December 9, 2023


Tech as it is.

What Should You Know About Injection Molding Undercuts?

biodegradable plastic injection molding raw material

biodegradable plastic injection molding raw material

Undercuts allow for the creation of intricate plastic part shapes for injection molding. Overmolding and insert molding are two examples.

Any depression or protrusion that prevents a part from being ejected from a mold is known as an undercut. It is a quality that the cavity and center by themselves are unable to capture. Because it is die-locked, the part cannot be expelled from the cavity.

Why Undercuts Are Used in Plastic Injection Molding

Undercuts are frequently used in the following ways:

  • Add snap-and-latch or interconnecting capabilities. That makes it possible for clamshell or housing layouts to assemble quickly.
  • Jot down any side ports or openings for cabling and button details.
  • Capture barb connections and vertical threads. Usually, medical devices use barb connectors.
  • Core out any thick pieces that the core and cavity couldn’t contain. It eliminates the potential for sinking and warping.
  • A thorough DFM will reveal the undercut elements’ issue zones. Therefore, building it as you desire and maintaining the design’s effectiveness is preferable.

Medical plastic parts

Medical parts frequently have undercuts. An essential medical component can be seen on the left. The internal screws detach from the part via a side action. By doing this, the internal hand load won’t become die-locked. The interior of the portion is completely covered in ongoing threads.

The area between the inlet holes necessitates a core-out, a unique undercut. Core-outs are an intentional feature of the design that maintains uniform wall thickness. For medical components, barbs are intricate undercuts required for tubing. Most producers will typically mold the barb parallel to the dividing line.

Design requirements

To achieve victory when creating undercuts, take into account the following conditions. Design criteria only apply to part features recorded by hand loads or side operations. Space is the first prerequisite. Create your component (button, porthole, etc.) with enough room to remove the hand load without being obstructed. The hand load would be die-locked if this didn’t happen. It won’t be feasible to remove it because of this.

Next, ensure your component is made so the molder may squeeze the hand load or perform an operation with the hollow or core. As a result, the parting line flash will be reduced thanks to a firm shutoff.

Image source

How to Successfully Undercut for Molded Parts?


Parting Lines

Moving the separating line of the mold so that it intersects the undercut is occasionally the simplest solution. A motor housing may require a square and round standoff as a detecting or locking function. You might be able to change the parting line and modify the draught angles to meet these standoffs due to the draught on the outside of the component. When dealing with many features, it may be possible to zigzag the dividing line to intersect each feature. However, part shape, material flow, and several other variables also influence mold direction and parting line location.


Think about a component with a tubular shape, like the hose-barb example. That is an excellent use for a vertical side-action, which may be utilized to create the lengthwise hole in components like these and related ones like control levers and screwdriver handles. In this instance, the longer axis of the piece is used to split the mold in half horizontally. The side action moves on an inclined pin at the same speed the mold closes when the molding cycle starts, causing both to be seated shut simultaneously. Plastic that has just begun to melt is blasted into the mold and left to cool.

Until the side action is withdrawn sufficiently for the undercut to be clear from the component when it is expelled, it travels on the inclined pin at the same pace as the mold opens.

Bump Offs

Lens coverings, container tops, and similar items with functions requiring them to spring into place can be easily molded using bump-offs. We depend on the plastic material to contract during ejection, liberating the element from the undercut.

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Hand-loaded Inserts

What about a plastic snap-together container for medical equipment, such as a handheld meter for measuring blood sugar levels in people with diabetes? It is divided into two equally sized halves and is about the dimensions of a deck of cards. For attaching an electronic circuit and other devices, the bottom half has an inner perimeter edge that runs the length of the piece.

The separation line cannot be altered because of several perforations along the outside of the box, and the lip is too high and pointed for a bumpoff. To stop any plastic from pouring into the mold, the worker physically inserts various metal bits known as hand-loaded inserts. That makes the ejection procedure easier because the operator can take the component out when the cycle is finished and use it for the subsequent batch.

Telescoping Shutoffs

Mechanisms in the form of clips and hooks are frequently made using telescoping shutoffs, also known as sliding shutoffs. These are commonly employed to secure the two sides of molded products together. In many instances, they can replace the requirement for bump-offs, inlays, and side actions, increasing the mold’s complexity.


Characteristics of a plastic object that is not parallel to the mold parting line are known as injection molding undercuts. As a result, they are created in a way that keeps them from being expelled from the mold in the opening location. Recessed portions, hooks, overhands, perforations, and any other design element that restricts mold release are design elements that may create injection molding undercuts.