Blow Mold Tooling Vs Injection Molding

Most of the products we use everyday require molding plastic parts. There are several processes to do so, depending on the type of application and desired type of part. Blow molding and injection molding are the two most popular ways to produce high quality and economical plastic parts. Let us first understand the two processes.

Blow Molding:

In the blow molding process, a plastic tube is heated and filled with air until it blows into a hot plastic balloon called a "parison." This parison is clamped within a mold and while air fills it into and forms the desired shape, the plastic is trapped in the mold. The weight of the plastic shot used in the mold is an important factor that determines the size of the machine, blow mold tooling and the associated costs for production.

The process is identical in intent to glass blowing. The process manufactures one-piece hollow objects in high volume. Plastic bottles for beverages are made using this process. Blow molding creates uniform, thin-walled containers in a very economical way. Prototype Blow Molds and Sampling are being experimented upon to increase productivity.

Blow molding has many advantages including:

• Costs are lower when compared to injection molding
• Machinery and blow mold tooling costs are lower
• One-piece construction that can achieve shapes that injection molding cannot

Injection Molding:

In injection molding, creating the mold is the key to building a perfect part. Injection molding molds are made of stainless steel or aluminum. These molds are injected with liquid polymers at high temperatures under extreme pressure and involve high upfront engineering to develop detailed tools or molds. Plastic parts are released after the molds have been cooled. This process is best to produce high-volume orders and mass production. High precision match between mold halves are required to control the material flow.

Plastic injection molding has many advantages:

• Detailed, highly engineered tools
• Multi-cavity mold options
• Accurate processing for large volumes of small parts
• Flexibility in change of material or color
• Efficient use of material and low scrap yield

The product produced poses the biggest difference between injection molding and blow molding. Blow molding is used to produce hollow, singular containers, such as bottles for beverages. Injection molding is designed to produce solid pieces, like plastic products. Blow molding creates a hollow product by, literally pushing air into the product and injection molding creates solid parts. Also, with blow molding everything has to be kept in mind; temperature, type of plastic, pressure/velocity, and speed, while with injection molding, the actual making of the mold is the most important part of the process. The mold and core relationship determines the wall thickness of a part made by injection molding. However, a blow-molded part’s thickness depends on how the material has been stretched; its wall thickness varies in places depending on how much it has been stretched.

Molding Procedure:

In injection molding, the plastic is sealed in the injection chamber and mold throughout the entire process. But in blow mold, the process begins when the mold is removed from the plastic, which means the product can be blown further to expand. There cannot be any air present in injection molding as it can create air pockets or bubbles. This might lead to weak spots in the product, making it defective and removed.

With the blowing method, air is vital as the blow mold tooling process revolves around air forced into the mold in order to push out and expand the product. Extrusion blow molding involves the plastic to be melted and extruded into a hollow tube. In stretch or injection stretch process, the plastic is molded into a pre-form and then heated and stretched with blow mold tooling.

Injection molding and blow molding are done for a variety of industries, producing small, medium, and multi-part products of large volume, using a wide variety of materials.