Knowledge about blow molding

In the blow molding process, the introduction of compressed air has two functions. One is to use compressed air to expand the molten tube blank and close to the cavity wall to form the desired shape; the second is to cool the blow molded product. Depending on the type of plastic and the temperature of the parison, the air pressure is also different, generally controlled at 0.2-0.7 MPA. For plastics with low viscosity and easy deformation, such as polyamide and fiber-based plastics, lower values ​​are obtained; for plastics with higher viscosity and modulus, such as polycarbonate and polyethylene, higher values ​​are obtained. In addition, the inflation pressure is also related to the size of the product and the thickness of the parison. Higher pressures are generally preferred for large volume and thin walled products, while lower pressures are used for small volume and thick walled articles. It is most suitable to make the product show a clear pressure on the shape, pattern and the like after molding.

2. Inflation speed

In order to shorten the blowing time, in order to facilitate the product to obtain a more uniform thickness and a better surface, the inflation speed (the volume of air flowing per unit time) should be as large as possible, but it should not be too large, otherwise it will bring the product Adverse effects, first, will create a vacuum at the air inlet, so that this part of the parison is indented, and when the parison is fully inflated, the invagination part will form a diaphragm; nextly, the parison of the die part has It may be broken by an extremely fast airflow, causing waste. For this purpose, it is necessary to increase the diameter of the blow pipe or appropriately reduce the inflation speed.

3. Blowing ratio

The ratio of the size of the article to the size of the parison is generally referred to as the inflation ratio, i.e., the expansion ratio of the parison. When the size and weight of the parison are constant, the larger the size of the article, the larger the blow ratio of the parison. The size of the inflation ratio is determined according to the type and nature of the plastic, the shape and size of the product, and the size of the parison. Increasing the inflation ratio can save material, but the wall thickness of the product is thinned, the strength and rigidity are lowered, and molding is also difficult. The blow ratio is too small, the amount of plastic consumed is increased, the effective volume of the product is reduced, the cooling time is lengthened, and the cost is increased. Therefore, the inflation ratio is usually controlled to 2-4 times.

4. Mold temperature and cooling time

In order to ensure the quality of the product, the temperature of the mold should be evenly distributed, and the product should be uniformly cooled during the cooling process, and the mold is generally kept at 20-50 degrees. If the mold temperature is too low, the elongation of the plastic at the jaws is lowered, the expansion is not easy, and the product is thickened in this portion, and the molding is difficult, and the outline and pattern of the product are unclear. The mold temperature is too high, the cooling time is extended, and the production cycle is lengthened. At this time, if the cooling is insufficient, the product may be deformed, the shrinkage is increased, and the surface is dull. The temperature of the mold depends on the type of plastic. When the glass transition temperature of the plastic is high, a higher mold temperature can be used; otherwise, the mold temperature is lowered as much as possible.

In order to prevent the polymer from deforming due to elastic recovery, the article is deformed. Blow molded articles typically have a longer cooling time. It is usually 1/3-2/3 of the molding cycle, depending on the type of plastic and the shape of the product. For example, a polyethylene with a lower thermal conductivity requires a longer cooling time than a polypropylene of the same thickness. The cooling time is generally extended as the thickness of the article increases. In order to shorten the production cycle and speed up the cooling rate, in addition to cooling the mold, the molded product can be internally cooled, that is, directly cooled into the product by various cooling media (such as liquid nitrogen, carbon dioxide, etc.), and currently A new heat pipe cooling technology has emerged.

5. Molding cycle

The cycle of blow molding includes the steps of extruding the parison, cutting the parison, clamping, blowing, cooling, deflation, opening the mold, and taking out the product. The principle of this cycle selection is to minimize the number of products that can be finalized. However, due to the poor consistency and accuracy of the operation, the cooling effect of the mold is poor, and the artificial lengthening cycle will directly affect the efficiency and benefit. This is a problem that must be paid attention to in industrial production.