Category Archives: Digital Workflow

The series “A Thousand and One Questions from the Lab” is a run of short videos produced by the technicians at Khai Nguyen Dental Lab since May 2026, answering the professional questions that come up daily at the bench. It began as a way for our own technicians to revise together and share practical experience, and to support technicians new to the trade. This page collects the episodes released so far and the technical content behind each.

Above: the series, episode 2 — handling a dark preparation.

Episode 1: how polishing differs from glaze firing

Released 26 May 2026. This is the question new technicians ask most often at the finishing stage. Glaze firing applies a thin glassy layer and fires it, giving a very high shine but softening the surface texture, and it can wear away over time. Mechanical polishing creates the shine within the ceramic body itself, preserves the surface texture the technician created, and can be restored after the clinician makes an occlusal adjustment.

Episode 2: handling a dark preparation

Released 2 July 2026. How to mask a dark preparation while still reaching a bright shade and keeping natural translucency. The episode covers what the technician actually does at the framework and layering stages: choosing an ingot of the right opacity, cutting back to leave room for the effect layer, and placing the masking layer against the preparation rather than thickening the whole restoration evenly.

Episode 3: what makes a single dark-preparation central incisor more manageable

Released 14 August 2026. A single anterior unit on a dark preparation is always a demanding brief. This episode sets out three core factors the clinician can prepare so that the success rate is higher: restorative space, material, and a design matched to the required effect.

Related articles on the site

• Minimum veneer thickness
• Dental ceramic materials comparison
• Dental photography for the laboratory

Suggest a topic

The series is built from real questions. If you have one you would like the laboratory to answer in a coming episode, send it through our contact channels or comment under the videos on the Khai Nguyen Dental Lab Facebook page.

Source: Khai Nguyen Dental Lab Facebook page, videos of 26 May, 2 July and 14 August 2026.

Written for dentists and technicians. Khai Nguyen Dental Lab manufactures to clinical prescription only and does not sell directly to patients.

Summary: the real question about CAD/CAM is not “which brand of machine” but “how much error, and measured where”. This article gathers the reported figures for marginal fit of CAD/CAM zirconia restorations, the clinical acceptability threshold used as a benchmark, and which stages of the digital chain actually determine the final error.

Where the 120 µm threshold comes from

The most widely cited benchmark is the proposal by McLean and colleagues: a marginal discrepancy below 120 µm is considered clinically acceptable for ceramic restorations. That figure sits within the 4–174 µm range that systematic reviews report for ceramic crowns generally.

It should be read correctly. It is an acceptability threshold, not a target. A well-controlled workflow produces considerably smaller gaps, and most of the clinical value of narrowing the gap lies in reducing cement dissolution over time rather than in the number recorded at cementation.

Milled zirconia: the reported figures

• For zirconia copings produced by CAD/CAM, studies report marginal discrepancies from about 10 µm to 160 µm, with most below 80 µm.
• For monolithic zirconia crowns, some studies report a narrower band, from 11 µm to 58 µm.
• A scoping review of 3D printing and CAD/CAM milling in prosthodontics records a mean marginal gap for milled zirconia of 123.89 ± 56.89 µm — and the size of that standard deviation is itself the point: variation between systems and workflows remains considerable.

Such a wide spread across studies is not a contradiction. It reflects reality: results depend on the CAD/CAM system, the measurement method, the number of measurement points, the material, and the preparation design — the last of which lies entirely on the clinical side.

Milling or 3D printing

For zirconia, milling remains the standard process in fixed prosthodontics. A study comparing 3D-printed with milled zirconia crowns found no significant difference in internal fit and marginal adaptation between the groups, but did find significant differences in trueness in the occlusal, axial and internal areas.

In other words: the two technologies can seat comparably, while their overall geometry is not identical. For cases demanding high morphological accuracy, that difference matters.

Where error actually accumulates

The mill is one link. Final error is the sum of the chain, and the first three stages are usually larger contributors than machining.

1. Input data

A finish line that is not clearly readable on the impression or scan file is the largest single source of error, and no downstream stage can compensate for it. An in vitro study comparing extraoral with intraoral scanning examines exactly this stage and its effect on the fit of milled and printed zirconia crowns.

2. Design

The die spacer set in software determines whether the restoration seats fully. Too tight and it binds, producing a marginal gap even though the internal fit is close; too generous and the cement layer is thick and prone to marginal dissolution over time.

3. Sintering

Zirconia is milled pre-sintered and shrinks on firing. The shrinkage factor is entered per blank batch; entering the wrong factor or mixing batches produces a global deviation that no earlier stage detects.

4. Machining and finishing

Worn burs round off fine detail, particularly at the margin. This is why a tool-replacement schedule based on running hours matters as much as machine selection.

What this changes for the clinic

• Preparation design affects fit as much as equipment does. A clear, square, continuous margin outperforms a thin bevel — see preparation for crowns and veneers.
• Ask the laboratory about cement space when one type of error recurs. It is a parameter the clinic never sees but which acts directly on the result.
• Material dictates the thickness threshold, and thickness dictates the reduction required. The materials comparison and 3Y-TZP vs 5Y-PSZ zirconia set out the groups.

FAQ

What marginal gap is good?

Below the widely cited 120 µm threshold is acceptable. Many studies of monolithic zirconia crowns report considerably lower values. Consistency between cases matters more than the absolute number.

How is marginal gap measured?

In research, typically by microscopy or sectioning with measurements at multiple points. Clinically, probing and bitewing radiographs are the practical tools, though far less sensitive.

Does a more expensive machine give better results?

Not automatically. The literature lists the CAD/CAM system as one factor among several, alongside material, measurement method and preparation design. A well-controlled workflow on a mid-range machine is usually more consistent than a loose workflow on a high-end one.

Is 3D-printed zirconia ready?

Studies show fit comparable to milled crowns, but differences in geometric trueness are still reported. For routine fixed prosthodontics, milling has the deeper evidence base.

References

• Three-Dimensional Printing and CAD/CAM Milling in Prosthodontics: A Scoping Review of Key Metrics Towards Future Perspectives. PMC12294912.
• Fit, Precision, and Trueness of 3D-Printed Zirconia Crowns Compared to Milled Counterparts. PubMed 36421402.
• An in vitro evaluation of marginal fit zirconia crowns fabricated by a CAD-CAM dental laboratory and a milling center. PMC6567516.
• Evaluation of Marginal/Internal Fit and Fracture Load of Monolithic Zirconia and Zirconia Lithium Silicate (ZLS) CAD/CAM Crown Systems. PMC8585271.
• Verification of extraoral versus intraoral scanning techniques: fit accuracy implications for 3D printed and milled zirconia crowns. PubMed 41545973.

Written for dentists and technicians. Khai Nguyen Dental Lab manufactures to clinical prescription only and does not sell directly to patients.

Summary: for a laboratory, clinical photographs are not supporting documentation — they are primary input data. Technicians build form and shade distribution from photographs more than from written descriptions or shade codes. This article covers the minimum photographic set, equipment that is genuinely sufficient, and three shooting errors that destroy the value of an entire set.

What the laboratory uses photographs for

Three things, each requiring a different kind of image.

• Form and proportion. Facial and smile photographs show how much tooth is displayed at rest and in smile, whether the occlusal plane is parallel to the interpupillary line, and how wide the buccal corridors are. Without this, form is guesswork against population averages.
• Shade and translucency. A shade code conveys base shade only. Internal shade distribution — the opaque dentine body cervically, the translucent incisal third, white spots, craze lines — is legible only from photographs.
• Substrate. For veneers and thin restorations the preparation shade dictates the opacity required. The preparation photograph is the most commonly omitted image and the one that causes the most remakes.

The minimum set

Extraoral

• Full face, lips at rest — tooth display without smiling.
• Full face, natural smile — smile line, buccal corridors.
• Close-up of the teeth and lips in smile, to relate the incisal edge to the lower lip.

Illustration: a close-up of lips and teeth in smile, cropped from below the nose to the chin, enough to relate the incisal edges to the lower lip without showing the face. Image: Liz20151222, CC BY-SA 4.0; Wikimedia Commons.

Intraoral

• Frontal with retractors, teeth in occlusion.
• Left and right lateral with retractors.
• Close-up of the anterior segment, shot perpendicular.
• Incisal edge from below or from the palatal aspect, to show thickness and translucency in the incisal third.

Illustration: a frontal intraoral view, the retractors pulling lips and cheeks fully away, the dark oral background making the teeth stand out. For the lab, take it with the teeth in occlusion; in this image they are slightly apart. Image: Propdental, CC BY-SA 3.0; Wikimedia Commons.

Shade photographs

• Shade tab in the same plane as the teeth, at the same distance, never overlapping the tooth.
• Preparation photograph with a stump shade tab.
• Cross-polarised image, if a polarising filter is available — it removes surface reflection and reveals internal shade structure.

Illustration. First image: a VITA classical A1–D4 shade guide, the kind placed beside the tooth for shade photographs. Second image: the cross-polarisation principle, the same mirror photographed with flash; with a polarising filter the flash reflection almost disappears, just as the filter removes reflection from enamel. Images: Matthew Ferguson 57, CC BY-SA 4.0; Esquilo, CC BY-SA 3.0; Wikimedia Commons.

Equipment: what is actually sufficient

A dedicated setup gives the most consistent results: body, macro lens, and either a ring flash or twin lateral flashes. Twin flashes render form better than a ring flash, because a ring flash flattens the surface and erases texture.

A recent phone remains usable for form and proportion. Its limitation is shade: automatic white balance shifts between frames, so one frame cannot be compared with another. That is precisely why the shade tab must appear in the same frame as the tooth — it gives the technician a reference to calibrate against.

Accessories are worth more than a camera upgrade: retractors, intraoral mirrors, a black contrast background, and a polarising filter.

Illustration: a macro ring flash and a cheek retractor. Images: Diego Delso, CC BY-SA 4.0; Metalhead64, public domain; Wikimedia Commons.

The evidence base: how accurate is cross-polarisation

Cross-polarised photography is not an optional flourish. It has been studied as a shade-matching method in its own right: a study on the accuracy of shade matching using cross-polarisation photography evaluates the influence of a polarising filter on digital shade matching.

A study comparing shade-matching performance in laboratory and clinical settings recorded markedly better results in the laboratory setting with a polarising filter (about 52% correct), while in the clinical setting overall accuracy fell to about 32%. That figure deserves attention: shade matching is difficult even with good tools, which is exactly why sending photographs to the laboratory is not a supplementary step but a compensation for that limitation.

Comparing three methods — spectrophotometer, intraoral scanner and cross-polarised photography — a study in the Journal of Esthetic and Restorative Dentistry found intra-instrument reliability highest for the spectrophotometer, followed by cross-polarised photography, then the intraoral scanner.

Three errors that ruin a set

1. Beauty mode left on

Smoothing skin also flattens tooth surface texture — the very thing the technician needs to see. Turn off every filter, including automatic sharpening.

2. Shooting off-axis

An angled shot distorts width-to-length proportion. A technician building to the proportions in the photograph will produce the wrong form.

3. Dehydrated teeth

Teeth dehydrate within minutes of retraction, brightening and losing translucency. Shade photographs must be taken in the first minutes, or the patient should close for a few minutes beforehand.

Sending images to the laboratory

• Send original files. Messaging apps compress images and strip detail exactly in the shade transition zones.
• Name files by content, or include one line of description per group.
• State clearly which frame is the target shade and which is the preparation.
• If the substrate is uneven between teeth, say which tooth is darker.

FAQ

What is lost without a polarising filter?

Accurate reading of internal shade distribution. Compensate with multiple close-ups from different angles and a written description of white spots or translucent zones.

How many frames are enough?

For a single posterior crown, three or four. For a multi-unit aesthetic case, the full set above is the minimum rather than the ideal.

Can photographs replace direct shade matching?

Not entirely. Photographs and shade codes complement each other: the code gives a starting point, the image gives distribution and texture. The hardest cases are single or paired central incisors, where the reference sits on the adjacent tooth — there, photographs are close to decisive.

References

• The accuracy of dental shade matching using cross-polarization photography. PubMed 31840142.
• Reliability of Dental Shade Selection Methods: Agreement Among Spectrophotometer, Intraoral Scanner, and Cross-Polarization Photography. J Esthet Restor Dent (PubMed 40070050).
• Evaluation of Dental Shade Guide Variability Using Cross-Polarized Photography.
• Comparison of visual analog shade matching, a digital visual method with a cross-polarized light filter, and a spectrophotometer for dental color matching.

Written for dentists and technicians. Khai Nguyen Dental Lab manufactures to clinical prescription only and does not sell directly to patients.

Summary: Khai Nguyen Dental Lab has added an imes-icore CORiTEC 350i PRO five-axis milling machine. Rather than a product announcement, this article answers what a referring dentist actually wants to know: what it changes in tolerance, in the range of materials we can take in one case, and in turnaround.

The imes-icore CORiTEC 350i PRO at Khai Nguyen: control screen above, milling chamber below.

Why the frame matters more than spindle speed

Error in a milled restoration comes less from software than from vibration. Every vibration during cutting is transferred onto the surface and onto the margin — exactly where precision matters most.

The CORiTEC 350i PRO uses a one-piece machine frame made of polymer concrete, a material that damps vibration far better than welded steel or extruded aluminium frames. That is why this class of machine holds its stability when cutting hard materials such as pre-sintered zirconia or CoCr alloy.

The machine weighs roughly 180–210 kg at 758 × 790 × 857 mm — a figure worth noting because most of that mass is the frame itself. Full specifications are on the imes-icore product page.

Simultaneous five-axis machining

Simultaneous five-axis is not the same as indexed five-axis. In simultaneous mode all five axes move together throughout the tool path, letting the bur approach the surface at an optimal angle at every point.

This matters concretely for three groups of cases:

• Preparations with slight undercuts and complex margins — tool paths follow the real geometry instead of approximating it.
• Restorations with angulated screw channels — an inclined channel needs an inclined axis to cut the internal wall cleanly.
• Anatomic occlusal morphology — grooves and fossae are reproduced more crisply instead of being rounded off by the bur.

The manufacturer reports that high-resolution dynamic servo motors with absolute measuring systems deliver up to roughly 20% faster machining than the previous generation at the same level of precision.

Material range: wet and dry milling on one machine

• Dry milling — zirconia, aluminium oxide, wax, PMMA, PEEK, composite.
• Wet milling — glass ceramics and lithium disilicate, CoCr, titanium, pre-milled abutments.

Wet milling on the CORiTEC 350i PRO: coolant jets aimed directly at the bur while it cuts.

What this means for a case you send us: a mixed case — say lithium disilicate veneers anteriorly with zirconia crowns posteriorly and an implant restoration — does not have to be split across separate machines and separate workflows. Fewer handovers means fewer places for error to accumulate.

Four features that affect consistency between cases

Auto-calibration

The machine runs its own calibration routine rather than depending on a manual schedule. In production terms this is the difference between “the first case of the month and the last one match” and “we have to remember to do it”.

Integrated temperature compensation

Chamber temperature changes cause mechanical expansion at the micrometre scale — enough to affect a margin. Integrated compensation corrects for this in real time.

Zero-point clamping system

Blanks are clamped to one defined origin, so coordinates do not have to be re-established at every blank change. One fewer manual step is one fewer source of error.

Automatic cleaning

Zirconia dust left in the chamber interferes with clamping the next blank. An automatic cleaning cycle keeps machining conditions consistent from case to case.

What changes for your cases

• A wider material range within one case. Mixed veneer, crown and implant cases run through one chain.
• Crisper morphological detail in grooves, fossae and transitions — relevant for monolithic restorations with no external layering.
• Higher throughput for multi-unit and rush cases.
• Unchanged: input data still decides the outcome. A margin that is not readable on the impression or scan file cannot be rescued by any machine. Where error actually accumulates in the digital chain is covered in our materials comparison and in our accuracy notes.

FAQ

Will restorations fit better?

Better in the part the machine is responsible for — cutting stability, margin definition, repeatability between cases. The part that depends on impression quality and preparation design is unchanged.

Does it affect turnaround?

Yes, mainly on multi-unit cases and cases needing several material types, because fewer transfers between machines are required.

Does anything change in how we send cases?

No. Requirements for impressions, scan files and case documentation stay the same.

Do you accept milling from a design file we supply?

Yes. Contact the lab to agree on file format, implant system and blank type before sending.

Written for dentists and technicians. Khai Nguyen Dental Lab manufactures to clinical prescription only and does not sell directly to patients.

Case produced at Khai Nguyen Dental Lab on the treating dentist’s prescription, published on Instagram @khainguyendentallab on 15 May 2025. No patient-identifying information is included.

Case details

Above: ten Amber Mill veneers on the holding stick before finishing.

Why the wax-up decides the outcome

In a multi-unit aesthetic case the central question is not “does the restoration look good” but “is it what the patient already agreed to”. A wax-up, whether in wax or digital, is a visual contract between three parties: dentist, patient and laboratory. Once the patient has seen and approved the form on an intraoral mock-up, the laboratory’s remaining job is to reproduce that design faithfully — not to improve on it unasked.

In this case the final restorations follow the original wax-up closely. That sounds simple but requires a disciplined workflow: the wax-up is scanned, used as the reference envelope when designing the veneers, and verified by superimposing the design file onto the wax-up file before milling.

Ten Amber Mill veneers in 1M1 to 1M2 on holders, milled from the approved wax-up data.

Laboratory workflow

1. Scan the approved wax-up and set it as the reference for the design.
2. Design all ten units against that reference, keeping outline, proportion and incisal length consistent across the arch.
3. Superimpose the design file onto the wax-up file to confirm that deviation from the wax-up stays within an acceptable threshold.
4. Mill from lithium disilicate blanks, crystallise, stain to the 1M1–1M2 range and finish the surface.

Ten veneers on the model: form and incisal length follow the approved wax-up.

Lessons for multi-unit aesthetic cases

• Always send the wax-up or mock-up scan if the patient has approved it. Without it the laboratory designs to general aesthetic norms, and the chance of missing the patient’s expectation is high.
• State explicitly if the wax-up needs adjusting: for example, “keep the form, reduce incisal length by 0.5 mm”. The laboratory will not modify an approved wax-up on its own initiative.
• Prepare through the mock-up, so veneer thickness stays even and does not have to be compensated by thickening the restoration.
• Use the wax-up as the acceptance standard. When the finished work reproduces the original design, quality control becomes straightforward: compare the restoration against the wax-up rather than judging it subjectively. Chair time at try-in also shortens, because the form was approved in advance.

See also minimum veneer thickness and our dental ceramic materials comparison.

Written for dentists and technicians. Khai Nguyen Dental Lab manufactures to clinical prescription only and does not sell directly to patients.

Part of the “1001 questions at the lab” series produced by the technicians of Khai Nguyen Dental Lab for young technicians and for dentists who want to understand how ceramic restorations are finished. The first episode was published on the lab’s Facebook page in May 2026.

“Glaze or polish?” is the question new technicians ask most at the finishing stage, and it is also what dentists wonder when one crown arrives mirror-shiny and another has the soft lustre of a natural tooth. The two techniques produce surfaces that differ in optics, roughness and longevity, and they are not always interchangeable.

Two techniques, two kinds of surface

Photo above: A Khai Nguyen technician polishing a ceramic crown mechanically: the gloss is created in the ceramic itself, independent of any glaze layer.

When to use which

Glazing suits

• Stained monolithic posterior restorations that need a sealed surface quickly.
• After staining, because stains need a firing to fix them and the glaze protects the colour underneath.
• Layered porcelain with fine porosity after the last firing; glaze seals it.

Mechanical polishing suits

• Anterior veneers and crowns that must keep hand-made surface texture (vertical striations, horizontal ridges, reflective planes); a thick glaze erases these details.
• Monolithic zirconia in direct contact with opposing enamel; smoothly polished zirconia causes less wear than worn glaze.
• Occlusal surfaces and contact points that the dentist may adjust at seating; chairside re-polishing restores the gloss afterwards.

Combining both

In most esthetic cases at Khai Nguyen the technicians combine the two: stain and fire a very thin glaze to fix the colour, then polish selectively to reduce gloss where natural teeth are not shiny (cervical zone, between striations) and keep high gloss on the reflective planes. The result is zone-dependent gloss like a natural tooth rather than uniform shine. For monolithic full-arch work in Aidite 3D Pro multilayer zirconia the process is shorter: stain with the Biomic kit and glaze, since the multilayer disc already provides the effects.

Before surface finishing: veneers built up with multi-coloured effect porcelains; a thick glaze at the last step would soften these details.

Three common finishing mistakes

1. Glaze too thick. A glass-like surface with no texture; on incisors it creates flat reflections that make the teeth look larger. Fix: a thin glaze or none, and polish instead.
2. Skipping polishing grades. Jumping from a coarse point to fine paste leaves long scratches under the gloss, visible under oblique light. Fix: work through every grit and check under a raking light after each.
3. Occlusal adjustment without re-polishing. The adjusted area exposes rough ceramic that stains and wears the opposing tooth. Fix: a chairside ceramic polishing kit matched to the material, or return the unit to the lab if the area is large.

What the dentist can write on the prescription

• “Soft lustre, keep texture” for anterior teeth; “high gloss” for posterior teeth if the patient prefers it.
• The material of the opposing teeth (enamel, ceramic, composite) so the lab chooses the least abrasive finish.
• A request for photos under oblique light before shipping if surface texture matters.

Read more: three finishing steps on LiSi Press veneers in A1, choosing zirconia discs by case. Send a case via WhatsApp +84 902 945 585.

Written for dentists and technicians. Khai Nguyen Dental Lab manufactures to clinical prescription only and does not sell directly to patients.

TL;DR: every current intraoral scanner can export an open STL, PLY or DCM file, and a lab can take that file straight into exocad. The real barrier is not “is my scanner compatible” but three small things: exporting the right format, sending complete data (both arches, bite, scan body for implants) and getting fast feedback when a scan has a defect. This guide lists how to export from each common scanner, the scan errors we see most, and how our lab responds.

The three file formats to know

• STL: a plain triangle mesh, no colour. The most common format; every CAD package reads it. Sufficient for crowns, bridges and inlays/onlays.
• PLY: like STL but with surface colour. Lets the technician see gingiva, margins and neighbouring teeth; export PLY when the scanner supports it.
• DCM: 3Shape’s format, mesh with colour and case information. exocad reads DCM from TRIOS directly.

Closed native formats (for example native iTero files) must be exported to STL/PLY through the manufacturer’s portal before sending.

How to send files from common scanners

For files over 100 MB (full-arch colour scans) use the case portal or a download link; email is usually capped at 25 MB.

Minimum data for a digital case

1. Scan of the prepared arch and the opposing arch.
2. Bilateral bite scan in maximum intercuspation.
3. Prescription: tooth number, restoration type, material, shade, due date. See the ordering process.
4. Photos: preparation with a shade tab; smile photo for esthetic-zone cases.
5. For implants: implant system, connection size, scan body type and a scan with the scan body seated.
6. For esthetic cases: a pre-operative or mock-up scan if you want the morphology preserved.

Seven common scan errors and how to avoid them

• Missing margin: retract and dry before scanning; rescan the area, not the whole arch.
• Holes on occlusal or proximal surfaces: auto-fill invents morphology; angle the tip to close them.
• Bite that does not align: patient not in habitual occlusion or one-sided bite scan; scan both sides and check on screen before sending.
• Too few neighbouring teeth: capture at least 2–3 teeth each side so the lab can build morphology and contacts.
• Saliva, blood, debris: creates false surfaces; dry and rescan.
• Scan body not fully seated or wrong type: the implant crown will be off-axis; photograph the seated scan body.
• Locked export: the lab cannot open it; choose open/unlocked export.

How our lab responds to a file

At Khai Nguyen every file is checked in the next working shift: margins, holes, occlusion and preparation depth against the chosen material. If there is a problem you receive an annotated screenshot with a suggestion (rescan one area, reduce further, or change material) before production starts. For esthetic cases you can request an exocad design preview to approve contour and contacts. In-lab time of 2–4.5 working days (depending on material and number of units) counts from file approval. Read more in turnaround time and shipping.

Frequently asked questions

Can I send digital cases without a scanner?

Yes. Ship the model or silicone impression; the lab pours and scans it on a desktop scanner and continues digitally. It only adds shipping time.

Is an older scanner that only exports STL enough?

Enough for crowns, bridges and inlays/onlays. For esthetic cases add photos to compensate for missing colour information.

Do you accept designs made in the clinic’s own software?

Yes. Send the design STL together with the original scan so the lab can check occlusion and thickness before milling or pressing.

See also the workshop on digital impression techniques and our guide to outsourcing to a Vietnam dental laboratory. To try a case: WhatsApp +84 902 945 585.

Written for dentists and technicians. Khai Nguyen Dental Lab manufactures to clinical prescription only and does not sell directly to patients.