PVC wpc foaming production process and equipment

Pvc wpc foaming production process:

PVC resin, wood powder, calcium carbonate, and additives are mixed by a 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 degrees. The material is discharged and used after natural cooling:

1. Directly use the mixed powder to extrude the product through the cone twin extruder. This is a one-step method. The advantage of one-step production is that no granulation is required, which saves investment in granulation equipment and maintenance costs of water, electricity, labor and equipment, and reduces material loss. But the product quality and appearance are not as good as the two-step method. Especially ecological wood is a one-step method that is imitating wood for indoor and outdoor decoration and needs to have the same wood grain as wood (one-step method can also produce wood grain such as density of 1.0 or 1.2 or more, but it increases cost).

2, The cooled powder is produced by two-step method, which is made by two-step method, which is granulated by cone and double and then extruded by single screw. The advantages and disadvantages are opposite to the above. The two-step method of ecological wood production is ideal.

2. Molding 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 good-shaped cells.

1. Mixing and feeding

The 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 level of water content affects the stability and products of plastics, especially wood plastics. The natural enemy of quality, such as affecting the expansion ratio, the water absorption rate of the product, the deformation of the product, the weather resistance of the product and other physical properties. Therefore, the moisture content is best controlled within 1%. Control the reasonable moisture of the fiber and filler-----add it to the high-mixer-----mix to 100 degrees-----add an appropriate amount of coupling agent----mix to 110-115 degrees-- --Turn off the motor and open the large cover of the mixer for 2-3 minutes---add PVC and stabilizer---mix to 80 degrees---add lubricant and other additives-- ---100 degrees plus processing aids, foaming agent------120-125 powder unloading-----the motor stops after powder unloading, then open the big cover for 2-3 minutes before mixing the next hand material (It is best to mix five hand materials to clean the mixer once) ----- The powder can be granulated in the cooling mixing tank to 40 degrees Celsius or added to the twin-screw production (the cooling mixing tank must have circulating cooling water). When mixing, you must pay attention to the temperature and time. For example, the temperature and time of mixing in a common cold pot are about 20-25 minutes, and the hot pot is about 12-15 minutes (temperature setting is about 125 degrees), if the time difference is 3 minutes or longer It may be a problem with the temperature sensing line or inaccurate temperature control, or serious wear of the horn knife of the mixer. This will affect the stability and color of the expansion ratio. When granulating, it is necessary to pay attention to the accuracy of the temperature control, whether the fan is running normally, the size of the current, the particle size, the surface, and the length of the particle, whether the material is stable, and the material must not be interrupted.

The mixing process affects the dispersion of each component by affecting the contact and reaction between different components, and then affects the material properties. When mixing, you should choose the appropriate feeding sequence, feeding temperature, and feeding time. Because the wood flour powder is fluffy, the phenomenon of "bridging" and "holding poles" is prone to occur during the feeding process. Unstable feeding will cause extrusion fluctuations and reduce the quality of extrusion. Therefore, the feeding method and feeding amount must be strictly controlled. Generally, forced feeding devices or starvation feeding are used to ensure the stability of extrusion. Extrusion and foaming of PVC/wood powder composite materials generally has two process routes: two-step method and one-step method: the two-step method is to pelletize and then shape; the one-step method eliminates the pelletizing process and uses surface modified wood Powder and PVC powder are mixed at high speed and then directly added and extruded. Research shows that the masterbatch method (two-step method) is beneficial to improve the mechanical properties of PVC/wood plastic.

2. Molding temperature

Extrusion process:

Mainly pay attention to the following aspects

a. The key to the temperature setting of each section of the extruder is the compression section, the metering section and the die. The accuracy of each temperature control such as inaccurate temperature control affects the foaming ratio and color. Foaming is mainly the setting of temperature in the compression section and metering section, and molding is mainly the mold temperature.

b, the normal operation of each section of the fan.

c, the size and stability of the host current. If the host current fluctuates greatly, it means that the dispersion of lubricant, processing aid, fiber and PVC components is not very uniform.

d. The speed of the host must be kept the same, because the speed will affect the foaming ratio and color.

Setting the extrusion molding temperature should take into account the physical function and chemical reaction of the material in the extruder barrel. The temperature of the feeding section must not only ensure that the material can be melted quickly, prevent the escape of decomposition gas, but also prevent the foaming agent from decomposing in advance; the temperature setting of the compression section and the metering section needs to take into account the decomposition temperature and decomposition rate of the chemical foaming agent, wood powder Factors such as scorching and PVC decomposition; the temperature of the machine head should keep the melt good fluidity and at the same time have sufficient melt viscosity to maintain the melt in the machine head under high pressure, so that it does not foam in the machine head . Yao Zhuping believes that low temperature extrusion should be used under fully plasticized conditions. Equipment such as screws and molding dies should also have low temperature extrusion characteristics to ensure that the cells have a good shape and a small diameter. The temperature of the feeding section should be controlled below 165℃, the compression section and homogenization section should be between 160~180℃, and the head and die should be set below 160℃.

3. Screw speed

  The influence of screw speed on extrusion foaming is mainly reflected in the following aspects: First, it affects the extrusion pressure. The higher the speed, the greater the pressure in the extruder, which is more conducive to nucleation, and the number of nucleated cells is also The more, the higher the foaming rate. However, when the pressure is too high, the nucleated cell growth is inhibited, which affects the full growth of the cells; the second is that the higher the screw speed, the stronger the shearing effect, and the too strong the shearing effect will easily cause the cells to merge or rupture, affecting the development Foam quality and the formation of low-density foam; third, the screw speed is too high or too low, so that the residence time is too short or too long, and it is prone to pre-foaming or insufficient foaming agent decomposition, which is not conducive to the formation of uniform and dense的cellular structure. Therefore, under the condition that other influencing factors remain unchanged, the screw speed has an optimal value, generally between 12~18r/min.

4. Extrusion pressure

  Insufficient extrusion pressure will cause rough surface and low strength of the product, and higher extrusion pressure can not only control the gas-containing melt in the die not to foam in advance, but also make the pressure difference between the inside and outside of the die The high rate of decrease is conducive to the nucleation of bubbles, the number of nucleated bubbles increases, and the foaming rate also increases, which is beneficial to obtain a uniform and dense cell structure. However, too high extrusion pressure is detrimental to cell growth. To obtain the appropriate head pressure, it can be achieved by adjusting the screw speed, head temperature and die shape.

5. Molding equipment

  The single-screw extruder mainly relies on friction to convey materials, and the mixing effect is poor. The wood flour stays in the barrel for a long time and is easy to burn. Therefore, it is greatly restricted in the extrusion of PVC/wood-plastic composites. In order to improve the mixing effect of PVC/wood plastic, the single screw used for PVC/wood plastic processing should be equipped with a mixing zone, or pelletized first, and then extruded with pellets, but this process consumes additives and reduces PVC performance.

Second, the shortcomings of PVC wood plastic products

A discoloration problem

There are many reasons for discoloration, such as: ultraviolet rays, temperature, humidity, air and the pH of the environment where the product is located.

How to prevent product discoloration:

a. Choose toners with good color fastness, migration resistance, high temperature resistance and heat resistance.

b Select the lubricant that is not easy to precipitate.

c Add antioxidants and ultraviolet absorbers to improve the weather resistance of products.

d Meet the processing requirements and reduce the processing temperature as much as possible.

e The surface of the product is sprayed with weather-resistant paint with good weather resistance.

f Co-extruded materials with good weather resistance.

B deformation problem

The deformation is mainly due to the low thermal deformation of the product. To improve the thermal deformation resistance, we mainly start with the following aspects:

a As far as possible, do not add or add less additives that affect material deformation such as low melting point lubricants, CPE, DOP, epoxy soybean oil, etc.

b Reasonable lubrication system and dosage.

c Reasonable production technology such as production speed, cooling water temperature.

d Reduce the internal stress of the product.

Reasonable design of e mold.

F placement and storage of semi-finished products.

g The type and amount of filling are reasonably matched to improve the related mechanical properties of wood plastic such as impact strength, bending strength, modulus, etc.

C flame retardant problem

The material composition of wood plastic has flame retardant properties. The key is to increase its oxygen index and reduce the amount of smoke during combustion. It can be filled with flame retardant, and chlorinated paraffin is used as plasticizer.