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The cutting stock problem is an optimization problem, or more specifically, an integer linear programming problem. It arises from many applications in industry. Imagine that you work in a paper mill and you have a number of rolls of paper of fixed width waiting to be cut, yet different customers want different numbers of rolls of various-sized widths. How are you going to cut the rolls so that you minimize the waste (amount of left-overs)?

Solving this problem to optimality can be economically significant: a difference of 1% for a modern paper machine can be worth more than one million USD per year.

Contents 1 Formulation and solution approaches 2 Illustration of one-dimensional stock cutting problem 2.1 Solution 3 Classification 4 Cutting stock problem in paper, film and metal industries 5 References 6 Further reading 7 External links

[edit] Formulation and solution approaches

The standard formulation for the cutting stock problem (but not the only one) starts with a list of m orders, each requiring q_j, j = 1,...,m pieces. We then construct a list of all possible combinations of cuts (often called "patterns"), associating with each pattern a positive integer variable x_i representing how many times each pattern is to be used. The linear integer program is then:

minimize

subject to and

, integer

where a_ij is the number of times order j appears in pattern i and c_i is the cost (often the waste) of pattern i. The precise nature of the quantity constraints can lead to subtly different mathematical characteristics. The above formulation's quantity constraints are minimum constraints (at least the given amount of each order must be produced, but possibly more). In this case waste minimisation is equivalent to minimising the number of utilised master rolls. The most general formulation has two-sided constraints (for which minimising waste is no longer equivalent to minimising the number of master rolls):

This formulation applies not just to one-dimensional problems. Many variations are possible, including one where the objective is not to minimise the waste, but to maximise the total value of the produced items, allowing each order to have a different value.

In general, the number of possible patterns grows exponentially as a function of m, the number of orders. As the number of orders increases, it may therefore become impractical to enumerate the possible cutting patterns.

An alternative is to use a Delayed Column Generation approach. This method solves the cutting stock problem by starting with just a few patterns. It generates additional patterns when they are needed. For the one-dimensional case, the new patterns are introduced by solving an auxiliary optimization problem called the knapsack problem, using dual variable information from the linear program. The knapsack problem has well-known methods to solve it, such as branch and bound and dynamic programming. The Delayed Column Generation method can be much more efficient than the original approach, particularly as the size of the problem grows. The column generation approach was pioneered by Gilmore and Gomory in a series of papers published in the 1960's.^{[1] [2]}. Gilmore and Gomory showed that this approach is guaranteed to converge to the (fractional) optimal solution, without needing to enumerate all the possible patterns in advance.

A limitation of the original Gilmore and Gomory method is that it does not handle integrality, so the solution may contain fractions, e.g. a particular pattern should be produced 3.67 times. Rounding to the nearest integer often does not work, in the sense that it may lead to a sub-optimal solution and/or under- or over-production of some of the orders (and possible infeasibility in the presence of two-sided demand constraints). This limitation is overcome in modern algorithms, which can solve to optimality (in the sense of finding solutions with minimum waste) very large instances of the problem (generally larger than encountered in practice^{[3] [4]}).

The cutting stock problem is often highly degenerate, in that multiple solutions with the same waste are possible. This degeneracy arises because it is possible to move items around, creating new patterns, without affecting the waste. This give arise to a whole collection of related problems which are concerned with some other criterion, such as the following:

• The minimum pattern count problem: to find a minimum-pattern-count solution amongst the minimum-waste solutions. This is a very hard problem, even when the waste is known^[5][6]. There is a conjecture that any equality-constrained one-dimensional instance with n orders has at least one minimum waste solution with no more than n + 1 patterns. No upper bound to the number of patterns is known either, examples with n + 5 are known.

• The minimum stack problem: this is concerned with the sequencing of the patterns so as not to have too many partially completed orders at any time. This was an open problem until 2007, when an efficient algorithm based on dynamic programming was published^[7].

• The minimum number of knife changes problem (for the one-dimensional problem): this is concerned with sequencing and permuting the patterns so as to minimise the number of times the slitting knives have to be moved. This is a special case of the generalised travelling salesman problem.

[edit] Illustration of one-dimensional stock cutting problem

A paper machine can produce an unlimited number of master (jumbo) rolls, each 5600 mm wide. The following 13 items must be cut:

Width	Rolls
1380	22
1520	25
1560	12
1710	14
1820	18
1880	18
1930	20
2000	10
2050	12
2100	14
2140	16
2150	18
2200	20

[edit] Solution

There are 308 possible patterns for this small instance. The optimal answer requires 73 master rolls and has 0.401% waste; it is believed that in this case the minimum number of patterns with this level of waste is 10 (one such solution is shown below and in the picture):

Repetition	Contents
2	1820 + 1820 + 1820
3	1380 + 2150 + 1930
12	1380 + 2150 + 2050
7	1380 + 2100 + 2100
12	2200 + 1820 + 1560
8	2200 + 1520 + 1880
1	1520 + 1930 + 2150
16	1520 + 1930 + 2140
10	1710 + 2000 + 1880
2	1710 + 1710 + 2150
73

[edit] Classification

Cutting stock problems can be classified in several ways^[8]. One way is the dimensionality of the cutting: the above example illustrates a one-dimensional (1D) problem; other industrial applications of 1D occur when cutting pipes, cables and steel bars. Two-dimensional (2D) problems are encountered in furniture, clothing and glass production. Not many three-dimensional (3D) applications involving cutting are known; however the closely related 3D packing problem has many industrial applications, such as packing objects into shipping containers (see e.g. containerization - the related sphere packing problem has been studied since the 17^th century (Kepler conjecture)).

[edit] Cutting stock problem in paper, film and metal industries

Industrial applications of the cutting stock problems for high production volumes arise especially when basic material is produced in large rolls that are further cut into smaller units. This is done e.g. in paper and plastic film industries but also in production of flat metals like steel or brass. There are many variants and additional constraints arising form special production constraints due to machinery and process limits, customer requirements and quality issues; some examples are:

• Two-stage, where the rolls produced in the first stage are then processed a second time. For instance, all office stationery (e.g. A4 size in Europe, Letter size in US) is produced in such a process. The complication arises because the machinery in the second stage is narrower than the primary. Efficient utilisation of both stages of production is important (from an energy or material use perspective) and what is efficient for the primary stage may be inefficient for the secondary, leading to trade-offs. Metallised film (used in packaging of snacks), and plastic extrusion on paper (used in liquid packaging, e.g. juice cartons) are further examples of such a process.

• Winder constraints where the slitting process has physical or logical constraints: a very common constraint is that only a certain number of slitting knives are available, so that feasible patterns should not contain more than a maximum number of rolls. Because winder machinery is not standardised, very many other constraints are encountered.

• An example of a customer requirement is when a particular order cannot be satisfied from either of the two edge positions: this is because the edges of the sheet tend to have greater variations in thickness and some applications can be very sensitive to these.

• An example of a quality issue is when the master roll contains defects that have to be cut around. Expensive materials with demanding quality characteristics such as photographic paper or Tyvek have to be carefully optimised so that the wasted area is minimised.

• Multi-machine problems arise when orders can be produced on more than one machine and these machines have different widths. Generally availability of more than one master roll width improves the waste considerably; in practice however additional order splitting constraints may have to be taken into account.

• There is also a semi-continuous problem, where the produced rolls do not have to be of the same diameter, but can vary within a range. This typically occurs with sheet orders. This is sometimes known as a 1½ dimensional problem.

Suppliers of such software to the paper industry include ABB Group, Greycon, Honeywell and Tieto.

[edit] References

[edit] Further reading

• Chvátal, V. (1983). Linear Programming. W.H. Freeman. ISBN 978-0716715870. 
• Hatem Ben Amor, J.M. Valério de Carvalho, Cutting Stock Problems in Column Generation, edited by Guy Desaulniers, Jacques Desrosiers, and Marius M. Solomon, Springer, 2005, XVI, ISBN 0-387-25485-4

[edit] External links

• European Special Interest Group on Cutting & Packing
• A web site, provided by the Argonne National Laboratory, where you can submit one-dimensional problems with up to 10 sizes can be found here.