With perforating tools with small bases, the hole stamps are frequently arranged at a 1.5 to 2 times the distance from each other to prevent damage to the tools.
This means the full hole patterns are not formed at the beginning of the hole pattern until after the second of third hub.
Equally, the completed hole pattern ends after the second last or third last hub.
The terms for this standard configuration customary to the trade are:
"Large beginning and end”
"double offset beginning and end” “not pushed out”
Start 1. Hub
With Figure 1, the operating process for such a configuration is diagrammed from the first to the last row of holes.
Theoretically, the missing rows of holes can be added during the punching process with almost all configurations, yet it must be agreed upon separately. [Image 2. hole pattern normal offset completed
Image 3. hole pattern symmetrically completed, i.e.
the hole pattern starts and finishes with the same row of holes]
The unperforated area between the outer edge of the sheet and the edge of the hole is what is called the border.
The key advantage when using perforated plates is that they can be manufactured with unperforated borders and zones. This way considerable costs can be saved in part if, for example, the manufacture of a frame can be left out.
A distinction is made between the front border and the side border [Image 4.]. It is customary for the types produced to usually be sheets with small unperforated front and side borders.
If an unperforated border is not needed, costs can frequently be saved by cutting through the perforation.
However, it is possible to manufacture practically any desired border size. By switching off and on individual stamps during the perforation process, even sheets with unperforated zones Images 5.-7.] or any other hole pattern contour [Images 8.+9.] can be manufactured.
Which design is more economical needs to be determined on a case to case basis.
It is thus important to lay down all requirements during the construction and enquiry phase of the perforated product.
If an unperforated border is required, it must be noted that this depends on the division of the holes. If the division is given, this shall have an influence on the theoretical size of the border.
In addition, the hole pattern can also stretch from the perforating process; it is not until after that when the actual pattern size is shown, (also see Images 13-14, p. 32 Section on tolerances).
Complex outer and inner contours are nibbled or laser-cut with a round stamp if straight cuts are not possible.
When nibbling, unevenness can develop on the cutting edge from the offset of the punching die [Image 10].
Finishing work by sanding the nibble contour can be agreed upon separately. Usually, no finishing work is necessary when the contours are laser-cut.
However, the kind of finishing work to be done has to be announced so that the right cutting gas can be chosen.
When perforated plates are used for technical purposes, e.g. as a sieve, but also for use of a decorative manner, i.e. cladding panels, the direction of the sieve [Images 11. + 12.] is of great significance. The direction of the sieve decides over the direction of flow of the sieving in which the best possible sieving result can be achieved.
Thanks to the correctly chosen sieve direction, the sieving cannot move along the unperforated bases, thus guaranteeing an optimal performance of the plant.
To help recognise this, the so-called machine direction, which is at a right angle to the direction of sieve, can be looked at.
The machine direction is the direction in which the arrangement of holes is recognisable in straight rows.
With elongated holes, no clear allocation is possible frequently due to the sole statement of the sieving direction, as it depends on the intended use. For this reason, when there is no drawing, the direction of the hole axis to the sheet outer measurement should always be stated with elongated or crossed holes. Clearly defined is the specification: Length of hole// to length of sheet [e.g. 20mm // 2000mm]
Plates with uneven borders, unperforated bases or an extreme length in combination with a narrow width tend to form sabres due to the different distribution of tension [Image 13.]. This happens all the more intensely, the more of the afore-mentioned criteria are fulfilled.
Above all, with perforated plates with a high percentage of perforations that are stretched extremely during the punching process, symmetrical hole patterns or wide borders on one side can promote an uneven stretching and cause sabre shapes [Image 13.], trapezoidal formations [Image 13a.] or bulges [Image 13b.].
When subsequently straightening this, it must be achieved that the less stretched areas such as unperforated borders experience the same change in length as the hole pattern from the punching process.
If this is not sufficiently successful, what will remain is unevenness of the outer edge and of the hole pattern.
The outer edges can be straightened by cutting them afterwards, yet the asymmetries in the hole pattern remain. Deviations in the unperforated border are thus unavoidable.
For the reasons mentioned above, an evenness of the hole patterns and edges should be ensured as early as at the construction phase.
The punching process causes great tension in the plate. The reason for this, on the one hand, is the different degree of deformation between the side of where the stamp enters and exits and, on the other hand, the change in the lengths caused by the stretching in the punched area. The consequence of this is that the plate has to be adjusted after punching by using a special straightener [Image 14.].
The purpose of straightening is that the less stretched areas such as the unperforated borders experience the same change in length as the hole pattern from the punching process.
Extremely wide unperforated borders, unperforated zones or strips can impede the full rolling out of these tensions and partially even make them impossible and need to be avoided during construction where possible.
Besides a deformation zone, the punched hole reveals a cylindrical cutting zone and, on the stamp exit side, a conical fracture zone [Image 15.].
The width of the hole (w) is measured in the cutting zone. The length of the cutting zone is about 30% of the thickness of the sheet, depending on the firmness of the material and the mould design. As approximate value, it can be assumed that the punched hole on the stamp exit side is 0.15-0.2 mm larger per mm sheet thickness than on the stamp entry side.
As with any punching or cutting process, the punching on the stamp exit side causes a more or less rough raw edge.
The thickness of this raw edge formation depends on the composition of the material to be punched and the die.
The raw edge and the tension developing during perforation correlate with each other so that a minor formation of a raw edge is often associated with great tension.
In order to achieve better evenness, a somewhat stronger raw edge formation is accepted in most cases.
With drafts and drawings, if no deviating specifications are known or have been noted down, the stamp entry side is in the top view, i.e. the punch burr is on the bottom of the drawn part [Image 16.].
This regulation applies to pre-treated plates (sanded, brush-finished, coated) in equal measure.
If the parts to be produced are not symmetrical or of they are bent or rounded off afterwards, the specification of the raw edge is absolutely necessary.
One speaks of marginal holes if the hole width and the sheet thickness are almost the same of the measurement from hole edge to hole edge is smaller than the sheet thickness.
Although the threshold values stated above are frequently fallen short of, they still are deemed an approximate value to achieve an optimal and simultaneously economic solution
During the punching process, with so-called marginal holes, stamps can break fully or in part. This can entail the risk that, in the event of a stamp breaking, the area of the material affected is not hole-punched - DIN 24041 2002-12 allows for a lacking percentage of holes of 2%. Where necessary, it is possible to rebore and remove the missing spots in return for payment for the effort. The boreholes inserted this way do, however, differ optically from the punched holes; for this reason, we advise against this type of rework in visible parts.
If holes in the border area are a must-have for you, then enquire about our possibilities. We will submit a solution to you where it is possible production-wise.
Almost all firm metallic, synthetic or natural materials are suitable for the production of perforated, embossed sheets and foils. SHS mainly processes different stainless steel, materials with zinc/aluminium coating, aluminium materials but also non-alloy steel, copper, titan, PTFE, PE, PVC.
The thickness tolerances are in line with the permissible deviations of the respectively used material.
The nominal dimensions of the standard sheets 1000x2000 (small format); 1250x2500 (medium-sized format); 1500x3000 (large format) as well as coils of 1000; 1250 and 1500mm in width and their sections are usually not given special processing treatment after the hole-punching and straightening.
Depending on the hole punched, tensions develop during processing that stretch the punched field and the outer contour. Above all in the longitudinal direction, greater tolerances need to be anticipated than the standards for unperforated sheets permit.
With fixed dimension lengths with a small tolerance zone, it is necessary to cut the sheets after punching and straightening to the desired finished dimension.
The changes in length in the hole pattern caused by stretching can no longer be rectified by this cutting process, so that possible deviations in the unperforated front borders occur.
With larger production lots, the stretching can be determined by trials and thus be taken into consideration from the offset by appropriate measures such as reducing the number of hole rows.
The former DIN 24041 (round holes); DIN 24042 (square holes) and DIN 24043 (elongated holes) were replaced by the new DIN 24041:2002-12 in December 2002.
When manufacturing plates from coil material, there may be deviations in the lot size that are due to the tolerances of the basic material and these cannot be prevented.
In order to ensure constant quality of the punching during processing, lubricants have to be applied in most cases.
This results in the following delivery conditions:
- Not degreased (all materials)
Delivery condition for parts made from steel, aluminium, stainless steel, copper and titan. With non-alloy steel, this grease coating also serves as corrosion protection.
- Pressure cleaned (materials resistant to corrosion)
Pressure cleaned parts are degreased, yet 100% degreasing cannot be guaranteed. This type of cleaning is not recommended for materials susceptible to corrosion because there is no subsequent passivation.
- Cleaned (all materials)
Parts are degreased and passivated in a special cleaning plant. This process is suitable for all metals as well as materials resistant to temperature, chemicals and liquids.
- Powder-coated (all materials)
By way of electrostatic or electro-kinetic application, coating powder is applied in the desired colour, structure, quality and thickness. After that, the polymerisation process is conducted by feeding heat.
The colours are named in line with the RAL colour register (other names are: NCS, Sikkens, British Standard or DB).
Using powder-coating, metallic effects can also be manufactured, with special processing even imitations of natural and artificial surfaces such as wood grain, granite or as per a graphic file provided.
Powder-coated parts are fat-free.
- Use of evaporating lubricants (non-alloy materials)
After evaporation, only a light film remains of the material surface that has no negative impact on possible further processing (coating, anodising).
- Blued/electro-polished (alloy steel)
Contaminations such as welding scales, oxide coatings, discolouration, extraneous rust, greases, oils and metallic components incorporated into the surface from mechanical processing can be removed by bluing.
The metallic pure surface achieved from bluing can form the passive layer which protects the stainless steel from corrosion. The surface achieved in this way is not, however, suitable for decorative use.
- Afterwards, with stainless steel, a smooth shiny surface can be achieved by an electrochemical removal of the tips of roughness.
- Anodised (aluminium)
The anode oxidation after processing is of great significance for high quality aluminium parts. Due to the electrochemical change in the edge layers, a surface more resistant to corrosion and wear than with untreated aluminium is achieved.
There is a wide range of colours available for decorative areas of application.
Anodised parts are fat-free.
- Pre-treated/coated basic material (all materials)
Material pre-treated on one side is used for material coated, sanded or brush-finished prior to processing unless other specifications are made.
In order to avoid damage to the surface during machine processing as much as possible, the material is often given protective foils. Due to the reduced adhesive area and the use of punching oil, the protective foil may peel off during processing. In order to prevent damage to the surface of wrong hole-punching caused by the peeling off protective foil, it has to be removed completely. The standard condition on delivery here is also without further processing “not degreased”.