NX Undercut Detection: The Short Version
NX undercut detection means testing a molded or cast part against its pull direction for faces that would block ejection. Siemens NX offers Draft Analysis, Check Regions in Mold Wizard and Moldability Validation, each run on demand. inteliScience, Siemens NX and NXOpen specialists, turns your guideline’s draft and pull-direction rules into a live check; the designer decides every fix.
A note on terms, because both meanings are in use: “undercut” here is the molding and casting sense (German “Hinterschnitt”), geometry that stops the part from leaving the mold along the pull direction. It is not the relief groove on a turned shaft (German “Freistich”), which English drawings also call an undercut and which DIN 509:2022 standardizes as “relief grooves” [13].
The Problem: The Toolmaker Finds the Undercut
Most undercuts are not modeling errors. They are decisions nobody made on purpose: a side window added in the last iteration, a snap hook copied from another cover, a rib modeled straight because draft was “for later”. They surface at the toolmaker’s DFM review or quote, when every fix is either tool cost or a design loop.
A side action is an extra mold component that moves separately from the two mold halves; a long-standing handbook figure puts its added mold cost at 15 to 30 percent [12]. Before release, the alternative is often three features in NX. After the quote, it is a change request against a price.
In a recent meeting with a DACH machine builder’s CAD team, we described two applications of an NX AI stack: undercut detection in molded parts, and checks that run continuously while the designer works. This article combines them. The team ranked design support inside NX ahead of Teamcenter work because it pays back fastest, and NX undercut detection is that kind of support.
What Siemens NX Offers for Undercut Analysis Today
For native NX undercut detection there are three routes, and all three are good at what they do: Draft Analysis on the Analysis tab, Check Regions in Mold Wizard, and Moldability Validation in the Molded Part Design add-on. They differ in who runs them, when, and on which license (checked against Siemens sources on 29 September 2026; dialogs can differ by release).
| Tool | Where it lives | What it reports | Who runs it | Limit to know |
|---|---|---|---|---|
| Draft Analysis | Analysis tab, Face Shape group [6] | Faces colored by draft against a draw vector, with a positive and a negative limit angle [7] | Any designer | Judges each face’s angle; it does not ask whether other geometry is in the way |
| Check Regions | Mold Wizard [1] | Face tab colors draft conditions and locates undercut faces and edges; Region tab shows parting loops [1] | Mold designer | Part of the Mold Wizard tool set (also on the Analysis tab, Molded Part Validation group); built for mold designers |
| Moldability Validation | Molded Part Design add-on [2][3] | Thin and thick walls, positive or negative draft, vertical walls, sharp corners, undercut regions [2] | Product designer with the add-on | Separate add-on through value-based licensing [3]; runs when invoked |
Molded Part Design begins with the Draw Direction command; its draft recommendation weighs design thickness, draw direction, assigned material and face depth [4]. In a March 2026 Siemens example it recommended 2.6 degrees for an ABS part [5]. Since the June 2024 release, Moldability Validation and Feature Validation also propose edits for failed results [3]. Third parties are moving in too: on 13 July 2026 CoreTech (Moldex3D) announced DesignSim, a DFM add-in embedded in NX 2312 to 2606 that detects undercuts as one of 17 checks [9].
Our position: if your designers already have Molded Part Design, use Moldability Validation before anyone writes custom code. The gap we see in practice is not detection quality. Each tool runs when somebody remembers to run it, reports in its own categories rather than your guideline’s rule numbers, and the decision (slide, lifter, redesign) ends up in an email instead of next to the finding.
Draft Check vs Undercut Check: Why One Is Not the Other
A draft check asks each face a local question: is your angle to the pull direction large enough? An undercut check asks a global one: can the steel that forms this face withdraw along the pull direction, or does another part of the same body stand in the way? A face with perfect draft on the inside of a side window is still trapped.
We’d advise against reading a clean Draft Analysis as “no undercuts”. It is the right first test and cheap to automate; this C# sketch flags faces below the minimum draft:
// Minimal NXOpen .NET sketch: flag faces whose draft is below a minimum.
// Samples ONE point per face (mid-parameter); production code samples a grid.
using System;
using NXOpen;
using NXOpen.UF;
public class DraftSketch
{
public static void Main(string[] args)
{
Session session = Session.GetSession();
UFSession uf = UFSession.GetUFSession();
Part work = session.Parts.Work;
ListingWindow lw = session.ListingWindow;
lw.Open();
double[] pull = { 0.0, 0.0, 1.0 }; // in production: read from the part
double minDraftDeg = 1.0; // in production: from the guideline
foreach (Body body in work.Bodies)
{
if (!body.IsSolidBody) continue;
foreach (Face face in body.GetFaces())
{
double[] uv = new double[4]; // umin, umax, vmin, vmax
uf.Modl.AskFaceUvMinmax(face.Tag, uv);
double[] param = { 0.5 * (uv[0] + uv[1]), 0.5 * (uv[2] + uv[3]) };
double[] pt = new double[3], u1 = new double[3], v1 = new double[3];
double[] u2 = new double[3], v2 = new double[3], n = new double[3];
double[] radii = new double[2];
uf.Modl.AskFaceProps(face.Tag, param, pt, u1, v1, u2, v2, n, radii);
// Solid body: the unit face normal points out of the body.
double dot = n[0] * pull[0] + n[1] * pull[1] + n[2] * pull[2];
double draftDeg = Math.Asin(Math.Min(1.0, Math.Abs(dot))) * 180.0 / Math.PI;
if (draftDeg < minDraftDeg)
{
face.Highlight();
lw.WriteLine(face.JournalIdentifier + ": draft "
+ draftDeg.ToString("F2") + " deg, minimum " + minDraftDeg);
}
}
}
}
}All members are in the NXOpen .NET reference [7]; for a solid body, the normal from UF_MODL_ask_face_props points out of the body [8]. The mid-parameter sample can fall outside a trimmed face, which is why this is a sketch, not a checker. For a color map instead of a list, NXOpen exposes the interactive command as GeometricAnalysis.DraftAnalysisBuilder, with draw vector and positive and negative limit angles [7].
The undercut test itself needs an obstruction test. The following is labelled pseudocode, not NXOpen code:
PSEUDOCODE: undercut test for one face F (not NXOpen code)
d = pull direction (unit vector), read from the part
for each sample point p on F (grid inside the trimmed boundary):
n = outward unit normal at p
if |n . d| < sin(min_draft): report DRAFT_TOO_SMALL(F, angle)
s = d if n . d > 0 else -d # direction the forming steel must withdraw
if ray(from p + eps * n, along s) hits the same body again:
report UNDERCUT(F, p) # trapped: slide, lifter, core or redesignNXOpen has the building block: UFModl.TraceARay wraps UF_MODL_trace_a_ray, which fires a ray at a list of bodies and returns each face it hits [8]. Tolerances, tangent rays and sampling density decide whether to trust the result, so test on parts whose undercuts you know.
Undercut Types and the Usual Fixes
Most undercuts in molded parts fall into a handful of types, each with a usual tooling answer and a design alternative. A good check names the type, because the type decides who joins the conversation: a slide is a toolmaker topic, a redesign is the designer’s.
| Undercut type | Example | Usual tooling answer | Design alternative |
|---|---|---|---|
| External side hole or window | Cross hole in a hose barb, housing window | Slide (side action) moving perpendicular to the mold opening [10][11] | Move the parting line through the feature, or open the hole into a slot [10] |
| Internal snap hook, clip or tab | Latch on the inside of a cover | Lifter, which releases internal undercuts [11] | Opening under the hook so a shutoff forms it; telescoping shutoffs need at least 3 degrees of draft [10] |
| Internal thread | Threaded cap or boss | Unscrewing mechanism or collapsible core [11][14] | Metal thread insert, or move the thread outside |
| Shallow bead or lip | Lens cover, container cap | Bumpoff (stripping), if the material is flexible, such as LDPE, TPE or TPU [10] | Reduce the depth, add a lead-in |
| Wall or rib without draft | Straight rib, vertical boss | None: this is a draft finding, not a mechanism | Add draft per guideline [5] |
| Internal lip on a low-volume part | Lip inside a device case | Hand-loaded insert, practical from about 0.5 in square [10] | Secondary machining [10] |
Mechanism choice has its own economics: flexing collapsible cores can run cycles up to 30 percent faster than unscrewing molds, according to a mold component maker [14]. For cast parts the tooling vocabulary differs, but the geometric test against the die opening direction is the same, so one rule set can cover both processes with different parameters.
Turning NX Undercut Detection Into a Continuous Rule Check
The native tools answer “are there undercuts?” when asked. A continuous check answers it every time the geometry changes, against your guideline’s numbers, and records what the designer decided. That is the difference between a command and a process.
This is how inteliScience builds it. The rules come from your guideline: pull direction per part, minimum draft per material and texture, allowed undercut types per part class, who approves a mechanism. If the guideline lives in Confluence, the check reads it through the API, not from a copied PDF. One rule set runs in three modes: live on modeling events (feature created, parameter changed), from a button, or as a batch over a folder.
The geometry test is deterministic and runs first. The AI layer phrases the finding in your rule’s words, cites it and lists the options (slide, lifter, collapsible core, redesign). The designer sees highlighted faces and a report, then accepts the undercut with a mechanism and a reason, redesigns, or waives the warning; every decision is logged. Nothing blocks, nothing changes automatically. An approved redesign can go to our NX Action Assistant for preview and explicit confirm. The model behind the wording is configurable per use case: a hosted model, or an open-weight model on your own hardware.
| Rule | Parameter (from your guideline) | Severity |
|---|---|---|
| Pull direction defined | Part attribute or named datum axis present | Error |
| Minimum draft, smooth faces | Guideline value per material, for example 1.0 degree | Warning |
| Minimum draft, textured faces | Guideline value per texture grade | Warning |
| Undercut face | Forming steel blocked along its withdrawal direction | Error, unless an approved mechanism is recorded |
| Approved mechanism | Attributes: type (slide, lifter, core), reason, approver | Info |
| Side-action directions | At most n distinct directions per part class | Warning |
| Rib or wall without draft | Draft below minimum on faces near-parallel to the pull | Warning |
Severities feed the report and the release gate. The live check itself only warns: a checker that stops designers mid-thought gets switched off.
This runs as a POC script on one part. What’s missing for production: rules read from your guideline instead of hard-coded angles, a pull direction stored with every part, event triggers inside the NX session, a measured false-positive rate, a decision log, and hosting on-premise or in the EU. We close that gap with the Live Model Check Assistant: it flags, cites the rule and suggests options, and the designer decides.
Rollout Checklist for a Live Undercut Check
Start narrow. A rule set that is right for one part family beats a generic one that is noisy for all of them. This is the sequence our NX and NXOpen engineers at inteliScience, working from Romania one hour ahead of Central European Time, follow with customers.
- Pick one part family and one process, for example glass-filled polyamide housings or die-cast covers.
- Write the rules as rule, parameter, severity and owner. The catalogue method is the same one we describe for checking company design rules automatically in NX.
- Store the pull direction with the part, as an attribute or a named datum. Without it, there is nothing to measure against.
- Replay 10 to 20 past parts with known toolmaker findings. Count what the check would have caught.
- Measure false positives on clean parts and tune sampling and tolerances before any designer sees a warning.
- Define “accepted”: mechanism, reason and approver as attributes the toolmaker can see.
- Go live for the family, with the same rules callable from a button and as a batch over legacy parts.
- Review the waivers monthly. A rule that is waived every week needs a new parameter, not more discipline.
Frequently Asked Questions
How do you detect undercuts in Siemens NX?
Define the pull (draw) direction first. Then use Draft Analysis for a quick color map, Check Regions in Mold Wizard to locate undercut faces and edges, or Moldability Validation if you have the Molded Part Design add-on. For NX undercut detection during modeling, the same test can run as a rule-based check on every geometry change.
Does NX Draft Analysis find all undercuts?
No. Draft Analysis colors each face by its angle to the draw vector, so it shows insufficient or reversed draft. A face with good draft that is hidden behind other geometry of the same part, such as the inside of a side window, needs an undercut test like Check Regions, Moldability Validation or a ray-based check.
Check Regions vs Moldability Validation: which one should designers use?
Moldability Validation is built for product designers: it sits in the Molded Part Design add-on and reports undercuts together with wall thickness, draft and sharp corners. Check Regions belongs to Mold Wizard and also shows parting loops, so it is the mold designer’s tool. If your designers have neither license, a rule-based NXOpen check is the practical route.
How much effort does a continuous undercut check take?
Most of the effort goes into the rules, not the code: pull-direction convention, draft values per material and texture, allowed mechanisms, approvals. For one part family that means workshops with senior designers and a replay against past toolmaker findings, quoted as a fixed-scope proof of concept once we have seen your guideline and example parts.
Can the AI decide whether an undercut is acceptable?
No, and it should not. The geometry test is deterministic; the AI explains the finding in your guideline’s words and lists the options. Whether a slide is worth its tool cost is an engineering and commercial decision; the designer makes it, the check logs it.
Next Step
Send us one guideline chapter on draft and undercuts, and one part your toolmaker sent back. We will show what a live check would flag on it, in your rule’s own words. Talk to our NX engineers about a live undercut check
Related Topics
- How the Live Model Check Assistant works
- NX AI assistants and chatbots overview
- What NX users expect from an NX AI
References
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