First, the basic principle of plastic extrusion
The plastics processing industry is a comprehensive and technical industry. It involves polymer chemistry, polymer physics, interface theory, plastic machinery, plastic processing dies, formulation design principles and process control. The extrusion theory mainly studies the movement and variation of plastic in the extruder. The relationship between the three physical states of the polymer in the extruder under different external pressures, the screw structure, the plastic properties, and the processing conditions under the action of a certain external force. Thereby reasonable process control. In order to achieve the purpose of improving the output and quality of plastic products. When the plastic polymer material is heated under a constant pressure, three physical states of glass state, high elastic state and viscous flow state appear in different temperature ranges. Generally, the molding temperature of plastic is above the viscous flow temperature.
Second, polyolefin pipe extrusion molding process control
The control parameters of the extrusion molding process include molding temperature, extruder working pressure, screw rotation speed, extrusion speed and traction speed, feeding speed, cooling setting and the like.
1. Pretreatment of raw materials
Polyolefins are non-absorbent materials and generally have a low moisture content to meet the needs of extrusion, but are sensitive to moisture when polyolefins contain water-absorbing pigments such as carbon black. In addition, the water content increases when the return material and the filler are used. Moisture not only causes the inner and outer surfaces of the pipe to be rough, but may also cause bubbles in the melt. The raw materials should usually be pretreated. Drying treatment is generally used, and a corresponding auxiliary agent having a dehumidifying function can also be added. Such as defoamer and so on. The dry temperature of PE is generally 60-90 degrees. At this temperature, the yield can be increased by 10% - 25%.
2. Temperature control
The extrusion temperature is a necessary condition for plasticizing the molding material and flowing the plastic melt. It has a very important influence on the plasticization of materials and the quality and output of products. The theoretical temperature window for plastic extrusion is between the viscous flow temperature and the degradation temperature. The temperature range is wider for polyolefins. It is usually processed above the melting point and below 280 degrees. To properly control the extrusion temperature, you must first understand the relationship between the temperature limit of the material being processed and its physical properties. To find out its characteristics and laws, we can choose a better temperature range for extrusion. Therefore, the following aspects should be considered in the temperature setting of each section: First, the properties of the polymer itself, such as melting point, molecular weight size and distribution, melt index, etc. Second, consider the performance of the device. In some equipment, the temperature of the feed section has a great influence on the host current. Again, it is smooth by observing the tube die by extruding the tube blank surface. Whether there are bubbles or not, it is judged.
The extrusion temperature includes the set temperature of the heater and the melt temperature. The heating temperature refers to the temperature provided by the external heater. Melt temperature refers to the temperature of the material between the front section of the screw and the joint of the head.
The barrel temperature distribution, from the feed zone to the die, may be flat, incremental, declining, and mixed. It depends mainly on the material point and the structure of the extruder.
The temperature is set on the machine head, in order to obtain better appearance and mechanical properties, and to reduce the expansion of the melt outlet, the temperature of the control body is generally low, and the temperature of the head is high. The high temperature of the head allows the material to enter the mold smoothly, but the shape stability of the extrudate is poor and the shrinkage rate is increased. If the temperature of the machine head is low, the material plastic is poor, the melt viscosity is large, and the pressure of the machine head rises. Although this will make the product too dense, the post-shrinkage rate is small, and the shape stability of the product is good, but the processing is difficult, the die swell is large, and the surface of the product is rough. It also causes an increase in the back pressure of the extruder, a large load on the equipment, and an increase in power consumption.
The temperature of the die set, the temperature of the die and the mandrel have an influence on the surface finish of the pipe. Within a certain range, the temperature of the die and the mandrel is high, and the surface finish of the pipe is high. Generally speaking, the temperature at the exit of the die should not exceed 220 degrees, the melt temperature at the inlet of the die should be 200 degrees, and the temperature difference between the inlet and outlet melts of the machine head should not exceed 20 degrees. Because the higher temperature difference between the melt and the metal will cause a shark skin phenomenon. Excessive melt temperatures result in die buildup. However, it depends on the actual situation.
Melt temperature refers to the actual melt temperature measured at the end of the screw and is therefore a dependent variable. It is mainly determined by the screw speed and the barrel setting temperature. The upper limit of the melt temperature for extrusion of polyethylene pipes is generally specified to be 230 degrees. Generally, the control is preferably around 200 degrees. The upper limit of the melt temperature for extrusion of polypropylene pipes is generally 240 degrees. The melt temperature should not be too high. The degradation of the material is generally considered, and at the same time the temperature is too high, which makes the tube difficult to shape.
3. Pressure control
The most important pressure parameter in the extrusion process is the melt pressure, that is, the head pressure. Generally speaking, increasing the melt pressure will reduce the extruder output and increase the compactness of the product, which is beneficial to improve the quality of the product. But if the pressure is too high, it will bring security problems. The melt pressure is related to the properties of the raw material, the screw structure, the screw speed, the process temperature, the mesh number of the filter, and the porous plate. The melt pressure is usually controlled between 10 and 30 MPa.
4. Vacuum setting
Vacuum setting mainly controls two parameters of vacuum degree and cooling rate. Generally, under the premise of satisfying the appearance quality of the pipe, the degree of vacuum should be as low as possible, so that the internal stress of the pipe is small, and the deformation of the product during storage is small.
5. Cooling
The cooling water temperature requirement for polyethylene pipe extrusion is generally low, usually below 20 degrees. When producing PPR pipes, the first stage temperature can be slightly higher and the latter stage is lower, thus forming a temperature gradient. It is also important to adjust the cooling water flow. The flow rate is too large, the surface of the pipe is rough, and pits are formed. If the flow rate is too small, the surface of the pipe will be bright and easy to break, such as uneven distribution, uneven wall thickness, or ellipse.
6. Screw speed and extrusion speed
Screw speed is a rework parameter that controls extrusion rate, throughput, and product quality. The speed of the single-screw extruder increases and the output increases. The shear rate increases and the apparent viscosity of the melt decreases. Conducive to the homogenization of materials. At the same time, due to good plasticization, the interaction between molecules is increased and the mechanical strength is improved. However, the screw speed is too high, the motor load is too large, the melt pressure is too high, the shear rate is too high, the die swell is increased, the surface is deteriorated, and the extrusion amount is unstable.
7. Traction speed
The traction speed directly affects the product wall thickness, dimensional tolerance, performance and appearance, the traction speed ratio must be stable, and the traction speed matches the pipe extrusion speed. The ratio of the traction speed to the extrusion line speed reflects the degree of orientation of the product. The ratio is called the draw ratio, and the value must be equal to or greater than 1. The traction speed is increased. When the temperature condition of the cooling set is constant, the traction speed is fast. The time for the product to stay in the sizing sleeve and the cooling water tank is relatively short. After cooling and setting, the inside of the product will retain more heat, which will cause the orientation structure of the product to be disoriented during the traction process. , thereby causing a decrease in the degree of orientation of the article. The faster the traction speed, the thinner the wall thickness of the pipe, and the greater the shrinkage rate of the product after cooling in the longitudinal direction. The slower the traction speed, the thicker the wall thickness of the pipe, and the easier it is to accumulate between the die and the sizing sleeve. Destroy normal extrusion production. Therefore, the extrusion speed and the pulling speed must be well controlled in extrusion molding.
8. On-line quality control and post-treatment of pipes
Polyolefins are crystalline polymers, and the performance of the newly laid pipe is different from the size and performance of the pipe product when it is delivered. The main reasons are. First, the crystallization occurs during the cooling of the polyolefin melt. The crystallinity and crystal form are related to the temperature and heat history and the time of placement. Second, the temperature of the freshly laid pipe is usually higher than normal temperature. Third, the stress inside the pipe that has just been off the line is large. In order to achieve the performance and dimensional stability, the general polyethylene pipe should be placed offline for 24 hours, and the polypropylene pipe should be placed for 48 hours, and the performance test can be carried out according to the corresponding standards.
Third, common problems and treatment in the production of polyolefin pipes
The polyolefin melt has viscoelastic properties. Two phenomena often occur in processing, namely, die swell and melt rupture. Here, it is not detailed. The following is a list of common anomalies, causes, and treatments commonly found in pipe production.
Frequently Asked Questions: The surface is dull and dull
cause:
Raw material moisture
2. The melt temperature is not suitable
3. The molten material extruded by the extruder is uneven
4. The sizing sleeve is too short
5. The molding section is too short
Recommended solution
cause:
Raw material pretreatment
2. Adjust the temperature
3. Increase back pressure, design a suitable screw structure with a thin filter
4. Lengthened sizing sleeve
5. Lengthen the molding section.
Frequently Asked Questions: Surface Spots
1. There is moisture in the raw material
2. There are bubbles on the tube in the sink
Recommended solution
Dry material
2. Eliminate air bubbles. Adjust the process temperature.
Frequently Asked Questions: The outer surface presents a bright and transparent block (commonly known as the eye)
cause:
1. The head temperature is too high
2. The cooling water is too small or insufficient, or uneven
1. Reduce the temperature of the head
2. Cooling water to open or clean the sizing sleeve
Frequently Asked Questions: regular markings on the smooth outer surface of the pipe
Cause: The pipe tends to adhere to the sizing sleeve to increase the cooling water flow to clear the waterway or slow down.
Cause: The corrugated sizing sleeve of the outer surface of the pipe is not centered, and the sizing box and the die are on the same axis.
Frequently Asked Questions: Rough inner surface
cause:
1. Raw materials are damp
2. The core mold temperature is low
3. The gap between the die and the mandrel is too large.
4. The die setting section is too short
Recommended solution:
1. Raw material drying, or pretreatment
2. Increase the temperature or extend the holding time
3. Change the core mold
4. Change the long section of the fixed section
Frequently Asked Questions: Corrugated inside the tube
cause:
1. The output of the extruder changes, and the material is unstable.
2. Traction and slip
3. Uneven cooling of the pipe
Recommended solution:
1. Reduce the temperature in the screw feed zone.
2. Adjust the traction air pressure.
3. Adjust the waterway
Frequently Asked Questions: There are pits on the inner wall of the tube
cause:
1. Raw material moisture is large
2. The filler is poorly dispersible, not plasticized, impurities
Recommended solution:
1. Preheating and drying of raw materials
2. Refueling, adjusting temperature, cleaning raw materials
Frequently Asked Questions: There are coke particles on the inner wall of the tube
cause:
1. The extruder head and the inner wall of the die are not clean.
2. Local temperature is too high
3. Die accumulation is serious
Recommended solution:
Clear mold
2. Check if the thermocouple is normal.
3. Clear the mold, properly reduce the temperature of the die
Frequently Asked Questions: Outside diameter or wall thickness changes at any time
cause:
1. Extrusion speed change
2. Traction speed changes or slips
3. Unloading is unstable (uneven material particle size is uneven)
4. Instability of the melt
5. Uneven cooling
Recommended solution:
1. Check the tractor
2. Appropriate pressure
3. Raw material sieving or granulation
4. Increase the material temperature, reduce the line speed, increase the die gap
5. Clean up the waterway
Frequently Asked Questions: Uneven wall thickness
cause:
1. The die is not centered
2. Uneven temperature of the die
3. The tractor, the sizing sleeve, the die is not centered
4. The sizing sleeve is too far away from the die
Recommended solution:
1. Adjust the die concentric
2. Adjust the temperature
3. Stay on the same axis
4. Close the distance
Frequently Asked Questions: Poor weld seam
cause:
1. The molding section is too short
2. Low melting temperature
3. Plastic dispersion in the die
4. The structure of the nose machine is unreasonable
Recommended solution:
1. Use a longer die forming section
2. Increase the temperature of the material
3. Clean the die
4. Replace or modify
Frequently Asked Questions: Tubes are damaged prematurely
cause:
Blisters
2. Bubble
3. Impurities
4. Poor dispersion of pigment or filler
Recommended solution:
Dry material
2. Dehumidify or lower the temperature
3. Clean the raw materials or use a filter
4. Adjust the temperature or replace the raw materials
Frequently Asked Questions: Premature Tube Damage and Brittle Destruction
cause:
1. Low material temperature
2. The temperature is too high, the raw materials are decomposed
Recommended solution:
1. Increase the temperature of the material
2. Clean the mold and lower the temperature
Frequently Asked Questions: Tube Cracking
cause:
1. The head temperature is low and the extrusion speed is fast.
2. The cooling water is too large
Warming up, slowing down
2. Reduce cooling water flow
Frequently Asked Questions: Tube roundness is not good, bending
cause:
1. Die, the center of the core mold is not correct
2. The temperature around the head is uneven
3. The cooling water is too close to the die
4. Cooling water spray is too strong
5. Cooling water spray is too small
6. The water level is too high
7. The tractor pressure is too large
Recommended solution:
Adjust the concentricity
2. Adjust the temperature
3. Adjust the cooling water position
4. Adjust the nozzle angle
5. Clean up the waterway
6. Drainage
7. Adjust the air pressure
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