Originally Published MDDI January 2002

ASSEMBLY

Reaping the Benefits of Continuous-Motion Assembly

Continuous-motion assembly engineering can be performed concurrently with new product design or to make existing manufacturing systems more efficient.

John Giacopelli

Increasingly, manufacturers of medical devices are choosing continuous-motion systems for the automated assembly and solvent bonding of a variety of products—cannulae, check valves, IV-set components, filters, and syringes, as well as such manufacturing-related processes as force measurement and leak and flow testing. Why? Continuous-motion systems offer several advantages over equipment that operates using intermittent motion, including improved product quality and output rates.

Both the transfer of components onto a continuous-motion production line and their passage along it are smooth, protecting fragile parts and assemblies against damage. The nonstop motion also reduces the acceleration required to transfer parts from the track to an assembly dial or other operating platform. For typical operations requiring 4 to 6 seconds to accomplish, speeds in excess of 400 parts per minute are feasible.

And most important, the accuracy of continuous-motion assembly is unsurpassed. Because the tooling never loses contact with the individual components, part alignment is maintained during assembly. This ensures that device specifications will be met consistently. Finally, considering the complex and unique tasks they perform, continuous-motion machines require comparatively little maintenance and experience minimal downtime.

Device manufacturers can maximize their production capabilities by designing new products with continuous-motion production in mind and by coordinating with an assembly equipment supplier in the early stages of the design and development process. Continuous motion can also be engineered into existing systems to speed manufacturing operations, improve product quality, and reduce costs.

Designing for Continuous-Motion Systems

Most device manufacturers recognize the importance of seeking information from their customers very early in the product development process, possibly even before design work has begun. Among the rationales for this practice is the fact that the earlier manufacturers identify design features that may compromise device performance, the less expensive and time-consuming it is to make changes. Like customers, automated continuous-motion machinery suppliers can be an invaluable source of information that should be considered and evaluated before the company commits to a final design.

Unfortunately, some manufacturers bring samples or prototypes to the equipment supplier after the design is complete and the production molds are tooled, expecting the vendor to create a system that will produce the device efficiently. They often discover that goal cannot be achieved. Perhaps just one component is the cause of a recurring assembly flaw.

The automation supplier can probably recommend a slight change to the part that can alleviate the problem, but, at that point, such a change may delay the product's market launch and be too expensive to justify, forcing the manufacturer to live with production equipment that will never operate as well as it might have. Product reject and rework levels will be high, necessitating stringent process controls and contributing to high manufacturing costs.

In contrast, by involving the equipment supplier from the start, a manufacturer can reduce the risk of design flaws, minimize the time to market, and hold down overall production costs. Throughout the design process, the equipment supplier can draw upon its staff's expertise to make recommendations about part features that can affect machine operation. The end result will be component assemblies that fully satisfy the performance, cost, and aesthetic requirements of the new product, and a production system that operates with the highest possible efficiency.

Several design features present special challenges for these systems. Simple changes in a component's design, however, can make a difference between a part that is difficult or easy to produce using continuous motion systems.

In general, springs are one of the most difficult kinds of parts to orient, track, and control during assembly. Tangling in the bulk hopper and feed system is a common problem.Slight changes in the winding, however, such as closed coils at the ends or in the middle, can make springs much less prone to tangling and greatly improve the feeder performance.

To improve tracking and pickup onto the assembly dials, the configuration of the end coils and the wire cut at the ends can often be modified without affecting the product's performance or overall cost. Such design changes can also significantly reduce jamming and subsequent machine downtime. Even the direction of winding can be a factor worth considering as a means to increasing assembly system efficiency.

Another factor to consider is the need to radially orient parts for assembly. One method is to feed parts onto the track in a random orientation and then rotate each part on an assembly dial, using the machine tooling to stop the part in the correct orientation. In such operations, the orientation feature on the part must be of adequate strength to stop the rotation and be located and toleranced according to the radial accuracy required.

In an alternative technique, an external feature is used to select the parts in the feeder and to maintain proper orientation as they pass along the track and through the assembly machine. This eliminates the need for extra tooling in an assembly dial, but may make the feed system more complex than necessary. The size and location of the external feature, how well the tooling can maintain orientation of the parts, and the effect of weight bias on feeding-system selection consistency should all be considered carefully when the continuous-motion equipment is being engineered, as well as during the product design process.

Figures 1–3 provide examples of part designs that address the issue of radial orientation. Figure 1 shows a square part with an offset hole. Without this hole, the centered part could not be oriented. Figure 2 demonstrates how a flat exterior surface can be used to effectively orient an otherwise round part in a feeder system, and Figure 3 shows how an internal feature can be used to orient an otherwise round part in an assembly machine.

Finally, such factors as maintaining tight tolerances on press fits and the likelihood of a warped or bent part can affect the performance of assembly equipment. Usually, these problems are subtle and do not change the cost to mold or manufacture the part significantly. Figures 4 and 5 illustrate how a simple design feature can improve the operational efficiency of the assembly process. The lead-in chamfer in the design shown in Figure 4 compensates for any misalignment during tube insertion, whereas the chamferless tubing shown in Figure 5 must be accurately aligned to be assembled successfully. Any misalignment will result in part damage or an out-of-specification assembly.

Redesigning Existing Systems For Continuous Motion

In addition to choosing continuous-motion equipment for their new products, medical device manufacturers can integrate such machines into existing automated production lines to assemble established products faster and more efficiently. Systems can be redesigned to manipulate and mechanically position components so that they can be assembled, leak-tested and vision inspected, and accepted or rejected at speeds up to 1000 parts per minute.

For example, one medical device manufacturer achieved significant savings in production time and space when its production line was redesigned. Previously, 18 indexing machines were required to add round urethane inserts to complexly shaped plastic tubes, apply alcohol, and slit the inserts. The system produced finished products at a rate of 40 parts per minute.

In the redesign, the indexers were replaced by two continuous-motion machines, each capable of producing 400 ppm, that manipulate and control the tubes in an upright position and transfer them in a carrier from dial to dial. As a result, the production rate increased to 800 parts per minute. The system also enables the device manufacturer to quickly and easily change over from one tube configuration to another.

Conclusion

Continuous-motion assembly equipment offers medical device manufacturers many advantages when compared with intermittent-motion systems. Companies that choose this technology for a new medical device can benefit from bringing the equipment supplier into the design process early. Given the opportunity to share their experience with product designers, the vendor's application engineers can suggest part design features that will maximize machine efficiency, minimize equipment and manufacturing costs, and reduce or eliminate delays to market. Cost savings and improvements in production rates and product quality can also be achieved by redesigning existing systems to include continuous-motion machines.

John Giacopelli is a vice president at Haumiller Engineering Co. (Elgin, IL).

Copyright ©2002 Medical Device & Diagnostic Industry