To be effective and commercially viable, the screen printing of objects such as aerosol cans or bottles requires extremely reliable and accurate machinery. In addition, the printing machine needs to be very carefully set up by the operator for each separate object – until now a lengthy and complicated manual process requiring accurate mechanical setting.
Retek Engineering is a manufacturer of rotary screen printing machinery for cylindrical objects, from small items such as pens up to large canisters such as fire extinguishers. The company has overcome the need for laborious setting procedures with a can-printing machine that is also faster and quieter than other machines on the market. This has been achieved by installing two Toshiba T1 Series expandable block style programmable controllers to regulate six different motors within the machine. The control system was sourced from TM Automation, whose engineering team oversaw all of the, initially complicated, programming.
Most printing machines of this type consist of a conveyor indexing system that transports the cans to the screen-printing head. This conveyor is steplessly variable between the machine’s capacity limits in respect of the lengths and diameters of cans of bottles to be printed. The fine control of the conveyor is achived via servo motors whose amplifiers are controlled by the Toshiba T1 series PLC. The programming interface is through an easy to use operator panel that has a touchscreen display.
The conveyor has `V’ attachments fitted to the chain guides, keeping it in a stable position as it is taken to the register. To find the can’s seam, the can is rotated past the sensor and then indexed back to it. The can’s dimensions relative to the seal are downloaded by hyperlink to the machine’s MMI – in this case a touchscreen ProFace GP370 – so the conveyor, register and printing head all `know’ the can’s dimensions and where to position its print.
Once the conveyor has indexed the canister, the component is lifted and registered, ready to print. The lifting mechanism is controlled by pneumatic cylinders and registered by a DC servo drive with the assistance of fibre-optic photoelectric sensors. The print head is also operated by pneumatic cylinders or, where applicable, directly by DC servo drives. On completion of print, the component moves onto a carousel that carries the parts through an ultraviolet curing system. This carousel is also controlled by a DC servo motor, synchronised by the Toshiba T1 series PLC.
After the print, the conveyor takes the can to a curing unit where the can is rotated under a UV lamp to cure the print. Again here, the machine relies on pre-programmed dimensions to ensure optimum drying. The whole UV system rotates so the finished object can be emptied into a bin or on to another conveyor via a pick and place unit, ready to go on to a separate filling process.
To complete this range of functions, the machine has six motors controlled by two Toshiba PLCs. A T1-28 PLC operates the drying system’s three motors – one to index the conveyor, one to index the UV drying carousel and the third to spin the container under the UV lamp. A T1-40 with a 32 I/O point (16 in and 16 out) expander unit controls the rest of the machine. This consists of a motor to raise and lower the conveyor, a motor to adjust the conveyor’s sides for length of the can in the setting mode and a motor to rotate the can at the register. Each of the motors is a low voltage brushless DC servo motor, run via an analogue card. All three control blocks are programmed via the Pro-face touchscreen.
By performing all of the programming via the MMI, various different modes are available: a cycle mode, a step mode and an automatic mode. The cycle and step modes allow the operator to undertake each movement individually, so he can ensure the first can in a run is properly set up before turning the machine over to an automatic run. Once set up, the machine can print and cure between 1200 to 1800cans/hour.
Toshiba International Tel: 0181 756 6000