By: Yamile Cornejo Meyer, Date: 02/13/2018
Summary
- Blow molding is a molding process in which air pressure is used to inflate soft plastic into a mold cavity. It is an important industrial process for making one-piece hollow plastic parts with thin walls, such as bottles and similar containers. Since many of these items are used for consumer beverages for mass markets, production is typically organized for very high quantities.
Background
- The technology behind blow molding originated with glass blowing. Enoch Ferngren and Willian Kopitke were the first to use the blow molding process with glass in 1938. There was not much demand for blow molded plastic products at that time.
- The technical mechanisms needed to produce hollow bodied workpieces using the blowing technique were established very early on. Because glass is very breakable, after the introduction of plastic, plastic was being used to replace glass in some cases. The first mass production of plastic bottles was done in America in 1939.
- In the United States soft drink industry, the number of plastic containers went from zero in 1977 to ten billion pieces in 1999. Today, even a greater number of products are blown and it is expected to keep increasing.
Application
- Bottles and containers are the most common blow molded products, but other items such as highway barrels, automotive components, double walled cases, toys, medical items, and structural panels are produced.
- Advantages
- 1. Low initial mold tooling costs
- 2. Flexibility of tooling: Molds can accommodate interchangeable neck finishes and body sections
- 3. Flexibility in production: Neck inner diameters can be easily controlled to varying requirements. Bottle weights are adjustable
- 4. Container sizes can range from less than 1 oz. to 55 gallons and up
- 5. Container shape is not restricted by blow-up ratios. Bottles can be long and flat or have handles
- 6. Wide selection of machine sizes: Molds can be geared to volume requirements
- 7. Quick production with high volume outputs
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1. Blow molding is limited to hollow forms, such as plastic containers and bottles, as air pressure is an integral component of the process.
- 2. Wall thickness is also hard to control, as the larger the product being built gets, the thinner the polymer has to be stretched.
- 3.Plastic products made by the process of blow moulding aren't biodegradable, which presents an obvious environmental hazard, especially when empty containers find their way to landfills
Process Details
- Blow molding is accomplished in two steps: (1) fabrication of a starting tube of molten plastic, called a parison; and (2) inflation of the tube to the desired final shape.
- Forming the parison is accomplished by either of two processes: extrusion or injection molding.
- (1) Extrusion is used to push a molten tube of plastic, called a parison, through a die
- (2) Compressed air is blown into the parison, forcing it against the cold walls of the mold
- (3)The molded piece is then cut and removed from the mold
- (1) Parison is injection molded around a blowing rod
- (2) Injection mold is opened and parison is transferred to a blow mold
- (3) Soft polymer is inflated to conform to a blow mold
- (4) Blow mold is opened and blown product is removed
- (1) Parison is injection molded around a blowing rod
- (2) Injection mold is opened and parison is transferred to a blow mold
- (3) The parison is elongated by a stretching rod
- (4) The stretched parison is then blown to the mold walls
- (5) After cooling, the part is released
Costs
- Semi Automatic Blow Molding Machine: $3,500-$50,000
- Fully Automatic Blow Molding Machine: $50,000-$350,000
- EXAMPLE: For a simple cone made of high density polyethylene that is 5” tall, has a narrow diameter of 1” and a maximal diameter of 3”, and has wall thickness of 1/8” the typical Unit Cost is $.30