Injection Moulding Design Guide

Injection Moulding Design Process

This design guide helps you through the design process for creating parts for injection moulding. We have built this design process over 30+ years of injection moulding design experience and often use these steps ourselves.

This is the process …

Keep in mind that injection moulding design is an iterative process. We will be iterating both the part and tooling design as the part emerges to optimise for cost and production. The simpler the part the easier this process will be!

Take a look at our case studies and project examples for some examples of what is possible.

Work out what you want to make

Before starting the design, have a think about what you want to make, what it needs to do, and how it will be used. This will get you thinking about how best to design the parts and tooling.

Consider...	Think about...	Impact on design
What does the product need to do?	– Key features / forms / function – Interactions with other parts – Critical dimensions and tolerances – Surface finish and aesthetics	Part geometry Tooling complexity
How will the product be used?	– Stresses on the parts – Environmental conditions (e.g. UV, moisture, temperature, chemical exposure) – Flammability, conductivity resistance, food, medical.	Part geometry Material selection
What are the expected production volumes?	Production volumes change how the tooling is built. Low-cost tooling may be suitable for small volumes. Larger volumes may justify optimised tooling to reduce part costs.	Tooling layout Tooling construction
Is the part in a set, or are there similar variants?	Tooling can be optimised with family tools or interchangeable inserts. Often parts can be designed with this in mind.	Part geometry Tooling layout
What is your budget for tooling?	Tooling costs vary from a few thousand to over £1,000,000. This cost is driven by part complexity and can be significantly influenced by part design choices.	Part geometry Tooling complexity Tooling construction

You don’t need to know all the answers, but it helps to have an idea when you start.

Sketch out a rough concept

Now we know what we want to build, start developing a rough concept. A “back-of-the-envelope” sketch is a great starting point. Aim to capture the rough shape, key features, and their orientation.

Once you have an initial sketch start to build a CAD model. The CAD needs to provide an initial concept which we will mature as we build it into the tool.

When building this concept think about where the split line is placed, and what direction the tool will be moving in as it opens. The split line (or parting lines) is where the cavity plates meet and indicates the direction the tool will open. We need to avoid undercuts where possible and consider that the split line will be visible on the part.

The split line can vary from a straight planar split at 90 degrees to the opening direction:-

To complex split lines creating a range of different features. Here a step in the split line allows a shelf to be created.

Here steps in the split line create a hole through the webs in the part.

TIP – When building the parts in CAD use direct modelling (also called Direct Editing) instead of parametric modelling. This enables easy and radical adjustments of features which are not explicit in the feature tree but are often critical to the tooling design. Most CAD softwares offer this option.

Build the concept into the tool

Once we have the initial CAD, build a mock-up of the tool around the part. This helps us to visualise how the tool will open around the part and how part features impact the tooling. With experience you will become able to visualise the part and how the tool is wrapped around it. This is an iterative process- visualising the tool enables you to develop features in the part which are simpler to incorporate in the tool.

Building a mock-up of the cavity plates.

We start by modelling cavity plates around the part:

Create a plane in the approximate location of the split line to represent the tool face.
Build blocks from this plane to mock-up the tool cavity plates.
Use the cavity function to create the shape of the part in the cavity plates.
Move the cavity plates away from the part to see the cavities.

This allows us to see how the tool opens around the part and what shape the tool needs to be to make the part.

Adjusting the part and the tool

We can then adjust the part and the tool to make both work together. Adjusting the design can be complex as we need to think about many factors together. However initially focus on the geometry…

Undercuts – tool movements need to be constructed around the features of the part to avoid undercuts, so the part can be ejected from the tool.
Split line – these can be planar, angled or stepped (with a shut-off angle).
Ejection – there needs to be adequate surface area to push on the part with the ejector system.
Draft – ideally there needs to be draft on surfaces perpendicular to the tool face to help eject the part.

If the part does not work in the tool, we have several options. We may need to:

Re-orientate the part in the tool
Adjust features of the part so they align with the tool movements.
Move the split lines by building up the faces of the tool.
Add side movements to the tool to create undercuts (e.g. lifters, slides, side cores).
Split the part into smaller parts which can be assembled after moulding.
Add post-moulding operations to finish the part (e.g. drilling, machining).

Remember to make adjustments of both the part and the tool together to avoid any conflicts!

By this point we should have a design such that it is reasonably mature and works with the tooling. We can then refine the final details of the parts.

Tips to reduce tooling costs

During the design we can often remove cost out of the tooling through smart design. Some of our tips…

Simple planar split lines are cheaper to produce as they reduce the amount of machining required
Allow for CNC milling in the tool geometry. This is cheaper than EDM as it requires fewer manufacturing steps.
Avoid excessively thin or complex features in the cavity plates. They need to be machined with CNC or EDM, and delicate features in the tool can cause issues in production.
Leaving space for round ejector pins to push on the parts means ejector holes can be drilled into the cavity plates
If a part has variants, consider using removable inserts in the cavity plates

Some examples:

Optimise the final details of the part

Now we have a concept for the part and the tool, we can finish the details of the part.
Some of the common areas of design we need to consider are…

FAQs

What is injection moulding design and why is it important?

Injection moulding design is the process of developing a product so it can be manufactured efficiently and reliably using the injection moulding process. It involves optimising part geometry, material choice, and tooling features to ensure both quality and cost-effectiveness.

Even small design decisions can have a major impact on tooling cost, cycle times, and part performance. In some cases, design choices directly determine whether a product is commercially viable.

At AAV Plastics, we draw on over 30 years of expertise in injection moulding and toolmaking to help customers achieve the best possible outcome. Our team provides design support from the earliest stages, ensuring products are optimised for performance, manufacturability, and long-term value.

If you’d like to explore how your concept could be improved for injection moulding, don’t hesitate to contact us for a free design review.

What materials can be used for injection moulded parts?

Almost any thermoplastic can be used for injection moulding, from common polymers like ABS to high-performance engineering plastics such as PEEK. Additives including UV stabilisers, glass fibre, or fillers can be incorporated to achieve specific mechanical, thermal, or aesthetic properties tailored to your application.

Selecting the right material is critical for part performance, durability, and cost-efficiency. Resources such as MatWeb, a comprehensive materials database, can help identify the most suitable polymer for your product.

At AAV Plastics, we provide expert guidance on material selection to ensure your parts are optimised for function, manufacturability, and long-term performance.

Why is draft angle important in moulded part design?

Draft angle is a critical feature in injection moulding design. It allows parts to be easily ejected from the mould without sticking, reducing the risk of damage to both the part and the tool. Proper draft angles also minimise tool wear and help maintain consistent part quality throughout production.

How can injection moulding design reduce tooling and production costs?

Effective injection moulding design can significantly lower both tooling and production costs by making smart choices in part geometry, material selection, and tool manufacturing. At AAV Plastics, we take a holistic approach, optimising the component, the mould tool, and the assembly process to deliver a fully cost-efficient, high-quality solution.

By considering manufacturability from the earliest design stages, we help clients reduce waste, improve production efficiency, and ensure long-term value.

Contact our team today to discuss how expert design can make your injection moulding project more cost-effective.

When should you consider using inserts?

Inserts are recommended when moulding components that require threads or complex features. Using a metal insert allows threads to be added without the need for expensive and complicated unscrewing tooling, reducing overall tooling costs while maintaining part functionality.

Inserts also enable design flexibility, allowing multiple variants of the same component to be produced efficiently. For example, a single casing can accommodate either a male or female fitting simply by changing the insert, without modifying the mould.

What are common mistakes to avoid in injection moulding design?

One of the most frequent mistakes is designing the component and mould tool separately. This approach can lead to unmanufacturable parts, costly tooling, or project delays. Designing the tool and component simultaneously ensures manufacturability, reduces costs, and streamlines production.

Another critical consideration is material selection. The material must be chosen before tooling design because shrinkage and other material properties directly affect mould dimensions and part accuracy. Failing to account for this can result in poorly fitting or out-of-tolerance parts.

At AAV Plastics, we provide expert guidance from concept through to production, helping clients avoid common pitfalls and optimise both design and tooling for performance, cost-efficiency, and long-term reliability.

Contact our team to ensure your injection moulding project is designed for success from the outset.

Ready to take your concept from idea to production?

At AAV Plastics, we combine over 30 years of expertise in injection moulding design, tooling, and manufacturing to deliver cost-effective, high-quality solutions. Our team works with you at every stage to ensure your parts are optimised for performance and manufacturability.

Get in touch today to discuss your project.