Adhesive Bonding Without Traditional Clamping
Adhesive bonding is a popular and proven method for bonding plastics used in certain medical devices (e.g., lab on a chip). While adhesives offer empirically qualified levels of strength, production managers lament the added time, effort and equipment required to fixture, clamp and unclamp adhesive bonded parts until curing is completed and joint strength is realized.
Whether a single adhesive joint takes minutes or seconds to cure (e.g., UV-cured adhesives may cure in 5–10 seconds), the clamping/unclamping operation represents a significant investment of time, labor and tooling. Automating the clamping/curing process is possible but requires additional time and expense that won’t pay for low-volume parts or a variable product mix.
Consider the clamping required for a flat medical device assembly, such as a lab on a chip. Following the application of adhesive to the upper and lower portions, this flat part must be secured in a clamp that provides equal pressure across the entire surface to ensure a proper seal and cure. Even with UV-light curing, which can cure a joint in seconds, clamping and unclamping the parts requires people or processes to complete the part. All of this handling, independent of clamps and tooling, adds to per-part costs for labor, cycle time, or both.
A New, More Cost-Effective Alternative
Emerson is developing a new and more cost-effective alternative to the use of traditional mechanical clamping for curing adhesive bonded parts.
The alternative is based on Branson PulseStaking technology, a variant of the traditional “hot-tip” thermal staking processes. PulseStaking employs a patented tip (temperature controlled with pulses of heating and cooling) that can precisely manage plastic melt temperature (see Figure 1). As a result, this process can stake or swage complex, closely spaced plastic features one at a time (or simultaneously with a multi-tip tool) without the risk of melting nearby structures. Thus, PulseStaking can handle a range of assemblies that ordinary hot-tip processes cannot.
Figure 1: The unique design of the PulseStaking tip combines an electrical heating element with a compressed air cooling system, enabling instantaneous heating or cooling of the tip — and the surrounding plastic — to precise temperatures during the forming process. Image courtesy of Emerson.
Using heat-staked plastics to replace clamps is a relatively simple process. It starts on the product drawing board, when designers are asked to augment new or existing adhesive-bonded product designs with features to incorporate micro-plastic rivets or studs as small as 0.065 inches.
Access holes beyond the fluid path are added and a plastic rivet is PulseStaked to replace the mechanical clamping fixtures (see Figure 2). The sizing and finished strength of the rivets are determined by weld evaluation, followed by pull-strength or other tests during the product design or redesign process.
Figure 2: Clamping a lab-on-a-chip device using a plastic, pulse-staked “clamp.” Image courtesy of Emerson.
The mating parts are then ready for volume manufacturing. During assembly, just after application of the adhesive, the mating parts are aligned and the uncured assembly is placed on an automated PulseStaking platform (see Figure 3), which simultaneously heat-welds the studs, permanently joining the mating parts in seconds. The need for clamps and the costs of labor, automation and tooling for clamping/unclamping operations are eliminated. The clamps are replaced by inexpensive plastic rivets or studs that are integral to the mating parts and sized to ensure consistent clamping strength and proper part curing.
Figure 3: A Branson PulseStaking machine, capable of making multiple, automated pulse-stakes in a single cycle. Image courtesy of Emerson.
The time and cost-saving benefits to manufacturers in a variety of markets—medical, electronics, automotive, appliances, and many more—are potentially substantial. Consider an adhesive-bonded part with an annual production volume of 1 million units. If each part requires an average of 15 seconds of labor time for clamping and unclamping, the labor expended equals 4,166 hours per year—about two full-time workers. Automating the clamping process with an in-line PulseStaking process could dramatically speed production, provide rapid payback, reduce work in process, improve production yields and quality, and allow for redeployment of valuable labor to other, less tedious tasks.