatment before processing. The recommended drying condition is a minimum of 2 hours at 80~90°C. The material temperature should be guaranteed to be less than 0.1%.
Melting temperature: 210~280℃; recommended temperature: 245℃.
Mold temperature: 25~70℃. (Mold temperature will affect the finish of plastic parts, a lower temperature will lead to a lower finish).
Injection pressure: 500~1000bar.
Injection speed: medium to high speed.