Preliminary Study on the Forming Process of PVC Straw Composite Foam Material

There are soft and hard points, so the temperature required for molding is not the date of the drafting:. 2007-y4reserved.http:// Straw is a waste of agriculture and a valuable natural resource. At present, the utilization rate and utilization level of straw in China are very low, and most of them are directly incinerated, which causes very serious environmental pollution and even has a serious impact on the safety of aviation and highway traffic. Polyvinyl chloride (PVC) is widely used in industry and daily life, and is the second largest plastic product in the world after polyethylene. The disposal of waste PVC is the key to solving the "white pollution".

PVC straw composite foaming material is a kind of continuous uniform distribution microporous hair which is made by mixing waste PVC straw fiber, auxiliary filler, auxiliary agent, modifier and foaming agent in a certain proportion and heating and melting by plastic molding equipment. Bubble composite. The composite material effectively utilizes the straw and waste PVC products to have a significantly reduced density compared with the unfoamed PVC straw composite material, and saves a large amount of raw materials, and the tensile elastic modulus and the flexural modulus are also lower than that of the unexpanded. The material has been improved a lot, and the sound insulation and heat insulation performance are better. It is also possible to adjust the formula design and manufacture different wood-like materials, such as imitation mahogany furniture decorative panels, shock-proof linings for equipment packaging, and the like. The thermal insulation, shockproof and seismic properties of wood-plastic foam materials have broad prospects for packaging applications.

At present, the molding of PVC straw composite foaming material has three methods of compression molding, injection molding and continuous extrusion molding. The equipment and process parameters required for the three molding methods are different, and the cost control and process route design are also different.

1 Molding molding In order to make the straw in the material evenly dispersed and the cell size is about the same, a two-step molding method is generally used in the molding, that is, the modified straw fiber, PVC and various auxiliary agents are first granulated, and then the particles are granulated. It is placed in a hot press for molding. The process of the process is as follows. The product obtained by the molding method has a smooth appearance and good physical and chemical properties, but the process is complicated.

11 The effect of temperature on the product The two-step molding process is the same. The molding temperature of soft PVC is generally selected at 160 ° C, and the temperature of rigid PVC is generally around 175 ° C. The material with different straw fiber content during molding has different temperature requirements. The higher the content, the higher the required temperature, but not too high, to prevent the foaming agent from foaming in advance. Before pressing, there must be a preheating stage. The temperature can be selected from 150~170C to ensure that the material completely fills the cavity and proper flashing, and it is easy to ensure the quality of the surface of the product and the size of the product. In the pressurization phase, the effect of temperature is relatively small as long as the decomposition temperature of the blowing agent is reached. The demoulding temperature is generally selected at 80~100 °C for 12 hours and the effect of pressure on the product. For molding time and pressure, different materials have different requirements.

The length of the molding time is generally judged according to the condition of the flash. For materials with good fluidity, the molding pressure can be selected as small points, and the corresponding molding time is also short. This is to avoid the uneven surface of the product caused by too much flash. Empty, affecting the performance of the product. After the pressurization, the pressure is maintained, and the purpose of the pressure keeping is to fully blend the materials in the cavity, so that the performance of the product is greatly improved.

2 Injection molding foam injection molding machine is another equipment for preparing PVC straw composite foaming material, and the process includes low pressure structure foaming and microcellular foaming. The low-pressure structure foamed injection molded product has a dense surface, small cavity pressure during the manufacturing process, small clamping force and low production cost, so it is widely used in practical industrial production, especially when molding some small parts. . In the preparation of PVC straw composite foaming material, a two-step molding method is also often used. First, the modified straw fiber is mixed with a chemical foaming agent, a nucleating agent, a crosslinking agent and the like, and is cut into a rubber particle. The rubber pellets were blended and granulated with PVC by twin-screw extruder to obtain foamable PVC pellets. Finally, the injection molding machine was used for injection molding. 14.21 Injection time and injection rate influence on the injection time of the product was affected. The important parameters of the bubble size and uniformity of the foaming material. If the injection time is too long, the melt flowing along the surface of the cavity will be prematurely cooled, which increases the flow resistance and energy consumption, thereby reducing the number of cells in the product. Excessive injection rates cause strong shearing of the melt and the cavity to give a rough surface. Proper injection rate is the key to improving the surface quality of the product.

22 The influence of injection pressure on the product The low pressure foaming method requires a low cavity pressure of the injection mold, generally 2~7MPa and the ordinary injection is 30~6 (MPa low pressure foaming injection generally adopts the low injection method, filling time Short, the bubble formation time is short, so that the bubble is smaller, and the bubble distribution is more uniform. If the injection pressure is higher, the stress-bearing melt will generate more cells when flowing through the channel; the injection pressure is small. The resulting cells are large and unevenly distributed.

23 The influence of mold temperature on the product The change of the mold temperature has an effect on the surface roughness of the product, the thickness of the surface layer, the production cycle, etc., especially the number of cells. When the mold temperature is high, the surface roughness of the product is low and the surface layer is thin, but the production cycle is long. Under the same conditions, the isothermal filling and non-isothermal filling of the melt have a great influence on the formation of bubbles. When the isothermal filling is not performed, the number of bubbles generated by the foaming of the melt is less than that of the isothermal filling. After the melt is filled, the bubbles generated at the center are more. 15. Therefore, the mold temperature can be used to control the cell size and distribution during actual production, and even local control can be realized by different methods in different parts of the same product. Different density.

3 Extrusion foam molding At present, the molding equipment for industrial production of PVC straw composite foaming material mainly includes single-screw extruder, conical twin-screw extruder and co-directional (distinct) rotating parallel twin-screw extruder. The material can be formed by two-step method and one-step method. The two-step method is the most commonly used process, that is, the dried and surface-modified straw fiber and PVC are first placed in a high-speed mixer, and after sufficient agitation, extrusion or granulation is performed by a single-screw or twin-screw extruder. The method can improve the dispersibility of the straw fiber in the PVC matrix, and the feeding after granulation is also relatively easy, but the whole process is complicated. The one-step method uses the surface-modified straw fiber and the PVC powder to be high-speed after mixing at a high speed.

31 The effect of temperature on the product The temperature of each section of the extruder is an important parameter affecting the foaming rate. The setting of the extrusion molding temperature should meet the needs of the physical and chemical reaction process of PVC and auxiliary materials in the barrel of the extruder. The temperature of the feed section should not be too high, and the chemical foaming agent should be prevented from decomposing in advance in the feed section. Extrusion temperature requirements can make PVC and Other materials melt quickly; the temperature of the compression section and the metering section should be high enough to ensure the normal decomposition of the chemical foaming agent and the full plasticization of PVC, but the temperature setting should not be too high; the temperature of the head is on the product. The mass has the greatest influence, the temperature inside the head is such that the melt maintains good fluidity, and sufficient melt viscosity is maintained to maintain the melt in the head under high pressure.

In general, the extrusion temperature should be set according to the melt decomposition temperature of the PVC, the decomposition temperature of the blowing agent, and the charring degradation temperature of the straw fiber.

Effect of extrusion pressure on the product The extrusion pressure directly affects the surface roughness, bending strength, foaming rate and the like of the product. The higher extrusion pressure can not only control the gas-containing melt in the machine head to not foam in advance, but also increase the pressure difference between the internal and external die of the die, thereby increasing the pressure drop rate and facilitating the increase of the foaming rate. A uniform and fine cell structure is obtained. 16. The proper die pressure can be achieved by adjusting the screw speed, the die temperature and the shape of the die.

The influence of screw speed on the product has a great influence on the extrusion pressure and the quality of the foam. 171. If the screw speed is too high or too low, the residence time will be too short or too long, and it is easy to cause foaming or foaming in advance. The phenomenon that the agent is not sufficiently decomposed is not conducive to the formation of a uniform and fine cell structure. Therefore, in the case of extrusion molding, an optimum screw speed is sought in the case where other influencing factors remain unchanged.

4 Development trend of molding equipment By designing new screw, new mixing equipment, optimizing screw combination and other methods to develop a one-step molding equipment, reducing cost and process difficulty.

Develop large-scale, wide-width, thick plate products and molding technology to improve the application of PVC straw composite foam materials, such as density and size can not meet the actual needs, and continue to expand the application of the material.

The introduction of computer simulation and control technology 181, from the simulation of the formulation effect to the determination of the various parameters of the equipment, all achieve computer expert decision-making, reduce the cost of research and development and improve efficiency.

Adding, out of the granulation of this sworker c equipment to her h bubble material can be irigl machine. i on Thttp:// to develop a universal equipment, so that various plastic straw composite hair 5 knot in the formation process of PVC plant fiber composite foam material, due to the different types of plant fibers and PVC models Differences, no matter which molding process is used, there are many problems that need to be considered and valued. The resolution of these problems will directly affect the performance of the products, so it is necessary to discuss the processing technology.

(Finish)

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