Influencing factors in the blow molding process

Author: MULAN - Plastic Molding Manufacturer

Influencing factors in the blow molding process 

During the blow molding process, the influencing factors mainly include inflation pressure, blowing speed, inflation ratio and blow mold temperature 

1. 1. Inflation pressure : In the blow molding process, compressed air has two functions: one is to use the pressure of compressed air to inflate the semi-molten tube blank and cling to the cavity wall to form the desired shape; It is used for cooling blow molded products. The air pressure depends on the type of plastic and the temperature of the parison, and is generally controlled at 0.2-1. OMPa. For plastics with low melt viscosity and easy deformation (such as PA, HDPE), take a lower value; for plastics with high melt viscosity (such as PC), take a higher value, and the parison wall thickness also takes a higher value . The blowing pressure is also related to the volume of the product. Higher inflation pressure should be used for large-volume products, and lower inflation pressure should be used for small-volume products. The most suitable inflation pressure should be able to make the shape and pattern of the product clear after molding.

3. 2. Blowing speed : In order to shorten the blowing time so that it is beneficial to obtain a more uniform thickness and a better appearance of the product, it is required to inject a large flow of air at a low air flow rate to ensure that the parison can be uniform and smooth in the mold cavity. Rapid expansion shortens the cooling time in the mold cavity and improves product performance. The low air velocity can also avoid the occurrence of a Venturi effect in the parison to form a partial vacuum and cause the parison to collapse. This can be ensured by using a larger blow tube.

5. 3. Inflation ratio : When the size and quality of the parison are constant, the larger the size of the product, the greater the inflation ratio of the parison, but the thinner the thickness of the product. The inflation ratio is usually determined according to the type and nature of the plastic, the shape and size of the product, and the size of the parison. As the inflation ratio increases, the thickness of the product becomes thinner, and the strength and stiffness decrease; at the same time, molding becomes difficult. Generally, the inflation ratio is controlled at l:(2—4) left and right.

7. 4. Temperatura : The temperature of the blow molding mold has a great influence on the quality of the product (especially the appearance quality). Usually the temperature distribution of the mold should be uniform, so that the product can be cooled evenly as much as possible. The temperature of the mold is related to the type of plastic, the thickness and size of the product. For different types of plastics, there are a few plastics (PC blow molding bottles) and the mold temperature should be controlled in sections. Production practice has proved that if the mold temperature is too low, the elongation of the plastic at the jaw will be reduced, and it will not be easy to inflate, making the product thicker at this part, and at the same time molding is difficult, and the contour and pattern of the product surface are not clear; the mold temperature is too high , The cooling time is prolonged, the production cycle is increased, and the productivity is reduced. At this time, if the cooling is not enough, it will also cause the product to be demolded and deformed, the shrinkage rate will increase, and the surface gloss will deteriorate. Generally, for plastics with relatively rigid molecular chains, the mold temperature should be higher; while for plastics with relatively flexible molecular chains, the mold temperature should be lowered. The cooling time of hollow blow molded products in the mold is longer, the purpose is to ensure that the products are fully cooled and not deformed when demoulding. Cooling time generally depends on the thickness, size, shape, and type of plastic of the plastic product. The thicker the wall thickness of the product, the longer the cooling time. The cooling time of 61PE products with large specific heat capacity is longer than that of PP products with the same wall thickness and small specific heat capacity.

9. 5. Molding cycle : The production cycle of blow molding includes the process of extruding the parison, clamping the mold, cutting the parison, blowing air, deflation, opening the mold, and taking out the product. The principle of this cycle selection is to shorten as much as possible on the premise of ensuring that the product can be shaped without deformation, so as to improve production efficiency.