Internal Results vs. Independent Testing Laboratory - Inkometer

S

Southern Cross

Involved In Discussions
#1
#1
Our product is tested with an instrument called an inkometer. The product is a resin solution used to manufacture printing ink. At the moment we test on our own machine, then send a sample to the independent lab. We are seeing multiple rejections of our product, requiring re-adjustment prior to sending the product to our customer. We have known about our problems for some time, finally I have time to try to address them. I've done a Gage R&R test last year and the total R&R was 23%. Not too bad for a process industry instrument. Nevertheless, we are still encountering rejections.

The specification at the independent lab is 23.5 to 25. Roughly 40% of the samples we send fall outside that range according the the independent lab. See the enclosed files for the graphical information.

At this stage I'm more interested in the instrument itself. It is used by placing 1.32 mls. of solution on the rollers. The machine runs at 800rpm for one minute (90oF) and then takes a reading of the force exerted on one of the rollers. I've been checking the instrument each day recently using the calibration weights and I'm seeing significant variations without even using a sample. Cp without a sample is 1.3.

I need to run an experiment to test for variations in result based on (1) temperature and (2) sample volume. The problem is that I can't easily control how much volume is placed on the instrument, so a standard full factorial DOE is not possible. What is the best way to approach this?

1.JPG

2.JPG
 
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J

justncredible

#3
#3
Ignore the sample volume on the rollers, you might instead use viscosity or some other type to monitor the ink sample.

From what you have said I would be looking at the test rollers for bearing wear and tension spring wear.

It has been some years since I worked in printing, and it was web presses. The bearings on the rollers gave us alot of problems. As did the glue. We used a hand roller to calculate the spread of inks, all very rough ways to try and control a process.

You might increase the sample number to smooth out the noise of the process. Do extra runs to get a decent picture of what is affecting the process as well. Make sure you track the humidity as well as temps, also the basis weights of the stock. Ink laydown and saturation also can play into a study.

Also I would send a extra sample to a diffrent lab and make sure the 2 samples from the labs match each other.

Printing vexxed me to no end......600 bags a min and I have no idea why they all came out looking fine..
 
Hershal

Hershal

Metrologist-Auditor
Trusted Information Resource
#4
#4
The previous reply is right on track. Also, to reduce the potential variation from your monitoring items, have the weights and any temperature monitoring device calibrated by an accredited cal lab and make sure you get the uncertainties.
 
S

Southern Cross

Involved In Discussions
#5
#5
We already use a rheometer to control viscosity and psuedoplasticity. What we have found over the last few years is that tack is independent of viscosity in our product.

The instrument is only a year or two old and it's the latest model. It is calibrated, with temperature, on a regular basis. Our balances are calibrated on a regular basis.

What I'm seeing at this stage, even just using the weights, there is a good deal of variation in results from day to day within a range of about 0.4 units. Not so good, but I'm going to have to live with it.

My interest at the moment is in trying to determine how much the temperature and sample volume change the result. I've been working in the industry for ~20 years and I've never seen anyone actually try to systematically work this out before.

Fortunately we're not the ink manufacturer or user. I don't have to concern myself with paper variation due to humidity etc. That's our customer's problem and they keep that information to themselves.
 
Stijloor

Stijloor

Leader
Super Moderator
#6
#6
Southern Cross said:
<snip>My interest at the moment is in trying to determine how much the temperature and sample volume change the result. I've been working in the industry for ~20 years and I've never seen anyone actually try to systematically work this out before.
Click to expand...
Could a Design of Experiment (DOE) be of help here?

Stijloor.
 
S

Southern Cross

Involved In Discussions
#7
#7
Stijloor said:
Could a Design of Experiment (DOE) be of help here?

Stijloor.
Click to expand...
That's what I'd like to do, but I won't be able to control the sample volume accurately. Best I can do is add some or take some away, then weigh how much is applied by difference. I'm thinking that multiple regression is going to be the best approach?
 
J

justncredible

#8
#8
How much temperature variation do you have in a normal day? Is the unit in a controlled lab?

I do think your on the right track, also track the humidity. Make sure it is well grounded, a ungrounded paper conversion press can produce so much static it will actually warp light. Make sure plenty of brushes contact the paper. You might also see if the pumps are shearing the ink. A viscosity test before and after might show shearing of the ink.

To find the interactions first you must figure out the gage issue. How precise and accurate is it? Will it be good enough to study the tackiness of the ink? How do they calibrate the gage? Do you have a GR&R for it?
 
J

justncredible

#9
#9
How much temperature variation do you have in a normal day? Is the unit in a controlled lab?

I do think your on the right track, also track the humidity. You might also see if the pumps are shearing the ink. A viscosity test before and after might show shearing of the ink.

To find the interactions first you must figure out the gage issue. How precise and accurate is it? Will it be good enough to study the tackiness of the ink? How do they calibrate the gage? Do you have a GR&R for it?

Get a thermometer and measure the weights on the unit at every degree change thruout the day. Springs are pretty wild and do things unforseen.
 
D

DsqrdDGD909

#10
#10
Southern Cross said:
That's what I'd like to do, but I won't be able to control the sample volume accurately. Best I can do is add some or take some away, then weigh how much is applied by difference. I'm thinking that multiple regression is going to be the best approach?
Click to expand...
There are some highly accurate adjustable micropipettes.
Capacity Increments Accuracy (CV%)
0.2 - 2.0mL 0.01mL ±0.7 ±0.5 @ 0.2mL
±0.5 ±0.2 @ 2mL
1 - 10mL 0.1mL ±0.5 ±0.3 @ 1mL
±0.5 ±0.15 @ 10mL
 
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