Manufacturers are looking at Shape Nesting to cut waste. Practical Shape Nesting.

The rapid growth in the use of glass instead of polycarbonate for conservatory roof glazing has substantially increased the impact of shape cutting on daily production for many sealed unit manufacturers. The impact is felt mainly by tougheners, as most roofs use toughened rather than laminated glass.

Lost profits

It is an attractive new market, with higher margins at a time when other markets are in the doldrums. However, manufacturers have quickly found that much of the potential profit is being lost through high glass waste. Since most of the solar control glass being used is very high cost, manufacturers have had to address this, by nesting shapes.

Traditionally shape nesting has been used for repetitive shapes, such as automotive products – wing mirrors, and ‘mirroring’ of shapes, such as standard rakes is common. However, conservatory roofs generally produce large and complex convex polygons. Since most glass processors no longer have 6000 x 3000 jumbo cutting tables, there are frequently occasions where a single shape occupies the whole of a 3000 x 2000 stock sheet.

Many of the modern glass cutting tables can read the output from CAD packages, and this has been used as a solution, with users carefully drawing shapes and then manually fitting them together. This is both time consuming at the design stage, and at the cutting table. It also carries a high risk of inadvertent breaking of pieces, and of generating layouts that cannot be broken out successfully.

Other solutions include the use of a light box to project shapes on to the glass, and this has been effective in the subsequent use of offcuts.

Glass requires guillotine type cutting, so solutions used for shape nesting in the steel, plastics, and leather industries cannot be used, as these would generate patterns that could not be broken out.

Finally, although conservatory roofs can use a high percentage of shapes, our research showed that most conservatory roofs have very few shapes. It is, however, understandable, that most manufacturers will always remember the difficult jobs.

Addressing practicalities

The solution that Jotika added to its glass optimisation software had to address three practicalities:

1. Nesting unique, dissimilar shapes as efficiently as possible, whilst ensuring that both shapes can be simply and successfully broken out.

Where there are a high number of dissimilar shapes, savings of 30-40% are achievable, compared with single shape per rectangle optimisation

2. Integrating nested shapes as part of an overall least waste solution by combining with rectangles

3. Working with the display limitations of automated glass cutting tables that currently assume only one shape per enclosing rectangle.

A limit was placed on the solution of only two shapes per enclosing rectangle. This solution can be used on all shape cutting tables, and Jotika have mostly been able to manipulate overhead display identifiers to show two rather than one piece ID.

An effective solution

This solution is efficient overall, lowers the risk of premature breakout affecting other pieces on the stock sheet, and does not require a degree in mathematics to understand at shop floor level, as may be required when interpreting CAD based solutions.

Use of colour lasers for diagrams allows the highlighting of nested shapes where no overhead display is available.

Where there are a high number of dissimilar shapes, savings of 30-40% are achievable, compared with single shape per rectangle optimisation, but daily benefits really accrue whenever shapes are cut, by lowering the impact of these shapes on daily production, and by de-skilling complex polygon shape cutting.

Jotika have already had discussions with leading cutting table manufacturers, to improve the quality of overhead display where multiple shapes per rectangle are displayed.

The success of this product for glass has already attracted interest from existing polycarbonate roof processors, and a new version aimed at this market will be available in the near future.