1. PVC wood plastic production process
PVC resin is mixed with wood powder, calcium carbonate and additives by high-speed mixer to the required temperature. According to the filling amount, the temperature is generally 115 ℃ - 125 ℃, and then cold mixed to about 50-40 ℃ for discharging and natural cooling
1. This is a one-step process to extrude the product directly through the conical double extruder with the mixed powder. The advantages of one-step production are no granulation, saving the investment of granulation equipment and the maintenance cost of water, electricity, labor and equipment, and reducing the material loss. But the product quality and appearance are not as good as the two-step method. In particular, it's difficult to use the same wood grain as the wood grain for the indoor and outdoor decoration of ecological wood (one step can also produce wood grain such as density of 1.0 or more than 1.2, but increase the cost).
2. The cooled powder is granulated by cone-shaped twin-screw extruder, i.e. two-step production. The advantages and disadvantages are opposite to the above. The two-step production of ecological wood is ideal.
2、 Forming process and equipment
Good processing technology and equipment should ensure that the materials and foaming agent are evenly mixed, and maintain a sufficiently high and stable head pressure, so that the die pressure is large enough and the pressure drop is fast enough to obtain a well formed bubble.
1. Mixing and feeding
Production process of raw materials: wood fiber is a material with strong water absorption. Generally, the water content is about 15% or higher, and the inorganic filler is also about 5%. The water content is the natural enemy that affects the production stability and product quality of plastics, especially wood plastics, such as foam ratio, water absorption of products, deformation of products, weather resistance of products and other physical properties. So it's better to control the water content within 1%. Control the fiber and filler with reasonable water content - add them to the high mixer - mix them to 100 degrees - add proper coupling agent - mix them to 110-115 degrees - open the large cover of the mixer when the motor is turned off for 2-3 minutes - add PVC, Stabilizer ----- mix to 80 ℃ ---- add lubricant and other additives ----- add processing aids and foaming agents to 100 ℃ ---- 120-125 powder unloading ----- after the powder unloading, the motor stops and then opens the large cover for 2-3 minutes, and then mix the next material (it is better to mix the five materials and clean the mixer once) ---- the powder can be granulated or added to the twin-screw production when the temperature is 40 ℃ (the cooling bucket must have circulating cooling water). When mixing materials, it must be observed that the temperature and time are about 20-25 minutes for general cold pot mixing materials, and 12-15 minutes for hot pot (the temperature is set at about 125 ℃). If there is a difference of 3 minutes or longer, it may be the problem of temperature sensing line or incorrect temperature control, or the abrasion of horn knife of mixer is serious. This will affect the stability and color of foaming ratio. During granulation, it is necessary to pay attention to the accuracy of temperature control, the normal operation of fan, the size of current, the particle size, surface and length of particles, and the stability of blanking.
The mixing process affects the dispersion of components by affecting the contact and reaction between different components, and then affects the material properties. When mixing, proper feeding sequence, temperature and time should be selected. Because the wood powder is fluffy, the phenomenon of "bridging" and "holding pole" is easy to appear in the process of feeding. The unstable feeding will make the extrusion fluctuate and reduce the extrusion quality. Therefore, the feeding mode and the feeding amount must be strictly controlled. Generally, the forced feeding device or hungry feeding is used to ensure the stability of the extrusion. PVC / wood powder composite extrusion foaming process is generally divided into two steps and one step: two steps are granulation first and then molding; one step is to eliminate the granulation process, and the surface modified wood powder and PVC powder are mixed at high speed and then directly fed and extruded. The results show that master batch method (two-step method) is beneficial to improve the mechanical properties of PVC / WPC.
2. Molding temperature
Extrusion process:
Mainly pay attention to the following aspects
a. The key to the temperature setting of each section of extruder is compression section, metering section and die. The accuracy of each temperature control, such as temperature control is not allowed to affect the foaming ratio and color. Foaming is mainly the temperature setting of compression section and metering section, and molding is mainly the mold temperature.
b. Normal operation of fans in each section.
c. The size and stability of the host current. If the current fluctuation of the host machine is large, it shows that the dispersal of the lubricants, processing aids, fibers and PVC components is not very uniform.
d. The speed of the main engine must be the same, because the speed will affect the foaming rate and color.
The physical action and chemical reaction of the material in the extruder barrel should be considered when setting the extrusion temperature. The temperature of the feeding section should not only ensure the rapid melting of the material, prevent the escape of the decomposition gas, but also prevent the decomposition of the foaming agent in advance; the temperature setting of the compression section and the metering section should take into account the decomposition temperature and decomposition rate of the chemical foaming agent, the burning of wood powder, the decomposition of PVC and other factors; the machine head temperature should ensure that the melt has good fluidity and sufficient melt viscosity, In order to maintain the melt in the head under high pressure, so that it does not foam in the head. Yao Zhuping thought that low temperature extrusion should be used under the condition of full plasticization. Equipment such as screw and forming die should also have low-temperature extrusion characteristics to ensure that the bubble has a good shape and smaller diameter. The temperature of feeding section shall be controlled below 165 ℃, the temperature of compression section and homogenization section shall be between 160 ~ 180 ℃, and the head and die shall be set below 160 ℃.
3. Screw speed
The effect of screw speed on extrusion foaming is mainly reflected in the following aspects: first, it affects extrusion pressure. The higher the rotating speed, the greater the pressure in the extruder, which is more conducive to nucleation, the more the number of nucleated cells, and the higher the foaming rate. But when the pressure is too high, the growth of the nucleated cell is inhibited, which affects the full growth of the foams. Two, the higher the screw speed is, the stronger the shearing effect is. When the shear effect is too strong, it is easy to merge or break the bubble, affect the quality of the foamed body and the formation of low-density foamed plastics. Three, the screw speed is too high or too low, so that the residence time is too short or too long. It is easy to produce early foaming or insufficient decomposition of foaming agent, which is not conducive to the formation of uniform and fine cellular structure. Therefore, there is an optimal value of screw speed, generally between 12 ~ 18r / min, when other factors are not changed.
4. Extrusion pressure
Insufficient extrusion pressure will result in rough surface and low strength of products, while higher extrusion pressure can not only control the gas-containing melt in the die head from foaming in advance, but also make the pressure difference between inside and outside the die head, so that the pressure drop rate is high, which is conducive to bubble nucleation, the number of nucleated bubbles is increased, and the foaming rate is also increased, which is conducive to obtaining uniform and fine bubble structure. However, too high extrusion pressure is harmful to the growth of the cell. In order to obtain the appropriate head pressure, the screw speed, head temperature and die shape can be adjusted.
5. Forming equipment
The single screw extruder mainly transports materials by friction. The mixing effect is poor. The wood powder stays in the barrel for a long time and is easy to be burnt. Therefore, it is greatly limited in the extrusion of PVC / WPC. In order to improve the mixing effect of PVC / WPC, the single screw used for PVC / WPC processing should be equipped with mixing area, or pelletized first, and then extruded with granular materials, but this process consumes auxiliaries and reduces PVC performance.
5. Solutions to problems
Serial number
Abnormal phenomena
Reason
Resolvent
Remarks
One
Yellow on board
Insufficient stabilizer
Increase stabilizer fraction
Insufficient external lubrication and large friction cause material decomposition
Increase external slip score
Temperature set too high
Lower temperature
Two
Yellow in the middle of plate surface
Mold local temperature set high
Reduce the corresponding temperature
Insufficient external sliding
Increase external slip score
High temperature in zone 5 of extruder
Reduce the corresponding temperature
Three
Uneven plate thickness
Unreasonable die lip gap
Adjust die lip thickness
Improper adjustment of choke block
Adjust choke block
Excessive external sliding
Reduce the external slip score
Insufficient internal slippage
Increase internal slip score
Unreasonable mold temperature setting
Adjust mold temperature
Four
Plate brittle
Extruder temperature set too high
Lower temperature
Extruder temperature set too low
raise the temperature
Unreasonable formula
Adjustment formula
Five
Plate surface is not smooth
Insufficient external sliding
Increase external slip score
Insufficient processing aids
Increase score
Inadequate dispersion of fillers or additives
Adjust the filler or auxiliary content
Mold temperature set too low
Increase mold temperature
Temperature setting of extruder and die is too high
Lower temperature
Six
There are stripes perpendicular to the extrusion direction
Viscosity of foaming regulator is too high
Adjust the brand of foaming regulator
Extruder temperature set too low
Increase extruder temperature
Mold temperature set too low
Increase mold temperature
Traction speed is too fast
Reduce traction speed
Seven
There are big bubbles in the plate
Insufficient melt strength
Increase the amount of foaming agent
Extruder zone 5 temperature too high
Reduce the corresponding temperature
High mold temperature at corresponding position
Reduce the corresponding temperature
Impurity
Pay attention to impurities in raw materials
Return material block
Feed milling
Eight
Plate bending
Unreasonable temperature setting of three rollers
Adjust the temperature setting
Large local temperature difference or air convection in the plant
Improve plant environment
Nine
Large foam cell
Extruder temperature set high
Lower temperature
Less foaming regulators
Increasing dosage
Improper lubrication and adjustment
Adjustment ratio
Ten
Sheet thickness is difficult to control and moves back and forth
Too slippery
Reduce the amount of external sliding
Mold temperature is not stable
Correct the thermometer and improve the control accuracy
Eleven
The core of the sheet has large blisters and the surface has small blisters
High host temperature
Reduce the temperature of the main engine
Improper lubrication adjustment
Adjustment ratio
Insufficient melt strength
Add amount of foaming regulator
2、 Shortcomings of PVC wood plastic products
A discoloration
There are many reasons for discoloration, such as ultraviolet, temperature, humidity, air and the pH of the product environment.
How to prevent product discoloration:
A color powder with good color fastness, migration resistance, high temperature resistance and heat resistance.
B select the lubricant that is not easy to separate out.
C add antioxidant and ultraviolet absorbent to improve the weatherability of the products.
D. reduce the processing temperature as much as possible to meet the processing requirements.
E the surface of the product is sprayed with weather resistant paint with good weather resistance.
F material with good weather resistance.
B deformation
The deformation is mainly due to the low thermal deformation of Vicat. To improve the thermal deformation resistance, the following aspects should be taken into consideration:
A. as far as possible, additives such as low melting point lubricants, CPE, DOP, epoxy soybean oil, etc. that affect the deformation of the material shall not be added or minimized.
B reasonable lubrication system and dosage
C. reasonable production process such as production speed and cooling water temperature.
D. reduce the internal stress of products.
E. reasonable design of die.
F placement and storage of semi-finished products.
In order to improve the mechanical properties of wood plastics, such as impact strength, bending strength, modulus, etc.
C flame retardant problem
The material composition of wood plastic has flame retardant property. The key is to improve the oxygen index and reduce the amount of smoke during combustion. Can be filled with flame retardant, plasticizer using chlorinated paraffin.
3、 Reference formula
Wood plastic standard: wood plastic decorative board (GB / T 24137-2009)
Wood plastic floor (GB / T 24508-2009)
Ly / T 1613-2004 extruded wood plastic composite