Perforated, laser-cut and expanded metal

At a glance

D hole diameter, P pitch between neighbouring centres, S square side, W hexagon across flats, w slot width, L slot length, Px and Py slot pitches. Formulas are checked against ISO 7805-2:1987; expanded and laser-cut patterns need the supplier's or the drawing's figure.

Open area: the sum, and the check

Open area is the share of a perforated field that is hole. ISO 7805-2:1987 defines it as the ratio of the total area occupied by the holes to the overall area of the perforated part of the plate, and gives three layouts: round holes in a 60-degree stagger, and square holes in line or at half-pitch stagger. For round holes in a stagger its figure 1 says open area is about 0.9 × (w/p)²; the exact constant is 0.9069, because each hole owns a rhombus of area (√3/2) × P², and a hole of diameter D has area πD²/4. Published calculators round the constant differently (0.905 and 0.9069 are both quoted), which is why they differ in the third figure.

• Worked row. A 3 mm sheet with 6 mm holes at 9 mm pitch in a 60° stagger: 0.9069 × (6/9)² = 40.3 % open. The same holes in line: 0.7854 × (6/9)² = 34.9 %. The bridge is the same, 3 mm, so the stagger gains 5.4 points for free.
• Weight. The perforated field weighs about (1 − open area) of the solid sheet: 59.7 % in the example, before the unperforated border.
• The border counts separately. Open area belongs to the perforated field. A panel with a blank margin for fixing has a lower open area overall; work the field and the border apart.
• Checked three ways. The formulas agree with a computer raster of one repeat cell, and with the open areas printed in ISO 7805-2 (its Table 1 and all 74 cells of its Tables 2 and 3 agree within the standard's own rounding to preferred pitches, the largest gap being 2.1 points).

To pick a pitch for a target open area, rearrange the formula: P = D × √(0.9069 / open area) for round holes in a stagger. For 6 mm holes:

A pattern is named to the codes in ISO 7806:1983, which applies to perforated plate whatever its material: R round, C square with sides parallel to the plate edges, CD square on the diagonal, H hexagonal, LR slot with round ends and LC slot with square ends.

Hole, bridge and thickness: the rules we could source

The only standard we found for perforated plate is for industrial screening, not decoration, and it is old, but it is the one place the thickness rule is written down. ISO 7805-2:1987, for plate under 3 mm thick, says in clause 4.2.4 that the chosen thickness shall be less than the size of the holes and also less than the bridge width (the bar). ISO lists it as confirmed in November 2022, and BSI publishes the same text as BS 6620-2:1991. Its scope is holes down to 2 mm square or 0.5 mm round, in five pitch-to-hole ratios from 1.25 to 2, which give open areas of about 58 % down to 23 % for round holes.

Put the two limits together, a hole at least as big as the sheet is thick and a bridge at least as wide, and they set a ceiling on open area. With the bridge equal to the thickness:

• A big open area needs a big hole. At the smallest hole the rule allows, open area stops near 23 % in a stagger. More open than that needs a thinner sheet or a larger hole.
• Practice from fabricators. A US perforator's 2017 advice is not to make the hole diameter smaller than the material thickness, to stay under 70 % open area to avoid distortion, and to keep side margins small to reduce distortion. Smaller holes, it says, need a conversation.
• Laser limits are the cutter's own. One online laser service's 2022 guide gives minimum hole diameters from 0.5 mm in sheet up to 1.2 mm thick, rising to 2 mm in 3 mm sheet and 6 mm in 8 mm sheet, and a minimum of 1 mm between shapes up to 3 mm thick and 2 mm above that, with different rules for aluminium. It says parts that break its tables can still be made but the cut is not guaranteed. Another cutter will differ.
• Which side is measured. ISO 7805-2 clause 6.1 measures holes and pitch on the punch side of the plate, so say which face shows.

The rules above are for sheets, where the sheet's job is to be looked at. If the screen carries load, or is a balustrade infill, it is for an engineer: see balustrade and handrail rules.

What the sheet keeps: width, weight and flatness

Open area says what has gone; it does not say what is left to work with.

• Width left. Along a line of holes the metal left is (P − D)/P of the width: 33 % in the example. That is geometry, not a strength rating, and the weakest line may not be the one you think. Anything that must carry load needs an engineer or the partner's data.
• Flatness. Perforating takes metal out of a sheet and lets it move. The US perforator says to discuss flatness requirements with the supplier and that open areas above about 70 % raise distortion. Thin sheet with a wide field is where waves show: see oil canning in sheet metal thickness and gauge conversion, and tolerances for visible metalwork for stating flatness separately.
• Movement. A large perforated panel still expands with temperature: see thermal expansion of metals.
• Finish and edges. Finish a perforated or laser-cut part as you would any visible metal: see stainless finish codes, and decide whether it is finished before cutting or after. We found no source on how edges cut by laser are dressed on visible faces, so ask.

Light, view and sound

Open area is the share of the face that is hole, seen square-on. Walk past a screen and the view changes, because the sheet has thickness. Seen at an angle θ, only the overlap of the top and bottom openings is open, and a hole of diameter D in a plate of thickness t is closed off completely at an angle whose tangent is D/t. This is geometry for round, straight-walled holes in a single plate, worked by script and checked against a numeric integral.

• The example at 45°. A hole as large as D = 6, t = 3 still passes 39 % of its area at 45°, so about 39.1 % × 40.3 % = 15.8 % of the sheet's area is seen through at 45°, against 40.3 % square-on.
• Privacy. A hole as wide as the sheet is thick closes at 45°, so a screen of such holes is opaque from a low or oblique angle; a thin sheet with large holes stays open to the side. Thicker sheet and a layer behind both help.
• Light from behind. The pattern against a lamp shows hot spots where holes line up with the sources, and the number depends on the spacing. Ask for a backlit sample at the working distance.
• Sound. A perforated face is mostly a skin; absorption comes from the material and the air gap behind it. We have no test data for a perforated metal face, so ask for the supplier's laboratory data for the whole assembly, not the facing alone.

Laser-cut screens, islands and expanded metal

Laser-cut patterns are not limited to repeating holes: any shape can be cut. The catch is the island. A closed shape cut right through, such as the centre of a ring or the counter of a letter, is a loose piece and falls out unless a bridge ties it. Either leave bridges in the drawing, or design the pattern so every piece of metal is joined to the rest. Ask the fabricator to flag any island before cutting.

Expanded metal is slit and stretched in one piece, not woven, and loses nothing to waste. Its parts are measured from bond to bond: SWD and LWD are the short and long way of the diamond, the strand is the metal between openings, and a bond is where strands meet. Standard (raised) expanded metal is as it comes from the press, with strands tilted out of the plane of the sheet, so its thickness is the base metal. Flattened expanded metal has been through a cold roll mill and has a smoother face, and its thickness is the overall finished thickness (a fabricator's terminology page). Tilted strands change what you see from different angles, so judge a raised sample from the viewing positions that matter. Open area comes from the supplier's table for the product; it is not a simple formula. Woven wire mesh is a different material: see metal mesh for furniture and joinery.

[Material], [  ] mm thick, perforated with [round (R) / square (C) / slot (LR)] holes, [D = __ mm], at [60-degree stagger / in line], pitch [P = __ mm], to give a calculated open area of [__] % over the perforated field. Bridge [P minus D = __ mm], not less than the sheet thickness. Unperforated border [__ mm] on [all sides]. Hole size measured on the [punch] side; visible face is the [die / punch] side. Flatness [  ] mm, judged against approved sample [ref. __].

Pattern as drawing [  ], supplied as [DXF / DWG], cut from [material], [  ] mm thick. Every enclosed shape is to be tied by the bridges shown; the fabricator is to flag any island before cutting. Minimum feature [  ] mm and minimum web [  ] mm, as confirmed by the fabricator for this material and thickness. Edges that show are to be dressed to sample [ref. __]. Finishing is [before / after] cutting.

Expanded metal, [standard (raised) / flattened], [material], SWD [  ] mm, LWD [  ] mm, strand width [  ] mm, thickness [  ] mm, open area [  ] % from the supplier's table for this product. Long way of the diamond [horizontal / vertical]. Edges [open / closed]. Sample to be approved in [the light] and from [the viewing positions] stated.

Before pricing, the fabricator is to state in writing the smallest hole, the narrowest bridge or web, and the largest sheet it can perforate or cut flat in this material and thickness, and where the pattern is likely to distort.

Standards and sources

• ISO 7805-2:1987, industrial plate screens, part 2, thickness below 3 mm (BS 6620-2:1991): text read from a public preview. ISO stage 90.93, confirmed 25 Nov 2022; BSI: current, under review, identical to ISO 7805-2.
• ISO 7806:1983, codification for designating perforations: preview read. ISO stage 90.93, confirmed 25 Nov 2022.
• ISO 7805-1:1984 (plate 3 mm and over): listing only: confirmed 25 Nov 2022. Text not read.
• A US perforator's purchasing guide (2017); an online laser-cutting service's guide (2022); a fabricator's expanded-metal terminology page: read; fabricator practice, not standards.

Questions we are asked

How do I calculate the open area of perforated metal?

Divide the area of the holes in one repeat of the pattern by the area of that repeat. For round holes of diameter D at pitch P in a 60-degree stagger it is 0.9069 × (D/P)²; in line it is 0.7854 × (D/P)²; for square holes of side S it is (S/P)². For 6 mm holes at 9 mm pitch that is 40.3 % staggered and 34.9 % in line. Calculate the perforated field and the unperforated border separately.

What is the minimum bridge between holes?

The only standard-based rule we found is in ISO 7805-2 for industrial screening plate under 3 mm: the sheet thickness is to be less than the hole size and less than the bridge width. Fabricators publish their own limits, and they differ, so ask the partner for the narrowest bridge it will punch or cut in your material and thickness.

Should the holes be staggered or in line?

A 60-degree stagger gives more open area for the same hole and pitch: 40.3 % against 34.9 % for 6 mm holes at 9 mm pitch, with the same 3 mm bridge. In line gives a grid look. Choose on appearance first, then check the open area you need.

How much light and privacy does a perforated screen give?

Open area is the square-on figure only. At an angle the thickness of the sheet closes the holes: a hole as wide as the sheet is thick is shut at 45 degrees, one twice as wide at 63 degrees. The example of 6 mm holes in 3 mm sheet passes about 39 % of each hole at 45 degrees. See the table on this sheet.

Will a perforated sheet stay flat?

Not necessarily. A US perforator advises discussing flatness with the supplier and keeping open area under about 70 % to limit distortion. Thin sheet with a large perforated field is where waves show. State flatness separately and approve a sample; see the sheet on tolerances.

What is the difference between perforated, laser-cut and expanded metal?

Perforated sheet has repeating holes, usually punched. Laser-cut sheet can have any pattern, cut part by part, and needs bridges for any closed shape. Expanded metal is one sheet slit and stretched into a diamond mesh with no waste. Woven wire mesh is a different product.