Hollow blow molding is to place the tube blank obtained by extrusion or injection molding in a mold, blow compressed air into the tube blank to make it close to the wall of the mold cavity, and then cool 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.

　　One, blow molding materials

　　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 the first 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 widely 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 has begun to be used for packaging of beer and other beverages containing carbon dioxide gas. . Linear polyester material 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, excellent 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. Due to the modification of its resin and the progress of processing technology, the use of polypropylene has also increased year by year.

　　 2. Process classification and characteristics

　　 Hollow blow molding technology

　　 1. According to the different classification of tube blank forming methods: 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.

　　2. According to different types of molding process:

　　Ordinary blow molding

　　Stretch blow molding: The product is stretched, with high strength and good air tightness. Extrusion-stretch-blow molding (referred to as extrusion-stretch-blow). Injection-stretching-blow molding (referred to as note-stretch-blow).

　　3, according to the different classification of the tube layer:

　　single layer blow molding

　　Multilayer blow molding: good overall performance, complex production, suitable for product packaging with high packaging requirements.

　　Three, influencing factors

　　1 Blowing pressure: In blow molding, compressed air has two functions. One is to expand the tube blank and close to the cavity wall to form the required shape; the other is to cool. According to the different types of plastics and the temperature of the parison, the air pressure is different, generally controlled between 0.2-0.7 MPa, and it is suitable to make the shape and pattern of the product show clear pressure after molding. For low viscosity and easy to deform, take the lower value; for the higher viscosity and modulus plastics, take the higher value; use higher pressure for large volume and thin-walled products; use lower pressure for small volume and thick-walled products .

　　2. Inflation speed: In order to shorten the blowing time and help the product obtain a more uniform thickness and better surface, the inflation speed (volume of air flowing in a unit time) should be as large as possible, but it should not be too large, otherwise It will have adverse effects on the product: First, it will create a vacuum at the air inlet, which will cause the parison in this part of the parison. When the parison is completely inflated, the indented part will form a diaphragm; the second is the die part. The parison may be pulled off by the extremely fast airflow, resulting in waste. For this reason, it is necessary to increase the diameter of the blowpipe or appropriately reduce the inflation speed.

　　3. Blow-up ratio: The ratio of product size to parison size is usually called 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 inflation ratio is controlled at 2 or 4 times.

　　4. Mold temperature and cooling time: In order 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 thickened in this part. At the same time, it will make the molding difficult 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 deforms after demolding and shrinks. 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. Usually 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, deflating, opening, and taking out the product (the subsequent trimming, matching, and packaging are separately included).

　　 Fourth, product features and applications

　　 Hollow blow molding technology

　　LDPE: Food packaging container

　　HDPE: commodity container

　　Ultra-high molecular weight PE: large containers, melting tanks

　　PVC: mineral water, detergent bottles

　　PP: Thin-walled bottle

　　PET: Beverage bottle

Polycarbonate bottles can be used to fill product packaging at high temperatures, but if polycarbonate bottles are used to contain carbonated beverages that require good barrier properties (to prevent the escape of carbon dioxide in the beverage) or edible oil (to prevent oxygen from entering the bottle, In order to avoid oxidation and rancidity of the edible oil), the product cannot be protected well and the ideal packaging effect cannot be obtained. The use of ordinary polyester stretch bottles to package carbonated beverages can effectively prevent the escape of carbon dioxide in the beverages (polyester stretch bottles have excellent barrier properties), and use them in the packaging of edible oils to delay their oxidative deterioration (poly The shovel stretch bottle has good barrier properties and can effectively prevent atmospheric oxygen from entering the bottle through the container wall), but it is used to package high-temperature filled juice products. When filled at high temperature, the polyester stretch bottle will cause serious problems. Deteriorate and lose use value. Polyethylene bottles can contain substances such as acids and alkalis and are not suitable for containing organic solvents such as benzene and toluene (the swelling strength of polyethylene bottles is significantly reduced or organic solvents escape through the container wall). Although nylon containers are not suitable for storing substances such as acids and alkalis, they are very suitable for containing organic solvents such as benzene and xylene. Multi-layer composite containers PE/PA/PE and PE single-layer containers are almost identical in appearance. Even experts in the plastics industry can hardly separate them based on the difference in appearance. However, the performance of the two containers The difference is very large, especially in terms of barrier properties to oxygen, carbon dioxide, nitrogen and organic solvents.