【Science Stickers】Lectures on the production process of PVC products

Part 1: Brief introduction of PVC processing technology

1. The role of raw materials (including physical properties, chemical properties, mechanical properties of raw materials and their role in PVC);

2. The formula of PVC ;

2.1. Synergistic reaction formula: The two raw materials together can play three, four, five, etc. functions in the formula, and the efficiency is significantly increased.

2.2. Addition reaction: the efficiency of the two raw materials is not increased or decreased when put together.

2.3. Counter-reaction: When the two raw materials are put together and added to the formula, its efficiency is not increased but decreased, which is equivalent to one or less than one effect, so its effect is obviously reduced. In fact, counter-reaction is just one kind of counter-reaction. The chemical reaction, in the roughest terms, is the acid-base reaction in chemistry;

3. Mixing process : Put the raw materials made by the formula into a device for heating and mixing;

4. The structure and extrusion process of the extruder;

5. Mould;

6. The operating skills and sense of responsibility of the employees .

Part 2: The structure and extrusion process of the extruder

1. The structure of the extruder:

The extruder is composed of a motor (that is, a driving device), a reduction box (reducer), a distribution box, a barrel, a screw (a part of the barrel and screw), a heating and cooling device, and an electric control device. The core part of the extruder structure is the barrel and screw, and the others are auxiliary devices, but it is not possible without these devices. These devices are fixed and vulnerable parts. The material and the mixed dry powder material are pushed into the barrel of the barrel through the feeder at a certain speed, and this material is naturally pushed into the barrel screw.

2. The role of each part of the extruder barrel and screw:

Zone one (pre-plasticizing zone): The role of zone one is the most important in the electric heating and extrusion process of the entire extruder. It is more important than other zones. The tasks it undertakes include:

①Dry powder material is compacted, sheared, and forwarded quantitatively;

②A process of pre-plasticization in advance. If the pre-plasticization in one zone is not reached, the plasticization degree of the entire machine will not be reached. In the entire extruder (excluding the mold), the temperature in the first zone is The highest is the highest point of temperature. If a zone does not reach pre-plasticization, the following situations will occur:

① Material taking off from the exhaust hole of the main engine,

②, the current is obviously larger

③ The product is very brittle.

Second zone (plasticizing zone): In this zone, the dry powder material transferred from the first zone has been compressed into blocks through pre-plasticizing in the first zone, and the compacted blocks are transported forward with the rotation of the screw Arriving in the second zone, the structure of the spiral bell changes in this zone. The spiral bell becomes 4~5mm thick and produces 9~11 spirals, and the two ends are disconnected, so the second zone fully reaches the standard plasticization degree. 90% of the total. Because there are many small grooves in the spiral bell, the purpose of mixing is achieved, so overall the second zone has reached more than 90% of the plasticization. If the material does not reach pre-plasticization in the first zone, it will have an adverse effect on the second zone:

①, the dry powder material is not plasticized,

②. Squeeze out the snail bell. The temperature setting of the second zone should be 1~2℃ lower than that of the first zone or equal to that of the first zone. It should be set according to the plasticizing ability of the extruder. If the plasticizing ability of the extruder is better, the temperature of this zone can be lower than If the plasticizing ability of the extruder is not good, the temperature of this zone should be equal to that of the first zone.

Zone three (homogenization zone): The function of this zone is to completely plasticize the materials that have not been completely plasticized in the second zone. The third zone must ensure that the plasticization reaches 100%. Therefore, the third zone of the extruder is also more important. The temperature of the third zone should be 5~6℃ lower than the second zone, and the maximum should not exceed 8℃. Because the barrel screw material is alloy steel, the rigid material has thermal conductivity, and the temperature is stepped. Too much difference will not help.

Fourth zone (quantitative conveying and extrusion zone): This zone does not undertake any plasticizing task. If the material is plasticized quite well, you can see in this zone that the screw floats and rotates on the center of the extruder barrel. Therefore, the task of the fourth zone of the extruder is to transport the plasticized melt quantitatively. If this zone bears the plasticizing capacity, it will have a very harmful effect on the extruder. The temperature of zone four should be lower than zone three, and the temperature difference between the two zones should be 5~6℃, and the maximum should not exceed 8℃.

    From the above point of view, the temperature of the extruder is from high to low, and the temperature in one zone is the highest. It is absolutely not allowed to go from low to high, and it is absolutely not allowed to be flat. But in general, the temperature difference between zone 1 and zone 4 cannot exceed 20°C.

3. The role of the confluence core: 

① The melt materials extruded by the two screws reach confluence and welding.

② Fine adjustment device of plasticization degree.

③The degree of plasticization can be judged by measuring the melt pressure and melt temperature through the sensor of the confluence core.

The function of the fine-tuning device for the degree of plasticization: when the degree of plasticization is a little bit low or the degree of plasticization is a little high, you don’t need to consider other problems of the extruder. You can adjust the plasticity by lowering or increasing the temperature of the confluence core. Degree. Decrease the temperature of the confluence core to increase the degree of plasticization, and increase the temperature of the confluence core to decrease the degree of plasticization. Poor plasticization means that the plasticization is still a bit short. There is a certain rule for fine-tuning. If the temperature of the four zones of the extruder is 170℃, the temperature of the confluence core can be set to 160℃ or 180℃, and the temperature of the confluence core is different. It can be higher or lower than the four zones by more than 10°C, so the temperature of the confluence core should be adjusted within 10°C based on the four zones as the standard.

4. The function of the die is to produce qualified products:

Here we are explaining that lowering the temperature of the confluence core increases the degree of plasticization. Increasing the temperature of the confluence core decreases the degree of plasticization. Our PVC polymer material has a characteristic. The higher the temperature, the faster the fluidity, but it is not infinite. For example, a square pipe has four heating zones. If the flow on the left side is slow and the output is less, heating this side will immediately increase the fluidity. Therefore, the more the heating, the fluidity and extrusion of the object The faster the volume, why the fluidity of the heated object is faster, because there is no resistance, it is smoothly squeezed out, in fact, we can regard the confluence core as a valve, when our water valve is fully opened , The water flows down smoothly. When the valve is half open or completely closed, the water does not flow or flows very little. We use the confluence core as a water valve. When the temperature is low, it is equivalent to closing the valve for a while. This is the truth. The temperature of the confluence core is adjusted to increase a certain degree of plasticization, but it is not complete, and it is used to increase the degree of plasticization in a small amount. Poor plasticization does not mean that there is no plasticization, it means that there is a certain defect, so when there is poor plasticization, we can lower the temperature of the confluence core. After the reduction, whether the plasticization is good, the flow of materials It's slow, and a pressure is generated, and the result of the pressure is an increase in the degree of plasticization.

Part Three: Degree of Plasticization

1. The influence of degree of plasticization on product performance:

The performance of PVC products is closely related to the degree of plasticization. The degree of plasticization is poor, the product is brittle, and the mechanical properties cannot meet the requirements; if the plasticization is too high, the product will appear yellow lines, and the mechanical properties are unqualified. The degree of plasticization is lower than that of PVC products. The processing process is very important.

① When the plasticization degree is 60%, the tensile strength is the highest;

② When the plasticization degree is 65%, the impact strength is the highest;

③. When the degree of plasticization is 70%, the elongation at break is the highest;

For the production of water supply pipe materials, the plasticization degree of 60-65% is the most appropriate. Because in this range, it can reflect the two properties of tensile strength and impact strength.

2. The influence of temperature on the degree of plasticization:

The polymer material cannot be melted when the temperature is lower than 80℃, and it is glassy. The material in the glassy state is hard and brittle, and the material cannot be processed in the glassy state; as the temperature rises to 160℃, the material is highly elastic However, the material still cannot flow in this area. It can only make the material softer and increase viscoelasticity. For PVC melt processing and fluidity, the temperature should be between 160-200℃, but for any The stabilizer, when the temperature is higher than 200℃, the material will decompose after being heated for a long time, so when controlling the degree of plasticization, the temperature can only be controlled between 160-200℃. Within the temperature difference range of 40°C, when the temperature of PVC is set between 170-180°C, the plasticization is better.

3. Methods to improve the degree of plasticization:

①. By increasing the temperature of the fuselage and screw.

② When the screw speed is normal, increase the feeding speed of the feeder to increase the plasticization

③. Increase the speed of the extruder when the rated speed of the extruder and the feed are satisfied.

④. Give the dry powder a good maturation period (12-48h). The role of the maturation period: 1. Eliminate static electricity and reduce pollution

2. Increase apparent density

3. Improve the degree of plasticization

4. The low-molecular-weight polymerization is uniformly dispersed to prevent unstable extrusion.

5. Increase the degree of plasticization by lowering the temperature of the confluence core.

4. How to judge the degree of plasticization:

①. The degree of plasticization is judged by the current of the main engine. Take the (65/132 production line as an example, the current of the main engine is appropriate to 46-52A. Because our company is a low-calcium product, 45-50A is appropriate. The premise is: screw speed 16~22r /min, the feed is full and matches the screw speed, and the temperature setting matches the screw speed and host current);

②. Observe the plasticization degree of the material through the vacuum exhaust hole of the main engine (that is, the material is filled with more than 60% in the middle of the screw groove, the powder in the screw groove is in the state of tofu and the material at the bottom of the groove is flattened);

③. The degree of plasticization is judged by the viscoelasticity of the melt material of the mold die (this method is more suitable for when it is just turned on);

④. The degree of plasticization is judged by the melt pressure and melt temperature of the confluence core (the disadvantage is that if the instrument fails or the confluence core sensor is burned by the burnt material, etc., the accuracy of the test result will be affected)

Part 4: Flaring process selection

     For PVC pipe flaring, the flaring temperature is generally 245±5℃. Regardless of the wall thickness of the pipe, the flaring temperature should generally not exceed 250℃, because the flaring heating needs to be slow to heat the pipe uniformly to eliminate stress and improve product quality. Good, so the flaring heating time varies according to the wall thickness, and it is also related to the ambient temperature. The difference between the internal and external heating temperatures cannot exceed 10°C.

Part V: PVC pipe extrusion die structure and process setting

1: Transition section function: fix the mandrel bracket, fix the shunt cone, and compress the total area (design function of the mold forming area and the cross-sectional area of ​​the transition section);

2: The function of the compression section: compress the material from thick to thin, increase its compactness; increase fluidity and pressure;

3: The function of the straight section: Insufficient length of the straight section will cause the phenomenon of mold release expansion, and also affect the pipe burst pressure test, low temperature drop hammer test, flat test, and tensile test are all unqualified; the length of the straight section = die Wall thickness*30-40 times.

Extrusion die material: 2Cr13, 3Cr13 (the hardness is generally 30-32), 2Cr2W8, 45# steel (the disadvantage is that the surface needs to be plated with Cr before use, which is easy to deform)

The temperature setting of the connecting section is 5-10℃ higher than that of the confluence core; the temperature of the preforming section is about 5℃ higher than that of the connecting section; the temperature setting of the transition section is generally 175-178℃, not higher than 180℃; the temperature of the compression section is higher than that of the transition section The temperature of the die is 5-8℃ higher than that of the compression section, and the temperature of the die can even be higher than the temperature of the first zone of the extruder.

Part VI: Several key parameters of extrusion die 

Compression ratio: The ratio of the total cross-sectional area of ​​the die molding to the total cross-sectional area of ​​the preforming section is called the compression ratio. Generally speaking, for pipes, the compression ratio is between 1:2.5-5 times, depending on the performance requirements of the product.

The length of the straight section: generally 25-40 times the wall thickness, which is related to the amount of calcium powder added in the raw material. If the amount of calcium powder is high, the length of the straight section is 25-30 times; calcium powder If the addition amount is low, take the high value, that is, 35-40 times. The length of the straight section of the mold is directly related to the mechanical properties of the product (burst pressure, tensile strength, flat strength and impact strength).

The compression ratio of the mold should match the length of the straight section, and the compression angle of the mold should also be appropriate (generally, the compression angle is 11-12 degrees). Generally speaking, an extruder can only be equipped with three sets of molds. The length of the mandrel should be 5-10mm longer than the die. This is to prevent the product from collapsing. The mandrel should be ventilated and cooled. This can solve the overheating of the inner cavity and prevent the internal and external temperature from being different and causing stress.

Part Seven: Raw Materials

The role of processing aids: reduce PVC melt viscosity, promote plasticization, increase fluidity, and increase melt viscoelasticity and strength. If the low calcium screw exceeds 6 parts of calcium, it will not be plasticized, and only better processing aids can be used to make up for equipment defects.

Classification of ACR processing aids: (National standard)

ACR201: methyl methacrylate (85%) + ethyl or butyl acrylate (15%)

ACR301: methyl methacrylate (80%) + ethyl or butyl acrylate (10%) + styrene (10%)

ACR401: Methyl methacrylate (50%) + ethyl or butyl acrylate (10%) + styrene (25%) + acrylic acid (15%)

Impact modifier: CPE is the English abbreviation of chlorinated polyethylene. Chlorinated polyethylene (CPE) is obtained by adding chlorine to high-density polyethylene after heating in the water phase reaction. When the chlorine content is 35%, the resistance The impact performance is better, and the compatibility with PVC is the best, and its addition amount is generally 7-8 parts.