PVC wood plastic extrusion production process

PVC wood plastic extrusion production process

 1. Process parameters
1.1 General information

Extrusion molding process generally includes molding temperature, screw speed and metering feeding speed, traction speed, extruder working pressure, exhaust and vacuum cooling and many other aspects. However, the extrusion process is also related to the formula system, extruder structure performance, product shape and mold design, product quality requirements, and utilities. Therefore, process control is very important for profile products with excellent appearance and internal quality. It is determined by repeated practice based on theoretical principles and practical experience.
 1.2 The influence of temperature on low-foaming products

(1) The influence of extrusion temperature

The melt temperature has an important influence on the bubble structure of low-foamed wood-plastic products and the density and surface properties of the foamed products. The melt temperature is affected by the K value of PVC, the composition of the formula and the shearing force acting on the material during the extrusion process. The gas pressure in the bubble during the foaming process is opposite to the melt structure; if the material temperature is too low, only a non-bending foamed structure can be formed, which is caused by the higher viscosity of the material; if the material temperature is too high, it will be The melt is too low and the bubbles are torn apart, and most of the bubbles are lost. The bursting of the bubbles gives the product a flawed surface and high density. The temperature range of a good foaming state is relatively narrow, about 180℃~~190℃

The temperature of the fuselage is generally sufficient to ensure that the material is plasticized before foaming. Temperature control is one of the key factors for the pros and cons of foamed products. Crusted foamed wood plastic profiles have very strict requirements on processing temperature. Excessive extrusion temperature will cause the material to foam too much, without strength, or even unable to form at all, resulting in material decomposition, charring of wood powder, and mold paste; If the output temperature is too low, it will often cause poor plasticization, and shrinkage marks will appear on the surface of the profile, resulting in deterioration of the performance of the profile. The temperature in the feeding section of the barrel should not be too high. The single-screw extruder has no exhaust device. If the temperature is too high, it will cause the foaming agent to decompose prematurely, and it will also cause the material to bridge and the feeding is not smooth, so the temperature is required to be slightly lower , Generally set at about 145°C. The temperature of the compression section and the melting section should be gradually increased. The temperature of the flange section and the temperature of the machine head determine the density, mechanical properties and appearance of the product. When the temperature of the flange and the machine head is too high, it will cause cell rupture, rough surface, low strength, paste on the machine head, non-skinning of the surface, or even failure to form at all; when the temperature is low, the surface is prone to unevenness and plasticization. bad. In order to make the host produce a certain pressure, make the foaming uniform. It is required that the temperature of the machine head should be slightly lower than the temperature of the end of the machine barrel. The temperature of the machine head should be uniform. Otherwise, it is easy to cause uneven wall thickness and profile bending. Extrusion temperature is generally controlled at the highest temperature at about 3/4 of the length of the screw in the barrel, and then gradually decreases.

 

Heating section

body

Flange

Nose

1

2

3

4

Temperature/℃

115-125

135-145

155-165

175-185

160-170

165-175

 




The molten material extruded from the die sometimes produces intermittent pulses in the discharge. This is because the melting temperature is too high and the gas is generated prematurely. The resulting pressure extrudes the molten material from the die. If the melting temperature is too high or too low, the surface of the extrudate will be rough, so the temperature is an important factor to control the foam. Sometimes the foaming agent that is not completely decomposed in the extruder can continue to decompose after coming out of the die. In order to obtain better products, it is very important to accurately control the temperature of the molten material at the exit of the die at a temperature suitable for normal pressure foaming.

The cell structure of extruded foam is mainly affected by melt pressure and temperature. Therefore, heat conduction, temperature, and die form are the keys to extrusion control, and die form is the key to extrusion control. The increase in melt viscosity and pressure of the die can prevent the phenomenon of pre-foaming in the die. Only after the foaming cavity or leaving the die, the pressure can be normally foamed.

(2) The influence of cooling water temperature

    The cooling water temperature of ordinary PVC profiles is lower than 15℃, while the skinned foamed PVC wood-plastic profiles are not easy to radiate internal heat due to the surface skinning. Generally, the cooling water temperature of the shaping mold is set at 5℃. Too high water temperature will cause insufficient cooling of the profile, high surface temperature, low hardness, and unevenness when pressed. The flow rate of the cooling water will also affect the surface quality of the profile. Too small a water flow will make the profile surface too soft and non-skinning, and sometimes irregular skinning will occur; too much water flow will sometimes cause the profile surface Shrink marks appear. Under the same conditions, the influence of the water temperature on the hardness and Vicat of the profile is as follows:

Barrel temperature

Flange temperature

Head temperature

Water temperature

hardness

Vicat temperature

Heat shrinkage rate

145-175

165-170

170-175

5

70

78

1.0

145-175

165-170

170-175

15

58

76

1.5

145-175

165-170

170-175

twenty two

50

75

2.2

 

1.3 The influence of pressure on low foaming profiles

Generally, the size of the bubbles and the density of the foam become smaller as the extrusion pressure increases, while the number of bubbles increases as the extrusion pressure increases. The foaming process is a process in which the viscosity rises and the melt expands, and the gas pressure of the foaming agent and the atmospheric pressure work together. The interaction between the melt and the blowing agent is an important factor in the foam extrusion molding process.

Foams are less fluid than compacted materials, and the products are required to have a dense appearance, so they can only be foamed after the melt leaves the top of the cavity. It is necessary to maintain a sufficiently high pressure in the extruder and mold cavity to make the blowing agent continuously dissolve in the foaming melt. Appropriate screw design can achieve this goal

 

Extrusion pressure/Mpa

Average bubble diameter/mm

Number of bubbles in 1cm3 foam/*104

Density of foam/g·cm-3

1.7

0.14

11.6

0.80

2.4

0.12

33.4

0.67

3.7

0.09

84.5

0.65

5.2

0.07

170

0.65

7.3

0.06

304

0.63

10.5

0.05

573

0.60

13.9

0.04

1060

0.62

19.3

0.04

1120

0.60

Once the melt of the blowing agent leaves the die, the melt pressure drops sharply, causing the supersaturated gas dissolved in the melt to separate from the two phases, forming a large number of bubbles. In order to form as many bubbles as possible, the nucleating agent needs to be evenly distributed in the melt.

In addition, increasing the screw speed increases the extrusion pressure; lowering the head and die temperature will increase the melt viscosity and the pressure.

1.4 The influence of main engine speed and traction speed

The size of the extrusion speed is closely related to the type of material, the size of the density and the shape and size, and is limited by the capacity of the shaping cooling device. At the beginning, the extrusion speed is generally 13-18r/min (screw diameter 65mm). After the extrusion is normal, it can be opened up to 25r/min.

When starting up, the speed of the main engine should be slow first and then fast, in order to avoid excessive pressure in the extruder and damage to the equipment. After the material is extruded from the machine head, the speed is gradually increased according to the pressure, and the speed is controlled to a range suitable for traction and cooling speed. Too high a speed will cause the product density to be too high, and the tractor cannot pull the profile, resulting in blockage of the shaping sleeve; it will also generate higher frictional heat in the barrel, increase the temperature of the material, increase the expansion of the mold, and increase the material Difficulty of cooling after moulding. If the speed of the main engine is too low, it will cause the surface of the profile to be uneven, the size is too small, the material stays in the barrel for too long, and the material decomposes. Another thing to pay attention to is to try to ensure the stability of the screw speed to avoid fluctuations in material flow.

The PVC wood plastic skinned foam profile adopts a crawler tractor, and the traction pressure is generally 0.1MPa. The traction speed is generally fixed during debugging, and the fullness of the profile is adjusted by adjusting the speed of the main engine. During production, the traction speed is first slow, and the cooling is normal, and the extrusion speed is normal before adjusting it. Under the same process conditions, too fast traction speed will cause the profile wall thickness to become thinner, and the profile size becomes smaller; too slow speed will often cause blockage, and the production efficiency is not high. Under the same conditions, the influence of the traction speed on the profile is as follows :

Host speed (r/min)

Traction speed (m/min)

Profile wire quality (kg/m)

Density (g/cm)

Profile appearance

14

1.14

2.05

0.75

flat surface

14

1.20

2.0

0.72

Good surface

14

1.24

1.95

0.64

Flat with slight shrinkage marks

1.5 The impact of material residence time

The residence time of the material in the extruder barrel and die is affected by the performance of the system material and the feeding speed. If the residence time is too short, the foaming agent is not decomposed enough, and the final product has a higher density; if the residence time is too long, the foaming agent decomposes too much in the barrel, and the foaming agent that can continue to decompose at the die decreases, resulting in foaming The foaming density of the bulk decreases and the density of the material increases. Therefore, the residence time should be controlled within an appropriate range

Residence time in the hopper of the rheometer (min)

Average bubble diameter (min)

Number of bubbles in 1cm3 foam/*104

Density of foam (g/cm3)

1.0

0.09

8.2

0.93

3.0

0.08

27.4

0.86

5.0

0.05

334

0.75

7.0

0.04

1120

0.60

10.0

0.09

134

0.47

 

2. Operation

2.1 Prerequisite

l The machine is installed correctly.

l The mold is installed correctly.

l The hopper is full of materials.

l All protection devices have been installed.

2.2 Operating matters

2.2.1 Preparation steps before starting:

1. Pre-dry the pellets of the extruded profile and further dry if necessary

2. Check whether the components and the water and air system are in a normal, safe and reliable state. The parts that need lubrication should have sufficient lubricating oil. The running part should be regularly filled with lubricating oil. The entire extrusion and shaping system should be initially aligned. Lock the position after the extruded product is normal

3. Preheat the head, body, and screw to the temperature required by the process. At the same time, open the cooling jacket at the bottom of the hopper and pass in cold water. After the extruder head reaches a certain temperature, heat the body again.

4. The machine needs to be kept at a constant temperature for a period of time before starting up, because the temperature indicated by the extruder temperature control instrument is ahead of the boring actual temperature. If the temperature is not kept constant for a sufficient time, the material will not be preheated enough, the viscosity of the material flow will be too high, and the screw will Axial overload, damage to equipment and even personal safety

5. When each component reaches the specified temperature, the connecting bolts of the machine head part should be tightened while it is hot, and the connection status should be checked to ensure that no material leakage occurs during operation.

6. Check the feeding hopper and the remaining materials. There must be no foreign matter, especially metal and other hard debris, so as not to damage the extrusion screw or hopper.

7. Put on a clean filter before driving, check whether the machine head meets the product variety and size requirements, and whether the parts of the machine head are clean

8. Clean the operation site, keep the main engine and auxiliary equipment and the operation table clean, and place the raw materials, products, and tools in an orderly manner

9. After each part of the whole machine reaches the predetermined temperature and is consistent with the thermometer test, it is stable for 0.5-1h and starts the trial run.

10. Start each operating equipment and check whether the operation is normal

 

Note: Once the heating temperature reaches the required temperature, all couplings must be tightened again to achieve the required coupling strength

2.1.2 Start-up steps:

1. Turn on the main motor.

2. Adjust the screw speed to 15-20 rpm, and observe the plasticization of the material and the discharge of each part of the die, and adjust the temperature of each part according to the extrusion situation until normal production.

3. When the extrusion is normal, start the vacuum system of the extrusion exhaust section.

4. Introduce the extruded material into the shaping die, water tank and traction device, adjust the shaping die to be concentric and concentric with the machine head, adjust the center height of the traction block, and press the profile with appropriate pressure, adjust the traction speed to make the product wall thickness basically consistent Claim.

5. Turn on the cold water, then turn on the vacuum pump, lock the setting box, make the product stick to the setting sleeve, observe whether the wall thickness and size of the profile are qualified, whether the product is bent or warped, and analyze the cause of the abnormal situation according to the actual situation , Make corresponding adjustments, and mark the products after normal production to distinguish test-machine products and finished products.

2.1.3 Shutdown steps:

1. Open the vacuum control valve.

2. Turn off the vacuum pump and remove the vacuum setting table.

3. Set all temperature controls to 120°C.

4. Close the hopper plug.

5. Let the host run idling.

6. Turn off all temperature controls.

7. Turn off the main switch.

2.1.4 Die replacement steps:

1. The temperature sensor and pressure sensor on the Panasonic die head and adapter (the temperature of the die head must be the operating temperature).

2. Unplug the heating ring plug on the die and adapter.

3. Remove and clean the die and adapter

4. Turn on the main motor.

5. Set the screw speed to 20-25rpm, so that the residual material is extruded from the barrel.

6. Turn off the main motor.

7. Install the adapter and die head.

8. Plug in the temperature sensor and pressure sensor, as well as the heating coil plug.

9. Set to the temperature required for production and start to preheat the die.

3 maintenance

3.1 General considerations

1. It is not allowed to run empty for a long time after starting up, so as not to scratch the screw or barrel

2. Ensure that the raw materials are clean, and prevent metal debris or other hard parts such as screws from falling into the feeding port, damaging the screw and barrel

3. When feeding, ensure continuous feeding. For this reason, there must be sufficient materials in the feeding hopper. If continuous feeding cannot be guaranteed, the machine must be shut down immediately. It is strictly forbidden to run empty without materials.

4. After the new extruder starts to run for several hours, it should re-grow into the V-belt to avoid slipping

5. During continuous long-term operation of the machine, check the lubrication and temperature rise of each part on time, pay attention to the operation of the equipment at all times, and stop and check immediately if there is any abnormality

6. If the production is interrupted, the main drive and heating will stop. When the power supply is restored, each section of the barrel must be heated to the set temperature for a period of time before starting.

7. When it is difficult to eject the screw after processing, a screw-type screw separator can be used instead of hammering to avoid damage to the screw

8. After the extruder is used for 500 hours, there will be iron filings or other impurities in the oil used in the gearbox. Clean the gears and replace the lubricating oil in the gearbox. In the future, the lubricating oil must be replaced according to the specified time in the manual. Generally, check the wear of gears, bearings and seals of the reduction gearbox once a year. The parts with severe wear should be replaced in time. The lubrication parts and requirements of the extruder are as follows:

 

3.2 Common faults and solutions

Serial number

Failure phenomenon

cause of issue

Approach

1

The main motor is overheated

Cooling fan failure

Troubleshoot the cooling fan

2

The main motor stalls and has a buzzing sound

Power phase loss

Check power
3

The gearbox is noisy

Gearbox is out of oil

Add corresponding lubricating oil

4

Abnormal noise in the gearbox

1. Impurities in the lubricating oil; 2. Damage to the bearing; 3. Damage to the gear shaft.

1. Clean the gearbox and replace the lubricating oil; 2. Replace the corresponding bearing; 3. Replace the corresponding gear shaft.

5

Gearbox overheated

1. The lubricating oil has impurities; 2. The gearbox is short of oil; 3. The ambient temperature is too high; 4. The extruded material is overgrown.

1. Clean the gearbox and replace the lubricating oil; 2. Add the corresponding lubricating oil; 3. Strengthen the ventilation; 4. Increase the extrusion temperature appropriately.

6

The heating zone cannot dissipate heat

1. The fan is faulty; 2. The thermocouple is inserted incorrectly.

1. Troubleshoot the fan; 2. Check the thermocouple.

7

Heating zone one zone keeps heating the other zone keeps blowing

Thermocouple is incorrectly inserted

Check thermocouple

8

Heating zone keeps heating

1. The solid state relay is broken down; 2. The thermocouple is inserted incorrectly.

1. Replace the solid state relay and check whether the heater is leaking; 2. Check the thermocouple.

9

The heating zone cannot be heated

1. The circuit breaker is disconnected; 2. The heating ring is damaged.

1. Close the circuit breaker; 2. Replace the corresponding heating ring.

10

Inaccurate temperature control in heating zone

1. The thermocouple model is wrong; 2. The thermocouple is damaged; 3. The thermocouple is in poor contact.

1. Replace with K-type thermocouple; 2. Replace thermocouple; 3. Install thermocouple correctly.

11

The extruder does not discharge

1. The main engine is not started; 2. The feed valve is closed; 3. The screw holds material.

1. Run the main engine; 2. Open the feeding valve; 3. See the screw holding material treatment.

12

Screw holding material

1. The barrel cooling water is not open; 2. The temperature in a zone is too high.

1. Turn on the cooling water of the barrel; 2. Decrease the temperature of one zone appropriately.

 

3.3 Common defects of profile and process adjustment

unusual phenomenon

cause

Solution

Rough appearance and low strength

①The heat preservation time is short before starting up

②Low extrusion temperature

③ Short mixing time or low temperature

① Extend the holding time

② Adjust the temperature of each section or increase the extrusion speed according to the quality defect characteristics of the parison

③ Change the mixture

The melt flow of the die is severe, and the profile is bent and deformed

① The temperature of the die is not well controlled

② Uneven distribution of materials due to die flow

③ Extrusion speed is too fast

① Adjust the set temperature of the die correspondingly

② Correspondingly adjust the vacuum degree and cooling water volume of the forming mold, increase the vacuum degree of the fast side of the forming mold and reduce the cooling water volume; reduce the vacuum degree of the full side of the forming mold, and increase the cooling water volume

③ Adjust the extrusion speed accordingly

④ Stop the machine and reassemble or repair the die depending on the bending and deformation of the parison

The die output is fast and slow, unstable

① The temperature of the die is high, and the expansion of the parison is large. When entering the setting mold, the bending deformation occurs under the restraint of the inner wall of the setting mold, the friction with the inner wall of the setting mold is reduced, and the moving speed increases. After the parison is bent and deformed, it bears The compression stress is released. Under the cooling effect of the shaping mold, the shrinkage is re-attached to the inner wall of the large mold, and the friction with the inner wall of the shaping mold increases, and the moving speed slows down. At the same time, when the product passes through the shaping sink, there is no deformed part and shaping block. The friction is large, the movement is slow, the friction of the deformed part is small, and the movement is fast

② The feeder speed is unstable

③ The traction motor has low horsepower, overload or unstable speed

④ The mixing time is short, the temperature is low, the material composition, ratio, pellet configuration, uneven mixing or high water content

① Decrease the set temperature of the die or the extrusion speed

② Check the feeder motor

③ Check the tractor motor

④ Check or replace the mixture

There are waves in the corners of the parison molding

① Uneven discharge

② The vacuum of the shaping die is unstable

③ Unstable power supply voltage

① Adjust the gap of the die

② Check the shaping die and adjust the vacuum pressure

③ Stable voltage

The parison has bubbles or the surface and inner wall are rough and foamy

① The exhaust hole of the extruder is blocked or the vacuum pump is faulty or the pipeline is leaking

② The melt pressure is low

③ The temperature of the extruder barrel is too high

④ The raw material has a high content of volatile components

⑤ The gap between the extruder barrel and the screw is too large

① Check and dredge the exhaust hole of the extruder, check and repair the vacuum pump or pipeline

② Improve the ratio of feed and extrusion speed

③ Reduce the set temperature of the barrel or the extrusion speed

④ Replace material

⑤ Adjust the gap between the extruder barrel and screw

Curved inner ribs of the parison

The temperature of the inner rib of the preform is high, and it is out of sync with the outer wall when the mold is vacuum-adsorbed and cooled

① Before the parison enters the setting mold, use a copper needle to poke a few small holes on the surface of the inner rib of the parison to make the inner rib and the outer wall open, and strengthen the cooling effect of the inner rib

② Lower the set temperature of the extruder

③ Reduce feeding or extrusion speed

Decomposition line on the surface of the parison

① High extrusion temperature

② The residual material in the machine head and die has not been cleaned up, and the machine head, uniform distribution plate, and die flow to the existence of combined shoulders or dead corners

③ Poor thermal stability of the mixture

① Lower the temperature of the extruder and reduce the extrusion speed

② Check the machine head and die, and clean or dispose of it

③ Check replacement materials

Scratches on the surface of the profile

① There is foreign matter on the working surface of the vacuum shaping die

② There are impurities in the material

③ The external cooling water has impurities

④ There are grooves or foreign objects on the working surface of the die

① Open and close the vacuum valve repeatedly to remove foreign matter or bring the cloth head into the shaping mold with the profile for cleaning

② Check the materials

③ Check the source of impurities in the cooling water and deal with it

④ Check and handle the working surface of the die

Low profile foaming ratio

① The temperature of the barrel is low, and the foaming agent is not completely decomposed

② The resin model is too high, the viscosity is large, and it is not easy to form bubbles

① Increase the temperature of the barrel

② Change the resin model

The inner or outer ribs of the profile are broken

① The temperature difference between the inside and outside of the melt interface is large

② Low melt pressure

③ The temperature of the die is low or high

④ There is a foreign body in the die of the machine head

① Increase or decrease the set temperature of each section of the barrel

② Improve the ratio of feed and extrusion speed

③ Increase or decrease the temperature setting of the die

④ Clean up foreign objects in the machine head, die, uniform distribution plate and guide sleeve

Difficult cutting, unstable cutting

① The feeding screw speed is unstable or the material density fluctuates

② Bridging of pellets

③ The angle of the feeding hopper is not suitable or the hopper is too shallow

④ The temperature of the feeding port is too high

⑤ The feeding port or feeding screw is blocked

① Stable working voltage and stable hot mixing process conditions

② Screen out special-shaped pellets to reduce moisture

③ The feeding port is at a straight angle to the forward direction of the material, and the hopper is deepened, and the material is loaded in time

④ Reduce the temperature of the feeding section or cool the feeding port

⑤ Clean the hopper or feeding screw

Product strength is low

① Inappropriate resin model or ratio

② The processing temperature is too high

③ The type or amount of modifier is inappropriate

① Check resin viscosity and reduce filler appropriately

② Depending on the plasticization situation, reduce the processing temperature appropriately

③ Adjust the variety or dosage of modifier

Product traction fracture

① Unsuitable traction speed

② Excessive resistance in the shaping mold

① Reduce the traction speed

② Correct the shaping mold or reduce the vacuum

Deformation of the product after traction

① Insufficient cooling

② The pressure of the tractor is too large

① Reduce cooling water temperature or lengthen cooling water tank

② Reduce the downforce of the tractor

There are wrinkles on the surface of the product

① Uneven discharging of the machine head

② The water temperature of the cooling sizing jacket is too high

③ Slow traction speed

① Adjust the position and temperature of the machine head

② Reduce the water temperature of the cooling setting jacket

③ Adjust the traction speed

Large product shrinkage

① Insufficient cooling of the shaping sleeve

② The compressed air pressure is too high

③ The traction speed is too high and there is over-stretching phenomenon

④ The head temperature is high

① Improve the cooling effect of the sizing sleeve

② Control the temperature of compressed air

③ Adjust the traction speed and cooling speed

④ Reduce the temperature of the machine head