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Process capability study in Chemical industry

Discussion in 'Capability - Process, Machine, Gage …' started by LBB M2C, Mar 9, 2021.

  1. LBB M2C

    LBB M2C New Member

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    I was hoping experts in this forum could help me out here.

    I have experience in automotive quality, where we dealt with components/parts that could be physically measured.
    So how do you apply the same quality control concept in a chemical industry, specifically polyurethane (raw material)?

    Our products are supplied in 2 parts of chemical (1 is Isocyanate, the other is a blend of polyols and additives).
    All we do in house is blend certain chemicals and supply them to our customer.
    So basically, we won't be able to see the final product until it gets to the customers' production line where the chemical reactions occur.
    Quality checks measure a few reaction timing from a sample foaming test in the lab.

    Does it even make sense to apply SPC to lab-measured attributes such as water%, reaction profiles (very subjective in measuring time), when the product itself is so sensitive to environmental elements (temp., humidity, etc.)?

    How do you even measure process capability in this case?
    What methods could be used for quality control?

    Any feedback would be greatly appreciated!
    Thanks in advance!
     
  2. Jennifer Kirley

    Jennifer Kirley Moderator Staff Member

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    Welcome to QFO!

    Can you validate with test samples? Not for every batch, but as a part of design and process validation?

    Industry experts I have read don't recommend the usual SPC methods for chemical manufacturing.
    I hope this helps!
     
  3. Miner

    Miner Moderator Staff Member

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    Things to consider: SPC stands for Statistical Process Control. Most chemical processes are already controlled by electronic process controllers. Adding SPC would be redundant and non-value added. The electronic process controllers would typically create false alarms (e.g., runs, oscillations, etc.) on a control chart. In addition, chemical processes are typically autocorrelated, which creates major problems for subgrouped control charts.
     
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  4. brownfield

    brownfield New Member

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    Does anyone have any experience using Student's t-test to compare batch-to-batch consistency?
     
  5. LBB M2C

    LBB M2C New Member

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    Thank you all so much for the feedback.

    The blending process is all manually weighed and loaded, so I'd say the process is far from controlled... except for the blending time and blade speed of the blender.
    After a batch is finished with the blending, we take a sample and conduct a lab test to review the actual foaming of the product.
    Cream time, gel time, rise time, etc. are being measured by a QC operator using a stop watch and visual inspection.

    Because the product itself is sensitive to temperature and humidity, I don't think it's possible to do an MSA because the samples will not be the same for each operator.
    Basically we're solely relying on the test result at the time of lab test, by that operator, and check if it's within 'spec', and certify the whole batch.

    I was hoping to find a way to objectively monitor our process.
    But for now, the only thing I see that can be controlled would be the weighing/loading of the raw material.
     
  6. Bev D

    Bev D Moderator Staff Member

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    SPC might help you in this case. you may have to think about what that means - you will probably need a "3D chart" which is designed for batch processes as well as strongly non-homogenous processes where you need to monitor more than 2 components of variation a free article for this is: The Three-Way Chart by Donald Wheeler.

    You also will be able to perform a useful MSA. The variation has nothing to do with the usefulness of an MSA. Try reading the second example in my paper "MSA for for Functional Tests" in the "practical quality engineering resources" in the resources tab.
     
  7. LBB M2C

    LBB M2C New Member

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    Thank you so much for sharing your expertise.
    For gauge R&R, however, I was under the impression that the samples set need to be identical for each operators.
    Otherwise, Appraiser variation couldn't be calculated correctly, am I wrong?

    I'll look into the article you mentioned, thank you.
    I still have much to learn and I appreciate your feedback!
     
  8. Miner

    Miner Moderator Staff Member

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    See my blog on R&R for Non-replicable Measurements, in addition to Bev's resources.
     
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  9. Bev D

    Bev D Moderator Staff Member

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    Student* did. The thing with t-tests is that they will detect non-homogeneity as a difference in means. Batch processes are notoriously non-homogenous. A homogenous process is one where the factors that determine the within subgroup (in your case within batch) standard deviation are the same factors that determine the average of each batch. You will likely determine that every batch has a statistically significantly different mean than the previous batch. That will not mean that the batches are “inconsistent”. You MUST understand how the math works and what the assumptions of these tests are.

    A better approach is to use the 3D or 3 way SPC chart AND determine how much difference between batches can be tolerated. A t-test cannot tell you that.

    *Student was the name that William Gosset used when he published his work that developed the t-test. Gosset used a nom de plume to keep his true employer secret and maintain a competitive advantage. Gosset was the chief brewer at Guinness. Happy St. Patrick’s day.
     
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