A museum rejected a case because the joints read as grey lines under gallery lighting. The acrylic was fine. The bond was solvent cement, and solvent joints always leave a visible seam once light rakes across them.
What separates a case from a box of acrylic
- Bond clarity. Polymerised joints for visible corners, solvent only for hidden ones.
- Edge polish. Flame or diamond polish on all visible edges; a sawn edge glows white.
- Lid engineering. A lift-off lid needs a register lip; a hinged lid needs a counter-sunk hinge that does not stress the sheet.
- Ventilation. Sealed cases trap humidity; most artefacts need a controlled vent or a silica chamber.
Two builds, two purposes
Retail case, 5 mm cast. 5 mm clear cast acrylic, diamond-polished edges, solvent-bonded base and polymerised front corners, lift-off lid with a 10 mm register lip. Suitable for merchandise where a small seam is acceptable.
Museum case, 8 mm cast. 8 mm cast acrylic with all four corners polymerised, a 20 mm rebated lid, an internal silica tray and a 5 mm vent with a filter. The heavier sheet resists the bow that a 5 mm lid develops over a metre of span, which is what makes a lid bind after a year.
Lighting changes the requirement
Under a spotlight, every internal reflection and every seam becomes visible. Cases built for lit galleries use a slightly matte interior back panel to reduce glare, and the front panel is specified at a lower iron content so the artefact does not pick up a green cast. That is a material choice, not a polishing choice.
Fabrication for display fixtures generally is covered in acrylic display stand, and the illuminated variant in acrylic sheet LED signage.
Frequently Asked Questions
Q: Why do acrylic display case joints show as grey lines?
A:Because solvent cement was used on a visible corner. Solvent joints always leave a seam under raking light; polymerised bonding gives a clear joint that reads as continuous acrylic.
Q: What thickness acrylic for a museum display case?
A:8 mm cast is the common museum build for cases over about 600 mm, because a 5 mm lid bows across a metre and starts to bind. Retail cases up to that size work well at 5 mm.
Send the case drawing, interior dimensions and lighting detail to oversales@rpplexiglass.com for a cast acrylic fabrication quote.