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Deep drawing of monitor cavities

 

As part of an international collaboration, a mid-sized machinery builder designs a production line for PC monitor cavities – from the blank to the washing plant

Successfully implemented forming plans quickly direct the attention of potential users to the supplier and its performance capabilities. Particularly so, when complex tasks were solved and implemented quickly. When the process and automation know-how is additionally accompanied by fast information exchange and flexible responses to customer needs, facilitated by the structure of a mid-sized supplier, all barriers for a successful project implementation have been removed. The fact that this customer benefit is also heavily relied upon in international transactions is demonstrated by the "Monitor Cavity"project.

 

Indian performance capabilities are frequently discussed in the context of the 'greencard' topic. The fact that this sub-continent also produces high-quality products, and that know-how transfer can also flow from West to East is less known. For instance, the majority of computer monitors used in Germany are equipped with hole cavities produced in India. In order to produce these products in accordance with the latest technical standard, and with reproducible accuracy, an Indian company has planned a completely new production line. The corresponding specification document has defined a press plant for cold and hot forming of the cavities, consisting of a triple-action hydraulic press with transportation, lifting, and turning mechanisms for automatic processing from the stacked blank to the cleaned cavity.

 

Swabian-Far Eastern collaboration

Dunkes was included in the list of press manufacturers requested to submit a proposal. The machinery builder from Kirchheim/Teck, a globally leading supplier for forming technology, has quickly and cost-effectively implemented many projects in terms of process and automation technology. This expertise also convinced the Indian client. It retained the Swabian supplier for the planning and fabrication of a plant to produce monitor cavities in 15",  21" and 25" sizes. While the majority of cavities are cold formed, only the higher-quality 21" cavities, made from Invar steel, are formed at 220°C  in a heated special- purpose tool. The respective tools were fabricated in Japan, tested by Dunkes at the Kircheim facility under production conditions, and optimised in close collaboration with the participating companies with respect to the key performance parameters involving the forming processes, cycle sequences, and accuracy requirements.

 

Triple action press ensures inherent accuracy

The centrepiece of the production line is a triple action hydraulic press of type HDZ-3-63-25-25 with a press force of max. 880 kN. Dunkes was able to modify a standard press for this special task without overly extensive effort. This also included that the top action drawing ram ( 630 kN) and the sheet metal holder (250 kN) needed to be controlled independently of each other based on the parameters force, speed, and travel. The ram is equipped with a rapid travel speed of 400 mm/s and a working speed of 36 mm/s at max. press force. The drawing cushion rapid travel is 200 mm/s, and its working speed at max. press force is 40 mm/s. 75 kW of drive power are installed for the ram, and 15 kW are installed for the die cushion. All motions are visually displayed on the control panel, and can be validated on demand during the set up as well as production, inclusive of their various interactions. The motions of the ram and die cushion can be mechanically coupled in order to attain accurate repeatability – an essential requirement when hot-forming the 21" cavities. In spite of its overall weight of 25 tonnes, the press has a remarkable inherent accuracy. The parallelism between the table bed and sheet metal holder, and between the table bed and the drawing ram is <= 0.06 mm. The angularity between these three components is also less than 0.06 mm. In order to accurately align the dies, the table bed was ground to a surface roughness of Ra <= 0.003 mm.

 

The largest die, which is used for hot forming the 21" cavities, weighs in at 3 000 kg with dimensions of 950 x 1 000 x 655. As is the case with the other tools, it is transported to the press with a tool truck designed by Dunkes. The tool is then pushed into the workspace with load arms and transport rollers, hydraulically lowered, automatically centred, and hydraulically clamped.

All motions and speeds of the press and the automation components are controlled from a central operator panel; data is entered on a touchscreen. In particular the controllable drawing ram, sheet metal holder, and die cushion simplify the part-specific adjustment of press forces, the upper and lower dead centres, and switching from rapid to working, or minimum travel speeds. When determining the various parameters, the operator can draw on the empirical values stored by the manufacturer. But the control also provides the operator with sufficient degrees of freedom to plan and optimise individual processing sequences, based on the specific situation and supplemented with his own empirical values.

 

Loading and unloading in 10 second cycles

The feedstock for the cavities are blanks with maximum dimensions of 422 x 364 mm made from perforated special-purpose steel. These are stacked in magazines to a height of approximately 300 mm and are fed to the press in a rail guided stacker cart. They are then automatically cycled to the feed level of the upper lip of the tool. Due to the consistent retrieval height, a gripper facility equipped with vacuum suction cups can deposit the 0.122 0.22 mm thick blanks in the forming tool with the shortest possible distance. Accurate positioning is ensured by a retainer pin equipped centring station. 3 magazines are available for the various blank sizes, of which respectively 2 units of the same size can be attached to the stacker cart with quick connect couplings. This permits one magazine to be loaded with blanks, while the other is processed.

 

All automation facilities communicate with the press control via an interface. This significantly simplifies changing from one cavity size to another. When the cavity type is entered into the control, all motion sequences are activated, and the various transport elements are automatically set to the new dimensions.

 

The cycle time is 8.5 seconds on average. During this time, a mechanical gripper retrieves the drawn cavity, rotates it by 180° and the places it in programmable positions onto a conveyor belt with the concave surface facing up. At an average of 660 mm/m, the speed of the conveyor belt is designed such that the drawn cavities are accurately positioned on the belt, and can be transported to a customer installed washing plant. During the random  visual inspection by the operator, the selected cavities are rerouted to a storage surface next to the press, where they can be inspected and reinserted into the cycle.

 

In spite of the compressed project schedule – Dunkes only needed 9 months from the conceptual design of the plant until shipment – the plant has been in malfunction-free 3-shift operation for several months. Approximately 8000 cavities of various sizes are produced on a daily basis. Any occurring problems are directly addressed by Dunkes specialists together with plant operators using the remote diagnostics facility. The distances and the language and cultural differences created no insurmountable barriers for the project as a whole. The German – Indian – Japanese collaboration under the leadership of Dunkes worked, and resulted in a successful outcome for all involved parties.

The front view of the press during final assembly shows the first unit of the transverse conveyor belt to transport the drawn cavities to the washing plant. The receiver for the transfer unit for the mechanical gripper and the vacuum gripper is arranged above.
The left side of the image shows a blank made from perforated special-purpose steel, and a drawn monitor cavity is shown on the right.
The back view shows the loading side of the stacker cart for the blanks, as it protrudes over the safety fence. The guide rails for the two grippers are shown above. A surrounding safety fence secures the press and the transportation facilities.