Stovall Life Science Inc., a manufacturer of laboratory instruments for thelife sciences market, was developing a new hybridization
oven for use inresearch and development by molecular biologists working on DNAand RNA applications. The company’searly oven concepts utilized a standard, coiled-resistance wire heating system.A major problem developed when the tightly wound spirals of heat-concentratedwire kept failing. Compounding this
reliability issue was the fact that thewire coil acted like a spring. When the element broke, it would touch anelectrically conductive component within the oven. The Life Sciences Market :What new? (1)Not only would the heatershort out, but the reliability failure became a hazard to the oven’s operator. Thecompany approached
Thermal Circuits of Salem, Massachusetts to help diagnosethe cause of the problem and develop a solution.The first order of business was to do a root cause analysis to determine whatwas causing the reliability problem. Thermal Circuits engineers examined thefailed units and determined the standard open-wire element was not sufficientfor the job. The existing wire element was running in excess of its thermallimitation. To correct the situation, Thermal Circuits recommended anetched-foil element. Whilehybridization ovens present very specific heating challenges, heatingassemblies play a critical role in a variety of medical instruments. Highlysensitive applications, such as blood analyzers, dialysis instrumentation,sterilizers, and incubators demand precise solutions to heating requirements.Utilizing etched-foil heaters in these devices provide the inherent advantageof distributing wattage infinitely and exactly, making the units more reliable.By eliminating wire failure, devices are safer, run cooler, and last longer. Etched-foilheaters are available with a variety of dielectric encapsulations, to meetvarying temperature requirements, as well as provide solutions for applicationsrequiring low outgassing and resistance to radiation, chemicals, and solvents.Etched-foil elements are able tooperate at very high densities due to the larger surface area covered by thefoil track. The element is created by acid etching a circuit in resistancealloy foil. This process ensures excellent circuit pattern repeatability andallows the design of complex heat distribution patterns within the heater area.By not confining the wattage to tightly spiraled coils, the etched-foil elementheating system runs cooler and lasts longer, still with the same power. Etched-foilelements also provide faster heat transfer resulting in longer heater life inhigh power applications. With shorter conductors covering a largersurface area of the element, the design of the etched-foil element inherently reducesthe stress on the element itself, making it less likely to break and increasingreliability. In addition, the etched-foil element adds an extra level of safetybecause the wire is covered with dielectric insulation, thereby, not allowingthe foil element to short out under any condition. Thermal Circuits engineersalso determined that performance, reliability, and safety in this applicationwould further improve if the new element was packaged as a forced convectionassembly. The new forced convection packaging was implemented by retrofittingtwo concentric cylinders to the hybridization oven’s existing muffin fan. Specifically,the smaller inner cylinder was constructed by forming the heating elementitself into a tube. The outer, larger cylinder has a dual nature. It directsthe flow of air, similar to ductwork, and provides a dielectric shell for addedsafety.Thermal Circuits final recommendation was to incorporate a thermal fuse as afail-safe measure to enable UL recognition of the hybridization oven. Thisadded feature protects the entire hybridization oven should any othercomponent, such as the fan or temperature-controlling system, fail or shouldthe airflow be restricted. The fuse was strategically located to trip in afault condition, causing the unit to shut off before the oven reaches atemperature outside its normal operating range. This measure greatly enhancedthe safety of the unit for a minimal cost. By solving a reliability problemwith a more thermally responsive and safer design, Medical device companies are always under pressure in two key areas: time andcost. Many different types of plastics can be used for injectionmolding, and once a mold is made, an extremely large number of pieces can bemanufactured with it. Because of this, injection molding manufacturing isextremely cost effective in high volume production. But the properties ofinjection molding (plastic resin is fed into the injection machine, heated, andforced at high pressure into the mold by a screw and cures on cooling), and themechanics of molding, dictate that designers pay particular attention tocertain principals to ensure successful manufacture. Much of the specifics of mold design have to do with the particularcharacteristics of thermoplastics. Thermoplastics are heated, melted, and theninjected at high pressure into the mold. They cure upon cooling and partsshrink during this cooling time. The variation in shrinkage that can occur mustbe a major consideration for a designer in order to achieve dimension control,hold tolerances, and avoid warpage. This shrinkage is due to a number ofvariables:order to avoid costly and time consuming pitfalls. Alteringthe mold to achieve the required dimensions can be difficult at times but isgenerally able to be accomplished. If a major mistake occurs with any of thethree areas listed above, it is possible that the mold might need majorrevision or to be built over again. on the stationary side of the mold. Asplastics cool, it adheres to the sides of the metal mold which can makeejection difficult or even impossible if adequate draft angles are notincluded.Parting lines must also be considered when determining the basic functioning ofthe mold, but are also very important to the cosmetics of the molded product. Partinglines will leave visible lines in the part that will obviously affect thevisible surface after assembly. It may be unacceptable for certain parting linelocations to appear on the exterior of a product, and design should bedetermined accordingly