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Hollow blow molding is to put the tube blank obtained by extrusion or injection molding into a mold, injecting compressed air into the tube blank to inflate it, and then adhering it to the cavity wall, and then cooling to release the hollow product. Forming method. This molding method can produce various packaging containers such as plastic 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 resins. Among them, the consumption of high-density polyethylene is the first. It is widely used in the packaging of food, chemical and process liquids. High molecular weight polyethylene is suitable for the manufacture of large fuel tanks and barrels.
Hollow blow molding classification
1, according to different molding methods
Extrusion - blow molding: simple production method, high output, low precision, and more applications.
Injection molding - blow molding: high precision, good quality, high price, suitable for high volume products.
2, according to different molding processes
Ordinary blow molding;
Stretch blow molding: The product is stretched, high strength and good air tightness.
Extrusion-stretching-blow molding (referred to as extrusion-pull-blow)
Injection molding - stretching - blow molding (abbreviation - pull - blow)
3, according to the number of tube blanks
Single layer blow molding;
Multi-layer blow molding: good overall performance, complex production, suitable for product packaging with high packaging requirements.
Blow molding device
1, needle blowing method
Process: Closing - piercing the parison wall - inflation - needle retraction - healing of the melt
Features: Suitable for continuous rotary blow molding, small containers, with handles.
2, top blowing method
Process: closed - bottom clamped - top opening - core ventilation - inflation - rotating tool to remove excess material
Features: core for blowing core, mechanism simplification
Factors affecting the blow molding process
1, blowing pressure
In blow molding, compressed air has two functions. One is to make the tube swell and close to the cavity wall to form the desired shape; the other is to cool. Depending on the type of plastic and the temperature of the parison, the air pressure is not the same, generally controlled between 0.2-0.7 MPa, and the most suitable is to make the product show clear pressure under the shape and pattern 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 Bad effects.
3, the inflation ratio
The ratio of the size of the article to the size of the parison is generally referred to as the inflation ratio. 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. The blow ratio is usually controlled at 2 or 4 times.
4, mold temperature
In order to ensure the quality of the product, 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 elongation of the plastic at the jaws is lowered, the swelling 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 not clear. The mold temperature is too high, the cooling time is extended, and the production cycle is lengthened. The product is deformed and the shrinkage is increased. 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.
5, molding cycle
The cycle of blow molding includes the process of extruding the parison, cutting the parison, clamping, blowing, cooling, deflation, opening, and taking out the product (subsequent finishing, matching, packaging, etc.).