Hollow blow molding is the extrusion or injection molding of the resulting billet placed in the mold, in the billet through the compressed air will be blown up, so that it is tightly adhered to the wall of the mold cavity, and then cooled off the mold to get the molding method of hollow products. This molding method can produce bottles, pots, buckets and other packaging containers, daily necessities and children's toys.

 First, blow molding raw materials

 Plastic varieties used for hollow blow molding are polyethylene, polyvinyl chloride, polypropylene, polystyrene, linear polyester, polycarbonate, polyamide, cellulose acetate and polyacetal resin. They are widely used for packaging of food, chemicals and handling liquids. High molecular weight polyethylene is suitable for the manufacture of large fuel tanks and drums, etc. Polyvinyl chloride is commonly used in the packaging of cosmetics and detergents because of its good transparency and air tightness. With the development of non-toxic PVC resins and additives, as well as the development of stretch blow molding technology, the use of PVC containers in food packaging is rapidly increasing, and has begun to be used in the packaging of beer and other beverages containing carbon dioxide gas. Linear polyester is a new material that has entered the field of hollow blow molding in recent years. Because of its products have a glossy appearance, excellent transparency, high mechanical strength and containers of goods preserved better, waste incineration and disposal without polluting the environment and other aspects of the advantages, so the development of packaging bottles very quickly, especially in the pressure-resistant plastic food containers are widely used. Polypropylene because of its resin modification and processing technology progress, the use of the amount also increases year by year.

 Second, process classification and characteristics

 Hollow blow molding technology

 1, according to the different classification of the billet molding method: extrusion - blow molding: simple production methods, high yield, low precision, more applications. Injection - blow molding: high precision, good quality, high price, suitable for large batch products.

 2、Classified according to different molding process:

 Ordinary blow molding

 Stretch blow molding: the product is stretched, high strength, good air tightness. Extrusion - stretch - blow molding (referred to as extrusion - pull - blow). Injection - stretch - blow molding (referred to as injection - pull - blow).

 3、Classified according to the different layers of the billet:

 Single-layer blow molding

 Multi-layer blow molding: good overall performance, complex production, suitable for packaging of products with high packaging requirements.

 Third, the factors affecting

 1 blowing pressure: blow molding, compressed air has two roles, one is to make the billet expansion and close to the wall of the mold cavity, the formation of the desired shape; the second is to play a cooling effect. According to the plastic varieties and the temperature of the billet, the air pressure is not the same, generally controlled between 0.2-0.7 MPa, suitable for products in the molding of the shape, pattern and other exposed clear pressure. For the lower viscosity, easy to deform take the lower value; for the viscosity and modulus of the higher plastic take the higher value; large volume and thin-walled products should be used with higher pressure; small volume and thick-walled products use lower pressure.

 2, inflatable speed: in order to shorten the blowing time, in order to facilitate the products to obtain a more uniform thickness and better surface, inflatable speed (volume of air flowing through the unit of time) to be as large as possible, but should not be too large, otherwise it will bring adverse effects to the products: First, it will be in the air inlet to create a vacuum, so that this part of the parison inward sinking, when the parison is completely blown, the inward part of the formation of diaphragm piece; secondly Secondly, the mouth of the mold part of the blank may be pulled off by the airflow, resulting in scrap. For this reason, it is necessary to increase the caliber of the blowpipe or appropriately reduce the inflation rate.

 3, blowing ratio: usually the size of the product and the size of the parison ratio is called the blowing ratio. When the size and weight of the billet must be, the larger the size of the product, the larger the size of the billet blowing ratio. According to the plastic species, nature, shape and size of the product and the size of the blank to determine the size of the blowing ratio. Usually the blowing ratio control in 2, 4 times.

 4, mold temperature and cooling time: in order to ensure product quality, the temperature of the mold should be evenly distributed, the mold temperature is generally maintained at 20-50 ℃. Mold temperature is too low, will make the plastic at the mouth of the plastic elongation is reduced, it is not easy to blow up, and make the products in this part of the thickening, and at the same time make the molding due to the difficulty of the products, such as the contour and pattern is not clear. The mold temperature is too high, the cooling time is prolonged, and the production cycle is lengthened. The products are deformed and shrinkage increases. Mold temperature depends on the variety of plastics, when the glass transition temperature of the plastic is high, you can use a higher mold temperature; Conversely, as far as possible to reduce the mold temperature. Usually with the increase of the wall thickness of the product, the cooling time is extended. Sometimes in addition to the cooling of the mold, can also be internal cooling of molded products, that is, to the internal products into a variety of cooling medium (such as liquid nitrogen, carbon dioxide, etc.) for direct cooling.

 5、Molding cycle: the cycle of blow molding includes extruding the parison, intercepting the parison, closing the mold, blowing, cooling, deflating, opening the cross, take out the products (and then trimming, matching, packaging and so on) and other processes.

 Product characteristics and applications

 Hollow blow molding technology

 LDPE: food packaging containers

 HDPE: commodity containers

 Molecular weight PE: large containers, melt tanks

 PVC: mineral water, detergent bottles

 PP: thin-walled bottles

 PET: beverage bottles

 Polycarbonate bottles can be used for high-temperature filling commodity packaging, but if the polycarbonate bottle is used to contain carbonated beverages that require good barrier properties (need to prevent carbon dioxide from escaping from the beverage) or edible oil (to prevent oxygen from entering the bottle, so as not to oxidize the edible oil, rancidity), can not be very good protection of commodities, do not get the desired packaging effect. If ordinary polyester stretchable bottles are used to package carbonated beverages, the carbon dioxide in the beverages can be effectively prevented from escaping (polyester stretchable bottles have excellent barrier properties), and if they are used to package edible oils, the oxidative deterioration can be slowed down (the barrier properties of polychu stretchable bottles are good, and they can effectively prevent atmospheric oxygen from entering into the bottles through the walls of the containers), but if they are used for the packaging of high-temperature-filled fruit juices, the polyester stretchable bottles will be severely deteriorated at high temperature filling. Stretch bottles will undergo serious deterioration and lose their use value. Polyethylene bottles can be filled with acid and alkali substances, but should not be filled with organic solvents such as benzene and toluene (the swelling strength of polyethylene bottles decreases significantly or organic solvents escape through the walls of the container). Although nylon containers are not suitable for storing acid and alkali substances, they are very suitable for organic solvents such as benzene and xylene. Multi-layer composite containers PE/PA/PE and PE single-layer containers, almost identical in appearance, even for connoisseurs in the plastics industry, it is difficult to separate them from the difference in appearance, but the two containers in the performance of the difference is very large, especially for oxygen, carbon dioxide, nitrogen and organic solvents barrier properties.