Choosing the Right Adhesive Processing Equipment
Processing equipment is often the forgotten part of adhesives application. Yet the right equipment can improve efficiency and reduce costs for medical device manufacturers. The factors to consider are outlined.
Henkel Loctite Adhesives Ltd, Hemel Hempstead, UK
Arguments for automation
Fast and reliable methods are essential in the production of disposable medical devices. Manufacturers of these products utilise a number of assembly techniques including, ultrasonic welding, solvent welding and adhesive bonding. These each have their merits, but bonding with specially created adhesives has proved to offer advantages for medical device manufacturing. Foremost among these is the ability to provide easy application and to present cure on demand.
Squeezing a tube or unscrewing a bottle and then applying the required quantity of adhesive to the parts in question is not suitable for large-scale production, which may involve hundreds or thousands of units. For volume manufacture, the use of dispensing equipment provides a number of clear benefits:
- There is less waste. Just the required amount of product is applied for each task, and savings of up to 50% can be achieved compared with manual applications.
- Adhesive is applied with precision, at a specified time, in the required volume, in a definite position and with continuously repeatable accuracy. The parameters set in trial runs can be matched exactly when full production starts.
- Pinpoint accuracy can be achieved at speeds well beyond the swiftest of human efforts. For example, a manufacturing facility may have a production target of one million components a year. This will entail a throughput of one completed unit every five seconds and is an unrealistic goal if manual operations are to be employed.
- There is a safer and cleaner working environment because automation removes the need for operator contact with the adhesive.
The argument for automated application is significant, but there are three important considerations when selecting dispensing equipment.
Adhesive type. The kind of adhesive being applied will often determine the type of equipment. For example, an ultraviolet (UV) adhesive, much favoured by medical device manufacturers, must be
dispensed from a container and through tubing that restricts natural light. That is because any light falling on the adhesive before it reaches the surfaces to be joined will bring about premature curing.
Viscosity. The viscosity of the adhesive will also determine the type of equipment that is required. To state the obvious: the thicker the adhesive, the greater the power needed to dispense the product. The best brands will be available in a variety of viscosities to suit different needs.
Dispense pattern. The application dispense pattern can vary ranging from single to multiple spots with sizes from 0.0008 mL upwards, to linear beads and coating within bores.
With those factors in mind, equipment suppliers will need to find the answers to a number of relevant questions before being able to advise manufacturers on equipment.
- What type of adhesive will be used?
- What is the size and shape of the parts to be joined?
- By what method do the parts proceed along the assembly line?
- What is the planned production rate?
- At what stage in the manufacturing process will the adhesive be applied?
- How long after the adhesive is applied do the parts need to be handled or tested?
Once those questions are an-swered, it should become obvious what kind of equipment is best. So, what are the options for medical device manufacturers?
Those associated with the medical industry will be familiar with the peristaltic pump principle often used in operating theatres. That principle can also be used in some types of adhesive dispensing equipment such as bench top dispensers. This equipment is perfect for single part adhesives with a viscosity that enables them to flow freely.
Adjusting the cam rotation speed allows the amount of adhesive to be regulated. A foot or finger switch on the dispensing nozzle is then used to control when the adhesive is applied. Another control prevents unwanted adhesive from dripping from the nozzle.
Using this equipment provides easy control of the dispensing action. It is suitable for small batch production and does not require an air supply. It offers a simple changeover procedure for different adhesives and low maintenance. However, that is just one method of automatic dispensing and suppliers will have others to suit different applications.
Keeping up the pressure
Many manual, automatic and semi-automatic dispensers are governed by a pressure–time system to regulate the amount of adhesive being dispensed. Here, the adhesive is put under pressure and the dispensing valve opened (in the case of manual systems, by means of a foot or finger switch) for a determined time period to supply the accurate amount of adhesive as a dot, bead or drop. These systems are usually capable of dispensing adhesives with viscosities ranging from water-thin to paste.
As mentioned above, dispensing systems offer precision and speed, but engineers often ask, “How can I be certain the adhesive has been applied?” One obvious way is by means of a visual check of every application; however, on a moving production line this is labour intensive and time consuming and, therefore, not viable. A more efficient method involves the use of a flow monitoring system that has been interfaced with the controller unit.
With this arrangement, all the critical references to the dispensing cycle are generated by use of a specially created “good” bead of adhesive. During the manufacturing run, the signal from a transducer is boosted by a pre-amplifier that interfaces with the control unit. This signal then compares the dispensed bead with the pre-set parameters. If a discrepancy is discovered such as air bubbles, pressure changes, needle touch down or the loss or clogging of nozzles, the operator is alerted to a potential problem. In reality, this is a fail-safe system because, when linked with the appropriate controller, no dispensing cycle will pass unchecked. It is suitable for many adhesive technologies, including cyanoacrylates, anaerobics and UV acrylics.
Once the adhesive has been dispensed, it will need curing. In the case of UV adhesives this requires a suitable light source. For relatively low volume dispensing, a simple light probe will provide sufficient UV light to affect a cure. Where throughput is more substantial, an oven designed for UV adhesives may be a better proposition. The key is matching the UV light wavelength and intensity with the adhesive that is to be used.
There can be no doubt that dispensing equipment is the efficient, cost-effective, clean and accurate way of applying adhesives. For applications where fully automatic adhesive dispensing is appropriate, equipment suppliers will often design and build systems to meet specific requirements. These can be integrated into existing production procedures or be completely self-contained. Ultimately, though, it is a case of talking to a trusted supplier and working together to create the most appropriate solution.
Steve Ginger is Application Equipment Manager at Henkel Loctite Adhesives Ltd, Technologies House, Wood Lane End, Hemel Hempstead HP2 4RQ, UK, tel. +44 1442 278 000, e-mail: firstname.lastname@example.org, www.loctite.co.uk