Productivity through process control and QA

Simon Gibbs, technical sales executive at adhesives specialist, Intertronics, discusses how manufacturers can improve productivity throughout the quality assurance process.

 

In a successful adhesives process, the adhesive hardens into a material able to bear the required forces throughout the product’s lifetime. If a manufacturer is experiencing poor bonding, and therefore a reduction output quality, it may be time to re-assess the process to improve output, and so productivity.

Manufacturers of products like defence-grade electronics for a fighter jet, need reassurance that every product is made to specification. In highly regulated industries, like aerospace, the manufacturer must be able to demonstrate that the process has been completed as expected and that every product at the end of the line is the same, within well-defined tolerances.

If rejects occur, the manufacturer can consider whether it is the adhesive choice, the amount or location of the material, or the curing process that has resulted in failure to meet the required specification; there are other factors as well. As we all know, the only way to improve something is to measure it, and some specific techniques will be helpful.

Is the material suitable?

Adhesive choice is highly dependent on the performance requirements of the product, under the environmental conditions it will be exposed to. For example, products in military applications may be required to withstand both high and low temperature extremes.

The meeting of international standards and demonstrated success in similar applications is a great help in narrowing down adhesive choice, but is not a substitute for full evaluation. Even if the data sheet suggests the selected material is up to the job, it is still essential to perform your own tests and trials to understand if the material will perform as expected for the lifetime of your product.

Has it been dispensed correctly?

It is possible to select a suitable adhesive and apply it incorrectly — material must be dispensed in the right amount, and in the right place. Systems and processes must be set up and understood so that there is confidence in their ability to meet the quality requirements, and that means choosing the optimal dispensing technology for the project.

For example, the use of a positive displacement dispensing methodology, such as the preeflow eco-PEN range, will provide repeatable and consistent dispensing, with an accuracy of ±1%, >99% of the time. Some quality control systems will require ongoing measurement of this.

Manufacturers can monitor dispensing output by monitoring system pressure, using a tool like the ViscoTec flowplus16. All other things being equal, the flow rate is directly proportional to system pressure, so measuring system pressure can verify that the correct amount is being dispensed, giving real-time process feedback. The sensor can detect issues that could lead to inaccurate application of the adhesive, like inconsistent material feed, clogging needles or nozzles, or air bubble presence in the fluid.

Variability and consistency in the location of the material dispense will always be improved by the use of automation over a manual process. Benchtop dispensing robots are relatively inexpensive and straightforward to implement, giving positional resolution of up to 0.001mm/axis. Precision will then depend on the accuracy of the jigging or tooling, and the manufactured tolerances of the parts themselves. Variations in these aspects can be mitigated by the use of a camera-based vision system, driving an automatic part alignment function. The system captures an image of two separate fiducial locations on the part and uses pattern recognition software to compare the live image against a stored image. Any difference found in X or Y position results in the dispense coordinates being automatically adjusted to suit.

The use of a fluorescing agent in a material in order to help with inspection is well established. For example, MIL-I-46058C, Insulating Compound (For Coating Printed Circuit Assemblies) has mandated that conformal coatings for printed circuit boards be fluorescent, a requirement that has existed since the 1970s and before. Fluorescing agents have been used in adhesives for quite some time as well, especially in those used in plastics bonding. The fluorescing agent responds to blacklight inspection equipment, and this response can be measured visually or with sensors, giving feedback on both adhesive quantity and location.

Many plastics naturally fluoresce blue under blacklight, which makes it difficult to inspect blue fluorescing adhesives because the contrast is not high enough for accurate inspection of the bond line. Ultra-Red fluorescing UV curing adhesives overcome this, as they fluoresce red under blacklight, but otherwise remain clear. The red fluorescence also does not absorb the same light energy wavelengths as those used to cure the adhesive, resulting in faster, deeper cures compared with the same adhesive with blue fluorescence.

Is it fully cured?

Uncontrolled curing processes can result in bonding issues. For best results, manufacturers should understand the specific conditions required to cure a given adhesive and whether their process environment meets these conditions. Some products cure with moisture; for RTV silicones, it is atmospheric moisture, for cyanoacrylate adhesives it is the moisture on bonding substrate surfaces. In both cases, control of relative humidity is important for consistent cure, and is a consideration for climate-controlled factories or cleanrooms.

If adhesives are to be exposed to heat for cure, then it is important to ensure that the adhesive bondline is exposed to the correct temperature for the full schedule. Products with larger mass will take time to get up to full temperature, and this ramp up period needs to be added to the schedule. If an oven is used for multiple tasks, and the door is opened frequently, then the internal temperature may be compromised, and thermal cures adversely affected. Measurement and recording of actual internal oven temperature is a good step for improved adhesive cure process control.

In a UV light curing process, the key to success is to ensure the adhesive receives the correct dose (a combination of light intensity and time of exposure). A UV radiometer is a necessary instrument for understanding this; there are specific models which can measure intensity (normally mW/cm²) or dose (mJ/cm²), and are designed for the relevant wavelengths and power. Variation in curing dose can come from ordinary degradation of the bulb, or contaminated lightguides, reflectors and optics. Distance between the curing lamp and the bondline will have a significant effect on the dose, so eliminate this as a process variable for best results.

For manufacturers using light curable adhesives, incorporating Dymax See-Cure Technology is a possible way to check quality. Colour change technology makes it easy for an operator, or a simple automated optical inspection system, to confirm that the joint has fully cured. Prior to curing, See-Cure adhesives are blue, but their colour disappears during the curing process, providing a visual confirmation that the bond is secure.

This technology can also be used to help establish where the adhesive has been applied, as the bright blue uncured adhesives make it easy to spot any voids in the bond line. For those looking to incorporate both red-fluorescence and colour changing properties, to check both cure and adhesive placement, Dymax Encompass adhesives provide both in one.

Productivity improvement is about increasing outputs and/or decreasing inputs, so higher yields will improve your assembly productivity. There are number of reasons why a manufacturer may be experiencing problems with an adhesive process. Begin with selection and understanding of the optimal products, and then use ongoing quality assurance process measurement and verification to maintain the desired quality levels. The best approach is to work with an experienced adhesives specialist, who will be able to assess your process and suggest practical ways to improve it.

www.intertronics.co.uk