Thursday, 11 July 2013

QUALITY CONTROL: AN OBJECT LESSON





Bean-to-cup coffee makers manufactured at De’Longhi can produce a cup of coffee by just feeding beans at the touch of a button. These machines include a boiler, grinder, brewing unit, and in most cases, a steamer. The machines are manufactured on assembly lines with each unit individually manufactured and tested before the finished system is assembled. 

In late 2006, De’Longhi began another process of evaluating that testing procedure, the goal being to implement a system to guarantee the quality of every finished coffee maker leaving its site. A main reason for achieving this was for complete product testing. There was also a desire to quantify the product quality. 

To perform assembly-line testing of the grinder, an Insitec online laser diffraction particle-size analyzer was used. This instrument is suitable for use on the shop floor. It can measure up to four complete particle-size distributions per second and can be installed on a bypass sample loop or a process line. Emulsions, slurries, suspensions, and dry powders can be measured in a range between 1 and 2500 ┬Ám. 


Devised Solution 

Previously assembly-line testing of the grinder was done manually, and it was nearly impossible to obtain 100-percent quality control. The manual test allowed a qualitative but not a quantitative assessment, so it was not effective. In light of this, the company determined that a diffraction particle-size analyzer might be suitable for the desired testing requirements and perhaps enable a wider evaluation of this technology. Initially the company considered off-line laser diffraction testing, but that solution didn’t meet requirements. 



Figure 1: The Insitec real-time particle-size analyzer sits beneath the workbench for the grinder. Real-time data are displayed indicating the grinder status and provides guidance for taking corrective action if required. 


Next, online particle-size measurement was considered for the testing solution. The Insitec analyzer system’s speed, the small amount of material it needed for testing, and the representative nature of data it produced suggested that its technology could satisfy the demands of the testing procedure. De’Longhi decided to purchase a dedicated Insitec system for production testing. 


Developing Online Testing 

The Insitec system was installed at a workstation on one of the assembly lines. Fresh coffee beans are poured by the assemblyman into the grinder, and the test sequence is initiated. The ground coffee flows out of the grinding chamber to the analyzer. The coffee sample is aspirated by the Venturi effect into the instrument’s measurement zone. 

A parameter measured during laser-diffraction particle analysis is transmission or obscuration of the light amount entering the sample. Measurement at the right obscuration is required to obtain consistent particle-size data. Obscuration can be used to trigger an analysis in routine operation. 

This setup ensures that measurement is triggered at the right concentration or flow of ground coffee because the coffee reduces the amount of light seen by the detector. Hence there is no need for manual intervention. The sample’s particle-size distribution data are displayed in real time on the screen toward the back of the workstation. 

Data presentation is precise. The particle-size data are not the only parameters measured. The system’s software calculates a median particle size for the coffee as well. 

The operator focuses on the green areas of the screen that indicate if the grinder provides acceptable performance, and if not, how many turns of the gear wheel are needed for correction. 


Refined Solution 

There was a need for initial effort to refine the analytical solution to this level of simplicity and effectiveness. Main tasks were to establish suitable specifications for each manufactured grinder type and develop a suitable interface. 



Figure 2: Only small sample sizes are needed to to test the grinders.


De’Longhi worked with Malvern application specialists to study the full potential of the software that drives the analyzer and enables its integration with other processing equipment. By doing so data are presented in an easy manner, and risk of operator error is minimized. Testing is done in just a few seconds, and the amount of coffee used for the testing is considerably less. 

Because the grinder testing is done with real coffee, the small sample size is a significant cost benefit. All the grinders are tested, and each one leaves the testing station with numerical test data verifying performance. 

Even though the company does not depend on the assembly line staff’s eyes or touch, they are trained to recognize powders in this manner. By knowing the particle size data, it has been possible for trainers to teach people to recognize the feel of specific blends and to demonstrate the sensitivity of the solution that the company has developed. Such efforts emphasize for all staff the need to control the particle size of the coffee in a statistically reliable way, and underline company commitment to manufacturing the highest quality products. At De’Longhi the Insitec systems are in almost constant use because development staff also use the system to gather experimental data to support research. Particle-size results help evaluate different grinder designs, with, for example, different internals, allowing the company to refine its product offering. 

— Sources: De'Longhi, Quality Digest, The Quality Insider.



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