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Types of Plastic Blow and Injection Molding

Each plastic part demands a unique manufacturing process, often varying significantly between products. Solid components are created using injection molding, while blow molding is employed to produce plastic items with hollow spaces, like bottles and containers.

Injection stretch blow molding merges the injection molding and blow molding techniques. Initially, the plastic is molded into a solid preform to form a threaded bottle neck. After cooling, the preform is transferred to a stretch blow mold machine, reheated with an infrared heater, and then expanded into a plastic bottle using compressed air.

Injection molding and blow molding are fundamental processes in plastic manufacturing. Items produced through these techniques serve a broad spectrum of industries, including automotive, food service packaging, electronics, medical devices, irrigation, dental, firearms, energy, and environmental sectors.

To enhance your understanding of the various manufacturing methods suitable for your project, we have assembled a summary of the primary types of injection and blow molding techniques utilized by plastics manufacturers.

The Injection Molding Process

Thermoplastic Injection Molding

Thermoplastic injection molding employs thermoplastic polymers, which turn into a liquid state when heated. Unlike thermoset plastics that solidify permanently upon cooling, thermoplastics can be remelted into a liquid after they have cooled into a solid form.


Overmolding, also known as two-shot molding, involves covering an injection-molded part with another material, such as metal, to enhance the product’s performance or durability. Thermoplastic elastomer (TPE), a rubber-like compound, is commonly used in overmolding. For example, TPE overmolds are used for the handle grips on toothbrushes. Overmolding can also seal products with parts made from multiple injection molds.

Insert Molding

Similar to overmolding, insert molding is an injection molding process that integrates two or more components into a single finished product. Insert molding involves placing a component inside the injection mold cavity and then injecting plastic material around it. Inserts can enhance a product’s strength and reduce the need for additional parts, thereby lowering the product’s weight.

Cold Runner Injection Molding

Cold runner molds use a sprue to fill the runners that deliver plastic resin into the mold cavity. In two-plate molds, the runner system and parts are connected, and an ejection system may be employed to separate the part from the mold. Cold runners can minimize waste by recycling and regrinding the material, although this can increase the overall cycle time. Cold runner systems are suitable for a wide range of polymers and allow for easy color changes.

Hot Runner Molds

Hot runner molds use a manifold to heat and direct melted plastic resin through a gate to fill the mold cavity. The two primary types of hot runner molds are externally heated and internally heated. Externally heated molds are ideal for polymers less sensitive to thermal variations, whereas internally heated molds offer better control of material flow.

Since hot runner molding doesn’t require the use of runners, it reduces potential waste material. Additionally, recycling and regrinding the material does not affect the total cycle time.

The Blow Molding Process

During the blow molding process, raw plastic material is shaped into a hollow tube with one open end, known as a parison. The parison is then pressed into a cooled metal mold, and compressed air is injected into it. Once the plastic cools and solidifies, the metal mold opens to release the finished product.

There are three types of blow molding processes:

1. Injection Blow Molding

In injection blow molding, a blow or core rod is utilized throughout the process. Initially, a parison is injected into a split mold cavity around the rod, forming a shape similar to a test tube. The core rod then transfers the parison to the blow mold machine, where forced air shapes the final product. The rod subsequently removes and ejects the finished item from the machine.

2. Extrusion Blow Molding

Extrusion blow molding can operate continuously or intermittently. In continuous extrusion blow molding, a parison is constantly fed into the mold, with each form being cut off by a blade as it is created. In intermittent extrusion blow molding, each new plastic form is expelled from the metal mold once it cools, and the next parison is fed into the mold only after the previous one is ejected.

Extrusion blow molds are generally more cost-effective and quicker to produce than injection blow molds, making them suitable for smaller production runs. Advantages include reduced tooling costs and shorter production times, while disadvantages often involve less control over wall thickness and increased scrap material.

3. Injection Stretch Blow Molding

Initially, the plastic is molded into a solid preform, forming a threaded bottle neck. Once the preform cools, it is transferred to a stretch blow mold machine, reheated with an infrared heater, and blown into a plastic bottle using compressed air.

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