A cut and bent leaf skin is still just sheet metal. The stage that decides whether a medical door survives ten years of disinfectant wipe-downs and trolley impacts — or rusts at the seams within two — is fabrication: welding, core installation, assembly, and coating. What separates a hospital-grade door from a generic steel door at this point in the line?

Welding and Door Fabrication

The formed skins, edge channels, and internal reinforcements are joined by TIG or MIG welding, with spot welding used for non-visible internal attachments. For medical doors the welding requirements differ from commodity doors in two ways:

  1. Continuous, crevice-free seams. Intermittent welds leave gaps where bacteria and cleaning fluid accumulate. Medical fabricators specify continuous TIG seams, then grind beads flush so no crevice remains — "zero pinholes, flush ground" is the stated acceptance criterion in sanitary welding practice.
  2. Low-heat control. Door skins of 0.8–1.2 mm distort easily. Stitch welding sequences, copper backing bars, and controlled interpass temperature keep the face flat; an overheated skin shows "oil-canning" ripples that no amount of filling hides under gloss powder coat.

Common weld defects (per ESAB's welding defects guide and ISO 5817 quality levels):

  • Porosity — gas cavities from poor shielding or contamination; under ISO 5817, porosity limits tighten from quality level D (general) to B (high). Medical door seams should be specified to level B or C on visible faces.
  • Undercut — a groove at the weld toe that thins the skin and traps contaminants; reject on visible seams.
  • Lack of fusion — invisible without inspection, weakens reinforcement attachments.
  • Spatter — cosmetic defect that must be removed before coating or it telegraphs through the finish.

Core Installation and Assembly

After welding and dressing, the core is bonded in:

  • Paper honeycomb — light, economical, used with high-strength adhesive lamination for standard ward doors; modern door lines integrate punching, flanging and gluing in a continuous process.
  • Rock wool / mineral core — for fire-rated assemblies.
  • PU foam — better thermal and acoustic performance, adds rigidity.

The adhesive must be applied over full contact area and cured under pressure; partial bonding creates drumming (hollow sound) and later delamination under closer loads. Edge channels are then closed, hardware reinforcements verified, and the leaf squared and dimension-checked before finishing. Frames are assembled similarly, with welded or mechanically jointed mitre corners ground flush.

Surface Finishing: Powder Coating Process

The standard finish is electrostatic powder coating over pretreated steel. The durable sequence is:

  1. Pretreatment — degreasing, rinsing, iron or zinc phosphate conversion coating, final rinse and drying (multi-tank lines, often 7 stages). Pretreatment is the foundation: the best spray booth and oven cannot compensate for poor conversion coating.
  2. Electrostatic application — powder sprayed onto the grounded part; typical dry-film target is 60–80 µm for medical enclosures (some systems specify up to ~180 µm over e-coat for exterior service).
  3. Curing — thermoset powders typically require 180–200 °C for 10–20 minutes at metal temperature — not oven air temperature. Undercuring leaves a soft, chemically weak film; overcuring darkens colours and embrittles the film.

For stainless steel doors, finishing is usually brushed or polished, followed by passivation (nitric or citric acid treatment) to rebuild the chromium oxide layer after welding.

Common coating defects: orange peel (wrong gun settings or cure profile), pinholes (outgassing from trapped contamination — often from welds that weren't properly cleaned), poor edge coverage on sharp corners, and adhesion failure traced back to skipped or exhausted pretreatment tanks.

Essential Manufacturing Quality Checks

Checklist for buyers auditing a fabricator, and for factory QC:

  • [ ] Weld seams continuous and ground flush on all cleanable faces; no porosity/undercut on visible surfaces (ISO 5817 level B/C).
  • [ ] Leaf flatness and squareness measured after welding, before coating.
  • [ ] Core bonding: no drumming; shear or peel check on sacrificial sample per batch.
  • [ ] Pretreatment tank chemistry logged daily (concentration, temperature).
  • [ ] Coating film thickness measured with a dry-film gauge; cure verified by solvent rub or hardness test.
  • [ ] First-article adhesion cross-cut test (ISO 2409) per coating batch.

Trade-offs and Limitations

Powder coat gives excellent chemical resistance to hospital disinfectants but any deep chip exposes steel; stainless removes that failure mode at higher cost. TIG is slower and costlier than MIG but gives the flush, sanitary seam medical doors need — a common mistake is accepting MIG spatter on "non-visible" areas that turn out to be inside the leaf's cleaning envelope. Investing in proper weld dressing and pretreatment pays for itself: coating failure in service is the single most expensive warranty claim in door manufacturing.

Sources

  • ESAB University — Welding defects guide (porosity, undercut, lack of fusion): esab.com
  • ISO 5817 — Quality levels for imperfections in fusion-welded joints: weldcalc.ssab.com