Start designing for Injection Molding and CNC machining

Both Injection Molding and Computer Controlled Numeric Machining are methods that can be employed by many industries and project teams to design a product.

A complex product design - image courtesy of 3D Hubs
A complex product design – image courtesy of 3D Hubs

Optimise designs for Injection Molding (IM) and Computer Controlled Numeric (CNC) machining using these guidelines to save time, reduce failures, and learn how to create features that maximise the functionality of the product designs.

Common IM Design Rules and Defects

Injection Molding, or IM, is often used for the rapid prototyping service of a product. Most of the defects that occur with IM are due to either melted material or inconsistent cooling rate of the product after production. With proper design, many common IM defects can be prevented, including these:

Wall Thickness – Design the thickness to be as consistent, or uniform, as possible using this chart as a guideline for the material type and measurements. Inconsistent wall thickness can lead to warping of the part as melted material cools. Inconsistencies lead to some areas shrinking faster than other areas during cooling, which often results in the part bending permanently because of internal stress. If areas of varying thickness are part of the product design, using a chamfer or fillet can be a great help, resulting in the heated material flowing evenly during production and filling the mold more smoothly.

Hollowed-Out Thick Sections – Hollow out any thicker sections adding ridges or “ribs” to the design structure to increase and distribute its strength and rigidity throughout the structure, while reducing the thickness of the walls.

Round Edges – Having a consistent wall thickness also applies to edges and corners. Sharp corners can result in stress concentrations producing weaker parts. It is critical that transitions be as smooth as possible to ensure the heated material flows easily and smoothly. For example, with interior edges, design for a radius of at least 0.5 x the wall’s thickness. For the exterior wall edges, add a radius equal to the interior radius plus the wall’s thickness. This helps ensure the thickness is constant throughout, even at the product’s corners. 

It's a good idea to round all edges – image courtesy of 3D Hubs.

CNC Machining Design Rules and Restrictions

When using CNC turning service, or Computer Controlled Numeric machining, the overall result of the cutting process itself can require multiple design considerations and can also represent risks due simply to the limitations of the basic cutting procedures. Design considerations should include:

Machining Tools Impact – The average CNC machining tool is round and typically employs either a flattened or rounded end. This results in a variety of possible shapes that can be produced but that are slightly limited as to what the cutting tool size and shape can accomplish with it. An example would be the inside, upright corners of a steel part being machined. These must always have a circumference-based measurement, no matter how tiny the cutting tools.

Intentionally Designed Pockets – Designed holes in the part being produced, often referred to as cavities or pockets, and which are beyond a certain depth, need to be machined with larger cutting tools. The cutting tools selected should have a span that can work on the inside edges with precision. Depth recommendations can be found in the article and tables describing design restrictions in more detail.

Designing Undercuts

Some features cannot be machined using standard tools since cutting tools cannot work across all of the surface areas, regardless of how a part is positioned or repositioned. This requires inclusion in the design of what is referred to as “undercuts,” a part of the machining process. For example, the cutting of several squares, brass pieces might be produced with rapid CNC machining, and its various sections, which are recommended to be designed with a grooved pattern. These grooves are often referred to as undercuts. Channels are cut into the brass, or the designated material, using specially shaped machining tools. Designing for these undercuts is critical to the success of both the finished product and the project budget’s integrity. However, undercuts should only be used when your design requires a 90° internal corner.

You should undercut the dimensions required to compensate for tool width – image courtesy of 3D Hubs.

You should undercut clearance on internal faces – image courtesy of 3D Hubs.

In the case of injection molding prices, once a mold is manufactured, parts can be reproduced at a low cost. Achieving the best results the first time with good design principles is critical however, as changes in the development at a later stage can be cost-prohibitive. In fact, for any project, the design process and proper preparation can save you both time and budget.

So, as we see, to properly design for either an IM or CNC machining method in a way that can be replicated in the manufacturing process is not difficult. It just takes a little organisation, planning, and adherence to the design guidelines offered. Keeping an eye on design restrictions and potential defects that could occur during the use of these two methods, is more likely to produce a satisfying final product within budget.