Quality improvement of cans for canned Food
Key words
Suppliers
Selection, Process Capability, Statistical Process control, Sigma Level of
Process, Defect per Million of Opportunities, DMAIC Introduction Onix is a
factory that provides metallic components, some of them are cans for canned
food. These cans are used by several clients that are big manufactures of
conserve meat and fish. In the last weeks, Onix is receiving a lot of
complaints about the quality of products supplied, most of them around the
sealed parts and nicks on the body. For that reason the factory will employ the
Six Sigma DMAIC procedure in order to find and eliminate the principal root
cause of the problem. Define
When the
Onix management group receives the mentioned group of complaints, immediately
activate its Quality Committee to investigate the main root causes of these
problems. This team find out two possible and antagonistic root causes of this
problem, as shown in the next chart.
|
Cause Type
|
Action
|
Possible Error
|
|
The fault is caused
by a natural wear of the machinery
|
The workers can
solve the problem fixing the machinery or replacing the affected parts
|
Error type 2:
Consider the raw materials as the problem and do not adjust the
machinery
|
|
The cans defects are
produced by defects with the raw materials, because the managers in the
effort of reduce costs, buy tinplate with bad quality. This problem in
unknown for the workers and the managers have no idea of damage produced.
|
There is nothing to
do for the workers, only the managers can solve this problem, changing the
shop policy.
|
Error type 1: when
the work line produces a defective can, the natural reaction of workers is to
fix the machinery; but the problem is on the material, so this fixing can
aggravate the problems, meanwhile the real root cause persist undetected.
|
After this
causes analyses, the work team has to answer the following question: What is
the real root cause of this problem? To solve this problem, what we must do
first is check the problems with suppliers, (the process stability and
capability). Measure In this
case we can’t check the process stability and capability before is discarded
the problems with the raw materials, because problems with raw materials
affects the process stability because it is a variation cause.
Specifically
for this problem, the quality characteristic that we must take into account is
the tinplate hardness; for that reason it was carried out specific controls for
that characteristic, which have to be higher than 90 N/m2. The data
sets obtained with the Vickers test are presented following:
|
Supplier 1
|
Supplier 2
|
Supplier 3
|
|
84,8949
|
97,697
|
85,7721
|
|
97,7484
|
87,119
|
89,4851
|
|
83,6844
|
100,368
|
89,3418
|
|
87,7264
|
94,259
|
88,3778
|
|
93,5217
|
89,360
|
88,0630
|
|
97,1654
|
98,981
|
89,6156
|
|
86,9147
|
99,737
|
89,5801
|
|
97,0011
|
98,848
|
87,6867
|
|
85,8637
|
103,940
|
89,7483
|
|
90,1963
|
76,806
|
91,1584
|
|
88,5920
|
108,281
|
88,8189
|
|
96,1181
|
95,032
|
91,2991
|
|
88,6737
|
84,369
|
89,8778
|
|
84,4503
|
103,696
|
88,5731
|
|
87,8797
|
84,386
|
90,4986
|
|
83,3948
|
112,514
|
87,1240
|
|
92,9444
|
91,732
|
88,4587
|
|
90,5980
|
100,175
|
90,8822
|
|
92,6927
|
103,243
|
88,3840
|
|
87,3310
|
93,394
|
89,3629
|
Analyze
In order to
have a better interpretation of the hardness results, we assess the supplier
capability, seeing that vendor 2 is the better, because have a good values of
process capability, vendor 3 can accomplish the harness requirements but only
with 3 sigma level and vendor 1 are not capable for our requirements.
Improve
After
cheeked the quality of raw materials, is needed to verify the process
statistical control and capability. For accomplish this goal the production is
started using only the best quality raw materials, provided by supplier 2.
During this production the work team verifies every lot of 10 000 units, with
the objective of do not supply any lot with 0.5 % of defects; the obtained data
is presented following.
|
Sample
|
Defects
|
|
1
|
21
|
|
2
|
24
|
|
3
|
16
|
|
4
|
12
|
|
5
|
15
|
|
6
|
5
|
|
7
|
28
|
|
8
|
20
|
|
9
|
31
|
|
10
|
25
|
|
11
|
20
|
|
12
|
24
|
|
13
|
16
|
|
14
|
19
|
|
15
|
10
|
|
16
|
17
|
|
17
|
13
|
|
18
|
22
|
|
19
|
18
|
|
20
|
39
|
|
21
|
30
|
|
22
|
24
|
|
23
|
16
|
|
24
|
19
|
|
25
|
17
|
|
26
|
15
|
Control
Parting on
the obtained data for the lot defects, is executed a quality capability and
process stability study, obtaining that in general the process is stable,
despite it have two points out of control limits, besides the process is
capable to accomplish the clients requirements . The 26 lots have 0.1984 % of
defects; that means 1984 defects per million. See next graphic. The Z
metric converts the average percent of defects in a capability index similar to
the other that can be calculated with continuous data. In most cases is desired
a Z value higher that 4, in this case is obtained 2.8, for that reason we can
assume that the process must be improved if we want to obtain world class
capability indexes.
The
tolerance limits show that is expected less of 95 % of lots with 27 defects. Conclusions It was checked that the principal problem was
the poor quality of raw materials, obtained from 3 different vendors that have
significant differences between the hardness of tinplate provided. It is needed
a rigorous control of raw materials to prevent this problem.
It is strongly
recommended to maintain commercial relationships only with supplier 2 until the
other suppliers can meet the quality requirements needed by Onix.
|