Feature Article

Building Better Bonds Using Light-Cure Adhesives

Posted in Medical Device Assembly by Camilla Andersson on September 14, 2011

The use of light-cure adhesives can optimise product assembly and reduce material waste, thereby achieving cost savings in the production process without affecting quality.

Medical device manufacturers have faced the most challenging economic conditions in decades during the last couple of years, and they have aggressively sought ways to reduce production costs without sacrificing product quality. Optimising assembly process efficiency and minimising material consumption are viable pathways for reducing manufacturing costs. Adhesives are well suited for many assembly operations that involve bonding and joining. In these applications, light-curable materials (LCMs) can help to increase productivity and reduce waste compared with traditional two-part, thermally cured or solvent-based adhesive systems.

Adhesives are routinely used to assemble catheters and can be used to bond engineering plastics such as polycarbonate, acrylic, ABS, nylon and various urethanes.

Determining the ideal adhesive performance criteria
Selecting the right adhesive to meet the performance, production and environmental demands of a medical device can be a daunting decision process. Many factors must be considered before choosing a method to bond and seal plastic, metal, glass or other substrates. Yet, the decision-making process can be greatly simplified just by asking the right questions.

Establishing application requirements for the adhesive or sealant and identifying ideal performance criteria are the foundation of an effective selection process. The following factors should be considered key when selecting an adhesive for a specific medical device assembly operation.

Chemistry. When bonding engineered plastics such as polycarbonate, acrylic, urethane, ABS, nylon or other resin systems to plastics, metal or glass, there are several adhesive chemistry options. These include light-curable acrylated urethanes, cyanoacrylates, one- or two-part urethanes and even a few hybrids such as thermally curable and light-curable adhesives or light-curable silicone systems. Bonding to silicones is often best accomplished with silicone adhesives, but under the right conditions, bonding to silicone elastomers also can be accomplished with urethanes.

Viscosity and thixotropy. Viscosity is the measure of a fluid’s resistance to flow. The lower a fluid’s viscosity, the greater the fluid’s ability or tendency to flow or spread over a surface or bond joint. As a point of reference in evaluating the viscosity of an adhesive, water has a viscosity of 1 mPas, whereas honey has a viscosity of 10,000 mPas. Potting or filling a groove moulded into plastic probably would require a low- to medium-viscosity fluid, as a low-viscosity material will self-level and fill the groove without voids or air bubbles.

An additional property for consideration is thixotropy. Materials that are thixotropic flow easily when placed under shear (during dispensing, for example), but exhibit a higher viscosity when the shear force is removed (dispensing stops). For example, ketchup, which has a viscosity around 10,000 mPas and is thixotropic, flows easily when dispensed, but stays in place on top of the hot dog. The thixotropic index (recovery) of a material is a helpful value. Typically, materials with values of 2.0 to 3.5 are very thick or gel-like and materials with values of 1.5 to 2.0 tend to slump. A thixotropic material would be an ideal candidate to fill large gaps or to use in applications where the adhesive is not allowed to flow into certain areas.

Adhesion. Once the adhesive chemistry is selected, an individual product within that adhesive class should be chosen based upon its adhesion to various substrates. The test criteria are defined by the specific performance expectations of the adhesive, based on the design of the components. Lap-shear testing or peel-force testing is common, as well as pressurisation-to-burst or leak testing. Accelerated ageing test criteria will depend on the storage and in-use conditions expected over the life of the device. Attempting to accelerate the accelerated ageing test by employing even more aggressive conditions should be avoided. Conducting the test at too high a temperature may inaccurately characterise the adhesive by creating additional cross-linking within the adhesive, which will cause a reduction in elongation properties.

Ease of processing. If multiple adhesive choices still remain, evaluate the adhesive based on its ease of processing. One-part adhesives require a simple dispensing system versus a metered mix system for two-part adhesives. Additionally, one-part systems usually do not require dispensing system purging or have issues with pot life. The ability to dispense and cure where and when needed make one-part acrylated urethanes ideal for many medical devices in which UV or visible light is required to reach some portion of the bond line. A careful consideration of the level of automation required for assembly (manual, semi-automated or fully automated) is also important. Can the system be stopped or shut off easily, or is there a shut down process that needs to be followed? Can the process be adjusted or qualified to handle lot-to-lot variations in viscosity or cure time?

Biocompatibility. Many adhesives companies classify their medical-related products as compliant either with ISO 10993 or USP Class VI, or some combination of both standards. These biocompatibility classifications are determined for fully cured adhesive samples under specific conditions. The medical device manufacturer still must run full testing on the completed device, since the cure method employed in assembly does play a part in passing the biocompatibility testing. Most materials are compatible with EtO as well as gamma sterilisation. Resistance to autoclave sterilisation is limited to a few cycles for cyanoacrylates, urethanes and even epoxies, and results can depend on the part configuration.

Quality enhancement. Since the quality and reliability of the medical device is of the utmost importance, the ability to ensure a high-quality adhesive bond is critical. Some medical device adhesives are formulated to fluoresce under a black light. This enables quality technicians to confirm that the adhesive covers the prescribed bond or seal area, and also to detect any leaks, air bubbles or voids. Fluorescing adhesive formulations are available in both blue and red fluorescing colour versions. Red is particularly useful in providing the proper contrast in situations where the surrounding plastics also fluoresce blue. An innovative, recent advancement that further ensures joint quality is the introduction of See-Cure technology. The uncured adhesive changes colour from blue to a clear/colourless aspect once the cure has been completed. A simple postcure visual inspection can thus determine completeness of the adhesive cure.

Cost. The true in-use cost of an adhesive considers all parts of the process including waste, downtime, start-up time, scheduled maintenance, the tight quality specifications needed to minimise variability within the process, number of workers required for a specific process and scrap rate. Two adhesives may differ only slightly in price per gram, yet one of them may deliver a 30% cost saving in process efficiencies.

Achieving process savings with light-curable materials
Cost savings in the assembly operation drop directly to the bottom line. These can be achieved in materials, assembly, quality assurance (QA) testing, inventory and even floor space.

Material cost savings can be achieved with light-curable adhesives. Consider a two-component epoxy adhesive that costs less than a one-component light-curable acrylic. The expected material cost benefit of the lower-priced, two-part system vanishes in a side-by-side comparison, which reveals higher two-part material usage caused by purging, improper mixing and general waste in the mixing system.

Light-curable materials reduce labour costs because the expenses associated with stacking and racking parts assembled with slow-curing adhesives are eliminated. Light-curable materials cure on demand, ensuring that the bonded medical device is immediately ready for the next step in the process. Assembly labour costs can be reduced by as much as 70% compared with two-part epoxy, two-part silicone or RTV silicone systems. Parts can sit on racks before and after their turn in the cure oven or to dry for as long as seven days. The labour associated with racking and loading the parts into cure ovens is eliminated when light-curable materials are used. It is also worth noting that, unlike with other adhesive products, there is no need to be concerned with environmental conditions, such as high humidity, having an adverse impact on your adhesive chemistry system.

On-demand cure enables a QA check of the bond line immediately following the cure, eliminating the need to retrieve defective parts. This immediate check minimises scrap and in-process work. The fluorescing of some adhesives allows for void or bubble detection. Manufacturers can then repair or scrap these parts before adding value by means of down-stream operations. On an assembly line, for instance, a high-speed camera can detect and confirm the postcure presence of adhesive on the bond line via the fluorescing of the adhesive under black light. Light-curable materials can reduce QA testing costs by as much as 50% compared with traditional adhesive chemistries.

The use of light-curable materials in assembly processes also reduces inventory costs by as much as 50%. On-demand cure and instant quality control testing eliminate excess inventory of slow-curing commodity adhesives. Light-curable adhesive systems avoid costly waste caused by improper mixing of two-component systems and they can minimise adhesive waste by eliminating the need to purge.

Floor-space savings also need to be factored in since light-curable material assembly processes do not require racking, batch process ovens or cooling racks. The light-curable material assembly process is compatible with just-in-time manufacturing or kanban production lines. Floor space savings enable expansion of the production operation and increase the dollar value of product produced per square foot of floor space.

The cost reductions in the aforementioned opportunity areas can lead to a 30% savings in overall process costs.

Light-curable materials offer numerous opportunities to increase productivity and reduce waste in assembly operations. Instant on-demand cure, automated in-line inspection, solvent-free formulations and a small process footprint are among the significant and positive effects the technology can bring to a company’s bottom line.

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