This article highlights ten things that machine builders should know about adhesives, in terms of both the bonding of machine components and the automated application of adhesives.
Modern adhesives are versatile, user-friendly and available in a range of types and strengths to suit a broad variety of applications. While product designers may be familiar with the possibilities offered by adhesives, machine builders are sometimes unaware of adhesives' potential, or they may harbour misconceptions - perhaps due to a poor experience in the past. This article presents ten things that machine builders should know about adhesives that could help them to build better machines more efficiently, or improve the automated application of adhesives in order to reduce cycle times, raise product quality or save costs.
Adhesives can be used for applications such as shaft-hub connections with tolerances specified to provide clearance for the adhesive, but sometimes there are concerns over concentricity or joint strength. In such cases adhesives can be used in conjunction with press-fit joints or heat-shrink joints to provide the necessary mechanical characteristics - it is even possible to use this technique to make joints that are at least as strong as laser-welded joints. Adhesives can also be used with non-sliding splined joints to prevent fretting and noise.
Typically machine builders fix clear polycarbonate panels to guards or doors by means of threaded fasteners, but these are time-consuming to install. A cost-effective alternative is to bond the window in position, which also offers the advantage of making the polycarbonate part of the structure (as with car windscreens), resulting in a stiffer guard or door that is less likely to vibrate. If the window has been designed to be replaceable if it is damaged, a lower-strength adhesive can be used in combination with a small number of threaded fasteners.
Joints in pipework and fittings (including those for pneumatics, hydraulics and process fluids) must be sealed. Machine builders often use PTFE tape for this, but there are two alternatives that can provide benefits. First, liquid sealants can eliminate any risk of PTFE particles becoming detached inside the pipework. Second, sealing cord can be applied to the thread; compared with PTFE tape, this has the major advantage of maintaining the seal even if the tightened joint is backed off slightly to provide the correct alignment.
Noise is an issue for machine builders in that noise levels must meet specified limits laid down in the Machinery Directive 2006/42/EC. In addition, the judicious application of sound deadening to thin panels can make a significant difference to the way a machine presents to a customer - rather like people assessing the quality of a car by the way the doors close. Self-adhesive pads can be applied, or simple rectangular pads can be adhesively bonded to unseen sides of panels.
Installing machines often requires that they are chocked in the correct position to prevent unwanted movement. Suitable compounds are available from companies such as Henkel to make the chocking procedure simple, quick and cost-effective, so the machine can be commissioned as fast as possible and the machine builder (or installer) spends no more time on the customer's site than is absolutely necessary.
Machine builders will be familiar with the application of threadlocking adhesives. Normally the threadlocker is applied to the fastener just before it is inserted and tightened, but sometimes this is inappropriate - for example, if a large component needs to have a number of fasteners carefully torqued down in a process that takes longer than the threadlocker's curing time. In such situations it is better to apply a threadlocker after the fasteners have been torqued, which is possible by using a grade with a very low viscosity so that it penetrates the threaded joint.
Most people would assume that adhesives only work reliably on components that are reasonably clean, so they might be surprised to learn that there are now adhesives that have been formulated to retain oil-impregnated sintered bushes such as the Oilite type. This can eliminate the need to press-fit the bushes and then ream them to the correct size - which saves the machine builder time and cost.
UV-curing adhesives have been available for many years and are popular for medical devices and similar applications. However, conventional UV lamps are relatively heavy consumers of energy and usually have to be left switched on because of the warm-up time. In contrast, modern LED-based UV lamps consume less energy when they are on, and also do not have a warm-up time so can be switched on and off as required, resulting in considerable energy savings.
In-process inspection is increasingly specified, particularly in industries such as automotive, but it can be difficult to check whether an adhesive has been correctly applied or whether a bead of sealant is complete and unbroken. Traditional vision inspection systems can struggle, especially with clear adhesives, but Henkel has developed products with an additive that is highly visible when illuminated by UV light. This makes vision inspection very straightforward and reliable, and the additive does not compromise the adhesive's shelf life, curing time or strength.
Components that are likely to wear due to continual contact with abrasive materials can add significantly to a machine's cost of ownership if periodic replacement is required. As an alternative, Henkel recommends layers of sacrificial polymer composites that enable the components to be refurbished and re-coated more cost-effectively than total replacement.
The application of adhesives, sealants and liquid gaskets can be automated to any level: simple dispensers can support manual assembly; metered dose equipment can be incorporated within assembly cells; or fully robotic systems can apply adhesives along complex two-dimensional or three-dimensional paths. For components that require an activator to be applied in order to accelerate the adhesive cure, the activator application can be automated as well. Machine vision can also be used to check that the adhesive - or sealant or liquid gasket - has been correctly applied before the joint is assembled.
Modern adhesives are certainly not a 'bodge'; a correctly designed joint with the right adhesive specified can be as strong or stronger than a conventional joint made with mechanical fasteners. If components need to be designed so they can be disassembled for repair or maintenance, then this can be taken into account at the design stage, perhaps by using a lower-strength adhesive and a small number of mechanical fasteners.
Machine builders can easily integrate automated equipment for dispensing/applying and inspecting adhesives, or Henkel can take full responsibility - including the design, manufacture, delivery and commissioning of turnkey automated systems. And don't forget that there are major benefits in sourcing the adhesive and application equipment from the same supplier, as there is no question over who takes responsibility should any issues arise during commissioning or operation.
* If you are wondering why there are 11 items not 10, it is just another example of adhesives delivering more than you might expect!