The Most Important Considerations When Designing Parts for Injection Molding

Published on June 21, 2023

Most plastic components that are made today utilize the injection molding process. During this process, the design phase is particularly important, as molding can be arduous and expensive. Yet it also requires a great deal of careful consideration as, once an injection mold is designed, it can’t easily be altered.

Before making a plastic injection mold, designers need to consider many alternatives. These include what material the mold should be made and whether the injection mold design process should be outsourced or remain in-house. As some molds are designed to produce a million components or more during their lifespan, it’s important to get the design phase right.

Considerations for Injection Mold Design

An integral part of injection mold design involves developing the actual mold itself. This stage of the design process is perhaps the most important, as beyond this, any changes or corrections will add to costs and delay production significantly. Rapid prototyping, mold flow simulations, 3D CAD (computer-aided design) interference checks, and other design tools make the process more accurate, but they can’t resolve every design mistake. That can only be accomplished once the first parts come out of the mold, which must then pass a rigorous inspection.

There are two primary components to a plastic injection mold. Designers must consider both the cavity half, also known as the A half, and the ejector half, also known as the B half. Injection mold designs normally are constructed so that when it opens the molded component remains in the mold’s B half. Once the runner and sprue are drawn from the A half, the molded component easily ejects from the B half.

The two halves of the mold work together in the following manner:

  • Plastic resin goes through the sprue (sometimes called a “gate”) into the A half as it leaves the molding machine.
  • Sealing firmly against the injection barrel’s nozzle, a sprue bushing enables molten plastic to flow from the molding machine into the mold’s cavity.
  • Directed by the sprue bushing, the molten plastic flows through machined channels – referred to as “runners” – into both faces of the mold’s two halves.
  • Entering the mold through one or more specially constructed gates, molten plastic flows into the cavity and solidifies into its finished form.

Plastic injection mold designs can be made to produce a single component each time or have multiple cavities that produce as many as a hundred or more parts simultaneously.


Generally, the materials from which a plastic part will be made are decided upon early in the design process. When large quantities of certain resins are purchased, an injection molding company can receive a discount to pass to customers. Besides the price of materials, there are other considerations.

Injection mold designers often need to consider the following:

  • Choosing a less expensive material with similar properties can bring costs down.
  • Properties like crystallinity, glass content, or viscosity can affect the materials used.
  • Though specific properties like chemical or abrasion resistance may make a resin desirable, the material may be difficult to mold, or required tolerances may be challenging to achieve.

If partnering with an injection molding company, it’s important that their input about materials is considered, as they’re the ones who will actually mold the components.


While it’s much easier with injection molds to design for looser tolerances, functional or aesthetic aspects sometimes make tight tolerances necessary. It’s integral that the mold maker should be included to ensure that the desired specifications can be reliably achieved. This may be a challenge, but it’s necessary that injection mold designers provide enough clearance to deal with any variation in tolerances.

Disparities in tolerance rely on several variables, which include tool design, processing controls, and materials used. Those designing the molds should consider how critical tolerance specifications need to be, as this will affect pricing, and they should be open to possible revisions if necessary. In such cases, it may even require designing the mold with extra clearance, which may involve machining material from the mold to make tolerances tighter. Other options for injection mold design may include machining the part once it’s molded, along with altering fixtures or gate locations.

Sink Marks

A common challenge injection mold designers face involves sink marks. While avoiding these imperfections in the design phase is challenging, it’s better to be proactive to lessen the need for post-production machining. Causes of sink marks due to injection mold design might include a low melting temperature for the resin being used, so the materials out of which components are made should be carefully chosen. Molds that are incorrectly designed or part geometry may also play a part in the appearance of sink marks.

Sink marks can be avoided by:

  • Balancing rib and wall thickness so that molten plastic flows with little or no resistance, filling up thinner areas first.
  • Creating a gradual slope at the rib base of about seven degrees, though this method only works when a gate is near the area.
  • Designing the hoop for the boss so that it has an inside diameter that’s half of the outside diameter so that it experiences nominal stress.
  • Increasing gate size to ease the packing and filling of the mold.
  • Making wall thicknesses that are uniform throughout the part usually fall between 0.5 to 5 mm (about 0.02 to 0.20 inch).
  • Relocating gates to ensure the flow of molten plastic goes unhindered.

Proactively identifying problematic areas during the mold design phase makes it easier to avoid sink marks, so it’s important that communication between stakeholders is clear.

Steel Safe Design

Making features “steel safe” means ensuring they have enough clearance to allow the machining away of excess material within the mold as a precaution to avoid having to weld material back into the mold. This ensures tight tolerances in molded parts with details like aligning features, interlocking parts, or snap fits. With an injection mold, designing these features is easy with CAD, though fabricating such features to align perfectly is difficult.

It’s much easier and cheaper to machine than weld an injection mold. Designers of parts should collaborate with those making the molds or tools early on to diminish the need to correct designs later in the process. Cooperation is key, as good planning will mean minimal delays or cost overruns. In certain cases, additional clearance won’t be a factor when components must come out exactly as they were designed.

Gate Location

Nearly every trait on a component made via injection molding relies on gate location. Close collaboration is desirable to ensure gate location and design don’t negatively affect the performance of the molded part. Ideally, injection mold designers should work with a tool maker and mold fabricator to correct gate placement.

Part characteristics of gate location effects include:

  • Appearance
  • Molded in stresses
  • Physical properties
  • Surface finish
  • Tolerances
  • Wall thickness
  • Warpage

Mold flow simulations can help injection mold designers plan and position where gates should go, though part geometry will also affect gate design. Additionally, each type of gate offers trade-offs. For example, while edge gates and fan gates are used for manual de-gating, the fan gate normally results in fewer flaws.

3D Printing, CNC Machining, or EDM for Making Molds?

There are essentially three ways in which to make an injection mold. Designers can utilize 3D printing, CNC (computer numerical control) machining, or EDM (electrical discharge machining). Each method has its advantages and disadvantages. Generally, 3D printing is only effective at lower production volumes while also constraining what materials and processes can be used. CNC machining works well for making molds at scale more economically. Though slower, EDM can produce injection mold designs that aren’t reproducible via conventional CNC machining.

3D Printing

The development of new, stronger materials for 3D printing has made it possible to use it to fabricate plastic injection molds. Designing 3D-printed molds in-house offers a means for creating functional prototypes cost-effectively and quickly. While this may allow companies to make useable components at a pace and quality necessary for prototyping, it’s unlikely to be used for large-scale production of injection molds.

CNC Machining

CNC machining is the primary means for fabricating accurate and complex injection molds. Design and manufacturing via this method is highly automated, using CAD and CAM (computer-aided manufacturing) software and computer-controlled cutting machines to produce a quality injection mold. Designers understand that CNC machining can achieve much smoother surfaces and greater precision than with 3D printing.

Electrical Discharge Machining

EDM uses an electrode made from copper or graphite to create a pre-hardened injection mold. Designs made via this method allow the production of shapes that conventional CNC machining cannot. Now widely used for making molds, the process involves positioning the EDM machine over the workpiece, which is then dunked into a dielectric fluid. An electrode is lowered from the machine toward but not touching the workpiece, where a controlled electrical source tears down and dissolves the metal or alloys opposite the electrode. Though the process is slower than CNC machining, it requires no additional heat-treating, and the fine finish it produces often means the mold cavity requires no polishing.

Spaulding Composites: Outsourcing Mold Design

Designing injection molds in-house has its benefits but requires a significant investment of time and resources. Most often, companies don’t have the facilities, equipment, software, and expert personnel to make an injection mold. Designing the molds also requires significant investment, so it makes sense to outsource injection mold design and production to a competent partner like Spaulding Composites Inc.

Spaulding’s capabilities in injection mold design and production include:

  • Understanding the entire injection molding process, from design and prototyping to production and after-production order management.
  • Evaluation of the number of molds necessary and best processes for the application.
  • Expertise in simplifying mold designs while maintaining a component’s necessary properties.
  • High volume, speedy production of premium components delivered on time.
  • Knowledge about the best material for the application and injection mold design.
  • Personnel with a wide range of expertise in materials and processes that can assist with developing and optimizing injection mold designs.

Spaulding can also provide an array of ancillary processes, including assembly, hot stamping, marking, over-molding, packaging, and precision CNC machining. To learn more about what we can do for you, contact the experts at Spaulding for more information.