Precise gripping proven a million times

High quantities, extreme quality requirements and keen competition are shaping the production of injection-moulded parts for the automotive industry. To ensure success when performing this delicate balancing act, Gevelsberg, Germany-based Denk Kunststofftechnik relies on Schunk grippers in its self-designed handling systems.

“Around 60% full automation, and the trend is rising rapidly; we are no typical plastic injection moulders,” states Konstantin Spenst, head of automation technology at Denk. “Automation is the only way to output between 500,000 and well over 2 million parts per year, and check them 100%.
“Precision and verifiability are decisive factors for us,” he adds. “The newer the system, the more steps are monitored. Recently we wrote a chain of 800 steps, but only 30 of them were paths. Everything else was just commands of pneumatic components or queries with alarms.”
Around 60% of the range was operator messages such as “end position not reached”, “part lost” or “part not correctly mounted”. What is decisive nowadays is that the user receives as much information as possible, because in the case of a malfunction, the reason must immediately be identified. This factor is particularly thanks to the precise monitoring of grippers, as only in this way can damage to the tools be avoided, along with system downtime.
At Denk, all parts are first monitored to see that they are correctly mounted. Presence monitoring is then deployed immediately before mounting the tool; ultimately checking immediately after removal and finally during storage. This methodology is the only way to prevent parts from being lost in the handling process, causing an expensive tool collision.
The gripping systems that are increasingly being conceived, designed and built independently by Denk since the beginning of 2010 are of double significance. Each system must ensure reliable handling, while at the same time facilitating reliable monitoring of the relevant process steps. Historically, the latter had proved to be a challenge, because not all gripping system components available on the market permanently achieve the required level of precision and process reliability, not even the high-end ones.
“With our gripping systems, precise monitoring is very important, as with 1 mm play in the gripper jaws and a closing stroke of 3 mm, reliable monitoring is not possible,” says Spenst. “It repeatedly transpired with grippers from various manufacturers that sensors delivered unreliable results because the play in the jaw guidance was so large. Over the course of time, we have realised that Schunk grippers can be queried very reliably, as they work very precisely.”
The Schunk MPG-Plus grippers for small components are particularly advantageous in this respect.
“For parallel grippers, not only an end point can be queried, but also a centre point, which can be set very easily,” says Spenst. “If a component gets lost, the end switch is set to zero and reports the loss. When handling metal parts, an inductive sensor usually monitors whether, and at what distance to the sensor, a metal inlay is gripped.”
Besides inductive monitoring, the Schunk MPG-Plus miniature gripper can be monitored by integrated and programmable magnetic switches, by means of which a high degree of flexibility can be attained. Compared with similar modules on the market requiring the same input, the gripper produces a significantly higher output – says Schunk – paving the way for ever smaller and more efficient systems.
The high-performance, individually adapted junction roller guide ensures high load bearing capacity of the entire guidance system, minimal wear and a long life span. Components made of special, high-performance aluminium are used internally. In order for engineers to be as flexible as possible in process and system designing, the module can be screwed through and tightened both on the side, as well as the bottom.
“The grippers must be as light and as small as possible,” stresses Spenst. “If I have a heavy gripper, I have to move slower. This costs cycle time and cycle time is real money.”
On average, the loss of one second of cycle time costs between €2000 and €4000 each year. To keep warehousing as low as possible, Denk uses the miniature gripper primarily in sizes 40 and 64, in order to dock gripping systems on to the tools in a stable manner when loading. For large strokes, the Schunk KGG 80 two-finger parallel gripper is used. This gripper even allows handling of T-nuts with adhered chips. For larger, round components, Denk uses the Schunk PZN-plus centric gripper, which due to patented multi-tooth guidance and manually ground base jaw guidance, combines a high level of precision with low wear.

For Spenst, the standardisation of the gripping system components is elementary: “We want to keep inventory as low as possible to ensure the universality of our systems, and to react flexibly in the event of damage. When we design grippers with Schunk, we know that we will be able to handle 2 million parts a year for seven years, without losing any precision or having to fit new grippers.”
For normal use without overloading the grippers, these cycle rates can definitely be achieved and sometimes even exceeded. The temperatures in the injection moulding machines are generally non-problematic for Schunk grippers. Only in critical applications do the designers at Denk contact the manufacturer and ask about maximum permissible values.
Despite the high level of gripping systems expertise, it is not possible for Denk to manufacture the module components in-house.
“We could certainly build parallel grippers, but they would cost 20 times as much and would still not achieve the level of quality provided by the Schunk modules,” says Spenst.
After the relaunch of the PGN-Plus-P parallel gripper, Schunk has now transferred the features of its flagship product to the three-finger gripper series. The Schunk PZN-Plus-P centric gripper is now equipped with a permanent lubrication unit in the multi-tooth guidance system which, under normal, clean working conditions, ensures lifelong maintenance-free operation.
With short strokes in particular, the continuous lubrication pockets are said to produce a fast and even distribution of lubricant along the entire multi-tooth guidance contour. By enlarging the supporting dimensions between the six load-bearing shoulders of the patented multi-tooth guidance system, higher moments can be accommodated and thus longer fingers can be used. In order to ensure fit accuracy, each individual base jaw is carefully ground (manually) and fitted to the individual housing. Moreover, the large piston drive area helps to maximise gripping force.
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