Major Types of Blow Molding + A Useful Glossary

There are several types of blow molding processes. Your product needs and specifications will determine which type of blow molding is best for you.

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Here is an introduction to the different types of blow molding to help you decide, along with a list of industry-specific words you may encounter as you create your product.

Injection blow molding

Injection blow molding is normally used to make bottles in large quantities. The method is actually two processes combined. The first step is to injection mold a plastic pre-form. The pre-form typically looks like a test tube, with the thread detail that will be used on the finished bottle neck molded into the open end of the tube.

The injection molded pre-forms are moved (sometimes over long distances) to the blow molding machine where they are reheated, placed into the blow molds, and blown into the finished shape of the bottle. The largest market currently uses the material polyethylene terephthalate (PET).

This method has both advantages and drawbacks. On the plus side, it’s ideal for making very precise bottlenecks, thanks to the core rod used in the injection portion of the process.

However, you can’t use injection blow molding to make more complicated items, such as complex bottle shapes with handles, most consumer goods, or industrial parts.

Stretch blow molding

Stretch blow molding is a variation of injection blow molding, which is also primarily used to make clear PET bottles. Two-liter soda bottles are one of the most common stretch blow molded products.

In the stretch blow molding variation of injection blow molding, the pre-form is both blown and stretched. The core rod increases the length of the pre-form as air is forced into it so that the product lengthens and widens at the same time to fill the mold cavity. The core rod stops lengthening near the opposite wall of the mold.

Stretch blow molding allows you to make the same bottle using less material than in regular injection blow molding. Though this provides an economic advantage, it is counterbalanced by the fact that stretch blow molding machinery is relatively expensive.

Extrusion blow molding

Extrusion blow molding allows the molder to make the most complex product shapes. The two most popular forms of extrusion blow molding are continuous extrusion blow molding and intermittent extrusion blow molding. Here’s a breakdown of the types of products that can be created with each.

Continuous extrusion blow molding

Used to make small, hollow products

Intermittent extrusion blow molding

Used to make more complex products

Milk bottles Automotive air ducts, fluid reservoirs, and filter housings Detergent bottles Sporting goods, such as balls, bats, coolers, lantern & stove kits, boat fenders and decoys Juice bottles Seating, furniture, and room organizers Motor oil bottles Playground equipment and toys, such as trikes, wagons, and play houses Plastic tubes Specific carrying cases for tools, musical instruments, and other shaped objects Medicine bottles Medical products, such as hospital beds, stretchers, walkers, and bedpans Small drums 55-gallon drums and large industrial parts

Both types of extrusion blow molding accommodate a wide variety of resin types. Polyethylene (PE) and polypropylene (PP) are the most commonly used.

Basic process for continuous extrusion blow molding:

• The extruder is similar to an auger that continuously mixes, melts, and feeds the resin under pressure. Extrusion dies create tube shapes of molten plastic. This process forms a continuous tube called a parison.
• The parison is then placed inside the product blow mold. The mold is usually halved and mounted on platens, which are opened and closed by clamp systems.
• Pressurized air is blown into the inside of the parison until it expands to fill the cavity of the mold, forming the desired product shape.
• The hot plastic product cools and hardens against the mold. Then, the mold opens, releasing the part.
• Finally, the extra plastic on the ends, called the flash, is removed for recycling.

Basic process differences for intermittent extrusion blow molding:

• The viscous resin from the extruder is pushed into an accumulator head, which is comprised of a chamber that collects a set amount of molten plastic resin and a pair of cylinders that expel the resin in a shot. The cylinders also control the shape of the plastic as it leaves the accumulator.
• When the volume of resin pushed into the accumulator reaches the capacity needed for the part, the molten plastic is forced through an extrusion die around a core, called a mandrel or pin. This creates a tube that determines the inner diameter inside of a perimeter ring, which creates the outer diameter.
• The dies are moveable so the wall thickness in the tubular parison can be changed as it is formed or shot from the accumulator head, allowing the maximum control of finished product detail.

You can see the intricate ways that your product specifications will determine what process will be best for you. There are also specialized derivations of extrusion blow molding, such as reciprocating screw blow molding, muiti-layer blow molding, 3D blow molding, and suction blow molding that provide additional capability for specific end markets.

Custom-Pak is an industry leader in extrusion blow molding. Learn more about our technology and services.

Glossary of Blow Molding Terms

Accumulator: The reservoir, ram piston, and die-control piston system where melted plastic resin from the extruder collects in intermittent extrusion blow molding. Once the accumulator has reached its set maximum, a ram forces the melted resin from the accumulator through the moving die, which creates the parison that will best conform to the final product shape.

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Clamp force: The maximum force in tons that the clamp holding the mold can exert. This force keeps the mold halves together against the force of air pressure inside the mold.

Clamp stroke (also called daylight opening): The maximum distance between the two clamp mold mounting plates (called platens) when fully open. This determines maximum mold shut height and defines how far the clamp can open and still provide clearance for removing the product.

Die: A tool for forming materials into a desired shape. In extrusion blow molding, a die is used to form the parison.

Die gap: The distance between the inner mandrel and the outer ring die, which defines the parison wall thickness and ultimately dictates the thickness of the walls of the final product.

Ejection: The removal of the finished product from the mold.

Extruder: The screw and barrel auger system where resin is heated, mixed, and pressurized until it becomes viscous.

Extruder capacity: The amount of resin, in pounds or kilograms per hour that the extruder can process.

Flash: Any excess plastic discarded from the perimeter of the product after it is removed from the mold.

Mandrel: The inner core pin half of the die that determines the interior circumference of the parison.

Mold: The metal shape of a manufactured product that fits on the platens of a blow molding machine and contains the plastic under pressure while it cools into the final part shape.

Parison: The tube of melted plastic resin formed by the blow molding die.

Parting line: The perimeter of the part shape where the two mold halves meet.

Pinch-off: The sharp part of the perimeter of the mold that defines the part shape when it closes on the plastic parison and separates the part from the excess flash.

Ring: The outer half of the extrusion die that determines the outer circumference of the parison.

Shrinkage: The difference in height, width, diameter, and other measurements on the finished product compared to the dimensions of the mold. Shrinkage is affected by the material, material thickness, and rate of cooling. It is an important factor in mold design.

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