Classifications of Blow Molding | Types of Blow Molding

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In Extrusion Blow Moulding (EBM), plastic is melted and extruded into a hollow tube called a parison. This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the desired hollow bottle, container, or part. Once the plastic has sufficiently cooled, the mold is opened and the part is ejected. There are two primary types of Extrusion Blow Molding: Continuous and Intermittent. In Continuous Extrusion Blow Molding, the parison is continuously extruded, and individual parts are cut off by an appropriate knife. In Intermittent Blow Molding, there are two methods: the straight intermittent method, similar to injection molding, consists of turning and stopping the screw to push the melt out; the accumulator method collects melted plastic, and when enough plastic has accumulated, a rod pushes it out to form the parison. This method can use a continuously or intermittently turning screw. Continuous extrusion struggles with precise wall thickness control due to the parison's weight, whereas the accumulator or reciprocating screw methods, using hydraulic systems, offer better wall thickness control through die gap adjustment with a parison programming device.

The process of injection blow molding (IBM) is utilized for producing a large number of hollow glass and plastic objects. The IBM process involves injection molding polymer onto a core pin, rotating the core pin to a blow molding station where it is inflated and cooled. This process, the least utilized among the three blow molding types, is typically used for making small medical and single-serve bottles. The process is divided into three steps: injection, blowing, and ejection.

IBM has two primary methods: Single-stage and Two-stage processes. The single-stage process is further divided into three-station and four-station machines. In the two-stage injection stretch blow molding (ISBM) process, plastic is initially molded into a “preform” using injection molding. These preforms, featuring the necks and threads of the bottles, are later reheated and blown into bottles using high-pressure air in metal blow molds. The preform is always stretched with a core rod during the process.

There are several blow molding processes available, each suitable for specific product needs and specifications.

Understanding the different types of blow molding can help you make an informed decision, along with familiarizing yourself with industry-specific terminologies that may be helpful as you develop your product.

Injection Blow Molding

Injection blow molding is typically used for producing bottles in large quantities. This method combines two processes. First, it involves injection molding a plastic preform, usually resembling a test tube with threaded detail on the open end. The pre-forms are then transported, sometimes over long distances, to the blow molding machine, where they are reheated, placed into blow molds, and blown into the final bottle shape. Polyethylene terephthalate (PET) is the most commonly used material for this process.

While injection blow molding is great for producing precise bottle necks, it is not suitable for creating complex items like bottles with handles, consumer goods, or industrial parts.

Stretch Blow Molding

Stretch blow molding is a variation of injection blow molding mainly used for making clear PET bottles, such as two-liter soda bottles. In this method, the preform is both blown and stretched. The core rod increases the preform’s length while air is blown in, causing it to lengthen and expand, forming the product to the mold cavity. Stretch blow molding allows for the creation of bottles using less material compared to standard injection blow molding, thus offering cost advantages. However, the machinery for stretch blow molding is relatively expensive.

Extrusion Blow Molding

Extrusion blow molding is ideal for creating more complex product shapes. There are two main types: Continuous extrusion blow molding and intermittent extrusion blow molding.

Continuous Extrusion Blow Molding

This method is used to make small, hollow products such as milk bottles, detergent bottles, and juice bottles.

Intermittent Extrusion Blow Molding

This process is for more complex products, including automotive air ducts, fluid reservoirs, industrial parts, and sporting goods.

• Continuous extrusion: Utilized for small products like milk and juice bottles and small drums.
• Intermittent extrusion: Suitable for more complex items such as automotive parts, medical products, and large industrial parts.

Both extrusion blow molding methods can work with a variety of resin types, with polyethylene (PE) and polypropylene (PP) being the most common.

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Basic Process for Continuous Extrusion Blow Molding:

• The extruder works like an auger, continuously mixing, melting, and feeding the resin under pressure. Extrusion dies create molten plastic tubes called parisons.
• The parison is placed inside the product mold, typically halved and mounted on platens which are clamped shut.
• Pressurized air is blown into the parison, expanding it to fill the mold and forming the desired product shape.
• The mold cools and hardens the plastic, then opens to release the part.
• Excess plastic, known as flash, is removed for recycling.

Differences in Intermittent Extrusion Blow Molding:

• Viscous resin from the extruder is pushed into an accumulator head, which collects a set amount of molten plastic. Cylinders expel this resin in a shot while shaping it.
• When the resin volume reaches the required capacity, it is forced through an extrusion die around a mandrel or pin, forming a tubular shape with controlled inner and outer diameters.
• Movable dies allow for changing the wall thickness of the tubular parison, providing maximum control over the final product.

These detailed processes will help you determine the most suitable blow molding method for your product specifications. Additionally, specialized techniques such as reciprocating screw, multi-layer, 3D, and suction blow molding offer capabilities for specific markets. Notably, the extrusion blow molding sector has numerous industry leaders, like Custom-Pak, which offers advanced technology and services.

Glossary of Blow Molding Terms

• Accumulator: A reservoir system in intermittent extrusion blow molding where melted plastic collects until it is expelled by a ram to form the parison.
• Clamp force: The maximum force the mold clamp can exert to hold the mold halves together against air pressure during molding.
• Clamp stroke: The maximum distance between the mold mounting plates (platens) when fully opened, affecting the clearance for removing the finished product.
• Die: A tool for shaping materials, specifically for forming the parison in extrusion blow molding.
• Die gap: The distance between the mandrel and die, determining the parison wall thickness and ultimately the product walls.
• Ejection: The process of removing the finished product from the mold.
• Extruder: A screw and barrel system where resin is heated, mixed, and pressurized until it becomes viscous.
• Extruder capacity: The amount of resin the extruder can process per hour, measured in pounds or kilograms.
• Flash: Excess plastic removed from the product after molding.
• Mandrel: The inner core pin in the extrusion die determining the parison's interior circumference.
• Mold: A metal shape holding the plastic under pressure while it cools into the final product shape.
• Parison: The tube of melted plastic formed by the blow molding die.
• Parting line: The perimeter where mold halves meet in the final product shape.
• Pinch-off: The sharp mold part defining the product shape and separating it from excess flash.
• Ring: The outer half of the extrusion die determining the parison's outer circumference.
• Shrinkage: The difference in product dimensions compared to the mold, influenced by material, thickness, and cooling rate, crucial for mold design.

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