Injection Molding Undercut Guide for Custom Plastic Parts

injection molding undercut design

Injection molding is one of the most precise and cost-effective plastic production techniques. Undercut injection molding is a byproduct of complex part geometry. A feature that is not typically intended to be on a part, but forms a result of intricate features, design, and mold partition. 

Undercut injection molding is a challenge to work around and this guide helps you identify design features that lead to undercuts, problems arising because of undercuts, and an overall injection molding undercut guide.

What is undercut in injection molding?

In plastic injection molding, undercuts are features of a fresh part that prevent part ejection from the mold. Undercut is simply the area where the object’s shape restricts the smooth linear ejection from the mold.

In plastic injection molding, undercut features are standard inconsistencies. Many part designs like snap fittings, threads, and barb fittings require molding. 

Here are a few design features that require undercut molding

#1 Threads

Injection molding Threads

Vertical threads are grooves commonly seen on bolts and nut joints, but can also be used on injection molded parts for fastening. Undercut molding in vertical threads can lead to the part being damaged after production.

#2 Side Holes

Side holes create undercuts that make the part removal difficult. In plastic injection molding, side cuts are holes on the side of the part that serves any design or practical function. An unpopular, route to solving these undercuts injection molding is to design a mold that opens in sections, allowing easy release of the part from different directions. 

#3 Custom Inserts

Undercut designs are a great way to accommodate custom inserts. They can also double as connectors instead of rivets.

#4 Snap Fits

snap fit joints

Many designers use plastic injection molding because it can eliminate screws from the assembly. Snap fits have complex shapes that utilize interlocking features to connect two parts. A great example of snap fits are battery covers and cam buckles. 

Snap fits are discussed in detail in this helpful guide.

#5 Barb Fittings

Barbs are just ridges on a fixed-diameter cylinder that can hold another cylindrical fitting covering it. The most common application would be a nozzle that holds a rubber tube or pipe. Barb fittings have to be undercut by design to form the lip or ridge that holds the tube.

5 Ways to Create Successful Undercuts in Molded Parts

Undercuts in general are not a flaw. Oftentimes, designing undercuts is necessary due to mold design. Designers need to work their way around these recessed areas and here are some ways undercut can be designed for the best undercut injection molding.

#1 Parting Lines

undercut in injection molding

Positioning the parting line in such a way that it intersects with the feature is the easiest solution to undercuts on external surfaces. Parting line separates the core (upper half) and drag (lower half) of the mold. When the mold’s parting line is repositioned with the part feature, it can be ejected from the mold without having to deal with an undercut. 

#2 Side-Actions


Side action core is sometimes called sliding side action core. These are inserts that slide in and out of the mold and have to be accommodated in the mold’s design stage. Side action core are only used when an undercut is absolutely necessary or undercut design is integrated into the plastic injection molding. 

Side action cores are inserted into the mold as the mold cycle begins and are removed before opening the mold. Side action features can induce a lot of tooling costs and increase the complexity of the work. Side action inserts move perpendicular to the mold’s opening and the side holes need to be designed to allow inserts from the other side of the part feature.

Side actions cannot be used when molding softer plastics. They are ideal for thermoplastics and rigid materials. Material flow properties and deformability needs are taken into consideration for such an expensive process. 

#3 Insert Bumpoffs

In injection molding bump off is simply the removal of the part from the mold. Bump-off inserts can be used when the plastic material is flexible or soft. Bump-offs are removed from the part after molding which leaves behind a small vacuum or space that allows little deformation of the part for ejection. 

Bump-offs require careful consideration for insert placement and whether or not they can be used despite working with a flexible material. Bump-off needs to be away from stiff design features like corners and the lead angle for the undercut must be between 30° and 45°. 

#4 Choose Hand-Loaded Inserts


As the name suggests, hand-loaded inserts are manual inserts that are fed into the mold. These metal pieces are carefully set by the machinists into the mold for an undercut that is difficult for standard bump-offs. Hand-loaded inserts can be 1 or more depending on the mold. The greater the number of inserts, the more time production takes as an operator manually places these inserts each time the mold is closed. 

They work as standard inserts or cores and stop the molten metal or plastic from occupying the space taken by the insert. Undercut injection molding generally increase production, but with manual placement, the cycle time changes, and per-piece cost increases. 

#5 Incorporate Shutoffs

Shutoffs can eliminate the use of insert, side action feature, and core altogether. Shutoffs employ hooks, latch features, and clips to join two halves of the part. Shutoffs require machining the mold to accommodate a telescope that runs through both halves and is able to shut off desired part features. 

Machining a sliding telescope or sliding cutoff is difficult as the tolerance has to be tight to stop the overflow of plastic past the feature. If the plastic squeezes into the sliding action, this can create unwanted friction and an overall poor product. 

Both surfaces need small draft angles to safeguard the mold from flash and damage.

Part Design and Secondary Operations

injection molding undercut

So far, all of the things mentioned are a way to work around the undercut problem. This increases cost and time. You can choose to avoid undercut by adjusting the design of your prototype. 

In most cases, it is possible to design tooling and part that does not require undercut, and in the aforementioned solutions, one of these design features was to reposition the parting line. Design For Manufacturing (DFM) catalog has many elements that help designers overcome production challenges. The objective is to simplify the design as much as possible for manufacturing.

Applications of Undercut Injection Molding Process

Undercut injection molding has broad uses in many industries. Many parts are plastic injection molded with undercut features that don’t require fasteners and can be easily assembled by non-technical users. 

Some of these industries are:

Medical Plastic Parts

Medical Plastic Parts

Medical devices are mass-produced using undercut molding. The complex design of many medical devices that need to allow capillaries, tubes, and sensors require undercuts. Medical plastic parts are complex in shape and small in size. So, plastic injection molding medical goods is ideal. Some examples include, needle hub assemblies, IV connectors, inhalers, insulin pens etc. 

Consumer electronics

Consumer electronics

Almost all consumer electronics have some kind of plastic. This plastic is usually rigid, protective, electrically insulated, and has a long shelf life. This makes undercut plastic molds with enough space ideal for all types of purposes.

From a design perspective, the plastic casing needs to have multiple undercuts, hooks, and latches for different wires, buttons, and electrical components. 

The most common use is in snap-fit casings for electronics, battery covers for toys, cosmetic boxes that have various undercuts for brushes and makeup, home appliances and much more. 

Design Requirements

While most of the time, undercuts are a byproduct of an existing feature, many times they are the required design. In such cases, designers pay extra attention to the molding and production stage for undercuts. 

Undercuts need to have enough space so that they don’t restrict the removal of the insert and core. When designing side actions, they should have loose tolerances in case they get trapped. Hand inserts must be spacious and safe for machinists to handle and bump off must be designed for flexible features. 

There are many other design aspects that are mentioned in detail in the design handbook or DFM.

Injection Molding Best Practices

I have discussed the problems that undercuts cause and the design solutions for undercut. Let’s now take a look at some of the experienced practices that can save you time and design rejections. 

Draft Angle

Draft angle applies to all vertical sides and edges to easily eject the part from the mold. Draft varies but generally lies between 0.5° – 3°.

Parting Line

Undercut design can be easily improved by positioning the parting line on the undercut feature. 

Wall Thickness

Wall thickness is one of the underlying features in injection molding and undercut design. Too thick a wall can increase material use and cost and too thin of a wall is difficult to fill. Nominal thickness controls material warpage and reduces stress in mold.


Corners provide stress and stiffness to the part. Undercuts cannot be designed to close to corners, as it leads to difficulty when positioning insert and bump off.


Undercuts are generally undesired features but they can effectively produce latches, hooks, and threadless assemblies. For this reason, the undercuts are used in the injection molding industry but require design consideration as they complicate the tooling and manufacturing.

Undercut design offers solutions to ejection problems and acts as a guide for different assemblies like snap-fittings. Plastic parts are deflective and with just the right amount of force can be deflected for ejection from the mold.

With this basic principle, inserts can be used to create room for deflection and part ejection. Whether or not force is used to eject the part, undercut design consideration must be used when manufacturing parts with injection molding undercuts.