Undercuts in injection molding are features that prevent a simple open-and-shut tool from releasing the part. Side actions, lifters, and unscrewing heads buy those features back — at the cost of steel, debug time, and flash risk. OEMs who discover undercuts after the quote are really discovering a second tool budget.
The DFM job is not “never have undercuts.” It is to spend actions only on features the product actually needs, and to pick the cheapest mechanism that will last the shot life. This guide maps undercut types, common mechanisms, cost tells, and what to freeze in the RFQ.
Start from injection mold design and a written DFM review before anyone prices sliders as an afterthought.
What counts as an undercut
If the part cannot slide off the core or out of the cavity in the mold-open direction, you have an undercut. Common sources:
- Windows and holes perpendicular to draw
- Snaps and hooks that catch on the core — snap fit design
- Internal grooves, barbs, and threads
- Logo or texture wrapping around a corner
- Overmolded or insert geometry that locks the substrate
Some “undercuts” are draft problems in disguise. Add draft and a parting-line move before you buy a slider. Living hinges are a material-and-flow problem, not a slider: living hinge design.
Mechanisms: pick the cheapest that will live
| Mechanism | Typical use | Cost / risk |
|---|---|---|
| Parting-line redesign | Feature can live on the split | Lowest if cosmetics allow |
| Hand load / pickout | Very low volume, simple core | Labor, inconsistency, lost inserts |
| Lifter | Internal undercuts, snaps | Wear, ejector timing, witness lines |
| Slider / side action | Windows, external hooks | Wear plates, flash, longer debug |
| Unscrewing / collapsible core | Threads, internal details | Highest complexity and maintenance |
Every action is a maintenance item: mold maintenance. Flash at slider shutoffs is a process and steel issue; do not “fix” it only with packing. Gate placement still matters so weld lines do not sit on a snap root — gate location, weld lines.
Cost and lead time impact
One simple slider can add weeks and a noticeable share of tool cost. Two opposing sliders plus texture is a different class of mold. Buyers who compare quotes without counting actions are comparing different tools.
- List every undercut on the 2D; do not hide them in the STEP
- Ask the maker to mark each: redesign vs lifter vs slider
- Price the tool with and without a redesign option when CAD can still move
- Expect T0 debug on actions — they stick, flash, and leave witness lines
See mold cost breakdown and build process / timeline. Steel choice still applies: sliders on abrasive GF nylon want harder, better-lubricated actions — mold steel selection and glass filled nylon.
DFM moves that delete actions
- Rotate the parting line so the window opens in draw
- Change a through-hole to a slot that can be cored
- Relax a snap undercut; use a separate clip if volume is low
- Move logos off wrap-around surfaces
- Split the part and ultrasonic weld instead of molding a locked box
Insert molding and overmolding add their own locks: insert molding, overmolding. Design the shutoff before you quote “simple two-shot.”
How Deuchi quotes undercuts
Deuchi marks actions on DFM before the tooling number. We will recommend deleting a slider when the feature is cosmetic vanity. We will not hide side-action cost inside a lump-sum mold price.
Send CAD to contact. Tooling frame: mold build.
FAQ
Are all side holes undercuts?
If the hole axis is not in the draw direction, yes — unless the parting line is designed so the hole is formed on the split.
Can 3D printed inserts replace sliders for production?
For prototypes or tiny volume, sometimes. For production shot life, you want steel actions and a maintenance plan.
Why is there a witness line on my snap?
Lifters and sliders shut off on a line. Call it out as acceptable or move the feature. Do not discover it after texture.
Do undercuts always mean a more expensive piece price?
They raise tool cost and can add cycle (delay for actions). Piece price impact is smaller than the tool impact unless the action is slow or fragile.
Next step: Contact Deuchi with CAD highlighting snaps, windows, and threads for an action-count DFM.