Problems and solutions in plastic extrusion
Section 1 Basic Principles of Plastic Extrusion
The plastics processing industry is a highly comprehensive technology-based industry. It involves polymer chemistry, polymer physics, interface theory, plastic machinery, plastic processing molds, formula design principles and process control, etc. Extrusion theory mainly studies the movement and changing rules of plastics in the extruder. The plastic in the extruder is under a certain external force, and the relationship between the three physical states of the high polymer in different temperature ranges, the screw structure, the plastic properties, and the processing conditions. So as to carry out reasonable process control. In order to achieve the purpose of improving the output and quality of plastic products. When plastic polymer materials are heated under constant pressure, they will appear in three physical states: glass state, high elastic state, and viscous fluid state in different temperature ranges. Generally, the molding temperature of plastic is above the viscous flow temperature.
Section 2 Polyolefin Pipe Extrusion Process Control
The control parameters of the extrusion molding process include molding temperature, extruder working pressure, screw speed, extrusion speed and traction speed, feeding speed, cooling and setting, etc.
1. Raw material pretreatment
Polyolefins are non-absorbent materials and usually have a low moisture content, which is sufficient for extrusion, but when polyolefins contain water-absorbing pigments, such as carbon black, they are sensitive to humidity. In addition, when using recycled materials and fillers, the water content will increase. Moisture not only causes roughness on the inner and outer surfaces of the pipe, but may also cause air bubbles to appear in the melt. Raw materials should usually be pretreated. Drying treatment is generally used, and corresponding additives with dehumidification function can also be added. Such as defoamers, etc. The drying temperature of PE is generally 60-90 degrees. At this temperature, the yield can be increased by 10% - 25%.
2. temperature control
The extrusion molding temperature is a necessary condition to promote the plasticization of the molding material and the flow of the plastic melt. It has a very important impact 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. Usually it can be processed above the melting point and below 280 degrees. To correctly control the extrusion molding temperature, it is necessary to understand the relationship between the temperature limit of the processed material and its physical properties. Only by finding out its characteristics and rules can a better temperature range be selected for extrusion molding. 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. Next, consider the performance of the device. For some equipment, the temperature of the feed section has a great influence on the current of the host. Again, by observing whether the surface of the tube blank extruded by the tube die head is smooth. Whether there are bubbles and other phenomena to judge.
Extrusion temperature includes heater set temperature and melt temperature. The heating temperature refers to the temperature provided by the external heater. The melt temperature refers to the temperature of the material between the front section of the screw and the connection between the machine head.
The barrel temperature distribution from the feed zone to the die may be flat, increasing, decreasing and mixed. It mainly depends on the material point and the structure of the extruder.
To set the temperature of the machine head, in order to obtain better appearance and mechanical properties, and reduce the expansion of the melt outlet, the temperature of the machine body is generally controlled to be low, and the temperature of the machine head is high. The high temperature of the machine head can make the material enter the mold smoothly, but the shape stability of the extruded product is poor and the shrinkage rate increases. If the temperature of the machine head is low, the plastic of the material will be poor, the viscosity of the melt will be high, and the pressure of the machine head will rise. Although this will make the product too dense, the post-shrinkage rate is small, and the product shape stability is good, but the processing is more difficult, the mold release expansion is large, and the product surface is rough. It will also lead to an increase in the back pressure of the extruder, a large load on the equipment, and an increase in power consumption. The setting temperature of the die, the temperature of the die and the core mold have an impact on the surface finish of the pipe. Within a certain range, the higher the temperature of the die and core mold, the higher the surface finish of the pipe. Generally speaking, the temperature at the die outlet should not exceed 220 degrees, the melt temperature at the head inlet should be 200 degrees, and the temperature difference between the head inlet and outlet melt should not exceed 20 degrees. Because the higher temperature difference between the melt and the metal will lead to shark skin phenomenon. Excessively high melt temperatures lead to die deposits. But it should be decided according to the actual situation. Melt temperature refers to the actual temperature of the melt measured at the end of the screw and is therefore the dependent variable. It mainly depends on the screw speed and the barrel setting temperature. The upper limit of melt temperature for polyethylene pipe extrusion is generally specified as 230 degrees. Generally, it is better to control it at about 200 degrees. The upper limit of melt temperature for polypropylene pipe extrusion is generally 240 degrees. Melt temperature should not be too high. Generally, the degradation of materials is considered, and at the same time, too high temperature will make it difficult to shape the pipe.
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 output of the extruder and increase the compactness of the product, which is conducive to improving the quality of the product. But too much pressure can bring security problems. The melt pressure is related to the properties of raw materials, screw structure, screw speed, process temperature, mesh number of filter screen, perforated plate and other factors. Melt pressure is usually controlled between 10-30MPa.
4. vacuum setting
Vacuum setting mainly controls two parameters of vacuum degree and cooling speed. Usually, 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. cool down
The cooling water temperature requirement in extrusion molding of polyethylene pipes is generally low, usually below 20 degrees. When producing PPR pipes, the temperature in the first stage can be slightly higher, and the temperature in the latter stage can be lower, thereby forming a temperature gradient. It is also very important to adjust the cooling water flow. If the flow rate is too large, the surface of the pipe will be rough, resulting in spots and pits. If the flow rate is too small, there will be bright spots on the surface of the pipe that are easy to break, such as uneven distribution, uneven wall thickness of the pipe, or ellipse.
6. Screw speed and extrusion speed
Screw speed is a heavy parameter to control extrusion rate, output and product quality. The speed of the single-screw extruder increases and the output increases. As the shear rate increases, the apparent viscosity of the melt decreases. It is conducive to the homogenization of materials. At the same time, due to the good plasticization, the force 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 expansion is increased, the surface is deteriorated, and the extrusion volume 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 must match the extrusion speed of the pipe. The ratio of traction speed to extrusion line speed reflects the degree of orientation that may occur in the product. This ratio is called the stretch ratio, and its value must be equal to or greater than 1. When the traction speed increases and the cooling and setting temperature conditions remain unchanged, the traction speed is fast. , the product stays in the sizing sleeve and the cooling water tank for a relatively short time, and there will be more heat remaining inside the product after cooling and shaping, which will cause the orientation structure of the product to be formed during the traction process. , resulting in a reduction in the degree of orientation of the product. The faster the traction speed, the thinner the wall thickness of the pipe, and the greater the shrinkage rate of the cooled product in the longitudinal direction. The slower the pulling speed, the thicker the wall thickness of the pipe, the easier it is to cause material accumulation between the die and the sizing sleeve. Disrupt normal extrusion production. Therefore, extrusion speed and pulling speed must be well controlled in extrusion molding.
8. On-line quality control and post-processing of pipes
Polyolefin is a crystalline polymer, and there is a gap between the performance of the pipe just off the line and the size and performance of the pipe product when it is delivered. The main reasons are as follows: first, crystallization occurs during the cooling process of polyolefin melt, and the crystallinity and crystal form are related to temperature, thermal history, and storage time. Second, the temperature of the pipe just off the assembly line is usually higher than normal temperature. Third, the internal stress of the pipe just off the assembly line is relatively large. In order to achieve performance and dimensional stability, general polyethylene pipes should be placed off-line for 24 hours, and polypropylene pipes should be placed for 48 hours before performing performance tests according to corresponding standards.
Section 3 Common problems and solutions in the production of polyolefin pipes
Polyolefin melts are viscoelastic. Two phenomena often appear in processing, namely die swelling and melt fracture. Here, it will not be explained in detail. The common abnormalities in pipe production, their causes and treatment methods are listed below.
Causes of abnormal conditions Recommended solutions
1. raw material moisture
2. Melt temperature is not suitable
3. The molten material extruded by the extruder is uneven
4. The sizing sleeve is too short
5. Die forming section is too short
1. Raw material pretreatment
2. Adjust the temperature
3. Increase the back pressure, use a finer filter, and design a suitable screw structure
4. Extended sizing sleeve
5. Extended die forming section. the
surface spots
1. There is moisture in the raw material
2. Bubbles in the pipes in the sink
1. Dry raw materials
2. Eliminate air bubbles. Adjust process temperature. the
The outer surface is bright and transparent block (commonly known as eye clear)
1. Head temperature is too high
2. Cooling water is too small or insufficient, or uneven
1. Reduce the temperature of the machine head
2. Open up the cooling water or clean the sizing sleeve
Regular markings on the smooth outer surface of the pipe The pipe tends to adhere to the sizing sleeve Increase the flow of cooling water to clean the waterway or reduce the speed
Deep corrugation on the outer surface of the pipe. The die of the sizing sleeve is not centered. Center it and keep the sizing box and the die on the same axis.
Rough inner surface
1. raw material wet
2. Low core mold temperature
3. The gap between die and mandrel is too large. the
4. Die shaping section is too short
1. Raw material drying, or pretreatment
2. Increase the temperature or extend the holding time
3. Changing the mandrel
4. Change to a die with a longer shaping section
corrugated pipe inner wall
1. The output of the extruder changes, and the feeding is unstable
2. Traction slipping
3. Uneven cooling of pipes
1. Reduce the temperature in the screw feeding zone. the
2. Adjust traction air pressure. the
3. Adjust the waterway
There are pits on the inner wall of the pipe
1. High raw material moisture
2. Poor dispersibility of fillers without plasticization, impurities
1. Raw material preheating and drying
2. Refueling, adjusting temperature, cleaning raw materials
There are coke particles on the inner wall of the tube
1. The inner wall of the extruder head and the die is not clean
2. Local temperature is too high
3. Die deposits are serious
1. Clear mold
2. Check whether the thermocouple is normal. the
3. Clear the mold and lower the die temperature appropriately
Changes in outer diameter or wall thickness at any time
1. Variation of extrusion speed
2. Changes in traction speed or slipping
3. Unstable feeding (uneven return particle size)
4. Melt Instability
5. Uneven cooling
1. Check tractor
2. Properly increase the pressure
3. Raw material screening or granulation
4. Increase material temperature, decrease line speed, increase die gap
5. Clean up waterways
Uneven pipe wall thickness
1. The die is not centered
2. Uneven die temperature
3. Tractor, sizing sleeve, die not aligned
4. The distance between the sizing sleeve and the die is too far
1. Adjust die concentricity
2. adjust the temperature
3. keep on the same axis
4. shorten the distance
Poor welding seam
1. The die forming section is too short
2. Low melting temperature
3. Dispersion of plastic in the die
4. Unreasonable machine head structure
1. Use a longer die forming section
2. Increase material temperature
3. Clean up the die head
4. Replacement or Retrofit
Pipe premature damage piercing
1. Blister
2. bubble
3. Impurities
4. Poor dispersion of pigments or fillers
1. Dry raw materials
2. Dehumidification or lower temperature
3. Clean ingredients or use a strainer
4. Adjust temperature or replace raw materials
Brittle failure of premature failure of the pipe
1. Low material temperature
2. The temperature is too high, the raw material decomposes
1. Increase material temperature
2. Clean the mold and lower the temperature
pipe cracking
1. Low head temperature, fast extrusion speed
2. The cooling water is too large
1. heat up, slow down
2. Reduce cooling water flow
The roundness of the pipe is not good, and it is bent
1. The center position of the die and core die is not correct
2. Uneven temperature around the machine head
3. The cooling water is too close to the die
4. The cooling water spray force is too large
5. Cooling water spray is too small
6. The water level is too high
7. Tractor pressure is too high
1. Adjust concentricity
2. adjust the temperature
3. Adjust the cooling water position
4. Adjust nozzle angle
5. Clean up waterways
6. Drainage
7. Adjust air pressure
