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Types of Plastic Blow Molding and Injection Molding

Nov. 21, 2022

Blow molding and Injection molding are the two core processes in plastics manufacturing. Products produced through these processes are used in a wide range of industries including 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 summarized the main types of injection and blow molding methods used by plastics manufacturers.

Blow Molding Process

Blow molding technology a rapidly developing method of plastics processing. The thermoplastic resin is extruded or injected into a tubular plastic blank, which is placed in a folio mold while hot (or heated to a softened state), and immediately after the mold is closed, compressed air is introduced into the blank, causing the plastic blank to expand and cling to the inner wall of the mold, which is cooled and demolded to obtain various hollow products. In the blow molding process, the plastic feedstock is formed into a hollow tube with an open end, called a form blank. The blanks are pressed into a cooled metal mold and compressed air is pressed into the blanks. As the molded plastic cools and hardens, the metal mold opens and discharges the product.


Injection Blow Molding

In injection blow molding, a blow mold or mandrel is used throughout the process. First, a mold blank is injected into a separated mold cavity around the rod. The resulting blanks look similar to test tubes. The mandrel transfers the parison to a blow molding machine where air is forced to form the final shape. The rod is then transferred from the machine and extruded into the finished product.


Extrusion Blow Molding

Extrusion blow molding can be continuous or intermittent. In continuous extrusion blow molding, the mold blanks will be continuously fed into the die and each model will be cut by the blade as it is molded. Intermittent extrusion blow molding expels each new plastic from the metal mold as it cools and feeds the model into the mold only after the previous model is expelled.

Extrusion blow molding molds are typically much cheaper than injection blow molding molds and can be produced in much shorter periods of time. Extrusion blow molding is suitable for small production runs. Advantages include savings in tooling costs and reduced production time, while disadvantages usually include less control over wall thickness and higher scrap volumes.


Injection Stretch Blow Molding

Injection stretch blow molding is a combination of injection molding and blow molding processes. The plastic is first molded into a solid preform to form the neck of the bottle with threads.

Once the preform has cooled, it is fed into a stretch blow molding machine. The preform is then reheated using an infrared heater and blown into the plastic bottle using forced compressed air.


Injection Molding Process

The injection molding process is a process in which molten raw materials are pressurized, injected, cooled, and separated to create a semi-finished part of a certain shape. The injection molding process of plastic parts mainly includes mold closing - filling - (gas-assisted, water-assisted) pressure-holding - cooling - mold opening - demolding, etc. -Opening the mold - Demolding and other 6 stages. Injection molding is used to manufacture larger quantities of plastic products, from large parts to small parts requiring micro-precision.

There are a variety of manufacturing methods for injection molding, such as thermoplastic injection molding, overmolding, insert molding, cold runner molding and hot runner molding.


Thermoplastic Injection Molding

Thermoplastic injection molding uses a thermoplastic polymer, which means it turns into a liquid state when heated. Unlike thermosets, which cool to a permanent solid, thermoplastics can be reincorporated into a liquid after cooling to a solid.



Molding Overmolding, or double injection molding, is a process in which an injection mold is covered with another substance, such as metal, to improve the performance or durability of a product. An example of an overmolded TPE application is the handle on a toothbrush. Secondary molding can also be used to seal products with parts made from multiple injection molds.


Insert Molding

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


Cold Runner Injection Molding

Cold runner molds use a sprue to fill the runner that injects the plastic resin into the mold cavity. In a 2-panel mold, the runner system and the part are joined together and the pair can be separated from the mold using an ejector system. Cold runners can reduce waste by recycling and regrinding the material, but this also increases the total cycle time. Cold runner systems can be used for a wide range of polymers and can easily change colors.


Hot Runner Molds

Hot runner molds use a manifold to heat the molten plastic resin and then feed the material into the gate to fill the mold cavity. The two main types of hot runner molds are externally heated and internally heated. Externally heated molds can be used for polymers that are less sensitive to thermal changes, while internally heated hot runner molds allow for better control of material flow.

Since hot runner molding does not require the use of runners, potential scrap is reduced and the recycling, regrinding and processing of virgin plastics does not affect the total run cycle time.



Which Molding Process Does Your Product Require?

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