If you've ever handled a bottle crate, it's likely you've been in contact with Schoeller. Not only have a billion Schoeller crates been manufactured throughout the world to date, but almost every crate not produced by Schoeller contains Schoeller patents. IMI visited Schoeller, Pullach, Germany, to find out about the technology it uses for injection moulding bottle crates.
Bottle crates of PP and HDPE are used all over the world, but mainly in countries with an efficient return system, or where return systems are required by law. Bottle crates not only are used for transport but also make a valuable contribution to marketing beverages at the point-of-sale. Whereas bottles are usually standardized, the variety of colours and shapes of bottle crates has increased steadily in recent years.
New PET bottles for carbonated soft drinks have gained in importance, and special crates have been required with a uniform format but with a variety of colours and decorative effects. So small runs must be produced economically. Efforts to reduce cycle times have also been important. This has culminated in the current single-purpose systems tailored to the production of bottle crates.
The Schoeller-Plast group is an international company based in Pullach near Munich, Germany. Last year it chalked up sales of DM 400 million (US$ 265 million) through the design and production of bottle crates and reusable boxes and the operation of return systems with transport packaging for vegetables, meat, and commodity goods.
Schoeller has 10 injection moulding shops in Germany, a mouldmaking company in Tachov in the Czech Republic, and subsidiaries and licensees in a number of countries. So far, more than a billion Schoeller bottle crates have been manufactured all over the world. Schoeller's activities have been recognized with numerous industrial awards in Germany, Belgium, and the European Union.
From Standard to Single-Purpose
Standard machines are not entirely suitable for manufacturing bottle crates, which require high plastication rates of between 200 and 300 kg/hour, clamping forces of between 5,000 and 10,000 kN, cycle times of about 35 seconds, and injection rates of between 100 and 200 mm/second. Bottle crates usually have wall thicknesses of between 1.5 and 3.5 mm. The shot weight is .6 to 4.5 kg with a tolerance of only .1 to .2 percent. Schoeller cooperated with Krauss-Maffei to optimize the C-Series standard machine for the production of bottle crates. The new machines are known as MCM (modular crate machines).
Bottle-crate production is subject to very stringent requirements for economy and flexibility, making it imperative to reduce the overall cycle time. To analyze all the processes taking place in a cycle, each machine component was examined in detail. Although efficiency measures taken saved only a few tenths of a second, cumulatively they reduced the cycle time from about 45 to 35 seconds. The combination of systematically improved modules and their coordination with one another, together with the parallel organization of machine functions-as well as the reduced wall thicknesses of the crates-was tackled in partnership with companies in the supply industry, explained Michael Rinderle, technical manager at Schoeller.
The joint developments were not restricted to machine technology. They also included improvements in the materials for bottle crates, carried out jointly with raw materials manufacturers. Together with Ciba-Geigy, the company developed suitable cadmium-free, yellow masterbatches that do not lose their colour even after repelletization or after being washed in the bottling plants. Rinderle was also not shy about approaching suppliers about making modifications to mould, hot runner, and nozzle technology.
Performance for Better Cycle Time
Here is the performance required of the machine and how the MCM units get there. At a cycle time of 30 seconds and weight per crate of approximately 2 kg, the required plastication rate is 240 kg/hour. The screws are up to 23D long. The standard screw for polyolefins, at 130-mm diameter, is 20D long. This screw drive has 10 percent greater torque and a higher speed, and plasticates 125g of polymer per second. The first torque level can provide a speed of 160 rpm for PP or 108 rpm for HDPE. The increased injection moulding pressure of 1,425 bar for the 130-mm screw and 1,553 for the 145-mm screw provides an additional reserve for rapid injection as well as adequate holding pressure. The clamping force is reduced shortly before the end of the cooling time, saving up to a second of cycle time.
Of course, the times of individual operations cannot be reduced arbitrarily without coming up against technical limitations. The MCM machines (Figure 2) carry out certain operations in parallel. Plastication is, therefore, independent of the machine movements. The opening and closing movements and ejection of the parts take place without any reduction in screw speed or slowing of movements.
Electric Screw Drives
An electric screw drive can help to save energy, and therefore money. The 10,000 kN machines from Krauss-Maffei, with hydraulic drive, have a connected load of 260 kW (160 kW for the pump and 100 kW for the heating). with an electric drive, it is 357 kW (147 kW for the screw drive, 90 kW for the pump, and 120 kW for the heating). Where a constant, reliable electrical power is not available, a hydraulic drive is preferred because of its lower peak power demand and lower connected load.
The bottle crate machines have no hydraulic ejectors. The core pullers have an independent control that emulates the movements of a hydraulic ejector. At Schoeller, demoulding is carried out by the stripper frame, which strips the crates synchronously with the opening of the mould (Figure 3). To avoid damaging the crates during stripping and the mould during the closing movement, the pullers are operated at two different speeds and pressures.
As Rinderle explained, it is essential to ensure that the crates have actually fallen out of the mould. A special photoelectric barrier monitors the moulds and ensures that in, for example, a 2x2-cavity mould, four mouldings have actually fallen out and nothing is obstructing the mould opening.
Generously dimensioned hydraulic lines and special hydraulic valves for the closing mechanism permit rapid buildup of pressure, thus saving .3 to .5 seconds. Another second can be saved by utilizing the backpressure of the plastication unit for the holding pressure in the mould feed area.
Cooling System Requirements
Because of the huge amount of heat to be dissipated from the bottle crates, a large amount of water has to be circulated. Feed lines were increased to 5 cm in diameter. To allow the system to heat up rapidly for the start of production, cooling can be switched on at any time between the first and 99th shot. In the event of interruptions to production, the cooling circuit is switched off to avoid condensation on the mould surface.
In view of the complicated design of the cooling system, would not multi-station machines be more economical than this thoroughbred single-purpose machine? Helmut Horl, head of Krauss-Maffei's technical service department for large machines, is resolute: "multistation machines only pay off for relatively large wall thicknesses of 5 mm or greater." The main problem is now adequate cooling of the plastic, since the plastication time has become shorter than the cooling time. Although Horl regards cycle times of 24 seconds as "feasible in the medium term" with further optimization of the part geometry, mould and machine, 20 seconds is probably asking too much.
For production in tropical countries, Schoeller and Krauss-Maffei have provided a high-performance cooling system, cooling for the electronics and chrome-plated tiebars.
Schoeller's in-house design department not only undertakes development work for the company's own production, but also accepts out-of-house jobs, which are tooled in the Czech Republic. Through continuous improvements in machine technology and development projects carried out in partnership with other companies, Schoeller is ensuring that it will continue to be the world leader in bottle crates.