The High-Speed Challenge
Time to market is critical in the medical device industry. Manufacturers depend on automation to increase production rates, decrease per-unit-cost and improve quality. However, high-volume assembly presents technical challenges to engineers.
In the medical device industry, "high volume" is usually much higher than what automotive and appliance manufacturers consider it to be. Indeed, the annual quantities of high-volume medical products are typically measured in the tens or hundreds of millions. Assembly rates for individual mach-ines can be several hundred parts per minute. That rapid throughput speed is usually the domain of a special class
of equipment known as continuous motion machines.
The majority of medical devices are disposable, single-use products. Most of their components are made from extruded or injection-molded polymer. Metal is used for springs and needles. Glass is used for vials and syringes. Many of these devices are involved in fluid management functions, such as intravenous (IV) systems, drug delivery and filtration. Assembly operations conducive to sealing, such as adhesive bonding and ultrasonic welding, are more popular than mechanical fastening.
Numerous technical challenges occur when automating the medical device assembly process. For instance, subtle variances in material properties can dramatically affect the performance of a high-speed machine. Many problems occur with automated part distribution and transfer. Components must be oriented and fed by vibratory or centrifugal bowls and then transported from the bowls rapidly along tracks or conveyors. Typically, the feed system must operate at least 25 percent faster than the required output of the machine to minimize production losses due to occasional jams or hang-ups.
Unfortunately, several factors undermine the smooth, uninhibited, high-speed handling of medical device components. These arise primarily from the nature of the components themselves, such as their unique geometries. Medical devices are prone to common feeding concerns, including shingling and orientation problems. They also tend to have odd-shaped and asymmetrical components that are sensitive to high-speed feeding.
Material properties also affect high-speed part feeding. First, there are the frictional characteristics of the material. For instance, the elastomeric parts commonly found in IV systems tend to have a high degree of stickiness that limits the feed rates of bowls and creates blockages in feed tracks. These frictional properties often vary because of ambient temperature and humidity. In addition, additives or other chemicals embedded in the plastic can migrate to the surface and cause dramatic changes in machine performance.
Rigidity also is important. Less rigid parts tend to jam in the feed system. Components such as tubing and sleeve stoppers are classic culprits, because of their flexible nature.
Static electricity is another common problem due to widespread use of plastic components. In a bowl feeder, the frictional contact of parts with each other, as well as with the bowl itself, can generate static charges that are not readily dissipated by simple grounding techniques, given plastic's nonconductive nature. These charges can cause parts to stick to each other or to elements of the feed system, particularly if their ratio of surface area to weight is relatively high.
Fragility is a common characteristic of some medical devices, such as glass syringes. It also can be a factor in plastic components. Breakage can occur as a result of rough handling in hoppers and feeders. And, in high-speed feed tracks, components can collide with each other.
Parts feeding challenges can be solved several different ways. Where friction or lack of part rigidity limits feed rates, multiple feeders can be used along with a special converging mechanism to gather the incoming parts and transfer them to the main machine. In addition, gravity can sometimes be used as a supplement to air or vacuum to help deliver parts in a feed tube or track.
Static electricity can be minimized in varying measures with the use of ionized air blown into the bowls or feed tracks. This neutralizes the charges built up.
When fragility is a concern, prefeeders can be used to limit the number of parts in a bowl at any given time. Metering cylinders are used in feed tracks to control part acceleration and reduce the risk of breakage due to high-speed collisions. Part nests can be lined with shock absorbing materials.
Another technique is to carry out the assembly on a robust carrier or "puck." It can ensure that the component does not continue to create problems if it must be transferred from point to point downstream in the assembly process.
Regardless of the particular solution employed, manufacturing engineers should supply the machine builder with representative samples of the full range of component variants that can be expected. Unfortunately, material problems are often unpredictable and often do not surface until the machine is at an advanced stage of construction. Extensive prototyping is extremely important.
High-speed medical device assembly applications typically use adhesive bonding and ultrasonic welding. However, the bonding methods favored for medical devices require a longer assembly time, which makes it difficult to achieve faster assembly rates.
Bonding applications require time to apply the adhesive, time to assemble the components to be bonded, and time to cure the adhesive. Ultrasonic welding requires time to bring the energy applicator to the parts, time to transmit the energy and flow the material, and time to permit the bond to form. In a high-speed process, where assembly rates of several hundred parts per minute are required, these times can be significant drags on throughput.
Bonding methods commonly used for medical device assembly have operational times that hinder high throughput. One solution is to replicate the long operations and place them in parallel assembly lines. However, delivering parts to a series of parallel stations can be a tricky matter in a high-speed process. Since many medical device components are inherently difficult to feed and transfer, parts handling problems make this an inappropriate solution.
In many cases, the bonding operation can be incorporated into a continuous motion assembly turret, allowing the effective paralleling of the operation while not exacerbating the already difficult feeding problem. In a continuous motion turret, a given operation takes place during the turret's rotation. The time available for the operation is a function of the rotational speed of the turret. Since the overall throughput of the turret is equal to the rotational speed multiplied by the number of stations in the turret, the rotational speed can be relatively slow while throughput remains high.
Numerous demographic factors indicate long-term growth for the medical device industry. For instance, aging populations in developed nations, increased demand for medical services in developing countries, and new advances in medical technology will spur future demand.
Medical devices will continue to be assembled with components that present challenges to high-speed feeding and handling. There also is an ongoing shift from PVC to other materials. However, this will be a slow transition, so assembly systems may have to handle PVC and non-PVC components interchangeably.
Ongoing pressure to reduce costs will demand even faster high-speed machines. Builders and buyers of capital equipment must work together to solve technical problems and jointly manage inherent risks.