Vacuum Metalizing
Vacuum metalizing evaporates aluminum inside a vacuum chamber so it condenses as a mirror-bright film on the part surface.
- Part
- Finishing
- Revised
- 2026-08-11
At a glance
- Family
- Additive
- Typical tolerances
- Metal film 0.05–0.15 µm (2–6 µin) is dimensionally negligible; the base and topcoats add roughly 0.5–1.5 mils (13–38 µm) total per coated surface
- Surface finish
- Mirror bright to satin, entirely determined by the molded surface and basecoat beneath the metal
- Typical volumes
- Hundreds to millions; batch chamber cycles favor high-volume decorative parts
- Lead time
- 2–4 weeks for first articles including basecoat qualification; days per lot in production
- Materials
- Plastic, Metal, Glass
What it is
Vacuum metalizing evaporates a metal — almost always aluminum — from resistively heated tungsten filaments inside a vacuum chamber, so the vapor travels in straight lines and condenses on the part as a mirror-bright film. The deposit is extraordinarily thin, on the order of 0.05–0.15 µm (2–6 µin), and has essentially no mechanical strength of its own.
That means the process is really a three-layer system, and the metal is the least of it. A sprayed and cured basecoat, typically 0.4–1 mil (10–25 µm), levels the substrate so the metal film has something optically smooth to sit on. The evaporated aluminum provides the reflectivity. A clear topcoat protects the soft metal from abrasion and handling and carries any tint. Cycle time in the chamber is minutes, the process runs at near room temperature, and it works on ABS, polycarbonate, other thermoplastics, glass and metal. It is the standard way to make automotive lamp reflectors, cosmetic closures and bright decorative trim.
How it works
- Basecoat. The part is sprayed with a UV-cure or solvent lacquer at roughly 0.4–1 mil (10–25 µm) and cured. Injection-molded surfaces carry flow lines, sink and micro-texture that would destroy a mirror finish; the basecoat fills and levels them. Reflector optics depend almost entirely on this layer.
- Load and pump down. Parts are fixtured on rotating planetary racks inside the chamber, which is then evacuated to a high vacuum. Pump-down is the longest part of the cycle.
- Evaporate. Aluminum wire clips on tungsten filaments are resistively heated until the aluminum melts and evaporates. In the vacuum, atoms travel ballistically to the part and condense. Deposition takes seconds to a minute; the rotation is what gives even coverage on a three-dimensional part.
- Vent and unload. The chamber is vented and the parts come out mirror bright, with a film so thin that it can be rubbed off with a finger.
- Topcoat. A clear lacquer is sprayed over the metal to protect it. Dyes in the topcoat give gold, bronze, smoke and other tints from the same aluminum deposit. Without a topcoat the finish will not survive handling.
Variants
Sputtering in place of evaporation gives denser films and access to target materials that do not evaporate cleanly — chromium and stainless for a true chrome look, or indium for the semi-transparent, electrically non-conductive "chrome" used on radar-transparent vehicle emblems. Thicker functional deposits are used for EMI shielding of plastic enclosures, where the requirement is surface conductivity rather than appearance.
Design guidelines
The basecoat, not the metal, determines the finish
A 0.1 µm aluminum film is perfectly conformal. Every flow line, weld line, sink mark, gate blush, ejector pin witness and micro-texture in the molded surface reads straight through. Getting a mirror requires a Class A molded surface plus a basecoat that levels what remains. Budget the mold polish accordingly.
Design for line of sight
Vapor travels in straight lines from point sources. Deep recesses, undercuts, the inside of ribs and any surface shadowed by another feature receive little or no metal, even with planetary rotation. Keep the decorated surface open and convex where possible, and do not expect uniform brightness inside a deep pocket.
Do not specify it for surfaces that are touched or abraded
The metal film has no wear resistance and depends entirely on the topcoat. It is well suited to reflectors, trim, closures and display parts, and poorly suited to handles, buttons, edges and anything subject to repeated contact or cleaning with abrasives. Where durability is needed, electroplated ABS or PVD is the appropriate choice.
Provide fixturing that does not shadow the show surface
Every part must be held. The contact area is unmetallized and often shows as a witness. Designate a non-cosmetic gripping location or a feature that later gets hidden by assembly.
Confirm the substrate is vacuum-compatible
Outgassing is the failure mode. Resins with high moisture uptake, plasticizers, or residual mold release contaminate the chamber and produce hazy, poorly adherent films. ABS and polycarbonate are the standard choices; nylon and other hygroscopic resins need drying and careful process control.
Do not rely on it for conductivity unless specified as such
A decorative aluminum film is far too thin for meaningful EMI shielding or grounding. Shielding requires a deliberately thicker functional deposit, specified by surface resistivity, not by appearance.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Metal film thickness | 0.05–0.15 µm (2–6 µin) | — | Optical layer only; carries no mechanical load |
| Basecoat thickness | 0.4–1 mil (10–25 µm) | — | Levels molded surface defects for the mirror |
| Topcoat | Always required | Never leave bare | Bare aluminum film rubs off with handling |
| Coverage | Line of sight, convex surfaces | Deep recesses shadow | Ballistic vapor transport in vacuum |
| Substrate | ABS, PC, PC/ABS, glass, metal | Avoid hygroscopic and plasticized resins | Outgassing produces hazy, non-adherent film |
| Fixture contact | 1 designated non-cosmetic area | — | Contact points are unmetallized
Cost drivers
Vacuum metalizing is a batch process billed per chamber cycle, but the chamber is usually the cheapest part of the routing — the sprayed basecoat and topcoat carry most of the cost.
- Base and topcoat application. Two spray-and-cure operations with their own booths, racks and labor. These typically exceed the metalizing cost.
- Chamber load density. Cycle time is fixed, so parts that fixture densely and present their show surface outward cost far less per piece.
- Mold and substrate quality. A Class A tool costs more up front but eliminates rework and basecoat build; a poor molded surface can make an acceptable mirror unachievable at any coating cost.
- Reject rate. Dust inclusions, fisheyes and basecoat defects are visible on a mirror surface at a glance, so this process runs higher inspection and scrap rates than a matte finish.
- Tinting and multi-color work. Each tint or masked area is another spray operation.
Cost-reduction tactics:
- Invest in mold polish and gating so the molded surface needs less basecoat leveling.
- Design the show surface to be convex and open so parts fixture densely with no shadowing.
- Use a tinted topcoat rather than a different metal to achieve gold, bronze or smoke.
- Keep cosmetic requirements to the surfaces that are actually visible in the assembly.
- Where the part is handled in service, price electroplated ABS or PVD instead — the rework cost of a scuffed metalized part exceeds the difference.
Questions
6 questionsHow thick is vacuum metalizing?
The evaporated aluminum film is only about 0.05–0.15 µm (2–6 µin) — thin enough to rub off with a finger before it is topcoated. The functional thickness in the system comes from the basecoat and topcoat, which together add roughly 0.5–1.5 mils (13–38 µm).
Why does vacuum metalizing need a basecoat and a topcoat?
The metal film is perfectly conformal and has no mechanical strength. The basecoat, typically 0.4–1 mil (10–25 µm), levels flow lines, sink and texture in the molded surface so there is something optically smooth to mirror. The topcoat protects the soft aluminum from abrasion and carries any tint.
Is vacuum metalizing as durable as chrome plating?
No. Chrome-plated ABS produces a genuinely hard, wear-resistant surface; vacuum metalizing produces a mirror protected only by a clear lacquer. It suits reflectors, trim, closures and display parts, but not handles, buttons, edges or anything cleaned with abrasives.
What substrates can be vacuum metalized?
ABS, polycarbonate and PC/ABS are the standard choices, along with glass and metal. Hygroscopic or plasticized resins outgas in the vacuum chamber and produce hazy, poorly adherent films, so they require drying and careful process control if they can be used at all.
Can vacuum metalizing be used for EMI shielding?
Only with a deliberately thicker functional deposit specified by surface resistivity. A decorative aluminum film at 0.05–0.15 µm is far too thin to provide meaningful shielding or a ground path.
Why are some areas of my metalized part dull or uncoated?
Deposition is line-of-sight from point sources in a vacuum, so deep recesses, undercuts and surfaces shadowed by other features receive little metal even with planetary rotation. Keep decorated surfaces open and convex, and expect a witness mark wherever the fixture held the part.