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  Post Number #17  
Old 18th April 2018, 05:20 PM
Wes Bucey's Avatar
Wes Bucey

Total Posts: 11,159
Re: Dimension control on injection molding parts

As I said, a long standing topic here on the Cove.
Here are some snippets from my posts over the years:
In 2015:
"Back in the day" when I owned and operated a precision contract machining company, we made "small" things for customers (nothing bigger than a human fist) and would ship BOTH First Article and our Inspection Report with space for customer to replicate the inspection with similar instruments as noted in the report. Oddly, after we incorporated that clause in our contracts, many of our customers welcomed the change and were very prompt in responding. (Part of the contract was approval of the exact inspection procedure and agreement both customer and supplier would use similar instruments and techniques.)

We were adamant, though, we would NOT ship without approval or written waiver. Often, we would not proceed with manufacture until the approval or waiver on any product which might have "subjective" attributes (color/finish/etc.) versus "objective" characteristics (dimension/hardness/chemical analysis.)

Every supplier must determine its "will not cross" lines. Ours were more like chasms that needed a bridge instead of a line that could easily be stepped over. We found it a commercial advantage in that customers valued the integrity.
In 2014:
As a supplier, I was obsessively customer-centric and did everything I could to make the customer's job of handling my products as trouble-free as possible. This obsession resulted in great customer loyalty. My goal was always to create a dock-to-stock mentality about our goods, by-passing incoming inspection if we possibly could.

NOTE: all our products were custom-made to a customer's specifications (sometimes the specs and designs were changed as a result of our concurrent engineering - discussed in this post.
Thus, the following discussion is really only valid for custom-made products.)

As a consequence, we agreed, as part of Contract Review with the customer, on EXACTLY what we would inspect, with what instrument, in what order (feature numbers on engineering drawings) and would report those on a checklist with each shipment.

Usually, this resulted in a columned check sheet with each feature number listed, the instrument used, the result, and a column for the customer to fill in with HIS readings.

If we had a sufficiently large run using SPC, the exact samples would be numbered and attached to the check sheet. Obviously, if desired, the customer could pull his own samples from the shipment as well.

This obsession to detail and precision convinced almost all our customers to rank us as dock-to-stock suppliers, skipping incoming inspection.

We ALWAYS included the engineering drawing of the revision we followed, with the feature numbers, so there would be fewer chances of a receiving inspector, himself, using an obsolete version of the drawing. None of our customers thought it burdensome to comply with our request for collaboration on inspection procedures and reporting. In a converse situation, I don't foresee a supplier objecting with a customer's request for something similar.

View this post
Inspection - Done right, it can be a marketing advantage
for links to a number of pertinent posts on inspection processes.
recapitulation of previous pertinent posts:
Over a number of years here in the Cove, I have written about my high tech contract machining business which I ran from 1990 to 2000. Since we may be shutting down the Cove in a few months, I offer you a last chance to review some of these to see if anything might be pertinent to YOUR situation, either as supplier or customer.

Regular Cove readers may be familiar with my frequent rants against Kwality Kops (those Keystone Kop types who relish their power to say gotcha when they detect a nonconformance.)

Ideally, there is no us versus them rivalry in the workplace. All should be happy to work toward a common goal of a more efficient, profitable environment.

I have written previously that our Quality guys did not perform routine inspections - they designed the inspection protocol for each part and trained operators, interfaced with customers and suppliers, and acted as court of last resort when issues arose.

In my own operation, I took the police action out of the quality function and had a much happier workforce. The following post is an overview description of what we did. Perhaps you might glean some insight into helping your organization work in cooperation instead of conflict.

I went into more detail in this series of posts: Wes Bucey on an efficient shop - empowerment (This is a single post (#18) in a longer thread - the url leads directly to the post - it is associated with a follow-up in post #20) Wes Bucey on quoting and empowerment (This is a single post (#20) in a longer thread - the url leads directly to the post)

In process and final inspection: Re: Inspection Dimensional Check Sheets - Over 500 part numbers (Inspection Dimensional Check Sheets - Over 500 part numbers)

Who inspects? I, too, ran a shop where primary responsibility... (Production duties vs. Final Acceptance Testing - New Quality Manager needs HELP!!!)

Control Charts In my high tech machining business (1990 -2000),... (Using Control charts on shopfloor filled by operators)

What makes sense? In point of fact, I have seen several operations... (Would you leave the Quality Assurance profession given the option?)

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  Post Number #18  
Old 18th April 2018, 08:50 PM
Marc's Avatar

Total Posts: 26,590
Re: Dimension control on injection molding parts

I apologise for getting this discussion off track. I've been known to do that...

Original Post (Key Question):
In Reply to Parent Post by Jafri View Post

I am thinking what should I ask them to ensure that these dimensions stay in tolerance after some passage of time?

Should I ask them for yearly CMM check, even if from outside? Or do customers usually just don't ask for such verification?
So - year to year dimensions, and some type of plastic or rubber material (the expected life of which isn't an issue in PPAP documents(?)).

In this scenario yes - Good prints is Job #1, but measurement method/equipment is extremely significant since often the material properties do not allow for "normal" measurement instruments such as a caliper.

This came to mind as I spent some time in injection molding, another situation where I was doing problem solving as that's what my contracts were about. Dimensional measurement came into play a lot. Rubbers, plastics and such are far different animals to measure than metals.

A lot of good posts. Excellent discussion! In a way, Thanks for the memories!
  Post Number #19  
Old 19th April 2018, 01:12 AM
Wes Bucey's Avatar
Wes Bucey

Total Posts: 11,159
Re: Dimension control on injection molding parts

In Reply to Parent Post by Jafri View Post


This is about one of our suppliers who will be sending us injection molding parts.
They are going to do First Article Inspection on all dimensions in 3 samples initially. Some of these dimensions are difficult to check without CMM, which they don't possess. Nor do they have any checking fixture.

I am thinking what should I ask them to ensure that these dimensions stay in tolerance after some passage of time?

Should I ask them for yearly CMM check, even if from outside? Or do customers usually just don't ask for such verification?

What's the real problem? You imply it is because supplier can't verify to the micron via CMM? Isn't the real problem essentially: Form, Fit, Function? As we learn later in this thread, Buyer doesn't have independent instrumentation to verify the dimensions either. This means buyer HAS to trust supplier or why is he doing business with him? Ultimately, buyer and supplier have to cooperate in finding a way to give assurance the mold and parts made from the mold remain stable over the term of the contract. FAI is useless if the instruments used to verify dimensions are incapable of accurately measuring the dimensions.

In Reply to Parent Post by Ninja View Post

Do you have a CMM?

If you are specifically concerned with tool wear and dimensional creepage...can they send ahead first and last article for you to verify?

Just thinking out loud...
Yep. My first thought. too. As we see later, the deal is more muddy.

In Reply to Parent Post by Golfman25 View Post

I think you both need to sit down and develop and agreed checking methodology. Gauging and fixturing may help.
Very sensible statement. It is easier when supplier and buy work as PARTNERS in produced an acceptable finished product.

In Reply to Parent Post by Jafri View Post

No, we don't have CMM either.
Yes, I'm concerned about too wear, and how the supplier will be able to find out about it before it starts making bad parts.

The supplier will do FAI 100% once, but I don't understand what is last article?!
Ah, now we are beginning to see some light. It isn't common, but it happens from time to time that designers don't take into account that their organization isn't equipped to perform sophisticated tests to assure products they buy meet the specifications in the designs. In such cases, they do something to assure themselves the product reaches them in ready to use condition. Usually this means selecting a supplier who has the capability and capacity to do that and the buyer will contract with an independent service with similar or greater capability of measurement and test to act as a surrogate in verifying the work of the supplier. I did it all the time for tests like metallurgy, chemistry, heat treating, x-ray diffraction.

IN SUMMARY: Marc is correct. We, especially I, went far astray on this topic. (Typical Wes - tell you how to build a watch when all you want is the current time of day.) Simply, buyer and supplier need to sit down together and work out a way to assure themselves they make and use a product which meets all specifications to form, fit, and function. If this means contracting a trusted third party, that may be what's required. If it means buying proper instruments and getting trained people to operate them, that's an economic decision for each party to make.
  Post Number #20  
Old 19th April 2018, 06:42 AM
Scanton's Avatar

Total Posts: 55
Re: Dimension control on injection molding parts

We used to get this from our bigger customers on a regular basis where we would receive a drawing to make a component, so we would make that component and then a second drawing would appear (with the same revision number) because they forgot a critical feature or two, and then another (with the same revision number) with a tighter tolerance that should have been on the drawing from the beginning. We would dutifully do the extra needed to satisfy the customers’ needs because they are a big customer and we need their business, and before you know it you are making a component “just for fun” for the next 5 to 7 years with any margin long gone through all the extra resources need to satisfy these “additional” requirements.

I recent years we have taken a much harder line with customers. If they change the drawing, we review this change and if it costs more to satisfy these changes because of additional resource or increased cycle time etc, then we re-quote at a higher price to pay for the additional resource required to make this component at this new revision level, simple. We also do not accept changes to a drawing without a change to revision level, you would be amazed how many times we have seen that from big business.

I agree with what many have said here, all of this should be sorted out at the beginning of the process, unfortunately it hasn’t been so you are going to have to work with your customer in order to get what you need at a reasonable price (or take your business elsewhere). If I was in their position and the job is high volume and running for years, I would requote the job at the latest revision drawing and amortize the cost of a CMM over the entire contract on X number of parts.

The biggest failure here has been your companies inability to communicate what it actually wants to its supplier, this should have been done at the quote stage.

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