A client asked last week whether he could lock the appearance first and deal with structure later. Yes — almost everyone does. The only condition is that on the day the appearance is signed off, somebody has already priced the mould.
That is the whole boundary between industrial design and mechanical engineering, and it is not drawn on the drawing. It is drawn on one question: can this shape be reproduced at a sane cost?
What Each Side Actually Sells
Appearance design sells how expensive the product looks at first glance, and whether it digs into your hand. Structure sells whether it can be made consistently, and whether it assembles and survives a drop.
Appearance outputs form, surfaces, CMF and grip feel, judged on "does this look worth the money, does it feel right in the hand." Structure outputs 3D structure, assembly relationships and mould feasibility, judged on "is every unit identical to the last one off the press."
Appearance changes mostly cost hours. Structure changes usually cost steel.
So the client's real question — can I settle the look now and worry about structure later — is fair, with one condition. Later is fine. The mould calculation cannot also be later.
Drawing the Expensive-Looking Line
Two 10,000mAh power banks on the same shelf. One is a brick, one looks like it belongs in an airport lounge. Inside: the same cells, the same protection board, close to the same bill of materials. The visible difference is how the outer contour opens up and closes down.
For handheld products, settle the overall width first. 62–68 mm lets most people sweep a thumb across the majority of the control surface without the web of the hand taking a hit. Past 72 mm, a good share of users have to change hands mid-task. Width comes first because it also sets the battery format, PCB size and speaker cavity.
Then split the thickness direction: 1/3 of the visual emphasis on the front face, the remaining 2/3 tapering toward the edges. That ratio separates "tool" from "consumer electronics." A brick carries full thickness to the corner; a product that reads as expensive has already given some away.
Fillets start the arguing: cheap on screen, expensive in steel. On one outline, R5 reads harder and more tool-like, R8–R10 noticeably softer and more consumer. Shift two or three millimetres and users cannot name what changed; they just say this one looks more expensive. That sentence is bought with radius.
Surface continuity is the last appearance lever. A G1 transition shows a kink where the highlight band crosses it; G2 runs through clean. Glossy parts hide nothing, so on a high-gloss shell the cheaper continuity is visible from across a room.
The Bill Arrives in the Mould
The main enclosure wall here is 2.0 mm. If appearance asks for an outer corner of R3.0, the inner radius lands at R1.0, and a tool can cut that. If appearance insists on R1.5 outside, the inner radius drops below 0.5 mm. The wall goes thin at the corner, stress concentrates there, and the part is prone to cracking on ejection or in a drop. Structure has two moves: add an insert, or send the surface back to appearance — parts and process, or days.
A case from our files: the battery bay of a blood pressure monitor, small inner radii plus uneven wall thickness. At T0 the sample came back with 0.3 mm of sink, visible on the outer face. The fix was not in the tool. Appearance redrew the corner larger, which moved the parting line, which meant a tool change. One number, traced back to the second week.
Draft angle is the next bill. On gloss, 0.5° is usually enough. Texture is not gloss: VDI 27 generally needs 1.5° or more, and the coarser the grain the more you give away. Hold a dead-vertical side wall and structure can only move the parting line or accept drag marks. Both are visible in the hand.
The parting line is the most typical trade. Appearance wants it hidden; structure knows the hiding places are limited. On handheld products it usually goes on the lower side edge, where a finger rarely sweeps. The thorough answer is double-shot moulding, which swallows the line but costs another tool and another process step — a tier up, not a tweak.
CMF Is the Cheapest Lever You Have
Same tool, three finishes: gloss, fine etch, coarse etch. In VDI 3400 the common fine step is VDI 27. The mood of the product changes completely and the tooling cost does not move at all. A PC+ABS alloy (Bayer T65 is what we reach for most) with an etched body and one polished edge gives depth without adding a part.
The catch: texture eats draft. Etch depth grips the steel harder, so the release angle has to grow — and that bill lands on the structure side, not CMF. Which is why CMF calls taken without a structural read come back six weeks later as a tool modification.
Different Scorecards, So the Arguing Is Normal
Appearance is accepted on colour, texture and gaps: ΔE within 1.0, assembly gaps within 0.15 mm, steps between parts within 0.1 mm. Structure is accepted on consistency: clip insertion and pull-out force, tolerance stack-up, whether the unit survives a drop without a crack.
Those are not the same exam. Appearance needs this one unit to look good; structure needs ten thousand. When a project manager asks why the two teams keep going at each other, that is the answer — different exams, both correct.
| Dimension | Appearance design | Structure design |
|---|---|---|
| Core output | form, surfaces, CMF proposals, appearance handboard | 3D structure, assembly relationships, mould feasibility, BOM |
| Basis for judgement | proportion, visual weight, grip feel, brand recognition | draft angle, wall thickness, tolerance chain, tool structure |
| What failure looks like | works but looks bad: too thick, cuts the hand, reads cheap | looks good but is hard to make: sink, drag marks, out-of-spec gaps |
| Cost of one revision | mostly hours, a few days to two weeks | possibly a tool change, tens of thousands of RMB, plus lead time |
| When to bring them in | at project kickoff | somebody should be watching from the first sketch |
A Pre-DFM Check Before You Sign Off
You can lock appearance first and do structure after; it is the most common sequence here. Before sign-off, run one manufacturability pre-check:
- Is the main wall thickness even? Uneven walls are where sink marks come from, and sink marks land on the face you spent three weeks making expensive.
- Convert every outer radius to the inner radius it implies. Anything landing below 0.5 mm inner needs a decision now, not at T1.
- On textured faces, is the draft there? VDI 27 and up needs 1.5°, not the 0.5° that worked on gloss.
Left to the structure phase, any of those three sends you back into the surfaces: the expensive rework, because it reopens decisions you thought were closed.
Our practice: appearance and structure sit together from the sketch stage rather than handing off like relay runners. By appearance's second form version, structure has had a rough look at whether it will eject and where sliders go. What that saves is not money. It is the sentence "this can't be changed now."
Sometimes it does not apply. For a project validating a market fast, appearance alone is the right buy: 3D-printed handboards, real reactions, tooling deferred until demand is confirmed. Be clear what you are buying — a good handboard is not a good mass-production part. There is a stretch to pay back, and it does not disappear because the print looked good.
On cost, in RMB: appearance only 5,000–12,000; appearance plus structure 15,000–40,000; full project 30,000–80,000. The spread is mostly structure delivering the complete 3D, following the tool, solving whatever trial shots throw up. You are not paying for drawings. You are paying for somebody still on the phone when the tool shop calls at eleven at night.
Sources: Compiled from Hezi Industrial Design's own project experience; no external citations.
Article by Ou Ruolan / Hezi Industrial Design — Dongguan, China. Originally published at hezidesign.com.
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