Injection Mold Design Guidelines And Design Options

Injection mold making process
Injection mold making process

The injection mold is a critical component of the injection molding machine and process. As such, enough attention must be paid to its design, and the available options adequately explored before a decision is taken. If you’re looking to understand injection mold design and structure, here is a quick guide to help.

What is Injection Mold?

An injection mold is the part of an injection molding machine where molten material is injected to form the required parts, as can be seen in this video. So it contains all the features that define the shape and size of a product, as well as the means to eject the part after cooling.

The injection mold structure is usually a two-piece unit, a stationary front half, and a movable back half. The two halves are made to lock together, with the molten material being injected into the cavity in between.

Injection mold, steel
Injection mold, steel

Injection Mold Material

The most common materials for injection molds are aluminum and stainless steel. While aluminum is relatively cheaper to use when designing a mold, stainless steel is generally the better option for most applications. Here is more about each injection mold material.

Injection Mold Steel Material

Steel is the most common material for injection molding. Steel molds can be hardened or pre-hardened with heat treatment to provide superior strength and hardness. Steel molds are also highly resistant to wear and tear, in addition to producing better quality parts.

Aluminum Injection Mold

A mold for injection molding can also be aluminum. Aluminum offers some advantages, such as superior temperature control, lower cost, and shorter lead times due to its ease of machinability. However, it is not as hard-wearing as steel and may also not suit some applications.

Injection mold parts
Injection mold parts

Injection Mold Parts

The injection mold assembly is composed of several components. Here, we will take a look at the major parts: These include injection mold parts that form the cavity, core, and other sections of the assembly. They include the following:

Injection Mold Sprue

This is the part that’s located next to the injection nozzle. It helps to feed the molten plastic into the mold. The injection mold sprue links to the runner system and must ensure efficient and uniform flow of the material.

Injector Mold Runners

The runners are channels that direct the melt from the sprue to the cavities. These injection mold channels can be heated types or cold, depending on the process requirements or options. An injection mold runner directly feeds the cavity and core, via what’s called a gate.

Injection Mold Gate

The gate is a small opening at the runner’s end that allows material into the cavity. It also helps regulate and control the flow. There are various injection mold gate types, such as edge gates, hot tip gates, sub gates, and direct sprue gates.

  • Edge gate: Located at the edge of a cavity and used for flat parts
  • Hot tip gate: Located on the top, mostly used for conical or circular parts
  • Sub gate: Located anywhere and offers greater control over the flow and process
  • Direct sprue gate: Connected directly to the sprue and commonly used for round parts

Injection Mold Cavity

The injection mold cavity is the actual space within the injection mold that receives the molten material. It is the shape and size of this cavity that will define the product being produced during the injection process.

Injection Mold Ejector Pins

Injection mold ejector pins are the parts that push out the product when it is finished. These, as earlier indicated, are found in the ejector plate, and must be strategically placed to ensure the product can easily be released without damage.

Injection Mold Cooling System

Composed of water channels or even oil, injection mold cooling system helps to cool the molten material, allowing it to harden in a defined shape and size – and within a set time. It must also be efficient enough to ensure uniform temperatures throughout the assembly.

Injection Mold Vents

Injection mold vents, which are usually small channels machined on the mold surface, help to allow the air trapped in the cavity to escape during the injection process. This helps to avoid any pressure buildup within the cavity and ultimately ensures good part quality.

Multi-cavity injection mold
Multi-cavity injection mold

What are the Types of Injection Mold?

Injection mold classification is based on 3 main parameters: the feeding system (hot and cold runner), number of cavities (single cavity or multi-cavity), plate (standard two-plate and three-plate molds). These types of injection molds are explained below.

Hot Runner Mold

Hot runner injection molding means the material is heated as it flows through. These produce clean parts and less waste (material doesn’t solidify inside them). However, they are more challenging to clean, being hidden from view and permanently attached.

Cold Runner Mold

In cold runner injection molding, on the other hand, there’s no heating of the material. These types of injection mold are usually exposed and easier to use or maintain. On the downside, they tend to produce a lot of waste that also needs to be manually removed.

Single Cavity Mold

A single cavity mold is designed to produce just one product at a time from each cycle of injection. This type of injection molding is typically used for low-volume production, such as prototypes and custom parts.

Multi Cavity Mold

A multi-cavity mold is designed to produce multiple products at once from each cycle of injection. These molds are best for mass production since they can quickly and efficiently create large quantities of parts in a single operation.

Family mold Design

The injection mold types are designed to produce multiple parts at once, but the individual cavities may be slightly different—producing different shapes, sizes or colors. These molds allow for greater flexibility in production and can simplify the manufacturing process.

Two Plate Injection Mold

A two-plate injection mold has two distinct halves. Although you can use the two-plate mold with any runner system, the single-cavity setup suits it best. It’s best used for shorter production runs or where smaller part quantities are needed.

Three Plate Injection Mold

A three-plate injection mold is similar to a two-plate mold but with an additional plate between them. The arrangement of the three plates allows for faster production. However, it also increases the initial tooling costs.

Stack Injection Mold

For faster manufacturing or higher production runs, the stack injection mold is used. In this type of mold design, multiple molds are stacked on top of each other. This allows for multiple parts to be created at once.

Injection mold design
Injection mold design

Injection Mold Design

During the design process, injection mold tool makers must consider a variety of factors such as material type, intended use of the part, and cost. The final design will depend on these factors, as well as the size and complexity of the part to be molded.

More importantly, the engineers must take into account these three factors when making an injection mold: Draft angle, wall thickness, undercuts.

Injection Mold Draft Angle

The injection mold draft angle is the degree of taper used on its walls to facilitate part removal. Generally, draft angles range from 0.25° to 0.5°, depending upon part geometry and material requirements.

Injection Mold Wall Thickness

Wall thickness is a critical factor in injection mold tooling. It should be uniform throughout the part, and not too thin to create voids or sink marks or too thick to cause warping and other injection molding problems.

Injection Mold Undercuts

Undercuts can prevent the part from being ejected without damage. They can be grooves, recesses, or projections on the part walls. Injection mold undercuts must be carefully designed and strategically placed, or removed unless they’re an absolute necessity.

How Long Does an Injection mold Last?

The injection mold life expectancy depends on the material used, how often it is used and how well-maintained it is. Generally, injection molds can last anywhere from a few thousand to well over a million cycles.

The life span may be shortened if the mold experiences frequent wear and tear, or if it’s not properly maintained.

The injection mold tool life is also affected by the material quality. For that reason, manufacturers must ensure that they are making their molds with the highest quality of material possible.

Cost of Injection Mold

The cost of an injection mold varies significantly depending on its size, complexity and number of cavities needed. Smaller parts can be produced more cost-effectively than larger parts due to their reduced tooling costs.

The type of material used also has an impact on the cost of injection mold, although prices may depend on your location on the globe. A steel mold, for example may cost $5 000 to produce, while a similar aluminum mold costs a fraction of that – around $1 000.

Additionally, the number of cavities in the mold also affects its cost; the higher the number of cavities, the higher the cost. Injection mold cost can range from a few hundred dollars to tens of thousands of dollars depending on these factors.


To conclude this injection mold guide, we must re-emphasize the importance of proper design and an understanding of the various mold requirements. That’s because with a well-designed injection mold, you can achieve long-lasting performance and high quality parts that are consistent across production runs.

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