External undercut
A clip, barb, or hole on the outside of the part is a side-action candidate unless you can reorient it or shut it off.
Undercuts
Injection Molding Undercuts: When a Feature Needs Side Actions, Lifters, or Redesign
An undercut blocks straight-pull ejection. Decide whether to eliminate it, shut it off, bump it off, or add a lifter or side action. DFM review comments on your CAD before quoting.
Design guideline
Decide the undercut before you add mold steel
An undercut is any molded feature that blocks the part from releasing in the mold-open direction. Side holes, snap hooks, internal threads, barbs, and recessed clips are common examples.
External undercuts often need side actions . Internal undercuts often need lifters . Shallow features on a flexible resin may bump off. NetProto does not set one undercut-depth rule. DFM review checks draw direction, resin, feature location, and volume before the tool is quoted.
Eliminate first when the function still works: reorient the feature, move the parting line, or add a through hole so steel can shut off. Cantilever snap fits and side holes are common sources. Deep threads may belong on the unscrewing molds page. The design guide lists every topic.
Roles
Which page answers which undercut question
Stay here for the decision. Open a related guide for the mechanism, the snap, or the CAD check.
| Page | Use it for |
|---|---|
| Undercuts (this page) | Decide if a feature needs action, and what type |
| Side actions | External undercuts: sliders, cams, hydraulic |
| Lifters | Internal lips, snaps, inward features |
| Cantilever snap fit | Snap geometry that may or may not need action |
| Side holes | Hole orientation versus draw direction |
| Draft angle | Lock pull direction and draft before you map undercuts |
| DFM review | CAD check of undercuts before quoting |
Checks
What DFM review looks for on undercuts
Qualitative checks. NetProto does not publish a universal undercut-depth limit.
Internal undercut
A lip, inward rib, or snap inside a pocket is a lifter candidate. Changing draw direction can turn it external.
Shallow and flexible
A shallow hook on a flexible resin may bump off the core. Rigid resins and deep hooks usually need steel motion.
Deep, threaded, or multi-direction
Deep threads or features that pull in more than one direction may need unscrewing or another mechanism. DFM review picks that from CAD.
Decision
Discover the undercut late versus lock draw direction early
Wrong: add steel after the quote. Right: decide redesign, bump-off, lifter, or slider first.
What goes wrong
- The undercut shows up after quote approval, so the tool plan changes late
- A slider is added when a shutoff or through window would release the feature
- An internal thread is molded when a post-mold insert would avoid the mechanism
- A rigid resin, a deep undercut, and no release path are left on the CAD
What to try first
- Lock draw direction and draft before you map undercuts
- Reorient the feature, move the parting line, or add a through window
- Use a lifter for an internal lip and a slider for an external clip
- Ask DFM review which mechanism fits the resin and volume
RFQ
Send these notes with CAD for an undercut review
The quote can start from CAD alone. These notes make the DFM pass specific.
Draw or pull direction you want locked
External versus internal undercut map
Flexible or rigid resin family
Annual volume, so review can weigh action complexity
Snap, clip, thread, and side-hole list
Cosmetic witness-line tolerance on action parting surfaces
Related
Related design guidelines
This page covers the undercut decision. Open a related guide for the mechanism, the snap, or the CAD check.
DFM review
How NetProto checks undercuts on your CAD before quoting.
DFM reviewDesign guide
All injection molding design guidelines by topic.
Design guideFAQ
Injection molding undercut FAQ
What is an undercut in injection molding?
An undercut is any molded feature that blocks the part from releasing in the mold-open direction. Side holes, snap hooks, internal threads, barbs, and recessed clips are common examples. This page decides whether to eliminate the feature, shut it off, bump it off, or add a lifter or side action.
External vs internal undercut: what is the difference?
An external undercut sits on the outside of the part and often needs a side action. An internal undercut sits in a pocket or behind a lip and more often needs a lifter. The same feature can switch type if you change draw direction. Open the side actions and lifters pages for those mechanisms.
When do you need a side action instead of a lifter?
Use a side action when the undercut is external and the steel must move sideways. Use a lifter when the undercut is internal and the steel must lift as the mold opens. Resin, volume, and how many directions the feature pulls change the call. DFM review picks the mechanism from your CAD.
Can undercuts be eliminated without moving mold steel?
Often yes. Reorient the feature to the draw direction, move the parting line, add a through window, or shut off the steel so the feature forms without a slider. Eliminate first when the function still works. DFM review will say if the remaining feature still needs an action.
Do flexible materials allow bump-off undercuts?
A shallow feature on a flexible resin can sometimes deflect off the core as the part ejects. Rigid resins and deep hooks usually cannot. There is no NetProto bump-off depth specification on this page. Call out resin family and feature depth on the RFQ.
How is this page different from side-actions, lifters, snap fits, or DFM review?
This page covers the decision: redesign, bump-off, lifter, or side action. Side actions and lifters own those mechanisms. Cantilever and annular snap-fit pages own snap geometry. DFM review is how NetProto checks your CAD before quoting.
Next step
Upload CAD for DFM review of your undercuts
Share geometry, resin, draw direction, and which snaps, holes, or threads must stay so review can decide redesign, bump-off, lifter, or side action before quoting.