Extrusion Blow Moulding: A Comprehensive Guide

 

Extrusion blow moulding is a widely used manufacturing process for producing hollow plastic parts. From everyday items like milk bottles and shampoo containers to industrial components like fuel tanks and drums, this versatile technique offers a cost-effective solution for mass production. This blog post delves into the intricacies of extrusion blow moulding, exploring its process, advantages, disadvantages, applications, and providing a specific example to illustrate its practical implementation.

To know various type of blow molulding technique please click here

The Extrusion Blow Moulding Process

The extrusion blow moulding process involves several key steps:

1. Melting the Plastic: Plastic resin, typically in the form of pellets or granules, is fed into an extruder. The extruder uses a rotating screw and heating elements to melt the plastic into a homogenous molten state.
2. Extruding the Parison: The molten plastic is then forced through a die, which shapes it into a hollow tube called a parison. The parison is extruded vertically downwards between the two halves of an open mould.
3. Capturing the Parison: Once the parison reaches the desired length, the mould closes, pinching off the bottom of the parison and sealing it within the mould cavity.
4. Blowing and Cooling: Compressed air is injected into the parison through a blow pin or needle. The air pressure inflates the parison, forcing it outwards to conform to the shape of the mould cavity. The mould is cooled, typically using water channels, to solidify the plastic part.
5. Ejection and Trimming: After the part has cooled sufficiently, the mould opens, and the finished product is ejected. Excess plastic, such as flash or trim, may be present and is removed in a trimming operation.

Types of Extrusion Blow Moulding

There are two main variations of extrusion blow moulding:

• Continuous Extrusion Blow Moulding: In this method, the parison is continuously extruded, and a cutting mechanism separates individual parisons before they are clamped into the mould. This technique is suitable for high-volume production of smaller parts.
• Intermittent Extrusion Blow Moulding: This method involves extruding a parison to a specific length and then stopping the extrusion process while the mould closes and the blowing process occurs. This technique is often used for larger parts or when variations in parison length are required.

Advantages of Extrusion Blow Moulding

• Cost-Effectiveness: Extrusion blow moulding offers a relatively low tooling cost compared to other plastic processing methods like injection moulding. This makes it particularly suitable for large production runs.
• High Production Rates: The process is capable of achieving high production rates, especially with continuous extrusion blow moulding.
• Design Flexibility: Extrusion blow moulding can produce a wide variety of shapes and sizes, including complex geometries and parts with handles or necks.
• Material Versatility: A wide range of thermoplastic materials can be used in extrusion blow moulding, including high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), and polyethylene terephthalate (PET).

Disadvantages of Extrusion Blow Moulding

• Limited to Hollow Parts: The process is inherently limited to producing hollow parts. Solid parts cannot be manufactured using this technique.
• Wall Thickness Variation: Achieving uniform wall thickness can be challenging, especially in complex shapes.
• Material Limitations: While a variety of materials can be used, some materials with high viscosity or low melt strength may not be suitable for extrusion blow moulding.

Applications of Extrusion Blow Moulding

Extrusion blow moulding is used to produce a wide range of products across various industries:

• Packaging: Bottles for beverages, food, personal care products, and household chemicals.
• Automotive: Fuel tanks, coolant reservoirs, air ducts, and bumpers.
• Consumer Goods: Toys, watering cans, and storage containers.
• Industrial Products: Drums, tanks, and large containers for chemicals and other liquids.

Specific Example: Manufacturing of a Plastic Fuel Tank

Let's consider the example of manufacturing a plastic fuel tank for an automobile using extrusion blow moulding:

1. Material Selection: High-density polyethylene (HDPE) is chosen for its excellent chemical resistance, impact strength, and ability to withstand temperature variations.
2. Extruder and Die Design: The extruder is designed to melt and homogenize the HDPE resin. The die is designed to produce a parison with the desired shape and dimensions for the fuel tank.
3. Mould Design: The mould is designed to create the final shape of the fuel tank, including any necessary features like mounting points, filler necks, and connections for fuel lines. The mould also incorporates cooling channels to ensure efficient solidification of the plastic.
4. Extrusion and Blowing: The HDPE parison is extruded vertically downwards between the open mould halves. Once the parison reaches the correct length, the mould closes, capturing the parison. Compressed air is then injected into the parison, inflating it to fill the mould cavity.
5. Cooling and Ejection: The mould is cooled using water circulation, allowing the HDPE to solidify and retain its shape. The mould then opens, and the finished fuel tank is ejected.
6. Trimming and Testing: Excess plastic is trimmed from the fuel tank, and various tests are conducted to ensure it meets the required quality standards for leak resistance, impact strength, and chemical compatibility.

Conclusion

Extrusion blow moulding is a versatile and cost-effective manufacturing process for producing hollow plastic parts. Its ability to handle a wide range of materials and produce complex shapes makes it a popular choice across various industries. By understanding the process, its advantages and disadvantages, and its diverse applications, manufacturers can effectively leverage extrusion blow moulding to create high-quality plastic products for a wide range of needs.