Medium pressure injection foaming molding machine
DATE：2022/4/12 8:36:00 / READ： / SOURCE：This station
Medium pressure injection foaming molding machine
The medium pressure method can significantly improve the surface quality of injection foaming products. Although the mold cavity pressure is higher than that of the low-pressure method, it is obviously lower than that of the high-pressure method, so it is called the medium pressure method. Its forming principle: the mold cavity is first filled with nitrogen, so that the plastic melt injected into the mold cavity cannot be foamed under the pressure of inert gas, and then it can be foamed through exhaust, discharge or mold expansion. The skin layer cannot be foamed due to low temperature, but only the core melt is foamed. As a result, the specific pressure value with uniform color and smooth skin layer depends on the critical pressure of forming bubbles in the melt in the mold cavity; Fig. 6-20 (2) when the nozzle valve 4 is opened, the plastic melt is injected into the mold cavity, and then the air supply valve is gradually opened to gradually reduce the air pressure in the mold cavity due to the injection pressure exerted by the screw on the plastic melt. Press the melt towards the mold wall: Fig. 6-20 (3) the screw retreats to make the body in the mold cavity return, the pressure in the mold cavity decreases, and the core rate of the melt in the mold cavity is high, so it is foamed and expanded, and part of the foamed melt is pressed back to the material; Figure 6-20 (4) close the nozzle, and the foamed products are cooled and shaped in the mold cavity.The defect of this method is that when the melt in the mold cavity is foamed and expanded, some foamed melt flows back into the barrel of the reciprocating screw, and when these melts are molded for the next product, they are first injected into the mold cavity, which will affect the surface quality of the product. Therefore, this structure has not been put into production in large quantities. Dow company uses TAF process in Japan. Its principle is shown in Figure 6-21. It is mainly used to process amorphous polymers. Physical foaming agent or chemical foaming agent can be used. Bulgana TM back pressure method makes up for the deficiency of allied method. TM back pressure method adds an injection barrel. The plasticized plastic melt is stored in the injection barrel first. During injection, the plunger presses the plastic melt into the mold cavity. When the plastic melt in the mold cavity foams and expands, the excess foamed melt is pressed back into the injection material. When the second product is injected, these foamed melts are finally injected into the mold cavity, so it only affects the interior of the product. Fig. 6-21 Dow TAF method and sprue system. Film coating composite production line www.handern.com com
First fill with pressure gas, close the gas valve, and the plasticized plastic melt (including foaming agent) is sent to the injection cylinder (i.e. storage cylinder) through the distributor; Push the original foaming melt in the cylinder upward. Fig. 6-22 (2) when the melt collected in the injection barrel reaches one injection volume, the distributor connects the channel between the injection barrel and the mold cavity, the injection plunger presses all the melt into the mold cavity through the distributor, and the gas is pressed back, as shown in the figure. Figure 6-22 (3) when the surface of the plastic melt entering the mold cavity has formed a solid smooth skin with the required thickness, open the air valve to discharge the pressure gas in the mold cavity, the plunger in the injection barrel returns, and the melt in the mold cavity foams and expands to press the excess foamed melt back into the injection barrel. Fig. 6-22 (4) open the mold and take out the cured air appliance. The plunger in the injection barrel compresses the foaming melt in the injection barrel and turns it into a reverse cycle. At lower temperatures, the reduction will rupture. Film coating composite production line www.handern.com com
Another structure foam molding method is similar to the principle of back pressure, but the process and equipment structure are different from the above. This method is also to inject the plastic melt containing foaming agent into the mold cavity under the back pressure of the gas, but it is not full. There is a thin tube in the center of the injection nozzle. The pressure gas passes through the thin tube into the interior of the melt in the mold cavity and presses the melt towards the mold wall. After the surface melt solidifies, the internal gas is discharged to foam the interior of the melt to obtain the structural foam. The exhaust can control the expansion speed of the melt in the mold, and the back pressure method can also produce foam products with relatively high density. The method is relatively simple. After the melt is filled into the mold cavity, the mold will not expand and the melt in the mold cavity will not return. Instead, the shrinkage of the plastic melt itself provides space for the foam of the core melt. Although the surface of the product obtained 281 by this method is relatively smooth, the relative density is large. The relative density can be reduced by 5% ~ 7% for amorphous polymers and 15% or more for bulk polymers. This method has been applied to the production of HDPE and PP structural foam products. The structure of the equipment formed by this method is relatively simple, as long as the air compressor, vent valve and mold with sealing device are added. The corresponding clamping force is 2.5 ~ 4 times larger than that of low-pressure method. Film coating composite production line www.handern.com com
Compared with other injection foaming methods, the molded foam products can be molded at low speed because the pressure of the gas can prevent the foaming of the melt until the air pressure is removed. The low melt flow rate limits the unstable sliding of the melt along the die wall, leaving no trace of plain flow. It reduces the shear and friction near the wall, makes the melt flow more uniform, the front of the material flow will not break, and can better maintain the dissolved gas. These are helpful to improve the quality of smooth surface of products. The uniformity of melt viscosity is one of the important factors affecting the uniformity of bubble body. The time of melt filling under the action of gas pressure directly affects the thickness of structural bubble skin. As shown in Fig. 6-23, the thickness of skin is proportional to the square root of the action time of gas pressure.
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