About Plastic Sheets
The methods of producing board and sheets include extrusion, calendering, lamination, etc. Extrusion is the simplest forming method. Sheets and board with a thickness of 0.25-20mm can be produced by extrusion molding. According to the thickness of the product: below 0.25mm is called film, 0.25-1mm is called sheet, and above 1mm is called board, but there is no obvious boundary.
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Application of plastic sheet
The main plastic raw materials for extruding sheet orboard are PVC, PE, PP, ABS, HIPS, PC, etc. The varieties of sheets and sheets include single layer and multi-layer, flat sheet and corrugated plate, foaming and non-foaming, single material and dissimilar material composite. with sheet。
Plastic sheets and boards are widely used and can be used to make containers, storage tanks, backing plates, and electrical insulating materials. Decorative panels, anti-skid pads, etc., which are widely used in vehicles such as automobiles, use non-toxic plastic sheets as packaging materials in the food industry and the pharmaceutical industry. Plastic plates and sheets are also widely used in industry, agriculture, commerce, construction and other industries.
Forming Equipment For Plastic Sheet Extrusion Line
The screw diameter of the single-screw extruder used for extruding plates or sheets is generally 65-200mm, and the length-diameter ratio L/D≥20. It is necessary to set a filter plate or filter screen at the head of the extruder screw. The pore size and number of holes, the mesh number and the number of layers of the filter should be determined according to the variety of raw materials and the thickness of the product. Non-vented single-screw extruder is suitable for extrusion molding of thermoplastics such as PE, LDPE, PP, PVC, etc. The vented single-screw extruder is suitable for extrusion molding of thermoplastics such as PS, PMMA, and PC.
The twin-screw extruder used for extruding plates and sheets is divided into two types: parallel twin-screw extruder and conical twin-screw extruder. The screw diameter of the parallel twin-screw extruder is 80-140mm, and the length-diameter ratio L/D≤21, the diameter of the small end of the conical twin-screw extruder is 35-80mm, and the twin-screw extruder is suitable for extrusion molding of PVC sheets and sheets.
A connector is generally used to connect the extruder and the head. The shape of the connector is mostly cylindrical, and the internal flow channel gradually transitions from a conical shape to a rectangle. The function of the connector is to compress the material evenly and transport it into the head .
The tube film head is a thin-walled tube head. The extruded tube film is cut in parallel with a knife and flattened to obtain a plate or sheet. This type of machine head is suitable for thin plate production. The advantages are that the thickness of the plate is uniform, the structure of the machine head is simple, and it is easy to process; Auxiliary devices, increased equipment investment.
At present, the heads for the production of plates or sheets are mainly flat heads. This type of head can produce plates (sheets) of various thicknesses and widths: the key to the design of a flat head is to make the material flow rate equal across the entire width of the head, so as to obtain plates (sheets) with uniform thickness and smooth surface. )material. According to the flow channel structure, it can be divided into branch pipe type head, fish tail type head, hanger type head and distribution screw type head.
1.The branch pipe type head is characterized by a cylindrical (tubular) groove parallel to the die lip in the head, which can store a certain amount of material, distribute the material and stabilize the pressure, so that the material flow is stable and round. The larger the diameter of the cylindrical groove, the more material can be stored. The more stable and uniform the material flow is. The resistance adjustment block can adjust the material flow rate and make the material outlet uniform. The branch pipe type head is the flat head with the simplest structure. The advantages are simple structure, small size of the head, and convenient operation; Plastic sheets, such as rigid PVC sheets (sheets), especially transparent RPVC sheets.
2.In order to obtain a plate with a smooth and flat surface, the surface roughness Ra of the die lip is not less than 0.8 μm and chrome plated. The branch pipe head is suitable for extrusion molding of SPVC, PE, PP, ABS plastic, PS and other boards and sheets
3.The branch pipe head has the form of single branch pipe head, double branch pipe head and so on.
Fishtail Head It is named after the inner flow channel of the head is shaped like a fishtail. The plastic melt is fed from the middle of the machine head, and is branched to both sides along the fishtail-shaped flow channel to reach the required width at the die. The advantages of this machine head are that the flow channel is smooth without dead angle, and there is no material stop part of the branch-type head. The structure is simple and the manufacture is easy. It is suitable for processing materials with high melt viscosity and poor thermal stability, such as RPVC and POM. and other heat-sensitive plastics. The disadvantage is that it is not suitable for the production of plates and sheets with larger widths and thicker thicknesses. Usually, the width of the board and sheet produced by the fishtail head is less than 500mm, and the thickness is generally not more than 3mm.
The hanger-type machine head is named after the inner flow channel of the machine head is shaped like a hanger. The structure is shown in Figure 6-5. The hanger type head combines the advantages of the branch pipe type and the fishtail type head. It adopts the branch pipe type cylindrical groove, which can stabilize the material, but reduces the cross-sectional area of the cylindrical groove and reduces the For the residence time of the material, the fan-shaped flow channel of the fishtail type head is used to make up for the disadvantage of uneven thickness of the plate. The expansion angle of the flow channel is larger than that of the fishtail type head, which reduces the size of the head and can produce more than 2m. It can form a variety of thermoplastic plates and sheets well, and it is the most widely used extruder head at present. The disadvantage is that the structure is complicated and the price is high.
1.The distribution screw head is equivalent to a flat head with a screw installed in the branch pipe of the branch pipe head. The screw is driven by a separate motor, so that the material does not stagnate in the branch pipe and distributes the material evenly. Over the entire width of the machine head, by changing the screw speed, the thickness of the sheet can be adjusted, and the uneven extrusion of the sheet can also be adjusted by the die lip.
2.The connection method between the distribution screw and the extruder has one end feeding type and center feeding type.
3.In order to ensure the continuous extrusion of the sheet, the extrusion volume of the main screw should be greater than the extrusion volume of the distribution screw, and the diameter of the distribution screw should be smaller than the diameter of the main screw. The distribution screw is generally multi-thread, and the number of threads is 4~6. The reason is that the extrusion volume of the multi-thread is large, which can reduce the residence time of the material in the machine head.
4.The advantage of the distribution screw head is that it reduces the residence time of the material in the head, which makes the extrusion of PVC sheets with poor flow and thermal stability easier. Obvious difference, long continuous production time, easier to change varieties and colors. The main disadvantage is that the material moves in a circular motion with the screw and suddenly changes into a linear motion, and wavy marks are prone to appear on the product; the structure of the machine head is more complicated, the manufacturing is more difficult, and the price is high.
1.It is supplied by two or more extruders, so that the extruded material is bonded into a composite plate or sheet immediately after being in the mold or out of the mold.
2.The feature of the co-extrusion sheet head is that two different colors or two different types of material flow through the A and B flow channels into the diversion area, the blocking area, and the retention area, respectively, into the die lip, and the pressure is used before the die lip is extruded. The two strands of material are bonded together to form a two-color (or double-layer) composite sheet, and through the combination of different materials (or colors), the characteristics of each material can be exerted to achieve the comprehensive effect of making full use of each material.
The molten material is extruded from the die head and immediately enters the three-roll calender, where it is calendered and gradually cooled. The three-roll calender can also play a fixed traction role, adjust the speed of the plate and the sheet at each point, and ensure that the sheet is straight. The three-roll calender is generally composed of upper, middle and lower rolls with a diameter of 200~400mm. The shaft of the middle roller is fixed, and the bearings of the upper and lower rollers move up and down with the help of the roller distance adjustment device, and the roller gap between the upper, middle and lower rollers is finely adjusted to adapt to plates and sheets of different thicknesses. All three rolls are hollow and jacketed for easy access to media for temperature control. The roll length should be slightly wider than the die width to allow the die lip of the die to be close to the nip. The surface of the roller should be chrome-plated, and the surface roughness R. is 0.20μm. In order to adapt to the change of the base output of the extruder and the different gaps of the machine head, the peripheral speed of the matt machine should generally have a large adjustment range, mostly 1 : about 2, the maximum peripheral speed is 2-8m/min.
4.Cooling and conveying device
The cooling and conveying device is located between the three-roll calender and the pulling device. It consists of more than a dozen round rollers with a diameter of about 50mm. The coming out sheet is cooled sufficiently to make it fully solidified and transported to the pulling device. The total length of the entire conveying cooling device depends on the thickness of the sheet and the specific heat capacity of the plastic. Generally, the total length of the cooling and conveying device for PVC and ABS plastic sheets is 3~6m, and the length of polyolefin is 4~8m.
The traction device is generally composed of one or two pairs of traction rollers, each pair of traction rollers is composed of an active steel roller and a passive steel roller covered with rubber. The two rollers are pressed by springs, and their function is to evenly pull the plate to the cutting device, prevent the accumulation of material at the calendering roller, and flatten the plate and the sheet. The pulling rate should be in sync with the calender roll rate or slightly less to account for the shrinkage of the plate and sheet. The speed of the traction roller should be able to realize stepless speed regulation, and the gap between the upper and lower rollers can also be adjusted.
6.Cutting and coiling device
The cutting of plates and sheets includes trimming and cutting. Disc cutters are used for edge trimming, and electric heat cutting, sawing and shearing are used for truncation. The latter two are used more, the sawing structure is simple, the power consumption is small, but the noise is large, and the sawdust is flying, and there are burrs at the cutting place, and the efficiency is low; the cutting method is not easy to produce burrs, and the cutting speed is fast. High efficiency, no noise and sawdust, good labor conditions for workers, but the equipment is large and heavy. Sawing and shearing can be used for both soft and hard boards. After the soft (plate) sheet is cooled by the conveying roller, it can be rolled into a cylindrical shape immediately, and then cut off and packaged.
The Molding Process Of Plastic Sheet Extrusion
Barrel and head temperature
The barrel temperature of the extruder should be determined according to the processed plastic raw materials, the characteristics of the extruder, and the form of the head. The temperature of the machine head is generally about 5~10℃ higher than that of the barrel. If the temperature of the machine head is too low, the surface of the plate will be dull and easy to crack; if the temperature of the machine head is too high, the material will be discolored, decomposed, and there will be bubbles in the product. The temperature of the machine head is generally controlled to be low in the middle and high on both sides. The temperature of the machine head is an important factor affecting the thickness uniformity of the board (sheet) material. The temperature fluctuation of each point of the machine head should be strictly controlled to prevent the temperature error from affecting the thickness uniformity of the board and sheet.
Three-roll calender temperature
The three-roll calender is a device for cooling, calendering and determining the thickness of the plate (sheet), and the process conditions directly affect the appearance quality of the plate. The temperature of the sheet extruded from the die head is high. In order to cool the sheet slowly and prevent the sheet from warping due to internal stress, the three rolls of the three-roll calender should be heated and a temperature adjustment device should be installed. If the temperature of the roller is too high, it will be difficult for the plate and the sheet to come off the roller, and horizontal stripes will appear on the surface; if the temperature of the roller is too low, the plate will not be easy to stick to the surface of the roller, and the surface of the plate will be prone to spots and dullness. The roll temperature should be high enough that the molten material and roll surface are in complete contact. In general control, the temperature of the roller is the highest, the temperature of the upper roller is slightly lower, and the temperature of the lower roller is the lowest. The three-roll temperature for forming several common plastic plates and sheets is listed in the table below for reference.
The three-roll temperature for forming several common plastic plates and sheets is listed in the table below for reference
|Calender temperature (upper roll)/°C||70~80||70~80||75~80||90~100||75~85||70~80||90~100||120~135||80~90|
|Calender temperature (middle roll)/°C||80~90||80~90||75~85||80~90||60~80||70~90||80~90||130~140||90~100|
|Calender temperature (lower roll)/°C||60~70||60~70||60~65||70~80||50~65||55~60||70~80||140~150||70~80|
2. Screw cooling
There are two main purposes of screw cooling: one is to facilitate the conveying of materials in the feeding section, and the gas (including volatiles) contained in the materials can overflow from the feeding hopper; the other is to control the quality of the products and prevent the materials from being decomposed due to local overheating . When the homogenization section of the screw is also cooled, a layer of molten material with lower temperature may be formed at the bottom of the screw groove of this section. It becomes “shallow”, so that the plasticizing effect is improved and the extrusion volume is decreased.
In the cooling system of the screw, the cooling medium passed into the screw is generally water or air. When cooling the screw, the feeding seat is generally cooled to prevent the temperature of the feeding port from being too high and affecting the feeding.
3. The relationship between sheet thickness, die lip thickness and three-roller spacing
Sheet thickness and die lip gap
When forming a sheet and a sheet, the die lip gap is generally equal to or slightly smaller than the thickness of the sheet or sheet. After the material is extruded, it expands and reaches the required thickness of the sheet or sheet through traction. In addition to adjusting the die temperature, the thickness and uniformity of the plate can also be adjusted by adjusting the resistance block of the die and changing the resistance at various places in the width direction of the die, thereby changing the flow rate and the thickness of the plate. The die lip gap can be adjusted by fine-tuning the thickness of the plate. When the thickness adjustment range is large, the resistance adjustment block should be adjusted. In order to obtain a plate with a uniform thickness, the die lip gap can be adjusted to be smaller in the middle and larger on both sides.
The length of the die lip flow channel of the die is related to the thickness of the sheet, generally 20 to 30 times the thickness of the sheet. Please refer to the table below.
|Sheet or Sheet Thickness||Die lip runner length|
Sheet thickness and three-roller spacing
The spacing of the three rollers is generally adjusted to be equal to or slightly larger than the thickness of the sheet, mainly considering the thermal shrinkage of the material. The spacing of the three rollers should be adjusted to be consistent along the width of the sheet. There is still a certain amount of material storage between the three-roller spacing, otherwise when the machine head discharges unevenly, there will be material shortages, large spots, etc., and it is not easy to store too much material. The material is brought into the plate to form “bone”-like stripes.
The thickness of the sheet can also be adjusted by the rotation speed of the three-roll calender. The stretching ratio of the sheet should not be too large, otherwise the sheet will be unidirectionally oriented, the longitudinal tensile properties will be improved, and the transverse direction will be reduced, resulting in the anisotropy of the sheet and affecting the sheet’s quality. Quality, the speed of the three rolls is generally controlled to suit the extrusion rate, which is slightly faster by 10%~25%.
4. Traction rate
The purpose of pulling is to keep the sheet in a “strained” state after it comes out of the cooling roll and is continuously cooled until it is cut. If there is no tension during cooling, the plate will deform; if there is no tension during cutting, the cutting will not be neat, and the traction tension is closely related to the performance of the plate. If the tension is too large, the plate will be cold stretched, and the plate will generate internal stress, which will affect the performance; if the tension is too small, the plate will be deformed and uneven because the plate has not been fully cooled. The pulling rate is basically equal to the extrusion rate, which is 5%~10% faster than the linear speed of the calender.
What Auxiliary Machines You Need For Your Extrusions?
The mixer has a hot and cool pot inside. The hot pot is used to pre-heat the raw materials and mix the raw materials evenly. The cool pot is used to cool down the temperature of raw materials.
It is used to recycle waste plastic profiles for crushing to reduce waste, and the recycled materials can be mixed into raw materials to make new profiles.
It is used in the grinding and processing of various plastic materials in the fields of chemical industry, waste recycling, plastic pipe profile manufacturing, plastic modification and other fields.
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