Also known as hollow blow molding, it is a rapidly developing plastic processing method. Blow molding technology began to be used to produce low-density polyethylene bottles during World War II. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology has been widely used. The volume of hollow containers can reach thousands of liters, and some production has been computer-controlled. Plastics suitable for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc., and the resulting hollow containers are widely used as industrial packaging containers. According to the method of making the parison, blow molding can be divided into extrusion blow molding and injection blow molding, and the newly developed ones are multi-layer blow molding and stretch blow molding.
Basic Information
The tubular plastic parison obtained by extrusion or injection molding of thermoplastic resin is placed in a split mold while hot (or heated to a softened state). After the mold is closed, compressed air is immediately introduced into the parison to inflate the plastic parison and cling to the inner wall of the mold. After cooling and demolding, various hollow products are obtained. The manufacturing process of blown film is very similar to that of hollow product blow molding in principle, but it does not use a mold. From the perspective of plastic processing technology classification, the molding process of blown film is usually included in extrusion. Blow molding technology began to be used to produce low-density polyethylene bottles during World War II. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology has been widely used. The volume of hollow containers can reach thousands of liters, and some production has adopted computer control. Plastics suitable for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc., and the resulting hollow containers are widely used as industrial packaging containers.
About blowing mould
Polymer
The most common blow-molded extrusion material is high-density polyethylene, the polymer that makes up most milk bottles. Other polyolefins are also commonly processed by blow molding. Depending on the application, styrene polymers, polyvinyl chloride, polyesters, polyurethanes, polycarbonates, and other thermoplastics can also be blow molded.
Recently engineering plastics have been widely accepted in the automotive industry. Material selection is based on mechanical strength, weather resistance, electrical properties, optical properties and other properties.
Mix
Various types of melt mixing equipment are used, from roll mills and batch mixers to single-screw and twin-screw extruders. Continuous mixers (extruders) are the most commonly used equipment because they provide consistent product quality and reduce operating costs. There are two types of mixing: Distributive mixing The materials are evenly distributed in the mix without the need for high shear stresses. This type of mixing is called extensional mixing or laminar mixing.
Dispersive mixing, also known as intensive mixing, involves the application of high shear stresses to break up agglomerated solids. When agglomerates of additives, for example, are broken up, the actual particle size becomes smaller.
Compounding operations often require both types of mixing in one process.
Extrusion
Polymer compounding is defined as the process of upgrading a polymer or polymer system by melt mixing. Compounding processes range from the addition of a single additive to multiple additive treatments, polymer alloys, and reactive blends. It is estimated that one-third of polymer production in the United States is compounded. Compounds can be customized based on the performance requirements of the final application. Compounded products have mixed properties, such as high gloss and excellent impact strength, or precision moldability and good stiffness.
Compounded polymers are usually pelletized for further processing. However, there is a growing interest in combining compounding with further steps, such as profile extrusion, which avoids reheating the polymer.
Blowing
Three-quarters of blow molded products are made by extrusion blow molding. The extrusion process is to force the material through a hole or mold to make the product.
The extrusion blow molding process consists of 5 steps: 1. Plastic mold (extrusion of hollow plastic tube ); 2. Close the mold on the mold, clamp the mold and cut the mold; 3. Blow the mold against the cold wall of the mold cavity, adjust the opening and maintain a certain pressure during cooling; 4. Open the mold and remove the blown parts; 5. Trim the flash to get the finished product.
Blow molding application range
Common products include daily necessities such as bottles, cans, baby products, sports equipment, etc.
Extrusion blow molding is a method for manufacturing hollow thermoplastic parts. Well-known blow-molded objects are bottles, barrels, cans, boxes and all containers for packaging food, beverages, cosmetics, medicines and daily necessities. Large blow-molded containers are usually used for packaging chemical products, lubricants and bulk materials. Other blow-molded products are balls, bellows and toys. For the automotive industry, fuel tanks, car shock absorbers, seat backs, center brackets and armrest and headrest covers are blow-molded. For the machinery and furniture manufacturing industries, blow-molded parts are shells, door frames, racks, pots or boxes with an open face.
Extrusion blow molding
Extrusion blow molding is a manufacturing process used to produce hollow plastic parts, typically bottles, containers, and other similar products. It is a variation of the blow molding process and involves the following steps:
Extrusion:
- The process starts with melting plastic resin (usually in the form of pellets) in an extruder.
- The molten plastic is forced through a die to form a hollow tube called a parison. The size and shape of the parison depend on the design of the die.
Molding:
- The parison is captured between two halves of a mold cavity.
- Compressed air is then blown into the parison, causing it to expand and conform to the shape of the mold.
- The plastic material cools and solidifies against the mold walls.
Part Ejection:
- Once the plastic has cooled and solidified, the mold opens, and the part is ejected.
- Any excess plastic material (flash) is trimmed off the final product.
Extrusion blow molding is commonly used for manufacturing plastic products with a hollow interior, such as bottles, containers, tanks, and drums. It is a versatile process that allows for the production of items in various shapes, sizes, and colors. The process is relatively fast and cost-effective, making it suitable for high-volume production.
Advantages of extrusion blow molding include:
- Cost-Effective: The process is well-suited for high-volume production, which can help reduce per-unit costs.
- Design Flexibility: It allows for the production of complex shapes and designs.
- Uniform Wall Thickness: It can create products with consistent wall thickness, which is important for structural integrity.
- Fast Production: The process is relatively fast compared to other manufacturing methods.
- Low Tooling Costs: Compared to other molding processes, the tooling costs for extrusion blow molding are often lower.
Overall, extrusion blow molding is a widely used manufacturing process in the plastic industry for producing a variety of hollow plastic products efficiently and cost-effectively.
Injection blow molding
Injection blow molding is a manufacturing process used to produce hollow plastic parts, primarily bottles and containers, by combining elements of injection molding and blow molding. This process typically involves the following steps:
Injection:
- Initially, a parison (preform) is injection molded in a mold cavity using a screw-type injection molding machine.
- The parison is a preform with a hole or opening at one end, which is created by injecting molten plastic into a split steel mold.
Blowing:
- The preform is transferred to a blow mold cavity via a core rod.
- Compressed air is then blown into the preform, expanding it to conform to the shape of the mold. This process forms the final hollow part.
Cooling and Solidifying:
- The plastic material is allowed to cool and solidify against the mold walls.
- Once the part has cooled sufficiently, the mold opens, and the finished product is ejected.
Injection blow molding is often used to produce small to medium-sized plastic containers such as bottles for beverages, personal care products, pharmaceuticals, and household chemicals. This process offers advantages such as:
- High Precision: Injection molding allows for precise control over wall thickness and part dimensions.
- Uniformity: The process can produce parts with consistent wall thickness and high accuracy.
- Good Surface Finish: Injection blow molding can yield products with a smooth and aesthetically pleasing surface finish.
- Design Flexibility: It enables the production of complex shapes and intricate designs.
- Material Efficiency: The process can be efficient in terms of material usage, leading to reduced waste.
Injection blow molding is particularly well-suited for applications where high precision, dimensional stability, and good surface finish are essential. It is commonly used in industries where high-quality hollow plastic parts are required in large volumes.
