Guide To Injection Molding


We developed this guide to educate the next generation of inventors, entrepreneurs, manufacturers and product managers in plastic injection molding technology. We’ve included the components, process, design process, and cost evaluation of injection molds in order to help you best understand them.

We consider ourselves experts in custom injection molding with a wide variety of capabilities, materials, and sizes. We design products, build tooling, manufacture parts, and perform a variety of secondary operations such as mold storage and maintenance for the molds we build. Our mold makers have years of experience with custom plastic injection molds and tooling.

If you haven’t already, we highly recommend you first read our guide to plastic injection molding  before you read this guide to injection molds. That guide includes the history, recommendations, and industry best practices for plastic injection molding.

What Is An Injection Mold?

An injection mold is one of the custom-machined tools used in plastic injection molding to mold molten plastic into plastic products. The mold determines the shape and size of the plastic product.

Examples include chairs, tables, packaging, household appliances, cases for consumer electronics, bottle caps, disposable cutlery, power tools, car parts and components, containers, combs, toothbrushes, musical instruments and parts, toys, medical devices, mechanical parts and gears, and many more!

The injection molding tool is complex and made up of many different components. Each part has a name and a specific purpose. We’ll breakdown an injection mold for you so that you fully understand what we’re talking about.

There are several types of injection molds. The most widely used is a two-plate tool, consisting of the injection mold (A Plate) and the ejector mold (B Plate).

Useful Terms About Injection Molds

  • The Parting Line: The divide between the plates of an injection mold.
  • The Sprue Bushing: An opening or entrance in the injection mold that connects the injection mold to the injection unit.
  • The Sprue: A passageway that connects the sprue bushing to the runners.
  • Runners: Tunnels or channels that connect the sprue to the gates.
  • The Gate: The entrance to the cavity image.
  • The Cavity Image: An empty space inside of the injection mold in the shape of the desired plastic product.
  • Ejector Pins: Circular pins used to eject the finished plastic product from the injection mold.

How Does An Injection Mold Work?

Molded plastics are made through a combination of the right materials at the right temperature, pressure, and timing.

An injection molding tool has several systems that that serve critical functions during the tooling process. There’s a distribution, cooling, and ejection system that turn molten plastic into a hardened plastic product.

After the plastic is heated by the injection unit, the injection barrel inserts molten plastic through the sprue bushing, down the sprue, through the runners, past the gate, and into the cavity image. Once the molten plastic fills the cavity image to its capacity, the cooling process begins.

As the molten plastic cools, it hardens into its desired shape. At the same time, the molten plastic shrinks and sticks tightly to the ejector mold (B Plate). When the molten plastic is finished solidifying, the mold opens and uses ejector pins to push the finished plastic product off of the mold. The injection mold then closes and the whole tooling process repeats.

For example, if you inject heated plastic into an injection mold in the shape of a comb, it will produce a plastic comb.

The Effect Of An Injection Mold

Due to the nature of using a machine to create plastic products, injection molds leave identifiable marks on plastic products. These marks are not typically desired but are unavoidable. 

The components of an injection mold, such as the sprue, runners, gates, ejector pins, and parting line, all leave marks. 

These marks can be caused by minor misalignment, natural wear and tear, air vents, moving parts, or dimensional differences of the mold. 

Some molds can be designed to hide the marks and some designers purposefully add marks to their plastic products, such as instructions, warnings, or date and time stamps.

How Are Injection Molds Created?

The perfect plastic product needs the perfect mold. Injection mold tools are created by cutting into a raw material (metal) and forming the desired shape and size of the product.

Designers use technologies like CAD, or computer-assisted design for part design, Mastercam for mold design, and 3D printing for prototyping and design verification.

Designers need to take account of the desired shape, size, and quantity of the product, the type and amount of raw material needed, which type of injection mold to use, the estimated shrinkage of the plastic, product finishes, and the cost limit when creating injection molds. It’s also important to consider design standards for building injection molds, such as draft angles, radiused edges, support ribs, and ejector pins.

Once an injection mold has been rigorously tested and prototyped, production can begin. When the production run for a plastic product is over, the injection mold will be replaced with a new mold for the next plastic product.

The Methods

There are three methods to creating molds: standard machining, computer numerical control machining, and electrical discharge machining.

Standard machining involves manually using machine tools to cut and shape the raw material (metal) into the desired shape and size of the mold. This type of machining requires annealing in order to soften the mold, followed by heat treatment in order to harden it again. Annealing is the slow heating and cooling of a plastic product to relieve internal stress. Historically, standard machining was the most common method of creating injection molds.

Computer numerical control machining involves programming a computer to cut and shape the raw material (metal) into the desired shape and size of the mold. As technology advanced, computer numerical control machining became the main method of creating molds because it could create complex plastic molds with more accurate details in less time than standard machining.

Electrical discharge machining involves using electrodes to erode and shape the raw material (metal) into the desired shape and size of the mold. At first, the surface of the metal is covered in oil. Then, an electrode made of copper or graphite is placed on the surface of the metal for many hours. Finally, a voltage is applied which causes spark erosion and creates the inverse shape of the electrode. This type of machining has grown very popular because it can form more difficult shapes without any heat treatment.

Type Of Metals

Most molds are created from hardened steel, pre-hardened steel, aluminium, or beryllium-copper alloy. Choosing which type of metal to use to create a mold comes down to basic economics.

Steel molds will last the longest and produce the most amount of plastic products out of any other type of mold. Since hardened steel is harder than pre-hardened steel, hardened steel molds are used for producing smaller plastic products or mass producing plastic products while pre-hardened steel molds are used for producing larger plastic products or a smaller amount of plastic products.

Thanks to modern technology, aluminum is becoming better at mass producing plastic products.

Beryllium-copper alloy molds are used in specific areas of molds that need fast heat removal or generate the most heat.

How Much Does An Injection Mold Cost?

Precision machine steel molds can cost hundreds of thousands of dollars. These molds should be placed in controlled environments maintained at the perfect temperature and humidity level for protection.

Building the injection mold the right way and with a trusted partner reduces costs in project delays or product defects. The mold may be the largest investment in the project. That means it’s even more important to get it right the first time.

The cost evaluation of any injection mold machine is based on the complexity of the mold’s design, specialized customization, the number of cavity images within a mold, and the type and amount of raw material needed. Injection molding is most economical for large production quantities.

The fewer the cavity images, the less work involved in the design, the lower the cost. More cavities may mean a higher cost long-term since it could require more maintenance over time.

The more complicated the design, the more work involved, the higher the cost. Examples include surface finishing, tolerance requirements, internal or external threads, fine detailing, and the number of undercuts.

When it comes to the raw material, steel is the most expensive metal used to create a mold while aluminum is much cheaper.

One way to save costs may be to partner with an injection company that both builds the molds and manufactures parts. That way you can bundle expenses and have a lower cost per unit.


Injection molds are complex tools with even more complex systems and designs. Thanks to advanced technology, they can be intricately designed at an affordable cost to fit any particular set of needs. 

If you’re ready to take the next step on your project, contact Murray Plastics today. Murray Plastics will fit the process to match your needs, whether it’s small plastic parts to large components. We firmly believe in U.S. manufacturing. We endeavor to source our materials and sell our products in Georgia as proud supporters of the Georgia Manufacturing Alliance. Our desire is to have Murray Plastics manufacture your products or components in the U.S. with a globally competitive cost and lead time.

About Murray Plastics

We can take your sketch on a napkin to a two- or three-dimensional design, build, prototype, and produce the product from start to finish.

With 20 years in business, Murray Plastics represents the combined expertise of industry professionals with proven results. Here are a few benefits you’ll get when partnering with us.

  • 24-hour production cycle
  • 51,000 square-foot warehouse
  • Local delivery
  • 20 years in business
  • Quality customer service


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