Injection molding is a manufacturing process used for producing parts in large volumes. The name injection molding comes from its resemblance with a syringe being used for injecting fluids into an object. The injection molding process makes use of molds and synthetic resins type materials. The material is first heated and melted and then poured or injected into the mold with the help of an injector nozzle. Once the material solidifies, it takes the shape of the mold, which was made to fit the object under manufacturing.  

There are several types of injection molding as given below;

  1. Thin Wall Molding – This particular type of injection molding technology produces a plastic part with a very thin wall and is often used in objects such as electronics, vessels, test apparatuses, tubes and other enclosures.
  2. Gas-assisted Injection Molding – A common problem with thick plastic injection molding is that as the material cools, there is a risk of distortion. So in order to overcome that, gas is shot in the plastic filled injection mold making the outside smooth while inside porous and hollow.
  3. Metal Injection Molding – Plastic isn’t the only material that is being used for injection molding, metal is another material under utilization for injection molding in applications such as cellular electronics, automotive, defense and aerospace industry.
  4. Liquid Silicone Injection Molding – As an alternative to thermoset injection molding, objects that need to withstand very high temperatures or chemical agents are made from injecting liquid silicone, which has a lot more strength than the more traditional alternatives.
  5. 3D Printing – Finally 3D printing is also a common type of injection molding, however instead of a mold, the material is melted and laid down in the form of successive layers.

Prototype and Production Injection Molding

The concept of prototype production via injection molding is that when you are building a product that needs to be produced in large quantities, it is always a good idea to get a feel of what the product will look like when it goes into mass production. While a digital design and a 3D printed prototype also does the job, it is not nearly as good as that of injection molding. Another important point to be noted here is that the production version of the product is going to be produced through injection molding anyway so it is always a good idea that the prototype is also produced through injection molding. Otherwise, there might be significant differences between the prototype and production model.

Tooling Required for Injection Molding

In this section, we are going to discuss some of the important tools required for injection molding;

2-Shot Injection Molds

2 Shot injection molds are used to produce complicated molded parts from two different materials. A highly specialized and automated process controls the injection of two different kind of resins usually co-polymerization of hard and soft material to create powerful molecular bond.

Single Cavity Mold

A single cavity mold features only a single cavity in which the material is injected and thus only produces one product per cycle.

Multi Cavity Mold

A multi cavity mold on the other hand has multiple cavities and multiple products can be molded per cycle.

Large Part Molds

As evident from the name, these type of molds are used for manufacturing large parts such as the ones used in aerospace, automobile or defense industry.

Precision Molds

Precision molds are used for high-precision plastic injection, primarily for high-end exacting applications including the medical and connector markets.

Thin Wall Molds

As discussed earlier, thin wall molds produce plastic parts with a very thin wall and are often used in objects such as electronics, vessels, test apparatuses, tubes and other enclosures.

Insert Molds

Insert molds are used for molding thermoplastic material around a performed component in order to create a finished part that is produced using a variety of materials.

Hot Runner Molds

A hot runner mold consists of two plates that are heated with a manifold system. In addition to this, in a hot runner mold the materials are kept in molten state during the runner process by a hot runner system until the material has flown into the mold cavity.

Cold Runner Molds

On the other hand, a cold runner mold consists of an unheated channel used for conveying molten material into the mold cavity after it has been dispensed from the nozzle.

Insulated Runner Molds

Insulated runners are actually oversized passages formed in the mold plate because of the plastic that freezes on the runner wall. However, this oversized passage allows an open molten flow path to be maintained because of its insulating effect.

Family Molds

A family mold is a type of mold with multiple cavities cut into the mold, which allows for multiple parts of an object to be molded in a single cycle using the same type of material.

Prototype Molds

Prototype molds as evident from the name are not final versions to be used in the manufacturing process. Instead, prototype molds are used to follow a phased approach to building your final mold. Prototype molds help with testing and troubleshooting of a mold before final production.

Bridge Molds

Bridge Molds are also not the final production version and instead are used as an intermediate stage between the prototyping and the production process. It can be defined as a stage in development when there is a need for molded parts but production tooling is unavailable.

Aluminum Molds

Aluminum molds are an alternative to steel tooling that was previously used for injection molding. Steel while more durable had a number of disadvantages over aluminum with the most important one being price. Aluminum molds are cheaper to build, light in weight thus making them portable, and easy to handle during molding. They also provide the molded product with a better finish.

SPI Mold Classifications and Standards

There are five different mold classifications done by the society of plastics industry or SPI. The basis for these classifications were important parameters like cycles of production, production level, use and investment required.



Production Level




1 million or more                  

Extremely high

Extremely high production and fast cycle times

Class 101 molds are the highest priced and made with only the highest quality materials


Not exceeding 1 million

Medium to high

Good for parts with abrasive materials and/or tight tolerances

Class 102 molds are fairly high priced and made with materials of high quality


Not exceeding 500,000


A very popular mold for low to medium production parts

Class 103 molds fall within common price ranges


Not exceeding 100,000


Good for limited-production parts with non-abrasive materials

Class 104 molds fall within low to moderate price ranges


Not exceeding 500

Very low

Prototype only

Class 105 molds are built inexpensively to produce a very limited number of product prototypes