Using Ppk to set up Control Plans - Injection molding

J

james r

#1
My company use's the process in from the injection molding machines to control the quality of the parts being made. During the new mold samples the process parameters are saved using the machine SPC. The SPC data is then used to come up with control plan limits. My problem with this is that we are using Ppk calulations during this process to justifiy that our limits are right. I say this is wrong becuase there are no USL/LSL for the process to calualte the Ppk against. I'm I right?
 
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#2
Re: Using Ppk to set up Control Plans

Hi James,

Welcome to the Cove :bigwave:

Being weekend, the response may be slow and your question may not be answered promptly. :)
 

Kales Veggie

People: The Vital Few
#3
Re: Using Ppk to set up Control Plans

James,

Make sure, I understand you correctly.

USL/LSL = specification limits, usually from a customer drawing?

UCL/LCL = control limits?

Does the machine have a control algorithm build-in to the PLC?

Control limits are established from the inherent variation of your process and are independent from the specification or acceptance limits. You also must find out if the measurement system is suitable to control the characteristic. (if the measurement system is not good, you might be measuring noise only)

What is the characteristic that you want to control with SPC? It sounds that you are controlling a machine parameter (temperature or pressure or so)
 
J

james r

#4
Re: Using Ppk to set up Control Plans

we collect spc on fill time, transfer pressure, zone one temp, cushion, and cycle time. There are no USL/LSL limits for thse settings. The real problem is that some of the limits that are put on the control plan seem to narrow (+/_ 50 psi) to hold in a molding operation resulting in a continuous weaking of the process when we get a new batch on material.
 

bobdoering

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#5
Re: Using Ppk to set up Control Plans

You are correct, Ppk will have to relate to specifications you develop. These are your process parameters, and what you are looking for from the SPC is changes from the norm as a signal that something is wrong and action is needed. To do this, you need to understand your process (material, machine and molds, in particular) to develop the appropriate understanding of the relationships of fill time, transfer pressure, zone one temp, cushion, and cycle time to successful product manufacture. I bring this up because some of these things are interrelated, and some are independent. Many are affected by lot to lot material variation - so you do not want to set up your specs on one lot, only to have the next lot blow your specs away.

The answer to developing specifications for your process is a DOE (design of experiments) to find out how much variation of each parameter is acceptable for the desired output. I am not sure what zone 1 temp is - if it is barrel or mold, but I would have barrel, nozzle and mold temps (do you use cooling water? Add it, too) in the DOE. Be sure to use several batches of material, also.

No, this is not a three hour job - and you may not be up to it. But, I suspect it is the most correct answer.

Oh, wait...the most correct answer is "It Depends." :cool:
 
J

james r

#6
Thanks for the help

Of all the machine settings the following are critical to monitoring the process. mat'l temp, transfer pressure, mat'l flow, and cooling. With the machine SPC we are able monitor the front barrel zone, xfer pressure, fill time and cycle time to control the critcals. A DOE would help to understand the relationship and interactions of these outputs. great idea!

What I need is a proof soucre to show that trying to us the Ppk does not suit our needs. Can you provide any help there.
 

Miner

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Admin
#8
One thing that you have not made clear is the origin of the spec limits used to calculate Ppk.

Reading between the lines, it appears that the SPC data may be used to establish the statistical parameters (mean and standard deviation). Some multiple of the the standard deviation (e.g., 4 or 5) are used to create tolerances that result in an acceptable Ppk, which is then used to justify the use of the tolerances. Is this correct?

If this is the process that you are using, then I would agree with you. These are definitely tolerances that the process is capable of meeting, but this does not assure that the final product meets requirements. In addition, the argument is a circular argument.

The correct approach is the one recommended by bobdoering. Perform screening DOEs that identify the critical process parameters that affect the final product quality. Then perform optimization DOEs or Response Surface DOEs that identify both the optimum target values as well as the tolerances required in the process parameters in order to meet the final product requirements.

Going even further you could perform robust experimental designs that help you identify levels that minimize variation. This could either be the Inner/Outer Array noise experiments popularized by Taguchi, or the more efficient Split Plot experiments (process factors are the whole plot, and noise factors are the subplot) advocated by Box. Note: the analysis of the Split Plot experiment is more complex. JMP can analyze a split plot experiment directly, but Minitab must be tricked into performing the correct analysis.
 
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