Max. Allowable M/C Hold Up Period Btw 2 spraying opn with same Powder Recipe&Set-up
12-03-2006, 03:30 PM,
#1
Max. Allowable M/C Hold Up Period Btw 2 spraying opn with same Powder Recipe&Set-up
Hi All,

This is a question pertaining the control of thermal spraying process. I hope someone who is in this line would be able to help to feed me with some info on different standard of practices in different shops around the globe.

What would be the maximum Machine hold-up period or time allowed between 2 spraying operation with the powder recipe/paramter, machine set-up be it in hardware or software conditions unchanged?

Assume that the previous spraying operation has a specimen attached to the part and the specimen has passed all QA test and criteria. And for continuous spraying operation, we need only one specimen to be representative of all the parts we spray in continuos mode. e.g: 1 specimen represents 8 parts sprayed in a day in continous motion. Once specimen has passed, all the part will be proceeded with post treatment or shipped, else a rework for all the parts will be disposed.

Yet, if I have finished all the parts, at 10:00a.m, without tearing down the gun or changing any thing, I have a part that comes in at 4p.m. There is a gap of 6 hours here. Do I need to spray the part with another specimen again? or I can take the advantage of the previous specimen result to continue to spray this part. If the answer is yes, then , again my question is how long maximum gap (in hours) is allowed or recommended, 6 hrs? 8 hrs? 12 hrs or without 24 hrs?

Thanks.

Regards,
Alex
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12-05-2006, 07:54 PM,
#2
RE: Max. Allowable M/C Hold Up Period Btw 2 spraying opn with same Powder Recipe&Set-
Hi Alex

Putting aside standard of practices and quality management procedures for a moment. Logically, if the set-up is not changed in anyway and the environmental conditions are reasonably constant, then there should be no difference in effect of say a 10-minute or 24 hour hold up time. You could argue that the probability of something changing would increase with time.

Taking Alex's example:
Quote:Assume that the previous spraying operation has a specimen attached to the part and the specimen has passed all QA test and criteria. And for continuous spraying operation, we need only one specimen to be representative of all the parts we spray in continuos mode. e.g: 1 specimen represents 8 parts sprayed in a day in continous motion. Once specimen has passed, all the part will be proceeded with post treatment or shipped, else a rework for all the parts will be disposed.

Yet, if I have finished all the parts, at 10:00a.m, without tearing down the gun or changing any thing, I have a part that comes in at 4p.m. There is a gap of 6 hours here. Do I need to spray the part with another specimen again? or I can take the advantage of the previous specimen result to continue to spray this part. If the answer is yes, then , again my question is how long maximum gap (in hours) is allowed or recommended, 6 hrs? 8 hrs? 12 hrs or without 24 hrs?

It does depend on your interpretation of "continuous spraying operation". If we are talking about spraying 8 parts simultaneously or 8 parts of jigged assembly, then yes you need to spray another test piece with additional parts. It is not time dependent, 30 seconds or 12 hours makes no difference. Additional sprayed parts are not related to original test piece. If we assume here "continuous" to mean no changes to a repetitive cycle of processing individual parts. Here the one test piece really is a check on the process set-up and operator, giving a level of confidence that all parts will be ok. In this case, time again should not really be a significant issue. Lets assume the operator takes 10 minutes to unload and load each part, which in effect should be no different to a 6-hour break. Also assuming here, that the operator is still on his shift. You could consider a change of shift or operator to be a change of conditions. So I would suggest no time limit is necessary within an operators shift time.

Sorry, getting a bit long winded, but I hope you see my reasoning.
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12-06-2006, 01:43 PM,
#3
Rolleyes  RE: Max. Allowable M/C Hold Up Period Btw 2 spraying opn with same Powder Recipe&Set-up
Gordon,

I agree with you on this point. The same operator within the same shift. And this definitely applies to manual /hand spraying operation.

However, if we are talking about automated computerized robotic spraying, the loading and unloading of parts by different operators, in any case will it contribute to the significant process change? I doubt so.

Further more, if the sample is sprayed prior to any production parts as a stand alone sample (for sprayability test), and the spraying parameter is set (including the angle and distance) as the same as any of the parts which may be different in geometry or configuration, but the whole idea is to affirm that the coating quality suffices all the test requirement. If this is the case, can I argue on this point and say the operator operating the machine, by identifying the correct CNC program number for each parts per operation sheets, making sure the loading and set-up is ok, dry running the program to make sure it is correct, and then igniting the gun, preheating, and press start/go to start spraying the part....etc. the S.O.P will be so standardized, so operator would not be a significant vital X towards whole coating outcome.

Thus, I can continue spraying parts by taking the advantage of the sample result for as long as possible if I do not alter all my recipe set-up and gun hard ware. Well, I recommend a maximum gap of 24 hrs is acceptable if to really put a control over this issue.

What's your comment? Is my reasoning making any sense? As the whole idea is to reducing sample spraying time, and the sample release turn around time, with the confidence level we have over the stabilization of process.

Thanks.

Regards,
Alex
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12-06-2006, 06:08 PM,
#4
RE: Max. Allowable M/C Hold Up Period Btw 2 spraying opn with same Powder Recipe&Set-
Hi Alex

If your not confined by your customer quality procedures, then really it is purely down to your own confidence level. I think the saying "failure can be expensive, quality need not be" or something like that is trueHappy0193 It will obviously depend on the nature of the part, being safety critical, high value or cheap mass produced etc.. Another point to consider, I don't think time is important, actual production throughput or gun-on time I would consider far more important. This will have direct effects on wear and tear of equipment hardware, which could lead to changes in quality.
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12-07-2006, 01:22 AM,
#5
RE: Max. Allowable M/C Hold Up Period Btw 2 spraying opn with same Powder Recipe&Set-up
Gordon,

Roger that. Thanks for your expertise opinion. We would not want to compromise on the quality of coating. The intension is to improve the cycle time while maintaining the high quality of coating integrity. Based on the logic and facts that the process performance is stable and the assumption of environmental factor is constant.

Shd there will any doubt or alteration to any of the aforementioned area e.g gun hardware or recipe set-up, we will not tolerate but to respray a new set of sample. And if the sample is rejected, we will call out all parts that were sprayed for rework.

Thank you again for your sharing of views.

Regards,
Alex
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12-08-2006, 01:13 AM,
#6
RE: Max. Allowable M/C Hold Up Period Btw 2 spraying opn with same Powder Recipe&Set-
Hi Alex

I really do appreciate your position on this subject and certainly have no arguments with your view. In answering many of the questions on this public forum I try to push my response for the benefit of other readers as well. I am often accused of arguing that black is white when I really believe black is black (argumentative ******). Anyway, I think it's probably good to occasionally try to argue a point of view from the other side.

Having spent a good number of years in laboratory environments (R&D rather than production in thermal spray) I have come across comments like ?necessary evil? for the existence of the laboratory from high management personnel. I use to worry about such comments, partially understanding that laboratories and skilled people are an expensive and so called non-productive burden, but customers do pay for that extra level of assurance, so I think it is a narrow minded view.

I know of many smaller thermal spray companies that do no have coating evaluation facilities. They rely on monitoring the process. They know from experience that if they maintain correct process parameters, they will rarely get poor coatings. In coating development, the reverse approach is used, where coating evaluation is used to establish what process parameters to set. Alex, I believe most of your coatings are for aerospace and you tend to use a more belt and braces approach and monitor both process and the resultant coatings (just as well seeing aircraft rely on many TSCs to stay in the air).

Sometimes we can over burden a process with too much quality management to an extent where both quality and production can suffer. So I think simplifying the quality and process procedures, but ensuring that all the important checks are in place must be a good thing.

Cheers
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