APW Perforated Metal Technical Information

Technical Requirements for Perforated Sheet

Perforated materials are more complicated than they look. Many details have to be considered, good co-operation and consultation between supplier and user is necessary. The following information aims to provide basic information on the technical requirements for perforated sheet, plate and coil.

Length, Width & Thickness

Standard sizes of sheets and plates are supplied without cutting after perforating and roller levelling. Tolerances may be larger than mill tolerances.

Squareness

Parellelism

During the process of perforating, stretch occurs in the perforated area. The amount of stretch is dependent on a number of factors (i.e. hole size, pattern, material type and material thickness).

Due to the considerable number of variables, it is not practical to set tolerances for the amount of stretch that may take place. On completion of perforating and during the flattening operation the stretch in turn creates distortion over the perforated area, especially when the side and end margins are part of the finished component.

Fig.1 shows an exaggerated view of the type of problem that can be experienced. The full lines show the profile of material and the perforated area.

The broken lines show the profiles after flattening, this profile can, of course, alter considerably depending on the previously mentioned factors and therefore may not be symmetrical along the centre line, as shown in Fig.1 and Fig.2.

When material has been cut to size as shown in Fig.2, the out of square problem can be seen (i.e. the edge of perforated area not running parallel to the edge of the component). If it is considered essential that the sides and ends of the perforated area should run parallel to the edge of the component, the customer is advised to contact the estimator prior to placing an order to discuss any problems which may arise.

Edge Bow

On perforated sheets with different side margins (e2>e1) it is possible to produce curving or edge bow.

This effect is the deflection (d) between one of the longitudinal edges and a straight line supported off the ends of the sheet.

The value of the deflection (d) will refer to the concave side. It depends from length, width, thickness, open area, type of material and in relation to e1 and e2.

If sheet are cut after perforation, the bowing refers only to the perforated pattern. Designers are requested to avoid different margins if possible.

Flatness

3 categories are possible:

a) unlevelled
b) roller levelled
c) quality levelled

Flatness tolerances of roller levelled plates or sheets with no or small unperforated margin to a maximum length of 2m.

The sheets are placed on a perfectly flat table. A ruler which does not flatten the material will give the degree of flatness, the measurement being from the highest point of the sheet to the table surface. Special arrangements will have to be agreed for sheets with large margins or blank areas.

Wavy Edge

The tensions that arise during the perforation may cause a deviation in the flatness of the edges, the so called "wavy edge", especially where perforated sheets and coils with unperforated wide margins or on both sides unequally wide unperforated margins are concerned.

Deviation within the permissible tolerances of the flatness will be guaranteed for perforated sheets with standard perforation and narrow equally wide unperforated margins on the long side (width of margin smaller than approx. 2,5% of the sheet width to a max. of 10mm).

Perforated sheets and coils with wider unperforated margins or with unequally wide unperforated margins on the long side, or special perforations, such as countersunk perforation, stamp, etc. tend to have considerably higher deviations from the flatness, especially when using soft materials (i.e. al, cu, cuzn). The admissible tolerances then will have to be agreed seperately.

Unit 12 Deacon Trading Estate Earle Street Newton le Willows Merseyside WA12 9XD Tel: 01925 295577 Fax: 01925 295588 Email: sales@apw.co.uk