Blow Molding vs. Injection Molding

Plastics professionals understand the basic differences between blow molding versus injection molding; however, many plastic buyers and users are not always sure of the difference.
The primary difference is the kind of product that is being manufactured. Generally blow molding produces products that are hollow, i.e., bottles and containers that need a hollow volume inside the object.

Injection molding on the other hand makes products that are solid plastic and may contain complex shapes, ie., cups/plates, and plastic toys. A cup may seem hollow, but its handle requires injection molding.

Blow molders (machines) start with a small tube of plastic (called a parison) which is dropped into the mold, and air is blown into the middle much like blowing up a balloon. The thickness of the walls is generally based on the volume and the amount of plastic used to start the process. The air blown into the parison pushes the material into the void in the die or tool. By comparison an injection molder, as the name implies, “injects” liquefied plastic material into a closed mold. The material fills the void in the mold to create a reverse image of the mold that is solid plastic. When the plastic hardens, the mold is removed, and the shape of the void remains.

From a technical perspective each manufacturing process is significantly different. Additionally, each has its own design and technical considerations and margins of tolerance. Design professionals should always be involved in the tool and die construction process. Consideration of the manufacturing process prior to product development is important to the product design itself.

All blow and injection molded product must have a form for the plastic to fill. This mold is called a die or tool. The tool can be a simple shape or very complex with several parts in order to complete the full product. Some tools are built to make multiple parts at one time, such as a bottle molding tool. The shapes are repeated in a series as exact copies. Other tools make just one product or part at a time.

The tools are usually made from steel or aluminum. They are precision-machined to form the shapes and crevices for the product and the place where plastic is injected into the molding tool. In blow molding the blow pin is normally on the top where the container opening is. This must also be removed and/or finished when the part is removed. In both cases when the plastic is cooling, the tool halves open to eject the new object inside.

Each process also requires considerations of the material to be used. Blow molded milk bottles must be inexpensive, flexible, and strong to prevent breakage. The aesthetics of the material may be a low consideration. The manufacturer of plastic dinnerware may require very close tolerance and a hard, shiny surface to make them desirable to the buyer. A plate may need hard rigid material or a firm yet flexible plastic more like a disposable paper plate.

The properties of the plastic material is a serious consideration for fit, form, and function (strength, temperature levels, and flexibility) and presentation, details, rigidness, wall thickness. Different plastic materials and/or formulations apply to each specific process: blow molding or injection molding.

Other types of plastic manufacturing processes do exist, including extrusion, forming, casting and compression molding.

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