By Dr Ing. Pavel SvejdaThe automobile industry is accelerating at full power out of the recent financial crisis. The worldwide demand for vehicles is increasing, as reflected in both in production figures and in new registrations. Alongside this, the willingness of the automobile industry to invest is returning as well. New painting plants are being planned and built, and a trend for these is becoming increasingly evident: more and more OEMs are concentrating on energy and material efficiency combined with compact and flexible painting and automation concepts.A paint shop that offers high flexibility, is particularly compact, and promises a high level of energy and material efficiency; is that too much to demand? The painting of cars, vans and truck cabs in varying numbers in one paint shop; is that even possible? What appears at first glance to be wishful thinking is now being realised in Nanchang in China.At the beginning of all plant design processes the range of products to be painted and the capacity required must be considered. In this particular case it is an SUV with 4 model variants, 26 units per hour and 15 sq.mtr of exterior surface to be painted. In addition there is a van with 20 different model variations, 30 units per hour and an exterior surface of 20 to 40 sq.mtr to be painted. With this type of vehicle the interior also requires automated painting, which, as it is a van with a large cargo volume, requires a vast expanse to be painted. Let's not forget the truck cab with two to three units per hour and a surface area needing to be painted that is about the same size as the van. It should become clear quite quickly that this enormous variety of vehicles cannot be processed on a single production line. A division of the line will be required on capacity grounds as well. How does the plant concept look?The standard layout of a paint shop is based on a sequential painting line. The vehicles can pass through continuously or in cycles. At each station only a part of the overall painting scope is completed. In principle, there is also the possibility to incorporate separate "parallel" painting stations to form a so-called box concept. The characteristic feature of this concept is to perform the highest possible painting scope, if not the complete scope, in one station. On closer inspection, both concepts provide very specific benefits. While the traditional painting line is particularly suitable for high throughputs, "the box concept" offers a flexible solution for a wide range of products.Back to the specific task. For the van the box concept seems to be the less expensive solution. But how does the actual layout look? Here's where the painting process comes into play. A so-called 3wet process should be implemented. 3wet means nothing more than applying all three paint applications, i.e., primer, basecoat and clear coat, consecutively, without intermediate drying. The entire film is cured in a dryer only after the last paint application. This is a compact process that can have particularly advantageous effects. With that said now the plant layout can be sketched. The vans first pass through a cleaning station. After that the primer application occurs on the exterior surfaces, here likewise consolidated in a throughput station. Then they are distributed to the individual boxes. In each box the complete base coat and clear coat application is performed for both the interior and exterior. Each box can be operated with a variable cycle time adapted to the respective painting task. In this way only three boxes are required for the given capacity of 32 vehicles per hour. A fourth box for future capacity expansion can be easily integrated into the layout.Is the most favourable solution really found with the concept described above? In order to answer this question quantitatively, a line concept is designed for the same task with the same boundary conditions. The first two stations, the cleaning and primer application are identical to the box concept. After that there is a series of zones that operate in stop and go operation. The van is then successively painted, at first inside, then outside, each time with base coat and clear coat.A limitation quickly becomes apparent. The number of stations required depends on the largest vehicle, in this case a van with 40 sq.mtr of exterior surface area and a share of 11% of total production. This means that the system is simply too large for the vast majority of smaller vehicles. The plant length, in terms of the paint spray separation system, is much bigger, and thus its energy consumption as well. The number of robots is higher and therefore also the loss through colour change. The actual comparison confirms the assumption: the box concept provides a solution with a reduced investment and lower operating costs in this case.There remains the question about whether the box or line concept is best for car painting. With four model variants, the variety is kept within limits. The painting scope and the number of process steps are similar for all variants. Interior painting is done manually. Recommended here as the most favourable solution is a line concept of the simplest design: with continuous flow and fixed six-axis painting robot.A modular box concept also brings advantages even when plants are to be designed for low quantities. Interior and exterior painting can be combined into one zone in this case. For top coat processes with waterborne base coats and clear coats based on organic solvents, two consecutive stations separated by an intermediate dryer are sufficient. Given sufficient cycle times, the entire top coat painting can be done with just two painting robots per zone. Even here the advantages are obvious: reducing by half the painting zones, the number of robots, the energy costs, and the colour change losses. The prerequisite for this is a compact and powerful atomizer that allows a variable spray pattern in a wide range and likewise can be used for both interior and exterior painting. Such innovative painting concepts have already gone into use recently. These may be viewed at renowned manufacturers of premium cars - in Germany, not in China.Back to the originally presented problem: designing a highly flexible and efficient paint shop for an extremely wide range of products. The optimal solution is two different layouts united under one roof; using a box concept for commercial vehicles with a high number of different variants, and a line concept for passenger cars with a relatively small range of models. Both approaches complement each other ideally.SourcesSchumacher, H.: Modular Paint Box Concepts ("Modshops") in Comparison to Conventional Sequential Paint Lines. SURCAR, June/July 2011.Schumacher, H., Svejda, P.: Process Chain "Painted Car Body". Strategies in Car Body Engineering 2011. Bad Nauheim, March 2011.Svejda, P.: Mehr Leistung bei geringerer KomplexitÃƒÂ¤t. JOT 3, 2010.(Dr.-Ing. Pavel Svejda is with DÃƒÂ¼rr Systems GmbH. Email: email@example.com)
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