Manufacturing processes can sometimes require refinement to ensure products of consistent high quality. These adjustments could be to material quality or to process conditions. This approach sometimes provides a totally satisfactory result, but often the improvement is unsatisfactory or further problems appear.
A solution, combining practical process experience, statistics and common sense is available. The technique often referred to as Taguchi experimentation was used initially in Japan. Through using an orthogonal array the effect of each variable can be measured in combination with other variables. Properly designed experiments enable the array to separate the effect of each variable within a matrix of manageable size.
The National Physical Laboratory has managed a programme on developing materials measurement methods for adhesives in which the Taguchi method has been applied to aid companies in the manufacturing process. This programme was sponsored by the Department of Trade and Industry. Analysis was conducted by Xyratex (formerly IBM Havant) with the assistance and expertise of PIRA.
The Taguchi approach allowed CPC International Inc. (Bestfoods) to focus on the problem of variable corner bond strength on their corrugated cardboard tray. These trays which carry 12 jars of mayonnaise are made at a rate of up to 50 trays a minute. Failure of trays which are the only support device during packaging can be extremely costly to CPC, the distributor and the retailer. It has been calculated that up to 14% of production line downtime is attributable to the tray maker. From the production figures mentioned above, the cost of failures can be estimated.
The use of the Taguchi method identified non-critical process parameters which provided scope for cost reductions and allowed the examination of production variables on corner bond strength. The Taguchi system also allowed other areas of production to be improved
The trays are erected from a flat sheet with the corner flaps bonded using hot melt adhesive. The adhesive is applied from dauber mechanisms, and the tray is erected by being pushed down a forming die.
Trays were being produced with variable bond strength, with intermittent problems of one or more comers failing either further down the packaging line or in distribution. In the worst case failure occurs immediately and has been found difficult to correct.
The objective was to produce trays with less than 0.1% bond failure.
Corrugated cardboard corner flap
Corrugated cardboard corner flap
Hot-melt adhesive
The high level of board warp was achieved by post conversion treatment of half of all sample variants. Crease stiffness was established by conversion creasing conditions.
Adhesive is applied from dauber mechanisms
Tray is erected by being pushed down a forming die.
Material variables include adhesive type, board warp, surface bond receptivity
Process variables include; spring setting, crease stiffness, adhesive placement, machine speed and pressure plate setting
Air cure
The bonding was carried out at the factory location
An initial list of 48 variables was identified by brain storming. A final list of the 8 variables thought most likely to influence the process was achieved with associated test levels.
|
Factor |
Experiment |
Level 1 |
Level 2 |
|
Adhesive type (open time /setting speed) |
A |
medium/fast |
fast/fast |
|
Spring setting (initial erection) |
current |
low | |
|
Board warp (end to end) |
C |
< 3mm |
10 mm |
|
Crease stiffness (cut:tie ratio) |
D |
50/50 |
30/70 |
|
Surface bond receptivity (wax pickstrength) (front/back) |
E |
14/20 (test/kraft) |
20/28 (kraft/kraft) |
|
Adhesive placement position (from flap) |
F |
1/4 |
3/4 |
|
Machine speed (trays per minute |
G |
28 |
36 |
|
Pressure plate setting (in die) |
H |
current |
heavy |
The bond strength was measured using a purpose built rig. The fast/fast adhesive was thought preferable for the process, but the supplier stated that it had lower bonding "power". Spring and pressure plate settings were not absolute, but the adjustment systems were graduated to give reference and repeatability. The high level of board warp was achieved by post conversion treatment of half of all sample variants. Crease stiffness was established by conversion creasing conditions. The 50/50 condition gave 1650mN. The 30/70 gave 2220mN for the kraft/kraft board but 1710mN for the test/kraft board
The analysis showed five variables to affect the result. these were ranked as below.
|
Factor |
Setting |
|
Adhesive |
Fast/fast |
|
Warp |
< 3mm |
|
Crease stiffness |
50/50 |
|
Adhesive placement |
3/4 |
|
Pressure plate |
heavy |
Non-critical factors were set to preferred conditions (spring-low: surface-kraft/kraft:, speed 36 per minute).
The objectives were met, and a considerable increase in bond strength obtained. Taguchi and other design of experiments systems have shown that they have a major part to play in developing a rational approach to process optimisation.
DTI MTS programme adhesives – Dissemination Guides ADH5CS2.DOC
NPL / ESR Technology Limited
Analysis was conducted by Xyratex (formerly IBM Havant).
Images courtesy CPS international http://www.agpak.com/packages-mn.html
