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Injection Molding Undercuts Explained

Updated: Aug 16, 2022

Injection molding refers to the manufacturing process of injecting the molten material into the mold to produce parts of various shapes and sizes. At the same time, injection molding materials may incorporate but are not limited to metals, elastomers, ceramics, and most usually thermoplastic and thermosetting polymers. However, the topic will swirl around plastic injection molding in this article, specifically covering the context of plastic injection molding undercuts, along with some standard vocabulary typically associated with molding plastic parts with undercuts.

Undercut in the mold


  • Core: A protrusion that forms a plastic part's inner surface or counterpart – the male side of the part.

  • Cavity: A void that forms a plastic part's outer surface – the female side of the part.

  • Parting Line: The parting plane where two halves of the injection mold meet.

  • Slider (Side Action): It is a side action that converts the vertical movement of the mold opening/closing into the horizontal direction.

  • Shutoff: Shutoffs use drafted walls sealing against drafted walls to eliminate the requirement of post-molding machining or removing functional geometry.

  • Bump-off: The slight undercut in part design that can be securely eliminated from a straight-pull mold without side actions.

  • Lifer: It is used to form the internal undercuts of an injection-molded plastic part. It also works for the ejection function.

  • Draft: The draft is a taper applied to the part faces while developing parts.

  • Fusible Core: Fusible cores are helpful when demoldable cores are difficult to use to mold internal cavities and undercut when injection molding the part.

  • Ejector: The ejector system in plastic injection molding is used to forcefully push and eject the final solid parts or samples out of the mold.

What Is Undercut In Injection Molding?

Undercuts can be characterized as any protrusions or recessed zones of a part parallel to a plastic injection mold's parting line, prohibiting the ejection of the part from the mold. There exist several major types of undercuts, and these are:

1. External undercut

2. Hole

3. Internal undercut

4. Radial undercut

5. Threaded undercut

External undercuts are located on the outside of the part, while interior undercuts remain on the inside of the part. Threaded undercuts occurs when threaded part is molded and usually it requires unscrew it to eject out. Holes and radial undercuts originate when some through holes are needed and must be placed in parallel to the parting line of the mold.

In any case, undercuts can be molded, but they need a side pull or side action, being an additional part of the mold that moves independently from the two halves. Furthermore, these undercuts can upsurge the cost of the molded part because of an additional 15-30% cost of the mold itself, increased cycle time, and the complexity of the injection molding machine.

Purpose of Undercuts in Plastic Injection Molding

The following are some of the general purposes to use undercuts in plastic injection molding:

  • Creating interlocking or snap & latch features and acquiring side holes or openings and ports for button and wiring features.

  • Gaining control of vertical threads and barb fittings utilized in medical devices and providing threaded and customized inserts that are not in the drawn line.

  • Coring out thick and impenetrable sections not secured by the core and cavity alone. Consequently, it helps reduce the chances of sink and warp.

Unavoidable undercuts - the hooks

How to Optimize Part for Injection Molding (Avoiding Undercuts)?

Redesign and upgrade the part to avoid undercuts whenever possible since they add to the mold's complexity, maintenance, and overall cost. Minor part design modifications may help prevent undercuts in the mold; it is simply a matter of choosing the proper workaround for the given part. Here are some ways to optimize the part to avoid undercuts in injection molding:

  1. Hole to Slot. Through holes or slots can be added through simple modifications in the sidewall of the mold, instead of a side-action mechanism. Creating a hole or slot in the mold is feasible to help eject the part without hooking; otherwise, the part might stick in the mold. It considers the metal in the mold to move across the part's hole and appropriately develop the underside of the undercut.

  2. Stripping. Similarly, stripping undercuts can benefit when the feature is sufficiently flexible to deform over the mold during the ejection process. When using stripping undercuts, ensure they are away from stiffening features (ribs and corners) and have a lead angle of 30-45 degrees. Stripping undercuts are discouraged in parts produced using fiber-reinforced plastics. On the other hand, more flexible plastics are acceptable.

  1. Moving a Parting Line. The most straightforward way of managing an undercut is to move the mold's parting line to overlap it with the part feature and modify the draft angles accordingly. In addition, this arrangement is appropriate for various designs with undercuts on an outside surface. The limitation of the parting line placement depends on the material flow, geometry, and other characteristics of the part.

  2. Shutoffs – Telescoping Shutoffs. Telescoping shutoffs, otherwise called sliding shutoffs, refer to another common injection molding technique and are frequently used to develop clip- and hook-style mechanisms. These are typically utilized for locking together the molded product's two halves and, much of the time can help eliminate the need for undercuts. Essentially, the telescope gets machined into the mold's one half and stretches out into the contrary side during mold operation while shutting off specific features.

How To Deal With Undercuts After Part Is Fully Optimized?

Even if the part is fully optimized, there may come situations when undercuts are unavoidable. In that situation, here are three of the ways how to deal with injection molding undercuts:

Hand-Loaded Inserts

A hand-loaded machined insert is embedded into the mold to avoid molten plastic from streaming into these areas. When the cycle is complete, the part gets ejected with inserts, where an operator is needed to take them out with the part for further utilization. Nevertheless, this manual intervention extends cycle time somewhat, in contrast with side actions that may run automatically.


If there exists a mild undercut, one can make an independent insert bolted into the mold. During ejection, the plastic momentarily extends over the insert but then resumes its required form. The bumpoff should be smooth with appropriate radial dimensions – the shape should not be too radical – and the material should be sufficiently flexible that it may slip past the bump with no tearing.

Side-Actions, Sliding Side-Actions and Cores (Angled Pin Slide)

A perpendicular side-action is suitable for round and hollow parts, and the mold gets split horizontally along the part's long axis. Once the molding cycle starts, the mold halves close together. The side-action slides on an angled pin through hydraulic actuators at the same speed to be correctly positioned simultaneously. Then again, when the molding cycle ends, and the mold opens, the side action slides on the angled pin at a similar speed until the side action is adequately withdrawn to allow the undercut to disengage from the part when ejected.

Collapsible Cores

collapsible core for injection molding tool

Collapsible cores produce plastic parts with internal undercuts as an alternate method. These cores get segmented with flexible elements collapsing inward during the initial ejection while releasing the internal undercut. Once collapsed, the part is easily pushed out of the core in the second phase of ejection. Similar to unscrewing molds, collapsible cores can produce threaded closures and fittings. However, dissimilar to unscrewing molds, the collapsible core molds can also create parts with internal features, including O-ring grooves, dimples, and holes in the sidewall of a piece. Ultimately, it eliminates the requirement of external side action.

Collapsible cores can be availed as pre-manufactured "blanks" in multiple sizes. Larger standard cores have diameters from 25mm to 90mm while offering a collapse of 1.20–3.75mm per side, which refers to the permissible depth of undercut feature. Smaller cores with diameters of 13–24 mm can also be availed with collapse distances of 1.32–1.50mm per side. These smaller "mini-cores" can only be used when the thread or undercut is interrupted – not continuous through 360°.

Lost or Fusible Core Molding

Several processes may allow the production of highly complex parts. E.g., parts with large internal undercuts, which cannot be released with the help of traditional injection molding technologies, can be produced using the fusible/lost core molding procedure. Typically, the fusible core molding process makes various parts, including valves, tennis rackets, pumps, and automotive air intake manifolds. Although the process becomes capital intensive, it has the ability to produce complex parts in one go. Eventually, it eliminates the cost and quality issues of secondary operations.

The fusible core molding process starts when a die-cast or gravity-cast metal-core is loaded into the injection mold. The rest of the process goes the usual way, meaning that the tool closes, and the plastic part gets molded onto the cast core. When the mold opens, the core and part are ejected together. A replicated core gets loaded into the tool for the following cycle. After molding, the core, which is usually a metal alloy with a low melting point, is melted from the plastic part. Melting can be carried out using several methods. But the hot liquid inductive heating method is chosen since the core can be dissolved quickly, and the oxidation potential of the metal is minimized.

The metal core melting can be achieved using various methods, in particular:

  • Circulation of hot fluid through the core (for hollow cores).

  • Immersion of the part and the core in a bath of hot fluid (swimming pool method).

  • Inductive heating (will probably cause oxidation of the metal).

  • Inductive heating in a hot liquid.

Once the core gets melted, an inspection process of the parts is accomplished using metal detectors to ensure complete fusion. Afterward, the metal alloy is recast to produce temporary cores for subsequent molding cycles.


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