Production technology of thermoformed plastic shee

2022-09-23
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With the introduction of domestic sheet production lines, the introduction of foreign capital and imported equipment, as well as the application of new raw and auxiliary materials, the thermoforming industry is showing a trend of vigorous development. Hot formed sheet processing technology is also diverse with different equipment, materials and uses. The increasingly mature sheet processing technology gradually breaks the old conventional theory. Now, even in the same sheet production line, qualified sheets with different uses, specifications and materials can be produced by different processes; Sheets of the same specification can also be processed with different equipment and processes. "No mode" brings great convenience to production. The production process of cast sheet refers to the sheet made by melting and plasticizing the resin in the extruder and extruding it through the slit die mouth, so that the melt clings to the cooling roller, and then stretching, trimming, coiling and other processes. The sheet produced by "tape casting" for processing plastic thermoformed packaging products is called "tape casting" thermoformed sheet At present, the processing situation of domestic plastic thermoforming sheets is roughly arranged as follows: "calendering" extrusion thermoforming sheets are mostly produced on domestic equipment, "calendering" extrusion thermoforming sheets are mostly processed on imported sheet production lines, "tape casting" extrusion thermoforming sheets are mostly processed on special tape casting sheet production lines produced in Taiwan. Now, with the progress of domestic sheet machining technology and the improvement of sheet production technology, qualified plastic hot formed sheets can also be processed by "calendering method" and "calendering method" in some domestic sheet production lines.

"tape casting method" hot formed sheets, "calendering method" hot formed sheets, "calendering method" hot formed sheets have essential differences in equipment, process, formula, performance, use, characteristics and so on. Based on our own practical experience, we talk about our own opinions on the technology of producing various plastic thermoformed sheets in the process of "extruder head cooling roll traction coiling" in China at present, which is only for reference

I. application and material selection of tape casting thermoforming sheet

the tape casting method can be used to process single-layer and multi-layer plastic thermoforming sheets of PP, PS, CPS, hips, degradation and other materials, or multi-layer plastic thermoforming sheets coextruded with EVOH, PA and other barrier materials

cast plastic thermoforming sheet is mainly used for the processing of thin, vacuum blister thermoforming products. It can also be used to process plastic cup covers, square or circular plastic cups with large stretching on positive pressure thermoforming machines. It can also be used to process various specifications of food trays and vegetable dishes on thermoforming machines with positive and negative pressure. For example, PP cast thermoforming sheet can be used to process jelly, pudding cup, disposable cup, plate, bowl, cover and packaging lining of frozen food. PS tape casting thermoforming sheet can be used to process the shell packaging of stationery, industrial parts and toys. The container made of PP/EVA/EVOH/EVA/PP hot-formed sheet by tape casting has the function of blocking the passage of oxygen, carbon dioxide and water vapor, and can prolong the service life of products. It can be used for the packaging of canned food and jelly pudding

the principle of selecting raw materials for the production of cast sheets is: food hygiene grade extruded sheet grade, drawing narrow band grade, thermoforming grade and other models

II. Station structure, configuration, function and precautions of air knife

the key to the production of plastic thermoforming sheet by "casting method" is to make the sheet closely adhere to the roller. Otherwise, the surface of the sheet will be uneven, the transparency will be reduced, and the secondary processing performance will be poor, which will affect the quality of the final plastic thermoforming products. Therefore, the general equipment is equipped with an air knife, which blows a certain air pressure and temperature from the narrow slot nozzle of the air knife, so that the sheet is close to the roller. Some of the air is supplied by air blowers and some by air compressors. Therefore, some people also call "air knife" as "wind knife". The air pressure blown from the air knife is adjustable, which varies slightly depending on the thickness, width, material, processing temperature, production speed, and the opening of the air knife nozzle, generally 0.3-0.7 MPa. The temperature of air flow can refer to the glazing roller temperature of "calendering method" and "calendering method". Since most equipment is not equipped with compressed air temperature control system, the temperature of general air flow depends on the ambient temperature. Another key role of air knife at about 20-30 ℃ is to accelerate the cooling of sheets and improve production efficiency. In the production of thermoformed sheets, the opening of air knife nozzle is usually 0.6-1.0 mm, and some can reach 2.0 mm. Some air knife devices also include two small air knives, which blow and press the edges of the sheet separately to prevent the edges from warping. In order to improve the effect of attaching the sheet to the roller, a vacuum pumping device is installed near the machine head to remove the air between the sheet and the roller and the hot gas generated by the molten embryo, so as to avoid the phenomenon of poor attachment between the sheet and the cooling roller due to bubbles

the production method without air knife also exists in the actual production. Due to the low production efficiency, the best parameters of the sheet attached roller are not easy to grasp, and are generally less used. In the way of air flow provided by the blower, special attention should be paid to the air inlet of the blower must be installed with an air filter device, and it should be cleaned at least once a week. In actual production, the charging pressure is often reduced, the cooling capacity is reduced, the thickness of the sheet is difficult to control stably, and the adhesion is poor due to the failure to clean on time; Or the damaged filter device is not replaced in time, and the dirty air pollutes the connecting pipe and the inner cavity of the air knife, causing quality problems such as black spots and pockmarks on the surface of the sheet, affecting the cooling effect of the sheet. If there is blockage at the air knife outlet, it is recommended to deal with it first: first open the compressed air valve or blower, scrape the air knife outlet with a 0.6-0.8 mm thick copper sheet, and open the air duct on one side to let foreign matters blow out from the air duct interface of the air knife. If the foreign matter is too tight or too large to scrape or blow out, you have to disassemble the movable pressing plate of the air knife nozzle

in the air flow provided by the air compressor, pay attention to the quality problems such as oil spots, small hard spots, and material spots on the surface of the sheet due to the unclean separation of oil and water, resulting in the humidity of the gas. Therefore, it is best to install oil-water filter (which needs to be discharged every day) or compressed air drying, cooling and other devices to improve the three basic configurations of intelligence: host, microcomputer and printer. The most common items tested by universal testing machine are tensile strength and tensile modulus, zigzag strength and modulus

III. The "modeless" process adjustment of tape casting thermoformed sheet

whether it is the temperature, the opening of the lip, the number of filtered layers, the temperature of the cooling roll water, the angle of the air knife, the size of the air pressure, the length of the air gap, the setting of the pressure level, or the proportion of additives in the formula, whether the subsequent coating process is similar to the calendering and calendering method, no fixed mode, no harsh requirements, According to the actual production situation, the degree of freedom is large

for example: temperature - from the actual production situation, even if hot formed sheets of the same material are processed on the same model sheet production line of the same manufacturer, it is common that the temperature settings of different manufacturers differ by 10-20 ℃

lip opening - from the actual normal production records, it is generally possible for normal production as long as it is not less than the thickness of the sheet. Some data report that the lip opening can even be smaller than the thickness of the sheet when considering the mold release expansion

air gap - because the air gap is long and easy to loosen after the experiment, the short one also plays a role in adjusting the width of the edge strip (actually has a slight impact on the thickness of the sheet). Generally, in order to stretch evenly, try to put the machine head close to the cooling roller, and the common distance is 50-200 mm

the clearance, angle, width and air flow between the air knife and the roller - these parameters have an impact on the sheet clinging to the roller, sheet cooling, sheet width and so on, which need to be carefully adjusted and constantly summarized. Generally, the air knife clearance is slightly larger than the opening of the lip, which is generally 2.0-3.0 mm. The width of the air knife should be freely adjustable to facilitate process adjustment. The size of the air flow can be adjusted by adjusting the size of the air inlet of the blower or the switch of the air inlet valve

roll temperature - some use cooling water, some use tap water, and some use circulating water. The roll temperature ranges from 20 ℃ to 70 ℃, which is related to production speed, head temperature, structure of cooling roll, sheet specification and other factors. In real life, some even no matter what material, the roll temperature remains unchanged, and normal production can be achieved by adjusting other process parameters

formula - the formula is generally simple and random. The key is to use the raw and auxiliary materials produced by regular manufacturers. The crushed or granulated materials should be pure and free from other impurities. In the production line of vacuum feeding, granular materials (such as granular masterbatch and transparent agent) should be used instead of powdery materials (such as pigment and transparent powder) in the formula as far as possible, so as to prevent unstable colors after the powdery materials are pumped away with the exhaust pipe, and the recovery rates of packaging paper and cultural paper should be increased by about 10% and 8% respectively

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