Polyethylene foam molding method

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There are many molding methods for polyethylene foam, mainly divided into extrusion, molding and expandable beads. Our country mainly uses extrusion and molding methods to produce polyethylene foam products. 1. Extrusion method The extrusion method is to extrude polyethylene molten material containing foaming agent from the die of the extruder head. When the molten material drops from high pressure to normal pressure, the gas dissolved in the molten material expands and foams. There are three forms of adding foaming agent:: ① Add thermal decomposition foaming agent to resin ； ② Inject low boiling point liquid foaming agent under pressure into the melting section in the middle of the extruder. ； ③ Expandable beads containing volatile foaming agents are prepared by dipping and then extruded to foam. The first is called chemical foaming, which is used in many low-foaming products, such as the preparation of electrical insulation layers of wires and cables. The third type requires the preparation of expandable beads before extrusion. The following focuses on the second foaming method, also known as the physical foaming method. 1. Specifications and formulas of raw materials The specifications and formulas of raw materials are shown in Table 6-4. Table 6-4 Specifications of raw materials and formulas. Raw material specifications formula/mass parts. Polyethylene foaming agent sodium bicarbonate citric acid. Melt flow rate 1g/10min, density 920kg/m3, melting temperature 114℃, softening temperature 90℃. Relative density 1.44, boiling point 36℃. Industrial pole. Industrial pole 100 10～20 0.1～0.3 0.1～0.3 2. Extrusion foaming process The extrusion foaming process is simple. Just inject a volatile foaming agent with a boiling point of -30～20℃ into the melting section in the middle of the extruder to extrusion foaming. Similar to extruded polystyrene foam, in order to make the bubbles fine and the structure uniform, a small amount of nucleating agent can be added, such as adding 0.01% to 1% organic acid, carbonate or acid carbonate, powdered silica, etc. The extrusion temperature is selected between 5°C lower than the melting temperature and the softening point. If the extrusion temperature is appropriately selected, a polyethylene foamed plastic with an expansion ratio of 20 times can be produced. 3. During the extrusion foaming process, the uniformly mixed raw materials enter the extruder from the hopper, are compacted by the screw rotation, and are melted and plasticized in the barrel. The foaming agent is injected into the molten material from the injection port in the middle of the extruder using a metering pump, and is completely mixed with the material through mechanical stirring. The molten material with uniform temperature is extruded from the mouth of the machine head, and is blown and foamed to obtain a polyethylene foam sheet with a beautiful pearl-like luster on the surface and a uniform internal cell structure. Some pigments can also be added to produce polyethylene foam sheets with various bright colors. In addition to sheets, polyethylene foam plastics produced by this method also include plates, tubes, rods, etc. 2. Molding method 1. The raw material generally uses low-density polyethylene plastic with a melt flow rate of about 2g/10min. Most use cumene peroxide as the chemical cross-linking agent, azodicarbonamide as the foaming agent, and tribasic lead sulfate as the activator. The recipe is as follows: Raw materials/mass parts Raw materials/mass parts Polyethylene 100 Dicumyl peroxide 1 Azodicarbonamide 20 Tribasic lead sulfate 4 2. Process flow The polyethylene molded foaming process flow is shown in Figure 6-2. Figure 6-2 Polyethylene molded foaming process 3. Operation process ① Ingredients. Weigh separately according to the formula and add ingredients in order. ② Mix. Mix the polyethylene resin in the mixer for 3 to 5 minutes, and control the temperature at about 110 to 120°C. After kneading the polyethylene into flakes, add azodicarbonamide and tribasic lead sulfate. At this time, the temperature drops to 70°C to 100°C, and kneads for another 10 minutes. Then adds dicumyl peroxide and kneads at the same temperature for 5 minutes to form flakes. ③ slice. Cut into pieces of required size according to the shape of the product to be processed. ④ Compression molding. Put the sheet material into the mold. At this time, deheat the mold to 160°C, pressurize it to 0.6MPa, and press for 12 to 15 minutes. ⑤ Mold foaming. There are 2 mold opening methods as follows. a. After the foaming agent is completely decomposed after the hot mold is opened, the pressure of the hydraulic press is released, so that the hot-melt sheet expands and pops up, and the foaming is completed within 2 to 3 minutes. The rapid expansion and foaming of molten materials is conducive to the formation of fine cells, but it cannot achieve a too high foaming ratio. It is difficult to achieve a balance between the gas pressure generated by the decomposition of foaming and the viscosity and elasticity of the material. Small resistance during foaming will cause the foam to crack, so special attention must be paid to controlling the elasticity of the melt to ensure normal production. b. Cold mold development: Cool the cross-linked foamed mold to about 65°C, open the mold, take out the foam block, and immediately send it to an oven at 120 to 170°C for heating for secondary foaming. ； The hot-pressed foam block can also be placed in a mold with a larger volume and heated and expanded for a second time. After cooling, the mold is opened to obtain a polyethylene foam product with a closed-cell structure and excellent mechanical strength. The foam sheet with high expansion ratio can be obtained by heating for secondary foaming under normal pressure. 3. Other molding methods 1. Extrusion-chemical cross-linking-chemical foaming process ⑴ Typical formula produces foam sheets with a density of 50kg/m3. Amount of raw materials/parts by mass Raw materials consumption/parts by mass Low-density polyethylene 100 Azodicarbonamide 10 Dicumyl peroxide 0.5 ⑵ Brief description of the production process Mix the chemical foaming agent, cross-linking agent and resin together evenly. During the extrusion molding process, heating first decomposes the cross-linking agent and cross-links the resin, and then decomposes the foaming agent to foam the resin to make foam products. 2. Extrusion-radiation cross-linking-chemical foaming process ⑴ Typical formula produces foam parts with a density of 50kg/m3. Amount of raw materials/Parts by mass Raw materials Amount/Parts by mass Polyethylene 50 Azodicarbonamide 10 ⑵ Brief description of the production process: Mix the polyethylene resin and the foaming agent evenly below the decomposition temperature of the foaming agent, then extrude the film or sheet at the same temperature, and then irradiate it with rays measuring 1 to 200Gy to cross-link it. Then heat it to above the decomposition temperature of the foaming agent to foam the resin to produce foam products. 3. Chemical foaming-chemical cross-linking-one-step molding process ⑴ Typical formula produces foam parts with a density of 200kg/m3. Amount of raw materials/parts by mass Raw materials consumption/parts by mass High density polyethylene 50 Calcium sulfite 60 Medium density polyethylene 50 Dicumyl peroxide 0.1 EVA copolymer 20 Azodicarbonamide 0.5 ⑵ Brief description of the production process: First, mix the foaming agent, cross-linking agent, filler and resin evenly, and then put them into pieces, granules or plates into the mold, pressurize (usually 5-21MPa), and heat to decompose the cross-linking agent. After the resin reaches a certain cross-linking degree, further increase the temperature to decompose the foaming agent. After complete decomposition, the pressure is suddenly released, the molten material pops out, and foaming is completed in 2 to 3 seconds. This production method requires less investment in equipment and can produce materials 3 to 15 times. 4. Chemical foaming-chemical cross-linking-two-step molding process ⑴ Typical formula raw material dosage/mass part raw material dosage/mass part low density polyethylene 100 dicumyl peroxide 0.8~1 light calcium carbonate 100~120 tribasic lead sulfate 1.5 azodicarbonamide 7.5~8 zinc stearate 2~2.5 ⑵ Brief description of the production process: After the foaming agent, cross-linking agent, filler and resin are mixed evenly, they are rolled into thin sheets and placed in a mold, pressurized (usually the pressure is 2MPa), and heated (140°C) to make the resin reach a certain degree of cross-linking and partially decompose the foaming agent. The pressure is suddenly released to cause the material to pop out to produce a pre-foamed slab. Put this slab into a predetermined mold while it is hot, pressurize (initial pressure 0.5MPa, final pressure 1MPa), and heat (160-180°C) to completely decompose the foaming agent, and the resin expands to fill the mold cavity. After cooling and demoulding, a high-foamed calcium plastic sheet is made. 5. Expandable polyethylene bead technology ⑴ Typical formula raw material amount/mass part raw material amount/mass part low density polyethylene 100 dicumyl peroxide 0.4~0.6 azodicarbonamide 12~17 zinc oxide 2~4 ⑵ Brief description of the production process First, mix each component evenly below the decomposition temperature of the cross-linking agent (140°C), calender into sheets, and cut into square particles. The particles are then extruded through an extruder and cut into hemispherical particles in hot water of 85 to 95°C. The particles are then put into a porous mold, and the resin is cross-linked under a pressure of 0.3 to 0.5 MPa. The foaming agent is decomposed under a pressure of 0.5 to 1.0 MPa. The resin foams and fills the mold cavity. It is cooled and demoulded to make foam products. This post was last edited by QXZ-1966 on 2009-2-28 15:04 ]