Injection molding has numerous applications across industries, producing intricate parts and products. Among the different features that facilitate high complexity, undercuts are important. These are the grooves, protrusions, hooks, or other similar shapes that help interlock or perform some specific function.
Like the shape, undercut injection molding is also complex and requires several considerations from designing to parts ejection. Furthermore, we will discuss what is undercut injection molding, types of undercuts, and effective tips for the process.
What is Meant by Undercut Injection Molding?
An undercut refers to a complex cavity, groove, hooks, protruded shapes, or recessed geometries. These types of features are typically associated with higher injection molding costs and process complexity, so it is better to include them in the design only when absolutely necessary.
The undercut injection molding is all about making the process easier by optimizing the mold and simplifying the undercut features. Some examples of undercut features are interlocks, barbed connectors, vertical threads, and side holes.
How to Produce Injection Molded Parts with Undercuts?
To produce parts with undercut geometry using injection molding technology, four methods are common in practice. Which one is best depends on the complexity of undercuts, size, and your end requirements. Let’s discuss them in brief;
Method 1: Shifting the Parting Line
Shifting the parting line of the injection mold near (or intersecting) the undercut features facilitates the draft angle for undercut geometry So, you can eject without any damage. On the other hand, you need to consider the geometry complexity and material flow while shifting the line.
Method 2: Insert placement Inside the Mold
Another method for undercut injection molding is placing the inserts inside accurate mold locations, the same number of inserts as the undercuts in the design. These inserts occupy the negative shape of an undercut and do not let molten material flow and solidify. After the process, the operator removes the inserts manually.
Method 3: Side Action
If the mold can’t be opened directly, a side action moves along with opening or closing that allows to ejection of the part with undercut features. It often features cam pin slides, where an angled pin withdraws smoothly. However, you need to consider the precise synchronization during mold opening and closing.
Method 4: Bump Offs Technique
A bump is a single insert that is placed inside the mold and while ejecting it forms a flexible room ( wiggle) where material bends or moves to form the desired undercut geometry. The part is removed from the mold. This method is suitable for flexible polymers and plastics.
Undercuts Injection Molding Quality Control
Undercuts are critical features in complex geometries, so their handling is associated with quality control. Unfeasible designs for processing and process handling can affect the accuracy and even parts failure.
Three major aspects should be considered during the injection molding quality control of designs with undercuts; Material efficiency, ease of assembly, and simplified tooling.
Undercut Tips for Injection Molding
Here are some undercut tips based on injection molding industry practices;
- Draft Angles: Use the proper draft angle to ease the pars ejection process. These angles can be complex based on the complexity of injection molding designs.
- Consider Appearance: Consider the aesthetic of desired injection molded parts, eg. the appearance of a parting line.
- Wall width and Thickness: The typical thickness for all injection molded parts is from 0.4 to 1.5 mm, apply these rules for undercuts also.
- Ejector Pins: Customize the ejector pin mechanism to make the easy ejection of the undercut section, or pin assist other mechanisms installed for undercuts.
Types of Undercut in Injection Molding
The undercuts can be categorized into different types based on their location, geometry, and functions. The following are the most common types of undercuts in injection molding;
- Internal and External Undercuts: The location of the undercut, is it inside the surface or on the surface?
- Through Hole Undercut: An undercut that passes from one side of the mold to another.
● Ribs Undercut: The protruding features on the part.
- Interlocks: Hanging length and grooved or other interlocking geometries.
Applications of Undercut Injection Molding
Undercut features eliminate the requirements for post-machining for critical assembly arrangements. For example, the interlocking undercuts streamline the assembly process and reduce the overall production time.
Different industrial or consumer items use undercut injection molding for complex and low-cost plastic parts.
| Industry | Applications | Examples |
| Automotive | Highly aesthetic and functional parts | Snap-fit clips, gaskets, vent covers, etc. |
| Consumer Goods | Durable and ergonomic products | Bottle caps, handles, and knobs. |
| Electronics | Protective and structural housings | Device casings, connector sockets, etc. |
| Medical | Complex devices with precise features | Syringe parts, medical device casings, etc. |
| Packaging | Reusable and secure closures | Twist-off caps, locking lids, etc. |
| Industrial | Robust assembly components | Seals, threaded connectors, and gear covers. |
