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Open Area Calculation and Formula
Open area is the total hole area on a perforated sheet. This is always compared to the sheet size and is evaluated by percentages. Calculating the open area for uniform and conventional perforated patterns is easy and intuitive. This is especially true if the perforated shape is round. On this page I will describe how to get the basic formula to calculate the open area based on the characteristic three pattern layouts: 60 degree pattern, 45 degree pattern and rectangular pattern.
The first step in calculating the open area of perforated pattern layout is to find the smallest uniform segment that constructs the pattern. So the smallest segment for a 60 degree perforated pattern is the equilateral triangle. Based on this, the percentage of open area can be found by comparing the area of the triangle against the partial hole in each triangle corner. Another example is when we are going to calculate the open area of the rectangular perforated pattern.
The smallest segment of a rectangular perforated pattern is a rectangular area. Using the same principle, we calculate the open area of this pattern by comparing the rectangular area against the segmental hole on each corner of the rectangular area.
According to IPA Handbook (www.iperf.org), an extreme open area proportion tends to increase distortion. Therefore, the handbook recommends staying below 70%. Also to be considered is the gap between each tool. The appropriate gap between the holes, according to Wilson’s Turret tooling maintenance manual (www.wilsontool.com) is 3mm, or at least 2 times bigger than the material thickness, whichever is greater. Although this gap rule applies specifically to cluster tools, I think this could also be applied to a single stroke punch tool.
The gap between the holes is very important to maintain the strength of the perforated sheet, as well as to make a better and smoother production process. Consult the machine manual book to find the best minimum gap between the holes according to particular material type. This will ensure that everything is good for manufacturing it.