How to improve accuracy and repeatability when dispensing
Simon Gibbs, product specialist at Intertronics, explains how increasing the precision in an adhesives process can improve manufacturing productivity.
Pushing up productivity in a bonding process often means improving yield or quality. An engineer will look at the production with one question – what gains can we get in accuracy and repeatability to make us more productive? Improving the precision of processes in small incremental ways can deliver a significant return on investment.
Precision or repeatability is consistently achieving the same results. Whilst measuring accuracy tells us how close we get to our requirements on average, measuring precision tells us how often we are actually achieving them. Your process could be accurate but not repeatable, getting the right result but not as reliably as you need. Improving process repeatability impacts on the robustness of your product by making sure that the operation is within specified tolerances, either all the time or with smaller variances. This brings higher productivity and more confidence in the process.
Poor precision in a process can decrease yield, adversely affecting rework or scrap rates. For example, some electronics require thermal management through the use of thermally conductive adhesives and potting materials. Precise dispensing of these is very important for their efficacy, because air entrapment or voids have significant impact on their heat transfer performance. Moreover, detecting this kind of defect in process is difficult, as it is often not visible, and may only come to light in subsequent testing or failure in the field.
Precision is particularly important in high value, low volume industries where scrap is expensive, like aerospace or motorsports, as well as in highly regulated industries like medical device manufacturing where processes are tightly validated, and markets have high expectations.
Improving your process
Application of materials by hand is almost always the least repeatable approach. While it can be done precisely, manual dispensing can be physically intensive and relies on a highly skilled workforce. Reliance on highly proficient operators is a risk, as they leave, fall ill or retire. Inevitably, the smaller in size the application is, the less likely that a human can unfailingly manually apply material to within the required tolerances. We are increasingly conscientious about workplace ergonomics and health & safety. A fully manual approach may involve higher rework levels, more waste, unwanted exposure to chemicals and a higher chance of RSI.
Dispensing systems can improve precision, by giving the user more control over the process. Different technologies offer different levels of control, repeatability, and accuracy – technology is available to make both dispensing amounts and location positioning highly precise. To determine which equipment is most suitable for process development, manufacturers should first be clear about what level of precision and accuracy is needed, and then choose accordingly.
One popular technique, which has been around since the 70s, is time-pressure dispensing, where a pulse of air in a syringe barrel forces liquid out through a needle or nozzle. The process can be controlled by the applied pressure, the timing of the air pulse and the diameter of the needle. These machines are a successful step for dispensing precision for innumerable companies, as evidenced by their ubiquitous presence in factories and ongoing demand. They perform to adequate accuracy in many cases, with improved benchtop housekeeping. However, they do not meet the high levels of precision demanded in a more sophisticated assembly.
A pneumatic dispensing valve can further improve precision, with the capability to actively turn the flow off and on like a tap. There are a broad range of valve types available, including needle valves, spool valves and diaphragm valves — the appropriate valve depends on material chemistry, cure mechanism and geometry of output. Dispensing valves can mitigate some of the variability of the flow from a time/pressure syringe dispenser, but output can still fluctuate with material viscosity change — something that may occur with ambient temperature change, for example.
Dispensing equipment based on some form positive displacement technology bring the highest levels of both accuracy and precision. The output from these is a specific volume, and they can preclude the effects of environmental and material variations on dispensed quantity. Jetting valves, for example, may be a good option for manufacturers looking for speed and/or the ability to dispense very small dots, or to dispense onto complex 3D surfaces, as well as high accuracy and repeatability.
The highest level of precision
At the higher end of precision, positive displacement dispensing equipment based on the progressive cavity pump principle can achieve high levels of repeatability and accuracy. A progressive cavity pump typically consists of a single-helix metal rotor and a double-helix hole in an elastomeric stator. This forms a sequence of small discrete cavities which progress through the pump as the rotor is turned, transferring the liquid. The output represents true volumetric dispensing, because the amount of material is directly proportional to the number of rotations of the rotor, and is not affected by material viscosity, input pressure or ambient temperature.
One example of a technology that works on this principle is the preeflow eco-PEN, which enables the user to dispense volumes as small as 0.001 µl within 1 per cent, 99 per cent of the time, offering a high level of repeatability and accuracy.
Positioning with precision
Dispensing valves and pumps improve control over the dispensed amount, but manufacturers looking for a very high level of precision will also want to have repeatability in the placement of the material. Manufacturers can repeat the application of materials with positional accuracy by incorporating a robot or other form of automation into their dispensing process. Once the dispensing technology has been specified, it can be mechanised. Options include rotary tables or simple 3-axis benchtop robots, usually at modest cost, right up to multi-axis robots with vision-based control and feedback.
We all understand that robots offer productivity benefits over humans because they are faster or more consistent, and work without breaks. But they can achieve results not possible manually, such as the application of an even bead of liquid gasketing around a complex profile. Return on investment calculations based on the usual automation benefits combined with precision- and accuracy-based productivity gains often show a quick payback.
Meeting material application tolerances using a robot depends on the dimensional consistency of the part, and the accuracy of the locating jigs and tooling, as well as the performance of the robot and the dispensing system. Where parts or tooling have some irregularity, automatic dispensing tip alignment, tip height sensing or camera-based vision systems can be used to improve precision further. For example, Intertronics offers a vision system for Fisnar ADVANCE Series Dispensing Robots which provides an auto-alignment function to guarantee fluid is being dispensed at the exact required location.
While different technologies offer different levels of accuracy and precision, which equipment is suitable for your application depends on the types of tolerances and what type of improvement you are looking for. Overspecification can be expensive, unless it delivers clear-cut benefits. An experienced adhesives supplier should be able to guide you through building your process with the level of repeatability and accuracy you need. A series of incremental changes may be a viable approach on the path to improved productivity.
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