Undercuts are common in custom injection molded plastic parts. A side hole, retaining hook, snap feature, internal groove or locking tab may be essential to the final assembly, but the same feature can prevent the molded component from being removed directly from the core and cavity.
For an OEM buyer, this matters because undercuts influence much more than the shape of the product. They can change the mold structure, tooling cost, maintenance requirements, cycle time, tolerance strategy and even the best location for the parting line.
The key question is therefore not simply, "Can this undercut be molded?" In many cases it can. A more useful engineering question is, "What is the simplest reliable way to produce this feature for the required material, tolerance and annual volume?"
That question should be answered during DFM review, before steel is cut. Once the mold has been manufactured, removing or relocating an undercut can require significant modification. Early review gives the product designer and mold engineer more options.
What Is an Undercut in Injection Molding?
An undercut is a feature that interferes with straight-line ejection of the molded part. In a basic two-part mold, the core and cavity separate along the mold-opening direction. Features that lock the plastic around the steel, or extend sideways relative to this movement, may prevent the part from releasing.
Typical undercuts include side holes, windows, hooks, clips, internal locking tabs, grooves, snap-fit features, recesses and certain thread forms. Some are external and visible from the outside of the component. Others are internal and hidden inside a housing or cylindrical feature.
An undercut does not automatically mean the design is poor. Many functional products need them. The important point is that the feature should justify the additional mold mechanism or should be redesigned if the same function can be achieved more simply.
Why Do Undercuts Increase Mold Complexity?
A normal core or cavity moves primarily in one direction when the mold opens. An undercut may require an additional steel component to move sideways or diagonally before the molded part can be ejected. This introduces another controlled movement into the molding cycle.
Additional mold actions require space, precision machining, wear surfaces and reliable alignment. They also add components that may need lubrication, adjustment or replacement during the working life of the tool. For high-volume production, these details can affect long-term maintenance as much as the original mold price.
This is why two visually similar plastic parts can require very different tooling. A housing with four walls and open features may use a straightforward core-and-cavity mold, while a similar housing with side-facing clips can require multiple side actions.
What Is the Difference Between External and Internal Undercuts?
External undercuts are located on the outside of a molded component. Examples include a side-facing hook, an external snap, a side hole or a protruding locking feature. These are frequently released by a slider or another side-action mechanism.
Internal undercuts are located inside the part. Examples include an internal retaining hook, an internal groove or a feature below an opening that prevents direct withdrawal of the core. Depending on geometry, these may be released by a lifter, collapsible feature, removable insert or another specialized solution.
The location alone does not determine the final mold design. The mold engineer must also consider undercut depth, surrounding wall thickness, available space, ejection direction, part size and how the feature interacts with other geometry.
Internal vs External Undercuts
When Is a Slider Used in an Injection Mold?
A slider is a mold component that moves laterally relative to the main mold-opening direction. It is commonly used for external undercuts, side holes and other features that need a sideways steel element during molding.
During the molding cycle, the slider remains in its molding position while the cavity is filled and packed. As the mold opens, the slider retracts away from the plastic feature. Once the undercut is cleared, the part can be ejected.
Sliders are widely used and can be very reliable, but they require sufficient mold space and appropriate guidance. Multiple sliders can increase the size and complexity of the tool. If the part has undercuts on several sides, the mold layout should be reviewed early because side actions may compete for space.
When Is a Lifter Used?
A lifter moves at an angle as the mold opens and the part is ejected. This combined upward and sideways movement allows the lifter to clear certain internal undercuts.
Lifters are useful for internal hooks and locking features that cannot be released by a simple straight core. However, they must be designed with adequate travel, support and clearance. The molded feature also needs to remain strong enough to survive ejection without distortion.
A lifter should not be selected simply because an undercut exists. Depending on the part, a slider, split insert, redesign or another method may be more appropriate. The best mechanism depends on the actual geometry.
Slider vs Lifter
Can a Flexible Plastic Part Be Stripped From an Undercut?
In some designs, the molded plastic can flex during ejection and temporarily deform enough to release from a small undercut. This approach is sometimes called stripping or bump-off ejection.
It can reduce mold complexity, but it should be used cautiously. Suitability depends on the resin, wall thickness, undercut geometry, strain during release, temperature at ejection and the number of cycles expected from the part. A design that releases successfully during a few manual trials may still create whitening, cracking or permanent deformation in production.
For this reason, there is no universal undercut depth that can safely be stripped for every material. The design should be evaluated for the exact resin grade and actual geometry, and sample approval should include repeated ejection and functional checks where relevant.
Can DFM Redesign Remove the Undercut?
Often, yes. Eliminating an undercut does not always require changing the function of the product. A small change in the direction of a hook, opening a slot to the edge, moving a hole to the parting line or separating one complex component into two simpler components can sometimes remove the need for side action.
This is one of the highest-value outcomes of DFM review. A simple geometry change made before tooling may reduce the number of sliders or lifters, shorten toolmaking time and make future maintenance easier.
However, redesign should not be performed only to minimize mold cost. The revised part still needs to meet strength, assembly, sealing, appearance and service requirements. A cheaper mold is not useful if the product becomes harder to assemble or less reliable.
DFM Redesign to Eliminate an Undercut
How Do Undercuts Affect Tooling Cost?
The cost effect depends on the number and type of moving mechanisms, mold size, machining requirements and expected production life. A single simple side action may have a limited effect, while several interacting sliders and lifters can substantially change the mold structure.
The buyer should therefore compare complete tooling concepts rather than only total mold prices. If one quotation is significantly lower, check whether the supplier has included every required side action and whether the proposed design can actually release the part.
Annual volume also matters. For a long-running OEM project, a robust slider system with replaceable wear components may be justified. For lower-volume production, another tooling strategy may be more economical. The right decision depends on total project requirements rather than a single feature.
How Do Undercuts Affect Cycle Time and Maintenance?
Moving mold components add actions to the production cycle. A slider may need to retract and return; a lifter must travel with the ejector system. The additional time for each action may be small, but it can become important in very high-volume production.
Maintenance is another consideration. Sliding surfaces experience repeated mechanical contact. Contamination, insufficient lubrication or wear can affect movement and alignment. A mold intended for long production runs should be designed so that wear components can be inspected and serviced.
This is one reason product designers should avoid unnecessary undercuts. Every moving mechanism should provide a functional benefit worth its added complexity.
How Do Tolerances Change Around an Undercut?
Dimensions formed by sliders or lifters may depend on the repeatable position of a moving mold component. The mold mechanism therefore needs reliable stops, guidance and support.
The customer should identify which dimensions around the undercut are functionally critical. For example, the position of a side hole used for assembly may matter more than the appearance of a hidden gate or minor witness line.
Avoid assigning extremely tight tolerances to every feature without a functional reason. Instead, communicate the mating component, datum structure and critical dimensions. This helps the mold engineer design the side action and inspection plan around the real assembly requirement.
How Do Parting Lines and Witness Marks Affect Undercut Design?
Where a slider or lifter meets the core or cavity, a visible witness line may remain on the molded part. This is a normal consequence of separate steel components meeting in the tool.
If the feature is on a cosmetic surface, the customer should identify the appearance requirement before tooling begins. In many products, moving the witness line to a hidden or non-cosmetic area is more practical than trying to eliminate it after the mold is built.
Parting lines also affect flash control. Moving interfaces require precise fitting, and wear over time can influence the visible line. Tool maintenance and realistic cosmetic standards are therefore part of the design discussion.
What Should Be Reviewed for Multiple Undercuts?
Parts with undercuts on multiple sides require careful sequencing and mold-space planning. A slider on one side cannot interfere with another slider, the ejector system, cooling channels or the runner and gate system.
The toolmaker may need to change the part orientation or parting line to create enough room for all mechanisms. This can also influence gate position, ejection marks and cosmetic surfaces.
For complicated components, a DFM review should clearly show the mold-opening direction and the proposed movement of each side action. A drawing that only marks "undercut" without showing how it will release is not enough for a reliable tooling decision.
What Information Should You Provide for an Undercut RFQ?
For an accurate custom injection molding quotation, send the 3D CAD file and a 2D drawing whenever possible. The 3D geometry allows the manufacturer to identify undercuts and evaluate the required mold movements. The 2D drawing communicates tolerances, datums and critical assembly dimensions.
Also provide the exact material or performance requirement, initial order quantity, estimated annual demand, cosmetic surfaces and how the undercut interacts with the mating component.
If the feature is a clip or hook, explain whether it is assembled once or repeatedly flexed during service. If it forms a side hole, explain what passes through the hole and how accurately it must align. Functional information helps the supplier evaluate whether the original geometry should be retained or redesigned.
Undercut DFM Checklist
What Should Be Approved Before Mold Manufacturing?
Before tooling begins, the buyer and supplier should agree on the final CAD revision, material, mold-opening direction, undercut release method, critical dimensions and cosmetic requirements.
If sliders or lifters are required, the DFM package should make their purpose clear. Any product changes made to remove an undercut should also be reflected in the final drawing revision before steel is cut.
After the first mold trial, sample review should include more than appearance. Check the undercut feature for damage, whitening, deformation, flash and correct assembly. For moving or snap features, functional testing should represent the actual product use.
Frequently Asked Questions About Injection Molding Undercuts
Do All Undercuts Require a Slider?
No. Some external undercuts are well suited to sliders, but internal features may use lifters or other mechanisms. Certain flexible parts may be stripped, and some undercuts can be removed through DFM redesign. The geometry and material determine the best solution.
Are Undercuts Always Expensive?
Not necessarily. A simple undercut with one straightforward side action may add only moderate complexity. Cost increases as the number of actions, precision requirements and mechanism interactions increase. The complete mold concept should be evaluated.
Should I Remove Every Undercut From My Design?
No. Functional undercuts can be completely appropriate. The objective is to remove unnecessary complexity, not necessary product function. If an undercut improves assembly, retention or product performance, a reliable mold mechanism may be the correct solution.
Can an Undercut Be Added After the Mold Is Built?
Sometimes, but it may require significant tool modification or even replacement of mold components. Adding a slider or lifter late in the project can be particularly difficult if the original tool was not designed with enough space. It is much better to identify the feature during DFM.
How Should You Decide Whether to Keep or Redesign an Undercut?
Start with function. If the feature is essential, determine the simplest reliable release method. If the same function can be achieved with geometry that releases in the main mold direction, consider redesigning it before tooling.
Then evaluate material behavior, tolerances, expected production volume, cosmetic requirements and long-term mold maintenance. The best design is not simply the one with the lowest initial tooling cost; it is the one that produces the required part consistently over the expected project life.
If your custom plastic part contains side holes, hooks, clips, internal locking features or other undercuts, send your 3D CAD model, 2D drawing, material requirement and expected quantity to amanda@tjswks.com. A DFM review before mold manufacturing can help identify the most practical tooling strategy and avoid unnecessary complexity.