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Why Do We Need to Perform the Pelletizing Process Before Plastic Injection?
Common plastic pellet making process
With the development of industry, all kinds of plastic products have entered thousands of households, and with it, plastic waste has also increased. All kinds of plastic packaging, plastic containers, plastic toys and stationery, plastic shoes, plastic appliance shells, vehicle bumpers, plastic pipes, industrial waste, etc. can be seen everywhere, causing great environmental pollution. The recycling of waste plastics is conducive to environmental protection and resource-saving. They can be recycled and pelletized, or pelletized after modification, and can be reused to produce plastic products.
The product quality will be higher after granulation, for the following reasons:
1. The particles and granular particles are uniforms, the crushed particles are not uniform, the material is unstable, the gap size and the melt density are different, and some will increase the difficulty of adjustment;
2. From the perspective of material impurities, when granulating materials are granulated, some impurities are difficult to enter the material after filtration, which is relatively clean, and it is not easy to crush the materials;
3. From the perspective of color, the color after granulation is relatively uniform, and the color difference or uneven dispersion often occurs with the toner or master batch. It is difficult to clean the hopper or barrel, and the granulation is easier to clean;
4. In terms of material quality, some plasticizers, diffusing agents, grafting agents, etc. are often added during granulation to ensure the quality of the materials;
5. Considering the aspect of material modification, when several materials are grafted in a small amount, the twin-screw stirring and mixing will be more uniform and the production will be more stable.
The difference between pelletizing and injection molding: these are two different processes. Pelleting is a process where some waste plastics are melted and extruded and cut into pellets using a cutting machine. During the granulation process, some additives can be added to improve the performance of waste plastics. Such as toughening agents. Injection molding is to form plastic products by melting plastic materials and then hitting the mold with pressure. Injection molding is made of plastic products, while granulation was made of raw materials!
Waste plastic granulation process and precautions
1. The characteristics of waste plastics
Waste plastics can be divided into three types according to the occasions where they are generated:
A. The corner waste generated in the production process is relatively clean, less polluted and contains impurities, such as substandard film, trimming in film production, leading part or unqualified product in pipe and profile production, injection Under filled parts in production, etc.;
B. Used plastic waste with a single material system, such as disassembled pipes, doors, and windows, packaging materials that have been strictly sorted according to resin types, or other waste plastic products;
C. Mixed waste plastics that are difficult to distinguish or cannot be separated at all, such as multilayer co-extruded composite films, coated plastic products, composite products of plastics and other materials, etc.
Different types of waste plastics have different characteristics. As far as the impurity content is concerned, the impurity content of the corner waste in the factory production is less than 0.1%, and the impurity content of the leftover material and other used products stacked for a certain period is 0.1%. ~0.5%. The impurity content of composite waste plastics mixed with aluminum, cloth, and paper is often greater than 10%.
For used waste plastic products, depending on the conditions of use, it will contain the effects of ultraviolet radiation, heat and oxygen aging, and the effects of pollutants. Different shapes of waste plastics have great differences in the bulk density of materials after crushing. The bulk density of the crushed materials of film, sheet, and flat wire is relatively small, which must be considered during the feeding process of waste plastic recycling and granulation. problem.
2. Pretreatment of waste plastics
Waste plastics from waste packaging, such as packaging bags, shopping bags, bottles, cans, boxes, and waste agricultural films, must be pretreated before pelletizing. The pretreatment process mainly includes classification, cleaning, crushing, and drying.
The work of classification is to classify various types of waste plastic products according to the types of raw materials and product shapes. Sorting according to the types of raw materials requires operators to have skilled knowledge of identifying plastic varieties. The purpose of sorting is to avoid the incompatibility of recycled materials and poor performance due to the mixing of different types of polymers; the classification according to the shape of the product is to facilitate waste plastics The crushing process can be carried out smoothly because the crushing equipment used for film, flat yarn and its fabric and some thick-walled, hard products are often unable to replace each other.
There are three processes for cleaning and crushing before granulation.
A. Wash first and then crush large-scale waste plastic products that do not have serious pollution and uncomplicated structure, such as car bumpers, dashboards, turnover boxes, plates, etc., should be cleaned and then crushed. First, immerse in water with detergent, then rinse with clean water, take it out and air dry. Large parts that cannot be put into the hopper of the crusher due to their large size should be coarsely crushed and then finely crushed to prepare them for feeding by the extrusion granulator. To ensure the quality of recycled pellets, drying should be carried out after finely crushing. Rotary dryers with heating interlayers are often used. Super heated steam is passed into the interlayers, which rotates while being heated, and the drying efficiency is high.
B. Rough washing-crushing-fine washing-drying process
For contaminated profiles, waste agricultural films, and packaging bags, rough washing should be carried out first to remove foreign objects such as sand, stones, and metals to prevent damage to the crusher. The waste plastic products are centrifuged and dehydrated after rough washing, and then sent to the crusher for crushing. After being broken, it is washed further to remove the sundries hidden in it. If the waste plastic contains greasy dirt, it can be soaked in a proper amount of alkaline water or warm washing liquid, and then stirred to cause friction and collision between the waste plastic blocks to remove the dirt. After rinsing, it is dehydrated and dried.
C. Mechanized cleaning
Before the waste plastic enters the cleaning equipment, it is crushed in a dry or wet crushing equipment. After drying, it is blown into a storage bin, and then the crushed material is quantitatively input into the cleaning tank by the screw feeder. Two counter-rotating blade shafts slowly convey the material through the cleaning tank, and the generated vortex rinses away the dirt on the plastic. The dirt sinks to the bottom of the cleaning tank and is removed at the bottom of the tank at specified time intervals. The cleaned waste material floats and is discharged by the screw conveyor. Most of the water is removed. The screw feeder quantitatively feeds the crushed material into the drying system. The drying system consists of a rotary dryer and a hot air dryer. The residual moisture of the material output from the drying system accounts for 1% to 2%. The cleaned material is sent to the storage bin and then sent from the storage bin to the extrusion granulation to produce pellets.
3. Extrusion granulation process of waste plastics
The performance of waste plastics is different from that of new resins. This is because they have undergone the thermal history and shearing history of the molding process, and have experienced the effects of heat, oxygen, light, climate, and various media during use. The mechanical properties of recycled materials, including tensile strength and impact properties, are lower than that of the original resin. The surface structure changes caused by cracks, the appearance quality is not as good as before, the color is yellow, and the transparency is reduced.
The performance changes of various materials are different. The change of polyolefin material is relatively small. Due to processing, especially the reduction of relative molecular weight caused by multiple processing, it can be compensated by a cross-linking reaction. Therefore, the processability can be kept constant to a certain extent. The situation of styrene co-polymer is different. The tensile properties are reduced once. After about four processing steps, the toughness decreases very seriously. Moreover, the effectiveness of the rubber phase impact modifier is also reduced due to cross linking. Although it is high-impact polystyrene, its impact toughness is not better than that of general-purpose polystyrene.
The performance of waste plastics can be improved by blending new materials or adding specific stabilizers and additives, such as adding antioxidants and heat stabilizers, which can reduce the adverse effects of heat and oxygen during the pelletizing process of waste plastics. In some mixed waste plastics, you can also add a compatibilizer appropriately, such as adding EPDM or EVA to the mixed waste plastics of polyethylene and polypropylene. Filling modification can also be carried out in waste plastic recycling and granulation, such as adding 10% to 35% of a filler, 3% to 6% of lubricant, and 2% to 4% of master batch in the waste PP film. The filler is a recycled material made of CaCO3 for injection products, which can effectively shorten the molding cycle, improve the rigidity of the product, increase the thermal deformation temperature, and reduce the shrinkage rate.
The lubricant improves the fluidity of the melt. Some engineering plastics can also be filled, reinforced, and alloyed in recycling. For some materials that are easy to absorb moisture, such as PA, PET, etc., during processing, moisture will cause degradation, reduce relative molecular weight, lower melt viscosity, and lower physical properties. Before processing, the water in the waste plastic should be removed and fully dried to ensure the quality of the recycled material.
The product quality will be higher after granulation, for the following reasons:
1. The particles and granular particles are uniforms, the crushed particles are not uniform, the material is unstable, the gap size and the melt density are different, and some will increase the difficulty of adjustment;
2. From the perspective of material impurities, when granulating materials are granulated, some impurities are difficult to enter the material after filtration, which is relatively clean, and it is not easy to crush the materials;
3. From the perspective of color, the color after granulation is relatively uniform, and the color difference or uneven dispersion often occurs with the toner or master batch. It is difficult to clean the hopper or barrel, and the granulation is easier to clean;
4. In terms of material quality, some plasticizers, diffusing agents, grafting agents, etc. are often added during granulation to ensure the quality of the materials;
5. Considering the aspect of material modification, when several materials are grafted in a small amount, the twin-screw stirring and mixing will be more uniform and the production will be more stable.
The difference between pelletizing and injection molding: these are two different processes. Pelleting is a process where some waste plastics are melted and extruded and cut into pellets using a cutting machine. During the granulation process, some additives can be added to improve the performance of waste plastics. Such as toughening agents. Injection molding is to form plastic products by melting plastic materials and then hitting the mold with pressure. Injection molding is made of plastic products, while granulation was made of raw materials!
Waste plastic granulation process and precautions
1. The characteristics of waste plastics
Waste plastics can be divided into three types according to the occasions where they are generated:
A. The corner waste generated in the production process is relatively clean, less polluted and contains impurities, such as substandard film, trimming in film production, leading part or unqualified product in pipe and profile production, injection Under filled parts in production, etc.;
B. Used plastic waste with a single material system, such as disassembled pipes, doors, and windows, packaging materials that have been strictly sorted according to resin types, or other waste plastic products;
C. Mixed waste plastics that are difficult to distinguish or cannot be separated at all, such as multilayer co-extruded composite films, coated plastic products, composite products of plastics and other materials, etc.
Different types of waste plastics have different characteristics. As far as the impurity content is concerned, the impurity content of the corner waste in the factory production is less than 0.1%, and the impurity content of the leftover material and other used products stacked for a certain period is 0.1%. ~0.5%. The impurity content of composite waste plastics mixed with aluminum, cloth, and paper is often greater than 10%.
For used waste plastic products, depending on the conditions of use, it will contain the effects of ultraviolet radiation, heat and oxygen aging, and the effects of pollutants. Different shapes of waste plastics have great differences in the bulk density of materials after crushing. The bulk density of the crushed materials of film, sheet, and flat wire is relatively small, which must be considered during the feeding process of waste plastic recycling and granulation. problem.
2. Pretreatment of waste plastics
Waste plastics from waste packaging, such as packaging bags, shopping bags, bottles, cans, boxes, and waste agricultural films, must be pretreated before pelletizing. The pretreatment process mainly includes classification, cleaning, crushing, and drying.
The work of classification is to classify various types of waste plastic products according to the types of raw materials and product shapes. Sorting according to the types of raw materials requires operators to have skilled knowledge of identifying plastic varieties. The purpose of sorting is to avoid the incompatibility of recycled materials and poor performance due to the mixing of different types of polymers; the classification according to the shape of the product is to facilitate waste plastics The crushing process can be carried out smoothly because the crushing equipment used for film, flat yarn and its fabric and some thick-walled, hard products are often unable to replace each other.
There are three processes for cleaning and crushing before granulation.
A. Wash first and then crush large-scale waste plastic products that do not have serious pollution and uncomplicated structure, such as car bumpers, dashboards, turnover boxes, plates, etc., should be cleaned and then crushed. First, immerse in water with detergent, then rinse with clean water, take it out and air dry. Large parts that cannot be put into the hopper of the crusher due to their large size should be coarsely crushed and then finely crushed to prepare them for feeding by the extrusion granulator. To ensure the quality of recycled pellets, drying should be carried out after finely crushing. Rotary dryers with heating interlayers are often used. Super heated steam is passed into the interlayers, which rotates while being heated, and the drying efficiency is high.
B. Rough washing-crushing-fine washing-drying process
For contaminated profiles, waste agricultural films, and packaging bags, rough washing should be carried out first to remove foreign objects such as sand, stones, and metals to prevent damage to the crusher. The waste plastic products are centrifuged and dehydrated after rough washing, and then sent to the crusher for crushing. After being broken, it is washed further to remove the sundries hidden in it. If the waste plastic contains greasy dirt, it can be soaked in a proper amount of alkaline water or warm washing liquid, and then stirred to cause friction and collision between the waste plastic blocks to remove the dirt. After rinsing, it is dehydrated and dried.
C. Mechanized cleaning
Before the waste plastic enters the cleaning equipment, it is crushed in a dry or wet crushing equipment. After drying, it is blown into a storage bin, and then the crushed material is quantitatively input into the cleaning tank by the screw feeder. Two counter-rotating blade shafts slowly convey the material through the cleaning tank, and the generated vortex rinses away the dirt on the plastic. The dirt sinks to the bottom of the cleaning tank and is removed at the bottom of the tank at specified time intervals. The cleaned waste material floats and is discharged by the screw conveyor. Most of the water is removed. The screw feeder quantitatively feeds the crushed material into the drying system. The drying system consists of a rotary dryer and a hot air dryer. The residual moisture of the material output from the drying system accounts for 1% to 2%. The cleaned material is sent to the storage bin and then sent from the storage bin to the extrusion granulation to produce pellets.
3. Extrusion granulation process of waste plastics
The performance of waste plastics is different from that of new resins. This is because they have undergone the thermal history and shearing history of the molding process, and have experienced the effects of heat, oxygen, light, climate, and various media during use. The mechanical properties of recycled materials, including tensile strength and impact properties, are lower than that of the original resin. The surface structure changes caused by cracks, the appearance quality is not as good as before, the color is yellow, and the transparency is reduced.
The performance changes of various materials are different. The change of polyolefin material is relatively small. Due to processing, especially the reduction of relative molecular weight caused by multiple processing, it can be compensated by a cross-linking reaction. Therefore, the processability can be kept constant to a certain extent. The situation of styrene co-polymer is different. The tensile properties are reduced once. After about four processing steps, the toughness decreases very seriously. Moreover, the effectiveness of the rubber phase impact modifier is also reduced due to cross linking. Although it is high-impact polystyrene, its impact toughness is not better than that of general-purpose polystyrene.
The performance of waste plastics can be improved by blending new materials or adding specific stabilizers and additives, such as adding antioxidants and heat stabilizers, which can reduce the adverse effects of heat and oxygen during the pelletizing process of waste plastics. In some mixed waste plastics, you can also add a compatibilizer appropriately, such as adding EPDM or EVA to the mixed waste plastics of polyethylene and polypropylene. Filling modification can also be carried out in waste plastic recycling and granulation, such as adding 10% to 35% of a filler, 3% to 6% of lubricant, and 2% to 4% of master batch in the waste PP film. The filler is a recycled material made of CaCO3 for injection products, which can effectively shorten the molding cycle, improve the rigidity of the product, increase the thermal deformation temperature, and reduce the shrinkage rate.
The lubricant improves the fluidity of the melt. Some engineering plastics can also be filled, reinforced, and alloyed in recycling. For some materials that are easy to absorb moisture, such as PA, PET, etc., during processing, moisture will cause degradation, reduce relative molecular weight, lower melt viscosity, and lower physical properties. Before processing, the water in the waste plastic should be removed and fully dried to ensure the quality of the recycled material.
Reference:kknews
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