Striking a Balance in Medical Molding

GUIDE TO OUTSOURCING: MOLDING

Three basic issues usually play a dominant role in the molding of medical parts: design, quality, and cost. The design informs the functionality and aesthetics of the device and is often at the core of the device's competitive advantage. But design can also play a major role in molded part quality and cost. Design features influence factors such as mechanical performance, dimensional control, cosmetics, mold cost, and part moldability.

Part quality not only has a direct bearing on device performance and potential liability, but it can also influence the perception of device quality and consumer confidence in the brand—two critical qualifiers for medical parts. Quality requirements can restrict part design and add to molding and quality control costs. In addition, efforts to reduce costs are often at odds with efforts to maximize part performance and quality. Because design, quality, and cost are related, the optimal molded part must strike the best balance among the three.

Design

The term molded medical parts refers to a broad family of device components as diverse as tubing connectors, blood oxygenators, monitoring equipment housings, face masks, and home healthcare devices—each with its own design criteria and challenges. For example, aside from the need to withstand periodic disinfection, housings for monitoring equipment hardly differ in basic design functionality compared with housings for typical consumer products. As a result, standard design guidelines are typically easy to apply.

Unique medical applications, such as blood-contact devices, often place very specific and demanding limitations on the part geometry, and they can be much more difficult to design in accordance with good molding and design practices. Understandably, many design efforts focus primarily on functional performance, while molding and manufacturing considerations take a backseat. However, functionality optimization can drive designers to violate good design practice.

Part design guidelines, as published in resin supplier literature, are often established based on decades of molding observation and experience. Designs that adhere to such guidelines typically experience fewer molding problems, retain better material performance, and mold more efficiently. Common violations of design fundamentals in medical parts include nonuniform wall thickness, sharp corners, and inadequate draft.

Good molding practice calls for the nominal wall thickness of a part to vary no more than 25%. As thermoplastics cool and shrink in the mold, thinner wall sections tend to solidify first and undergo less shrinkage than sections with thicker walls. These varied solidification times and shrinkage levels create stresses within the part, particularly at the boundary between two thicknesses. Elevated stresses can reduce chemical resistance and mechanical performance and can lead to premature part failure.

Shrinkage-induced stresses along the boundaries of long thickness transitions can cause a component to warp and distort. Small, isolated areas of increased thickness can be difficult to pack during molding, and they may exhibit sinks or voids. This is especially true if the location of the thick feature and gate result in thin-to-thick filling, in which flow from the gate must pass through restrictive thin sections before reaching the thick feature. Process adjustments to correct thickness-related problems can narrow the processing window and lengthen the molding cycle time and cost. The common solution is to maintain a uniform wall thickness.

Sharp inside corners act as stress concentrators and can dramatically reduce mechanical properties such as impact strength and fatigue resistance. Polycarbonate, for example, is often chosen for its superior toughness and impact performance. To retain its toughness, design guidelines call for inside corners to have a 0.01-in. minimum radius, particularly for corners subjected to impact or fatigue loads. Drawings that call out a maximum permitted radius are insufficient because they allow the mold maker not to add a radius. When no radius is added, inside corners typically end up with a radius of 0.005 in. or less. The radius should be called out as a range, such as 0.010–0.015 in.

Draft is the angle or taper added to the mold steel to allow the part to be removed from the mold without excessive force or damage. Allowance for this draft must be incorporated into the part design. Inadequate draft can lower part quality and molding efficiency.

As the part is separated from the mold, dragging of the part surface along insufficiently drafted cores can induce surface stresses and lead to stress cracking during sterilization or in-use exposure to chemical agents and applied stresses. Ejection damage due to inadequate draft adds to scrap and quality control costs. In addition, steps to reduce ejection stresses, such as adjusting ejection speed, material, or cycle time, can increase part cost. From a performance standpoint, draft is often not desirable; but to facilitate efficient molding, provisions should be made for at least some small amount of draft.

Medical part molding presents design challenges beyond those covered by the standard guidelines. Core shift, weld joint design, and long-term loading issues, for example, are particularly problematic in medical design. Options to improve moldability are often overlooked because the costs associated with poor moldability are difficult to quantify. However, if the true costs were known, more-informed decisions could be made.

Quality

Quality priorities vary by application. Complex, fluid flow–based medical devices, such as blood oxygenators, pumps, and filters, rely heavily on design to achieve proper flow dynamics and performance efficiency. Quality efforts in such devices must include shape optimization. Tight tolerances may be more critical to function in other devices. Because appearance is often perceived to be a reflection of part or device quality, aesthetic attributes such as uniform gloss, clarity, fit, and finish can be important quality concerns.

The prevention of part failure stemming from issues such as mechanical loads, molding deficiencies, or environmental exposure is a priority for every molded medical part. Most medical devices also require part-to-part consistency and the absence of defects, however they may be defined.

Optimizing functional performance is at the heart of quality and design efforts for complex medical devices, and it can define the market position of a device in a competitive marketplace. Trying to attain peak performance can also place excessive strain on the molding and manufacturing processes to consistently produce parts and assemblies that meet other quality requirements. The result can be a high-performance device that fails to reach full business potential because of quality-related production costs and delays. Ideally, manufacturers should consider and anticipate potential quality problems at the earliest stages of product development when corrective adjustments are easier and cheaper to implement.

Quality problems first become apparent when concepts and ideas finally take form as molded parts. Because the problems show up during molding, molding is often erroneously identified as the cause of problems. As mentioned earlier, design choices can reduce moldability and introduce quality issues. Mold design and construction directly influence molded-part quality. A good mold produces parts to specification under normal production conditions, over a broad processing range, and over the intended life of the mold. Plastic can penetrate gaps between poorly fitting mold components and leave a thin web of unwanted material, called flash, along parting lines and mold lines. Likewise, inadequate wear management in moving components of the mold can lead to misalignment and flash.

Runner systems in multicavity molds need to be balanced to reduce cavity-to-cavity part variations. Hot runners can contribute to molding and quality problems if they are not designed appropriately for the material being used. Molded medical parts, especially transparent medical parts such as filter housings, syringes, and dialysis canisters, require hot-runner systems that are free of stagnant flow pockets where material can linger and degrade. High-temperature plastics such as polycarbonate work best with externally heated hot runners with flow channel diameters sized to avoid excessive pressure drop. Many potential quality problems can be avoided by soliciting input from various sources such as the molder, the hot-runner manufacturer, the quality control department, and the resin supplier.

Resin selection determines many of the key quality and performance characteristics of molded medical parts. Basic material characteristics, such as toughness, clarity, and shrink rate, are important for many medical devices. Mechanical toughness and impact resistance are also key material properties for many medical devices. Figure 1 shows the broad range of Izod impact performance for a variety of common medical plastics.

Plastics in many medical devices must also meet special medical criteria such as those related to biocompatibility, sterilization, and resistance to medical chemical agents. Biocompatibility testing is expensive and time-consuming, so it is often prudent to choose a resin grade with a proven history of biocompatibility. Compatibility with various sterilization methods is also important. For example, gamma sterilization tends to yellow or color-shift many plastics. For this reason, special medical grades of polycarbonate resin, for instance, have been developed that shift from a transparent purple tint to a pleasing smoked tint after sterilization. In addition to avoiding yellowing, the color change also can function as a quality control to identify those devices that have undergone gamma sterilization. Selection of the right material is critical to maintaining molded-part quality.

Excessive molded-in stresses can reduce material performance and lead to part failure. Experimental techniques, such as solvent stress testing and photoelastic strain measurement, provide ways to check relative stress levels in molded polycarbonate. In solvent stress testing, sample parts are immersed for a set time in solutions that have been calibrated to generate stress cracks when surface tensile stresses exceed a known stress threshold. Experience or experimentation dictates the maximum stress that the part may have to endure to avoid premature failure over the intended part life. The effects of annealing, a postmold heating method used to relax stresses in molded parts, are often verified with such testing. Photoelastic techniques take advantage of polycarbonate's tendency to exhibit double refraction when under stress. When viewed between polarizing filters, the part reveals color bands that can be interpreted to determine stress levels throughout the part (see Figure 2). Plastics that work with such techniques offer valuable tools for verifying material and molded-part quality.

Cost

Various direct and indirect costs contribute to the true cost of molded medical parts. The obvious direct cost is a molder's quoted piece price, which is largely influenced by a part's design and quality specifications. The part cost also includes components tied to material, mold, and molder costs. These elements can spawn indirect costs. Although often difficult to quantify, these indirect costs are real and often quite significant.

The direct cost contributors are relatively easy to identify. The part design determines the volume of material per part and the complexity and size of the mold cavity. It can also limit the material and molder options. As such, part design directly affects the material, mold, and molding components of cost. The molding resin cost per part represents a direct cost. Accountants may differ in the way they handle mold costs, but mold cost is really a direct cost that adds a fixed amount to the cost of every part. Molder direct costs include press rate and profit, as well as costs associated with quality control, material handling, and secondary operations. These easily quantified direct costs are the most common targets for cost reduction.

Indirect costs tend to result from interactions with other cost elements and are more difficult to isolate. Choices made in part design, material selection, mold design and construction, and molder selection can affect part quality and molding efficiency, which in turn affects part and device cost. For example, a plastic that exhibits low levels of molding shrinkage, short mold-cooling time, and compatibility with various welding and bonding methods (e.g., polycarbonate) can be more cost-effective overall than a cheaper plastic that molds less efficiently, incurs higher quality-related costs, and is more expensive to weld or bond (e.g., polypropylene). Additional money spent on improved mold quality, better mold cooling, and hot-runner systems can improve molding efficiencies and reduce overall part costs.

Hot-runner systems, which deliver molten material directly to the part cavity, add to the initial mold cost. However, they eliminate regrind and scrap costs associated with conventional cold-runner systems. Mold cooling enhancements, such as high-conductivity inserts and cooling channel designs that closely conform to the part shape, add mold cost but can reduce the mold cycle time and reduce costs overall. Investment in a well-constructed mold can yield more parts over the life of a mold with fewer rejects and production delays. The cost-optimization process must consider not only the separate initial costs, but also the influence of the initial cost choices on the overall program cost over time.

Beyond such normal indirect costs are the more serious and less-tangible costs associated with the choices that influence medical part molding. The cost to cover warranty repairs or liability expenses, though difficult to predict, is easy to measure. Damage to the reputation of a product or company is much tougher to quantify, yet can be much more costly. In the medical industry, where the cost of failure is particularly high, a premium is placed on quality, reputation, and reliability. Cost-cutting measures that jeopardize these important attributes are risky. Failure to protect intellectual property can also be costly. A savings in mold or molding costs may not compensate for the loss of know-how or market edge.

In medical device molding, it is usually wiser to cut molded-part costs in ways that do not sacrifice quality or productivity. Instead of substituting a cheaper, lower-performance molding resin, computer-aided engineering (CAE) methods can be used to optimize wall thickness and remove unneeded material (see Figure 3).

Part-design consolidation and standardization can reduce mold and assembly costs. Investments in high-quality molds with advanced mold cooling and hot-runner systems can cut costs by reducing cycle time and material usage, and such investments can also reduce quality-related costs. CAE simulation techniques can save money by optimizing the part and mold design and by correcting potential problems. Selecting a reputable medical molder with the proper facilities, molding equipment, inspection equipment, processing expertise, and quality control procedures can save money in the long run by reducing quality problems, downtime, and delayed deliveries. These kinds of cost savings can actually improve part quality.

Conclusion

Competition and pricing pressures make it increasingly important for molded medical parts to strike the best balance among design, quality, and cost. This is best accomplished when the relationships among the three are considered at all stages of part and design development. Too often, design elements progress one after another, with molding and fabrication considered late in the development process when opportunities to correct problems and optimize the system are limited. To prevent this, the first step should be to pull together a team knowledgeable in the capabilities, limitations, and costs associated with molding for medical devices. Such elements are critical to the performance, reliability, and manufacturability of a device.

In addition, manufacturers must pay attention to both the direct and indirect costs of molding. The goal should be to cut costs without sacrificing quality and productivity, which may involve investing more in one area to realize a greater savings overall. The greatest benefits are obtained with a big-picture team approach that focuses more on cost interactions and overall costs.

Mark Yeager is principal engineer for Bayer MaterialScience. He can be contacted at [email protected].

Copyright ©2008 Medical Device & Diagnostic Industry