Views: 0 Author: Site Editor Publish Time: 2025-11-28 Origin: Site
Introduction: Hollow blow molding is one of the most commonly used plastic molding methods, and also one of the fastest-growing. Plastic products are widely used in various industries; it can be said that hollow blow molded products have become integrated into our lives. This article provides a brief overview of hollow blow molding, offered for reference by our procurement partners in the premium supply chain.
Blow Molding Raw Materials
Plastics used for blow molding include polyethylene, polyvinyl chloride, polypropylene, polystyrene, linear polyester, polycarbonate, polyamide, cellulose acetate, and polyacetal resin.
High-density polyethylene (HDPE) is the most consumed material, widely used in food, chemical, and liquid packaging. High molecular weight polyethylene is suitable for manufacturing large fuel tanks and drums. Polyvinyl chloride (PVC), due to its good transparency and airtightness, is widely used in cosmetic and detergent packaging. With the development of non-toxic PVC resins and additives, as well as the advancement of stretch blow molding technology, the use of PVC containers in food packaging has increased rapidly, and it has begun to be used in packaging beer and other carbonated beverages.
Linear polyester material is a new material that has entered the hollow blow molding field in recent years. Due to its advantages such as glossy appearance, excellent transparency, high mechanical strength, good preservation of contents, and no environmental pollution when waste is incinerated, it has developed rapidly in the packaging bottle industry, especially in the use of pressure-resistant plastic food containers.
Due to advancements in resin modification and processing technology, the usage of polypropylene has been increasing year by year.
Extrusion-blow molding: a simple production method with high output but low precision, and it has many applications.
Injection blow molding: high precision, good quality, high price, suitable for large-volume production.
Stretch blow molding: The product is stretched, resulting in high strength and good airtightness.
This includes extrusion-stretching-blow molding (abbreviated as extrusion-stretching-blow molding).
There are two methods: injection stretch blow molding (abbreviated as injection stretch blow molding).
1) Blowing pressure
In blow molding, compressed air serves two purposes: first, it expands the tube blank to fit tightly against the mold cavity wall, forming the desired shape; second, it acts as a cooling agent.
The air pressure varies depending on the type of plastic and the temperature of the preform, and is generally controlled between 0.2 and 0.7 MPa. The most suitable pressure is one that allows the shape and pattern of the product to be clearly visible after molding.
For plastics with low viscosity and easy deformation, use lower values; for plastics with high viscosity and modulus, use higher values; 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 achieve a more uniform thickness and better surface in the product, the inflation speed (the volume of air flowing per unit time) should be as high as possible, but not too high, otherwise it will have adverse effects on the product: First, it will create a vacuum at the air inlet, causing the parison in this part to sink in. When the parison is fully inflated, the sinking part will form a diaphragm. Second, the parison in the die part may be broken by the extremely fast airflow, resulting in scrap.
Therefore, it is necessary to increase the diameter of the blowpipe or appropriately reduce the inflation speed.
3) Blow-up ratio
The ratio of the product's dimensions to the preform's dimensions is usually called the blow-up ratio. When the preform's dimensions and weight are constant, the larger the product's dimensions, the larger the preform's blow-up ratio. The blow-up ratio is determined by the type and properties of the plastic, the shape and size of the product, and the preform's dimensions. Typically, the blow-up ratio is controlled at 2 to 4 times.
4) Mold temperature and cooling time
To ensure product quality, the mold temperature should be evenly distributed, generally maintained between 20-50 ℃ . If the mold temperature is too low, the extensibility of the plastic at the clamping point will decrease, making it difficult to inflate and causing the product to thicken in this area. This also makes molding more difficult, resulting in unclear product contours and patterns. If the mold temperature is too high, cooling time will be prolonged, extending the production cycle. The product may also deform upon demolding, and shrinkage will increase.
The mold temperature depends on the type of plastic. When the glass transition temperature of the plastic is high, a higher mold temperature can be used; conversely, the mold temperature should be reduced as much as possible.
Generally, the cooling time increases with the increase of the product wall thickness. 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 introduced into the product for direct cooling.
5) Molding cycle
The blow molding cycle includes processes such as extruding the parison, cutting the parison, closing the mold, blowing air, cooling, venting, opening the mold, and removing the product (subsequent finishing, fitting, and packaging are separate).
