Undercut Injection Molding: Ways to Avoid Undercuts in Molded Parts

In some cases, manufacturers may need to apply undercuts in various designs and shapes to
provide specific functionality to different parts. However, the undercut parts in injection molds
are usually complex and costly to fabricate, justifying the need to avoid undercut designs to
facilitate injection success. Here, we will examine the various ways to avoid undercuts in molded

Short Overview of Undercut Injection Molding

Undercut injection molding applies in obtaining features such as hooks, protrusions, grooves,
slots, or indentations for specific functionality. For instance, experts use undercuts for parts that
need side holes, custom inserts, interlock features, vertical threads, barb fittings, etc.
But then, injection molding undercuts require a high level of proficiency. This is because
undercuts on molded parts are extremely difficult to rip or eject directly from the mold without
causing any damage and ensuring the best performance. So, most manufacturers often apply
prototype injection molding when designing and fabricating undercut parts. This prototyping
process remains a pivotal step for the functional testing of injection molding undercuts. Besides,

It helps experts to mitigate the risks and exorbitant costs associated with undercut injection

Design Requirements for Undercut Parts in Injection Molding

Undercuts have varying geometry that proves challenging in most cases. Moreover, as the
undercut part gets deeper, it becomes more complex to eject the molded part without

Check the requirements to help ensure your success when designing injection
molding undercuts below:

1. Space

You must design your undercut feature with enough space to remove the hand load or tool
without impairments. Below are some crucial rules of thumb for holes:
 The undercut hole should have a minimum diameter of 1.27mm
 The gap should have a height of less than twice its diameter
2. Draft Angle

Draft angle is the degree of taper of the side wall of the mold or the angle of clearance designed
to aid the removal of a part from a mold. As such, the undercut part must have a sizeable draft
angle (minimum of 1 to 2 degrees) to decrease the linear force you need to remove the undercut

3. Stiffening features

The undercuts must be located away from stiffening features such as corners and ribs. More so, it
It would be best to fabricate the undercut parts made from soft plastics rather than those made
from fiber-reinforced plastics.

4. Lead Angle

Finally, the undercut should have a lead angle of at least 30 to 45 degrees. Note that the lead

angle is between the helix and the plane of rotation.
Top Tips to Avoid Undercuts in Molded Parts

While there are a few resolutions for undercut designs, several techniques can help avoid
undercuts in injection molding. Let’s take a closer look at the best ways to avoid undercuts in
molded parts.

1. Utilize Side-Actions Features

Side-action cores are inserts that slide into the mold as it closes and slides out before it opens.
When the molding cycle begins, the side action slides in and out until the side action feature is
retracted far enough for the undercut to be free from the part when it is ejected. Experts limit
side-action features to 213.84mm in width by 60.38mm in height and ensure that each travel
does exceed the maximum of 73.66mm.
When using side -actions, keep the following in mind:
 Ensure that there’s enough space for the core to move in and out
 Apply the feature to the other side of the part
 The sliding side-actions must move perpendicularly

2. Adjust Parting lines

To a large extent, adjusting the mold’s parting line to intersect it with the part feature remains the
simplest way to deal with an undercut. This is because the draft on the outer part helps move the
parting line between the two halves of the mold and adjust the draft angle accordingly. Thus, the
split line created at the undercut intersects with the desired feature. This solution is suitable for
many designs with undercuts on an external surface. Sometimes, experts use this approach to
eliminate the need for secondary operations. However, a downside to this method is that pivotal
factors such as part geometry and material flow mainly dictate mold orientation and parting line

3. Pick Hand-Loaded Inserts

Hand-loaded inserts comprise multiple pieces of machined metals. In this method, the pieces get
hand-loaded into the desired section of the mold cavity, thus preventing the flow of the molten
material to these areas. Once the molded part gets formed, the two halves of the mold will
separate, and then you can retrieve the inserts for reuse on the next part. In addition, you ensure
that these parts have adequate sizes for easy handling. Generally, experts use pieces with a length
of 12.7mm.
Unlike the side-action features, this technique extends the cycle time of the part production. Even
so hand-loaded inserts remain well-suited for prototyping and low-volume parts production.

4. Use Bump offs

Bump-offs often apply features flexible enough (mostly soft plastics) to deform over the mold
for its safe removal during ejection. Thus, bump-offs are used for components such as lens
covers and container caps. To use bump-offs, experts will fabricate a plastic insert that matches
the exact dimensions of the pocket required and then get bolted into the mold. During ejection,
the insert pops back over the undercut feature by undergoing a slightly brief deformation while
retaining the molded part's finished shape.
Therefore, you must use smooth and well-radiused bump-offs to ensure a hitch-free ejection

process. Besides, most manufacturers use materials that can slip past the bump without tearing.
They include low-density polyethylene, thermoplastic elastomer, and thermoplastic

5. Integrate Shutoffs

Sliding or telescoping shutoffs are helpful tricks to deal with undercuts on the internal region of
the part for snap-fit or on the side of the parts for holes or handles. Similarly, shutoffs are machined metal used to create a clip and hook-style mechanism. This method involves
applying the shutoffs to the core half of the mold, thus sticking out to produce an overhang. It
creates the underside of the cantilever or hook, and the part under the undercut gets
Shutoffs eliminate the need for side actions and bump-offs, adding more complexity to molded
parts. For this reason, shutoffs offer an excellent way to simplify mold designs and reduce
operating costs. To overcome damage to the mold surface due to the potential metal-on-metal
rubbing effect, you must draft each surface with a minimum of 3 degrees vertically.

6. Part Designs and Secondary Operations

You can use CAD to draft the model for the desired part, including the undercut design. With this
design draft, you can clearly define or avoid the undercut part alongside other moldability
concerns. You must ensure that the parts have sufficient draft angles for easy ejection from the


Applying undercut designs to injection molded parts requires a high level of expertise to achieve
injection success. Hence, prototyping injection remains a crucial phase of your undercutting
project. Above all, it is beneficial and cost-effective that you eliminate the need for undercuts in
your component design.

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