Cycle Optimization

In the deployment of Wexxar higher speed case formers like the WF30, we have encountered user reactions to the speed of individual motions in the case former cycle as appearing to be insufficiently fast for the expected case delivery rate. As a result, machine adjustments are manipulated with the result that individual motions are unnecessarily accelerated in order to provide what appears to be a faster machine cycle. In reality, this makes the machine ‘hurry up and wait’, albeit with added stress on the accelerated machine components, leading to excess noise and premature wear. Wexxar has achieved a higher case delivery rate by overlapping and synchronizing individual machine motions as much as possible, which actually allows some cylinder motions to so that they arrive ‘just in time’ to perform the required action. The result is a smoother, quieter machine cycle with less wear and tear on components that doesn’t sacrifice the formed case delivery rate. It is the machine equivalent of working smarter, not harder.

Wexxar has implemented a number of industry best practices with regard to PLC programming cycle time optimization for each of our WF Series machines. These practices are meant as a guideline for optimal machine tuning conditions for salespeople, engineers, and end users. The intent of this initiative is to reduce machine downtime, to maximize the service life of machine mechanisms, to educate machine maintenance personnel, to differentiate between solenoid valves and servo-valves, to clarify the factors influencing mechanism speeds, and to introduce advanced feedback features under development.

A PLC permits WF Series machines to perform multiple operations at once, which allows service technicians and machine maintenance personnel considerable control over mechanism speeds and the degree of overlap between machine functions. Maximizing overlap within the case forming cycle results in faster cycle times, which allows a higher CPM rate.

The machine’s PLC also allows overlap between cycles. A machine running at 30 CPM without external overlap will form one case every 2 seconds, or 2000 ms. External overlap permits cycles to run as long as 4000 milliseconds (including 1200 ms internal overlap and 800 ms external overlap per cycle) without reducing the machine’s throughput.

The cumulative time savings gained through the utilization of internal and external overlap allows the machine’s mechanisms to run slower while preserving the output rate. This prolongs the service life of machine components. Components on machines operating at 30 CPM with a cycle time of 2000 milliseconds will wear out much faster than machines that are calibrated correctly.

The graph below shows a WF-30T machine with a cycle time of 2830 ms, but with an average cycle time of 2000 ms due to overlap. Manually adjusting the solenoid valves (or adjusting the servo settings) will result in revised live on-screen feedback, enabling maintenance personnel to calibrate the pneumatic functions more precisely. If the times for each action within a cycle are added up sequentially (without internal or external overlap), each cycle would take 4060 ms. Overlap allow machine operators to reduce cycle times by up to 50% by employing optimal overlap programming strategies.

Wexxar machines incorporate two types of pneumatic function. Forward motions are performed under load, while return motions operate under a ‘no-load’ condition. Examples of load-bearing forward motions include ‘Injector Up’, ‘Jaws Open’, ‘Mast Down’, and ‘Magazine Pusher Out’. The faster a load-bearing pneumatic function is performed, the more likely there is to be a case jam. Moreover, slowing the pneumatic movements down will also reduce wear and tear on the machine. Therefore, it makes sense to maximize the time allocated to load-bearing pneumatic operations.

The opposite is true of ‘no-load’ operations, such as ‘Injector Down’, ‘Jaws Close’, ‘Mast Up’, and ‘Magazine Pusher Back’. Time lost due to maximizing the duration of load-bearing operations can be regained during the return stroke of any cylinder. Return strokes can be calibrated to run as fast as reasonably possible without resulting in case jams or causing premature component failure.

Due to the high CPM rates required by the WF30, servo-valves are used in place of solenoid valves for the mast and jawplate cylinders in order to precisely control the speed and deceleration of the cylinders. This allows a ‘soft’ stop during load-bearing cylinder motions, which greatly reduces case jams in the forming area and allows a prolonged service life. Servo-valves also permit the machine operator to program faster return strokes. The upwards motion of the mast assembly can be programmed to be faster than the downward stroke, and does not require the ‘soft’ stop deceleration that the downward motion requires.

End-position controllers permit pneumatic cylinders to stop or alter their speed mid-stroke and to precisely control the speed of the cylinder at each stage of its stroke, whereas solenoid valves rely on exhaust air flow control to stop the cylinder. Solenoid valves do not allow any mid-stroke speed or acceleration adjustments, while servo-valves allow for multiple mid-stroke adjustments. The speed of servo-driven pneumatic functions at each stage of the cylinder motion can be precisely set by adjusting the machine parameters on the HMI touchscreen such that mechanisms can be sped up or slowed down without having to manually adjust solenoid valve settings.

Case height, width, weight, corrugate thickness, flute size and scoring all affect the speed of the pneumatic cylinders. A significant increase in case size or weight makes it more difficult to lift, rotate and fold the cases. For tall or wide cases, the load-bearing ‘mast down’ and ‘jaws open’ strokes must be slowed down to prevent case jams. Case corrugate thickness (especially double or triple wall corrugate) will also add weight and make the case less pliable, making it more difficult to fold the flaps correctly and to perform other pneumatic functions.

Wexxar is currently adapting the PLC and touchscreen to provide precise pneumatic function speed feedback down to the nearest millisecond, visually representing internal and external overlap and providing numerical values that can be used to fine tune the machine. The touchscreen will also show optimal values that can be compared to the actual mechanism settings. This feature is especially valuable because the human eye cannot resolve time differences under 300 milliseconds. Because many pneumatic functions are performed in under 300 milliseconds, maintenance personnel can rely on machine feedback to calibrate solenoid valves much more precisely than they would otherwise be able to.

The machine will also be programmed to alert the machine operator when individual pneumatic operations fall outside of preset parameters, preventing the need for expensive service work and allowing the machine to be optimized in the field, thereby avoiding downtime due to machine component failure.