What is hollow blow molding?

Hollow blow molding
Hollow blow molding is placing the tube blank obtained by extrusion or injection molding in a mold, blowing compressed air into the tube blank to make it close to the cavity wall, and then cooling and demold to obtain a hollow product The molding method. This molding method can produce various packaging containers such as bottles, pots, barrels, daily necessities, and children's toys.
Plastics used for hollow blow molding include polyethylene, polyvinyl chloride, polypropylene, polystyrene, linear polyester, polycarbonate, polyamide, cellulose acetate, and polyacetal resin. High-density polyethylene consumes place and is widely used in food, chemical, and liquid packaging. High molecular weight polyethylene is suitable for manufacturing large fuel tanks and barrels. Polyvinyl chloride is commonly used in the packaging of cosmetics and detergents because of its better transparency and air tightness. With the development of non-toxic polyvinyl chloride resins and additives, as well as the development of stretch blow molding technology, the amount of polyvinyl chloride containers used in food packaging has increased rapidly, and they have begun to be used for packaging beer and other carbon dioxide gas beverages. Linear polyester is a new type of material that has entered the field of hollow blow molding in recent years. Because its products have the advantages of glossy appearance, transparency, high mechanical strength, and good preservation of the contents of the container, and the advantages of not polluting the environment during waste incineration, it has developed rapidly in packaging bottles, especially It is widely used in pressure-resistant plastic food containers.
Extrusion-blow molding: simple production method, high output, low precision, and more applications
Injection-blow molding: high precision, good quality, high price, suitable for large batch products.
Stretch blow molding: The product is stretched, with high strength and good air tightness, including extrusion-stretching-blow molding (referred to as extrusion-stretch-blow) and injection-stretching-blow molding (referred to as note-stretch-blow) two methods.
Blowing pressure
Compressed air has two functions in blow molding: one is to expand the tube blank close to the mold cavity wall to form the required shape, and the other is to cool the tube.
The air pressure is also different depending on the type of plastic and the temperature of the parison. It is generally controlled between 0.2 and 0.7 MPa. Suitable pressure is the pressure that can make the shape and pattern of the product appear clear after molding.
For low-viscosity and easy-to-deform plastics, take the lower value; for higher-viscosity and modulus plastics, take the higher value; for large-volume and thin-walled products, use higher-pressure; for small-volume and thick-walled products, use lower pressure.
2) Inflation speed
To shorten the blowing time and help the product to obtain a more uniform thickness and a better surface, the inflation speed (the volume of air flowing in a unit of time) should be as large as possible, but it should not be too large, otherwise, it will cause damage to the product. Adverse effects: It will create a vacuum at the air inlet, which will cause the parison in this part to sink. When the parison is completely inflated, the indented part will form a diaphragm; secondly, the parison in the die part may be The extremely fast airflow breaks, causing waste.
To this end, it is necessary to increase the diameter of the blowpipe or appropriately reduce the inflation speed.
3) Blow-up ratio
The ratio of the size of the product to the size of the parison is usually called the blow-up ratio. When the size and weight of the parison are constant, the larger the size of the product, the greater the inflation ratio of the parison. The blow-up ratio is determined according to the variety and nature of the plastic, the shape and size of the product, and the size of the parison. Usually, the blow-up ratio is controlled at 2 or 4 times.
4) Mold temperature and cooling time
To ensure product quality, the temperature of the mold should be evenly distributed, and the mold temperature is generally maintained at 20-50°C. If the mold temperature is too low, the extensibility of the plastic at the jaws will be reduced, it will not be easy to inflate, and the product will be thicker in this part, at the same time, it will be difficult to form, and the contour and pattern of the product will be unclear. The mold temperature is too high, the cooling time is prolonged, and the production cycle is prolonged. The product is deformed after demolding, and shrinkage increases.
The mold temperature depends on the type of plastic. When the glass transition temperature of the plastic is higher, a higher mold temperature can be used; otherwise, the mold temperature should be reduced as much as possible.
Generally, as the wall thickness of the product increases, the cooling time is prolonged. Sometimes in addition to cooling the mold, the molded product can also be internally cooled, that is, various cooling media (such as liquid nitrogen, carbon dioxide, etc.) are passed into the product for direct cooling.
5) Molding cycle
The cycle of blow molding includes the processes of extruding the parison, cutting the parison, closing the mold, blowing, cooling, venting, opening, and taking out the product (the subsequent trimming, matching, and packaging are separately included).