New plastic brings quality leap for valve systems

Aventics changed the plastic used for its dialysis unit valve systems. However, the pneumatic nutrunners previously used exceeded the specified torque values. Threads were stripped and the reject rate rose drastically. The solution was a Microtorque screwdriver. Using this tool, the new material can be assembled in a secure process

“Pneumatic components which are robust and compact are becoming increasingly important in the life sciences sector,” says Daniel Gebert, who is responsible for quality management at Aventics GmbH in Laatzen. “The health sector increasingly needs solutions for low-pressure applications with a high degree of electrical and functional integration,” he adds. As an example of his company’s extensive product portfolio, he describes a dialysis unit valve system: “Components like this combine precision with extreme reliability. We are continually improving our products.”

Change in plastic resulted in loose screws

In the course of the continuous further development of its products, Aventics decided to use a new type of plastic for the base plate of the system. However, in the pre-series stage, the screws often stripped their threads. Daniel Gebert launched a problem solving campaign; the engineering team took a very close look at the entire application and soon found the cause of the problem. The pneumatic straight nutrunners previously used were simply not suitable for the new base plate material. As a result of their design, tools of this type can only tighten screws with a fixed torque value and at a fixed speed. “Because of the inertia of the tool, the threads were simply stripped when the self-tapping screws were tightened,” Gebert says. Although the new plastic was more suitable from the chemical point of view, it was also more sensitive.The plastic component previously used had been thicker, tougher and stronger, and was not damaged if the extremely low specified torque value of 18 cNm (= 18 centi-Newton-metres or hundredsof a Newton-metre = 0.18 Nm) was repeatedly exceeded. The change to a new material gave Aventics cause to rethink the tightening systems previously used with a view to ensuring a secure assembly process.

medical pneumatic valves assembly process

“Compared with hand-held pneumatic straight nutrunners, the Microtorque system is much quieter and more precise,” says Aventics employee Frank Straubel. (Photo: Atlas Copco Tools)

“We had already used pneumatic tools for 20 years and had never had any problems before,” Daniel Gebert explains, “but we had also never had such a complex, iniaturized assembly application before.” The application calls for the installation of 22 miniature valves complete with filters and sleeves on a base plate with an overall size of about 12 cm x 14 cm using a total of 44 self-tapping M1.6 screws with Torx-T2 heads. The assembly process is followed by 100% inspection. “If a defect that requires reworking or replacement is detected, the additional labour cost for each individual component is already about €20,” Gebert reports. Aventics was pleased to take up the suggestion by Atlas Copco Tools to use a newly developed Microtorque spindle in this application. The spindle, which is digitally monitored and current-controlled, surprised the Aventics team with its wide working range of 12.5 to 50 cNm (corresponding to 0.125 to 0.5 Nm) as well as its ease of operation and versatility.

Sophisticated tightening strategy outwits the material

Although the self-tapping screws with a diameter of 0.93 mm only have four and a half thread turns, the Microtorque spindle, type QMC41-50-HM4, completes a three-stage tightening process over this very short distance. Daniel Gebert describes the process in detail: “At the beginning of the tightening cycle, the screwdriver insert turns at a reduced speed to insert the tool into the Torx-T2 screw head. Following the completion of this insertion stage, the system switches to the rotation angle stage and the screw is tightened at 500 rpm until the head makes contact with the material surface. As soon as this point has been reached, the spindle speed is reduced drastically to 210 rpm and the screw is tightened to the final torque value of 18 centi-Newton-metres in a torque-controlled stage.” As the rotation speeds can be freely programmed in all stages, the Microtorque system can adapt to a wide variety of tightening applications. Optical and acoustic signals on the screwdriver and the controller give the operator an overview of assembly results at all times. “Following the changeover to controlled, monitored tightening, the reject rate fell practicably to zero,” production planner Ayhan Horoz reports. Up to the late autumn, the system had completed 80,000 assembly cycles without any problems.

Keeping a close eye on quality with detailed tightening plots

The newly developed system from Atlas Copco can do even more, Gebert emphasizes: “The Microtorque controller is extremely compact and the graphics functions are just fantastic. We only need to connect an ordinary PC and the controller can print detailed curves and tightening plots. “Visualization is clear and unambiguous. Through the analysis of tightening plots, Aventics can now interpret the tightening operation in much greater detail and understand what happen precisely in each stage of the process. “That was just not possible with pneumatic tools.”

Tightening plots indicate quality of components supplied

aventics assembly process

Using a controlled Microtorque screwdriver system, Frank Straubel assembles pneumatic components for the life sciences sector. He tightens valve systems using 44 extremely small M1.6 screws. The number of tightening operations is automatically counted. (Photo: Atlas Copco Tools)

As an additional benefit of the screwdriver system, Ayhan Horoz reports that it provides assistance for assessing the quality of components purchased from suppliers. “The graphs give a precise indication of fluctuations in insertion resistance. This can occur if the diameters of pre-drilled threaded holes in the injection-moulded components are outside the specified tolerances. During screw insertion, the Microtorque screwdriver detects changes in friction values in the micro-range.” Aventics can compensate for such fluctuations by adjusting the tightening parameters or by rejecting the components concerned and submitting complaints to the suppliers if the values are too far outside the specified range. Thanks to this approach, even minor fluctuations in screw coatings or the material properties of plastic parts are revealed.

Operators appreciate ergonomic benefits

aventics

Aventics Quality Manager Daniel Gebert (left) and production planner Ayhan Horoz regularly exchanged experience with the experts from Atlas Copco during the trial phase with the new Microtorque spindle. “Thanks to the three-stage tightening process, we have reduced our rejection rate drastically and are now producing more precisely than ever before,” says Gebert. (Photo: Atlas Copco Tools)

Apart from the significant improvement in process security and technical performance capabilities, acceptance by the workforce is also a key factor. Compared with pneumatic nutrunners, the Microtorque system is virtually silent. In addition, the small QMC41-50-HM4 spindle is supported by a linear arm, which further relieves the burden on the operator. As a result, operators soon welcomed the new equipment. “Thanks to the three-stage tightening process, we achieve hightorque precision. As a result, we apply the correct pressure, which is so important for tightening, from the first to the last tightening operation,” says Daniel Gebert, who purchased the versatile Microtorque system following successful trials. “We are now carrying out assembly work better than ever before.”