A diagnostic instrument maker needed a 1.2 mm thin-wall cosmetic enclosure that stayed flat enough to seal against a gasket. Gating to four balanced drops, splitting core and cavity temperature control and correcting the steel against measured shrinkage brought flatness from 0.45 mm to 0.09 mm, with warp under 0.12 mm across the 210 mm face.
Key takeaways
- A cosmetic enclosure is judged by three things at once: flatness, surface appearance and a sealing face — and the three fight each other on the same wall.
- Thin walls freeze fast. Cooling time scales with the square of wall thickness, so a 1.2 mm wall wants a shorter hold and a tighter process window than a 2.5 mm one.
- A gasket land is a flatness feature, not a cosmetic one. Specify it as a datum and measure it in the constrained state, or you will over-correct the steel.
- Textured cosmetic tools hide sink and flow marks that a polished tool exposes. With a light texture you also buy margin on appearance without touching the geometry.
- Correct the steel only after fill and cooling are balanced. Two measured rounds took the residual bias out; a third was not needed.
The project at a glance
Part: upper enclosure shell for a bench-top diagnostic analyser, 210 × 140 × 38 mm, shallow draw with a perimeter gasket groove
Material: PC/ABS blend, light grey, UV-stabilised
Nominal wall: 1.2 mm throughout, with a 2.0 mm boss at four screw posts
Critical requirements: flatness within 0.15 mm across the 210 mm face in the constrained state; a matte A-surface with no visible flow marks or sink; gasket groove depth within ±0.05 mm
Volume: 12,000 parts a year, 2,000-part batches
Tooling: one 1-out-of-2 production mold, hot runner, hardened 1.2344 (H13) inserts
Delivered from: RCH Plastic, Dongguan, China
The challenge: a thin cosmetic wall that also had to seal
Enclosures are usually the easy part of a project. This one was not, because three requirements pulled in different directions on the same 1.2 mm wall.
Thin wall, short cooling window
Cooling time rises with the square of wall thickness, so a 1.2 mm wall freezes in a fraction of the time a 2.5 mm wall needs. That is good for cycle time and bad for process tolerance: with less melt in the cavity, small changes in injection speed or mould temperature move the flow front and the packing pressure noticeably.
The four screw posts make it worse. At 2.0 mm against a 1.2 mm wall they are 167% of nominal, well past the 50–60% guidance, so they freeze last and sink first on the opposite face — which here is an A-surface.
Cosmetic surface and a sealing face on the same part
The visible face is judged by eye under reflected light. The mating face is judged by a gauge. A modification that helps one usually hurts the other: polishing out a flow mark changes local heat transfer, and adding relief to fix flatness leaves a witness line on the sealing face.
Gasket groove tolerance against a warping shell
The groove is 1.4 mm wide and 0.9 mm deep. Its depth has to hold ±0.05 mm against a shell that wants to bow. If the shell bows after ejection, groove depth varies with position even though the steel is correct, and the gasket either leaks or over-compresses.
What we did
The sequence matters as much as the changes: balance fill first, balance cooling second, cut steel last.
1. Four balanced drops instead of one central gate
The original concept used a single central hot-drop. On a 210 mm face that gives a flow path of roughly 105 mm to the longest edge and leaves the corners as the last-filled regions, which then pack differently from the centre. We moved to four balanced hot drops, each feeding a quarter of the shell, cutting maximum flow length to about 62 mm.
Balanced fill is the precondition for balanced packing, and balanced packing is what makes both cavities shrink by the same amount. Weld lines between adjacent drops sit along the display-window rib, on a non-visible feature.
2. Split core and cavity temperature control
The core side of a shallow shell carries far more heat than the cavity side, because the core sits inside the part while the cavity only touches the outside face. On the original layout the measured core-to-cavity difference was 11 °C, which bows the part toward the hot side.
We rebuilt the core with baffled circuits following the shell contour and put core and cavity on separate controllers — core at 60 °C, cavity at 75 °C, a deliberate reverse bias that offsets the natural imbalance. Final difference: 3 °C.
3. Light texture to buy appearance margin
A matte finish is a gift on a thin cosmetic wall: it scatters reflected light, so shallow flow marks and micro-sink that would show on a polished tool become invisible. We specified a VDI 3400 grade equivalent to roughly Ra 2.4 µm on the A-surface, applied by bead blasting the cavity insert.
It is deep enough to hide appearance defects but shallow enough not to interfere with ejection on a 38 mm draw — with 1.5 ° of draft on the shell walls and 2 ° on the textured display recess, where the texture drags more.
4. Re-size the screw posts and core them
The four posts were cored down from 2.0 mm to 1.5 mm wall with a 3.2 mm central hole for the self-tapping screw. That brings the post-to-wall ratio inside the rib rule and removes the sink mark that had been showing on the A-surface above each post.
5. Correct the steel against measured shrinkage
With fill and cooling balanced, what remained was a repeatable bias — the right condition for correcting the tool. We shot a 30-piece sample across both cavities, measured on a CMM using the sealing face as datum, and calculated effective shrinkage in each principal direction. Two rounds of core machining took the bias out. A third was not needed, which is the usual sign the process was stable before the steel was touched.
The result
Trials ran at our partner molding plant, since we build tools in-house and run trials and production through vetted partners. Each round was delivered as a documented package: samples, trial photos, a trial video, the process parameter sheet and a CMM dimensional report, all issued against the mold number.
Three rounds ran. Trial 1 verified fill balance across all four drops with a short-shot series. Trial 2 ran the production resin with a gate seal study to set hold time, then measured a 30-piece sample on a CMM in the constrained state. Trial 3 ran 500 consecutive parts to confirm the process window at the extremes of the recommended settings.
| Parameter | Before (supplier tool) | After (RCH tool) |
|---|---|---|
| Flatness across the 210 mm face (constrained) | 0.45 mm | 0.09 mm |
| Warp across the face (free state) | 0.62 mm | 0.12 mm |
| Gasket groove depth variation | ±0.14 mm | ±0.04 mm |
| Core-to-cavity temperature difference | 11 °C | 3 °C |
| Gates / max flow length | 1 gate / 105 mm | 4 balanced drops / 62 mm |
| Screw post wall vs nominal wall | 167% (2.0 mm) | 125% (1.5 mm, cored) |
| Visible sink above posts | Present on all four | None |
| Cycle time | 31 s | 26 s |
| Scrap rate | 14% | 1.8% |
| Process capability on groove depth (Cpk) | 0.81 | 1.48 |
The flatness gain is the headline, but the cycle time and scrap result matter as much commercially. Scrap fell from 14% to 1.8% on a 2,000-part batch — roughly 245 additional saleable parts per batch. Cycle time dropped from 31 to 26 seconds, a 16% reduction, because the four-drop layout shortened flow length and let us cut injection pressure while keeping fill balance.
Our answer: how we build a cosmetic enclosure
RCH Plastic has been building injection molds in Dongguan since 2015, with more than 1,500 molds delivered to customers in North America and Europe, at 20–30 molds a month. Our answer to a cosmetic enclosure starts before steel: we model the part in UG and run it through Moldflow, then return a written DFM report within 3 working days at no cost, so gating, cooling and the cosmetic call-outs are settled on screen rather than on the press. Five mold designers averaging 8 years work alongside three project engineers and twelve fitters averaging 6 years — 49 staff, 37 in production and 12 in the office.
The machines that hold the geometry are 13 CNC machining centres with travels up to 1,600 mm, 6 EDM machines including Sodick mirror-finish and twin-head units, 3 wire EDMs, and CMM inspection on trial samples. Our machining tolerance is ±0.005 mm on critical mold dimensions; the molded part tolerance that tool can then hold is ±0.01 mm on critical dimensions, subject to the resin and the part geometry.
Tools go up to 1,600 × 1,000 × 500 mm and 15 t, with up to 48 cavities, in steels including 1.2344 (H13), 1.2343 (H11), 1.2738HH and S136, on LKM, DME or HASCO bases, with hot runners from HRS, Yudo, Husky, Incoe or Mold-Masters. Lead time to T1 is 35 days on a simple tool, 42 medium, 50 complex, and 30 days when the drawing is finalised and components are in stock. Tool life is up to 1,000,000 shots depending on steel grade and maintenance, with a 12-month warranty. We make molds in-house and run trials and production through vetted partner factories, which keeps our engineering on the tool, where cost and lead time are decided.
What carries to your project
Five things from this project transfer to any cosmetic enclosure, in any resin.
- Specify the sealing face as a datum, not as a cosmetic surface. The two are measured differently, and confusing them is how a supplier over-corrects the tool chasing the wrong number.
- Balance fill before you balance cooling. Cooling changes cannot compensate for a gating layout that packs one region harder than another.
- Treat a matte texture as engineering, not decoration. It buys appearance margin on thin walls for almost no cost.
- Core every screw post and hold it near 50–60% of the wall. Post sink is the most common cosmetic defect on enclosures, and it is designed out, not processed out.
- Correct steel only after two identical trial rounds. If two rounds disagree, the process is still drifting and the correction will bake the drift into the tool.
None of this is specific to diagnostic instruments. Any shallow cosmetic shell — a handset housing, a pump cover, a control panel fascia — is judged on the same three things and fails for the same reasons. The order never changes: model the flow, balance the cooling, stabilise the process, then cut steel.
Related guides in this series
- Injection molding defects and quality guide — the full defect and quality library
- Injection molding design guide — wall thickness, ribs, bosses and draft rules
- Injection molding tolerances guide — what a tool can actually hold, by feature
- Medical device plastics material selection guide — resins for clinical enclosures
Working on a cosmetic enclosure?
Send the 3D model, the resin grade and a photo of the face that will be seen, and tell us how the sealing face is measured today. We will come back with the likely failure mode, the first three things to check, and whether the fix is a process change, a tool modification or a new tool. Ask our engineers for a free DFM and flow review.