Michael J. Braeckel
Medical-device designers looking to cut costs, improve performance, and simplify processing should look at replacing metal and plastic components with parts made of liquid-crystal polymers (LCPs). LCPs eliminate costly machining, fill thin-wall designs, shorten molding cycles, and provide higher yields of stronger, more precise parts. With their design and processing advantages, LCPs can justify raw material costs which may be higher than that for other resins.
Raw material cost can be deceiving
How can medical designers capitalize on the design flexibility and cost savings possible with LCPs? To start, a fundamental concept must be understood: The cost-effectiveness of a resin lies in the finished cost per useable part, not in the price per pound of plastic.
Cost per part is a function of the raw material chosen, the design, and the processing requirements. One of these elements alone won’t tell the whole story. In one medical device, for example, raw material cost is roughly $0.07/part when molded in modified polyphenylene oxide (PPO) at $4/lb. When molded in Vectra LCP, the raw-material costs are around $0.37/part. But LCP, the higher-priced material, has processing advantages that ultimately generated a significant cost saving per useable part.
Vectra LCP processes much faster than PPO, so the cost of running equipment is lower because of faster throughput, which helps recapture some of the difference in material costs. However, in this case the molding accuracy and creep resistance of LCP generated the major savings.
PPO parts require annealing which involves lots of manual handling and temperature adjustments. The time and labor of this extra step adds $0.58 to the manufacturing cost of each piece and yields useable parts just 85% of the time. With proper design, LCP parts are ready to use as-molded and need no annealing. Looking at the entire picture, the overall cost using LCP is at least $0.30/part lower.
To understand the cost advantages possible with LCPs, one must consider the molding behaviors of different resins, which are determined in part by their crystal structure. Amorphous and semicrystalline polymers both have random microstructures when melted. Amorphous polymers retain that random orientation when they cool. Parts molded in amorphous resins consequently have good impact strength but relatively poor stiffness. To compensate for that poor stiffness, engineers often increase the molecular weight of the amorphous polymer. This not only boosts its load-bearing strength, but also increases its melt viscosity and makes it more difficult to mold in thin-wall sections.
Semicrystalline polymers also have a random microstructure when melted. However, as they cool, they form highly ordered crystalline regions surrounded by an amorphous matrix. The organized structure improves load-bearing strength and chemical resistance, but reduces impact strength. Compared to amorphous resins, semicrystalline polymers are generally easier to mold because they have much lower melting points, lower melt viscosity, and flow better in thin-wall sections.
On the molecular level, LCPs in the melt stage have rigid rods that remain ordered like uncooked spaghetti. The rods limit tangling of the molecular chains and instead molecules slide over one another under shear, giving the melted LCP a very low viscosity and letting it easily fill thin walls and intricate details. The resin consequently needs little pressure to make it flow. And because LCPs are highly ordered in the melt phase, they require little or no time to crystallize as they cool to a solid. Furthermore, LCP in the mold is stiff when still hot and can be ejected quickly. So compared to semicrystalline resins, LCPs offer even better processibility and higher mechanical strength.
LCPs, with their low heat of fusion, negligible time to crystallize, and high stiffness when hot, can have molding cycles half as long as those of ordinary thermoplastics. The PPO part in the earlier example, for instance, had a cycle time of 29 sec. The LCP alternative cycles in just 16 sec. Faster cycles save molding machine time, labor charges, and tooling cost per part.
The melt viscosity of LCPs is far lower than that of most competitive materials. At 1,000 reciprocal sec (a measure of shear forces and a function of how it is injected into a mold) Vectra A130 has a melt viscosity of 600 poise. By comparison, a medical grade of polycarbonate at 1,000 reciprocal sec has a viscosity of 2,900 poise, while the viscosity of a medical grade of polysulfone is 9,000 poise.
The lower melt viscosity of LCPs translates into higher flow in thin-wall sections. For example, Vectra A130 will flow 12 in. into a tool 0.5 in. 3 0.031 in. And it will flow over 50 in. into a tool 0.5 3 0.125 in. Many competitive materials flow only 60% of those lengths in such thin sections. Plus, some resins simply cannot fill a 0.031-in.-thick cavity at all.
With LCPs, a part 13.5-in. long 3 0.035-in. wide can be molded, excluding draft. To fill that length, most competitive materials require a width of at least 0.080 in. With 60% thinner walls, LCP can cut the amount of resin used and the material cost per part.
More important than simply filling thin sections, LCPs have the mechanical properties to make thin-walled parts strong and stiff. Unlike most resins, LCPs get stiffer as they get thinner. That’s because as LCP fills a mold, surface molecules align with the flow and form a skin comprising about 15 to 30% of the part thickness. This reinforcing layer gives the part high flexural and tensile strength and modulus. And the thinner the part section, the greater the proportion of reinforcing skin, resulting in improved mechanical performance.
To get better mechanical performance out of plastic parts, designers commonly increase wall thickness. But thicker walls use more resin, and make bulkier parts. With LCPs, however, higher mechanical performance in thinner walls reduces material cost.
If specific stiffness is calculated as flexural modulus divided by resin cost adjusted for density, LCPs outperform other plastics. In terms of “psi per penny,” LCP performs about twice as well as less costly amorphous materials and compares favorably in strength and stiffness with more costly high-performance carbon-fiber-filled nylons and other long-fiber-filled composites.
For example, a medical component molded of 40% carbon-fiber-filled nylon, is more expensive per pound than Vectra A130 LCP just for the resin. Using LCP in the same part would save about $0.3 cents/part without redesigning the part with thinner walls.
LCPs also cycle faster than most competitive materials. Cycle times with carbon-filled nylon are about 26 sec. LCP can reduce that to 16 to 22 sec, saving $0.06 to $0.14 cents/part. Carbon fiber is also abrasive and requires frequent tool repairs. The direct cost of such maintenance must be added to the cost of downtime and scheduling inefficiencies.
With its low viscosity, LCPs typically cause far less mold wear than alternative materials. Reduced maintenance costs and more favorable tool amortization can save almost $0.09 cents/part. For one major medical manufacturer, switching from carbon-filled nylon to LCP generated total savings of roughly $0.14 cents/part for annual savings of $34,000.
With amorphous and semicrystalline polymers, molded parts have some of their volume occupied by random, coiled molecules. As the plastic cools, the polymer contracts and crystallization significantly reduces the gross volume. By comparison, LCP’s rigid polymer rods always remain aligned in the flow direction. Gross part dimensions change little with processing conditions. LCPs commonly shrink less than 0.001 in./in. of part length in the flow direction. Consistent, precise part dimensions eliminate costly secondary operations such as the machining commonly required with metal castings. LCPs also produce little or no flash, so they need no expensive deflashing steps common to some other plastics.
The repeatability possible with injection-molding LCPs minimizes variation from part to part. Precise parts keep rejects and assembly problems in check. Manufacturers of electronic connectors report that LCP gives them 98% yields of acceptable parts, or better.
The maker of one surgical instrument specified glass-filled LCP for a two-piece body. The body halves had to be molded to tolerances of ±0.005 in. over 6 in. without flash. Strict cost goals and surface finish requirements prevented trimming or any operations which could expose glass fibers. The slightly textured housing material also had to accept custom colors. Compared to alternative materials, LCPs provided the right combination of dimensional stability, flash-free molding, and colorability needed to meet cost goals.
LCPs can generate dramatic savings by replacing metal. They replicate complex geometries without costly machining, and let designers consolidate parts. Combining several parts eliminates the expense of separate tools while reducing assembly steps and losses in yield at each operation.
The alignment of rigid, rod molecules results in a low coefficient of thermal expansion (CTE), minimizing shrinkage as parts cool. Low CTE also minimizes expansion when parts are sterilized or soldered. In most metal-forming processes, tight tolerances on critical features require some machining. Even with computer-controlled machining, the overall process is still operator-intensive and therefore more expensive and less efficient than injection molding. In plastics, designers can also color-code parts to simplify assembly and enhance aesthetics without using secondary finishing operations.
In one dramatic example of the savings possible with plastics, surgical manufacturer Pilling-Weck used LCP in the functional half of a ligating-clip cartridge. The mechanically demanding part requires extremely precise dimensions. Suppliers estimated a rough cost for the machined metal component at $50 to $60 apiece. By comparison, injection-molded parts cost the manufacturer just $0.84 apiece.
In another example, designers of the Medi-Jector needle-less syringe made the device lighter, easier to use, and less expensive to manufacture with LCP. The latest generation of the Medi-Jector uses critical load-carrying plastic parts that cost 50% less than the metal components they replaced. And the switch from greased metal parts to self-lubricating plastic components made the precision spring-loaded device easier for users to wind up before each injection. It also gave the syringe maker more opportunities to modify the device for different user populations without the machining costs associated with metals.
When the developers of a surgical skin-stretcher sought to convert a stainless-steel prototype into an affordable, disposable instrument, they chose LCP. Most surgical instruments have stainless-steel parts fabricated by cutting, welding, and machining. Each part requires a different combination of manufacturing processes and unique costs. Although the ancillary costs are difficult to generalize, most stainless-steel fabrication processes are more expensive than those associated with die casting. But we can estimate die-casting costs to provide some comparisons between metals and LCPs.
Assuming a molded part uses $1 worth of LCP, molding costs usually average less than half the material cost - in this case $0.50. Total cost of the plastic part can therefore be estimated at roughly $1.50.
To draw a rough comparison between molded plastic and machined metal parts, assume there is a metal part with similar raw material costs. Most stainless steels cost roughly one-quarter as much as LCP but are four to five times as dense. When price is adjusted for density, material costs are actually very similar. Diecasters estimate the cost of forming is roughly 1.35 times that of the metal and includes the expense of the process, tool wear, and associated factors. Finishing the part, including trimming, deflashing, and drilling the casting, costs another 1.4 times the metal’s cost.
With the cost for the LCP and metal being similar, the final metal part costs 3.75 times the basic material cost, more than double that of a molded LCP component. Using the same assumptions, consider a part designed in metal. Even with the part designed to be so thin the material costs are halved, the finished piece still needs forming and finishing, an additional 2.75 times the material costs. With these assumptions, a metal part with all its fabricating costs would still cost more than the LCP part even if the metal were free.
For the success of any project, it is important to focus on the final cost of the part. When the designer exploits the inherent strengths of LCP, the plastic enhances productivity and improves part economics. Compared to amorphous resins, LCP provides short molding cycles and therefore.