Types of Plastic Blow and Injection Molding

Each plastic part requires a specific manufacturing process which can be significantly different from one product to another. Injection molding is used to manufacture solid components while blow molding is used to make plastic products with hollow areas, such as bottles and containers

Injection stretch blow molding combines the injection molding and blow molding processes.The plastic is first molded into a solid preform, to create a threaded bottle neck. Once the preform cools it is fed into a stretch blow mold machine. The preform is then reheated using an infrared heater and blown into a plastic bottle with forced compressed air.

Injection molding and blow molding are two core processes in plastic manufacturing. Products made through these processes are used in a wide range of industries such as automotive, food service packaging, electronics, medical devices, irrigation, dental, firearms, energy and environmental industries.

To help you better understand the different manufacturing methods available for your project, we have compiled an overview of the main types of injection and blow molding methods used by plastics manufacturers.

The Injection Molding Process

Injection molding is used to manufacture higher volume quantities of plastic products ranging in size from large components to small components requiring micro-precision accuracy.

There are many types of manufacturing  methods categorized under injection molding, such as thermoplastic injection molding, over molding, insert molding, cold runner molding and hot runner molding.

Thermoplastic Injection Molding
Thermoplastic injection molding uses thermoplastic polymer, meaning it changes to a liquid state when heated. Unlike thermoset plastics that cool into permanent solid, thermoplastics can be remitted into a liquid after cooling into a solid.

 Overmolding
Overmolding, or two shot molding, is a process that covers an injection mold over another substance, such as metal, to improve the performance or durability of a product. A rubber-like compound, called thermoplastic elastomer (TPE), is a commonly used overmold material. An example of a TPE overmold application is the handle grips on a toothbrush.  Overmolds can also be used to seal products with parts made from several injection molds.

Insert Molding
Like overmolding, insert molding is an injection moldingprocess that combines two or more components into a single finished product. Insert injection molding inserts a component into the injection mold cavity and plastic material is filled around the insert. Inserts can add strength to a product and eliminate the need for additional parts to reduce the product weight.

Cold Runner Injection Molding
Cold runner molds use a sprue to fill the runners that inject plastic resin into the mold cavity.  In 2 plate molds, the runner system and parts are attached, and an ejection system may be used to separate the pair from the mold.The cold runner can reduce waste by recycling and regrinding the material, but this can also increase the total cycle time. Cold runner systems can be used for a wide variety of polymers and can allow for easy color changes.

Hot Runner Molds
Hot runner molds use a manifold to heat melted plastic resin and then send the material through a gate to fill the mold cavity. The two main types of hot runner molds are externally heated and internally heated.  The externally heated molds can be used with polymer that is less sensitive to thermal variations while internally heated hot runner molds allow for better control of material flow.

Because  hot runner molding  does not require the use of runners, potential waste material is reduced and  the recycling and regrind and process of virgin plastic does not impact the total run cycle time.

The Blow Molding Process

During the blow molding process, the raw plastic material is shaped into a hollow tube with one open end called a parison. The parison is pressed into a cooled metal mold and compressed air is forced into the parison. When the formed plastic cools and hardens, the metal mold opens and expels the product.

There are three types of blow molding processes:

1. Injection Blow Molding
In injection blow molding, a blow or core rod is used throughout the process. First a parison is injected into a split mold cavity around the rod. The parison that is formed looks similar to a   test tube. The core rod transfers the parison to the blow mold machine where forced air creates the final shape. The rod then transfers and extrudes the finished product from the machine.

2. Extrusion Blow Molding
Extrusion blow molding can be continuous or intermittent. In continuous extrusion blow molding, a parison will be constantly fed into the mold and each form will be cut off with a blade as it forms. Intermittent extrusion blow molding expels each new plastic from the metal mold when it is cooled and the parison is fed into the mold only after the preceding parison is expelled.

Extrusion blow molds are generally much less expensive than injection blow molds and can be produced in a much shorter period of time. Extrusion blow molding is appropriate for smaller runs. Advantages include cost savings on tooling and shorter production time while disadvantages usually include lesser control of wall thickness and greater amount of scrap material.

3. Injection Stretch Blow Molding
Injection stretch blow molding combines the injection molding and blow molding processes. The plastic is first molded into a solid preform, to create a threaded bottle neck.

Once the preform cools it is fed into a stretch blow mold machine. The preform is then reheated using an infrared heater and blown into a plastic bottle with forced compressed air.

What Molding Processes Does Your Product Require?
What type of manufacturing process is right for your next project? Our engineers have decades of experience and understand what type of material and mold is right for you. Contact us to discuss your new plastic design or product idea today and we will walk you through the manufacturing process that is best for your project needs.