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Design options with plastics

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jmw

jmw

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Jun 27, 2001
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At the risk of being shot down for not googling properly, I have a query way outside my field.
I am trying to discover the penalties of poor product design.

I have a fairly simple component that is to be moulded and the final component should meet very precise dimensional properties and should not distort.
But when designing the part I have two options.
For simplicity suppose we have a hollow cylinder where the target internal and external diameters must be met with very precise tolerances. The cylinder also has slot through the wall extending along the entire length.
It is a mass produced item.

The question is to know if there are any penalties if I make the part with a constant diameters along its length or if I should introduce a slight taper to both the internal and external dimensions.
Is there a cost penalty?
Does it affect the choice of plastics and/or fillers?
Does it affect any aspect of post moulding management?

JMW
 
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In my opinion draft will not be necessary because of the "C" rather than "O" shape and the relatively short cores.

I think I actually have some samples of Davies Kent? (it's been a long time) water meter mouldings from the 70s in my examples kit.

Regards
Pat
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Water meters come in all sizes.
The most common are the 1/2" and 3/4" which are approximately depicted here.
This is thus the dominant design.

If for the larger meters we must now scale all the parts up, how does increasing size affect the discussion, if at all?

JMW
 
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Pud:
imho, the whole assembly could probably usefully be redesigned for modern materials.
Click to expand...
But what and why?

My original thought was that if the nutating disc meter were in any way easier or cheaper to make by virtue of its geometry then there would be justification for changes to the design of the rotary piston meter.

Hence my query about drafting angles.
But these designs long ago "matured" (locking in design compromises necessary to 19th Cent. manufacture) and designers act like the "Guardians of the Sacred form" to the extent that these 19th cent compromises are still there despite the shift to plastic.

And since, from the above discussion, there are no benefits that would allow better quality control or cheaper manufacture, cheaper plastics or less sensitive "curing" or temperature control, then changes cannot be justified.
Note that I refer to changes that relate to the differences between the designs but I am open to changes that would affect both.

So perhaps it was my mistake to ask about drafting angles, which in my ignorance I had thought might make a difference.
And despite the sacredness of the original design, it is the easiest thing in the world to include whatever drafting angles you want into the rotary piston design without affecting its function or performance.

But what you say suggests there is some other issue that could prove useful?
You don't need to be specific if this is commercially advantageous to you, I'll settle for a yes or a no.

My objective isn't to improve either design. I was just looking for any cost advantages to a new single element PD meter design.

No technical or commercial advantage?
Then it remains a "Nice, but so what?" design.

I need some sort of compelling technical or commercial advantage to overcome "ye olde fear of cross-capture" (or for some smaller company to risk investing in a new design on the chance that may allow them to grow market share significantly). Though I think there are some valuable performance benefits, there is nothing that compels management attention like cost savings.


JMW
 
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jmw

I know this is a broad generalisation, but typically many of the advantages of plastics are lost and many of the disadvantages highlighted when we simply drop plastics into a design in place of existing metal.

Also if the original designers made the design in late 19th century, I doubt they can complain about or resist changes. The inertia against change must be a tradition passed down through generations. How that helps you I don't know. My typical response to such situations is, if we don't make our current designs obsolete with new and better models, our competitors surely will. Unfortunately I have been proved right on to many occasions.

Regards
Pat
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In this case the only change made would appear to be to make the measuring chambers smaller. I'm pretty sure this was necessary to pursue the low flow rates and at the expense of faster rotation at higher flows with consequent increased long term wear.
There are no other changes. I suspect manufacturing may have wanted them but been denied them.

But, oh Pat, to have such reasonable managers...

The "if we don't obsolete our own products our competitors will" might just as well be in a lost language for all the comprehension managers have.

It worked in our favour once when a competitor dragged their heels introducing new technology and a major client (the third biggest user on the planet) approached us.
They had 90% of the market and had held that for over 40 years. They saw a new product as costing them a lot of money to introduce and not improving their margins or market share in anyway. Indeed, moving from a mechanao like product to a no moving parts modern sensor meant a big hole in their after market sales of spares and services.

The day their MD discovered we had produced a new product that so outperformed them and had so much more functionality that even their intended new product couldn't come close, he was at a system integrators and apparently he went grey and practically collapsed into his chair. Then they told him the really bad news; we were only a few $ more expensive than them.
They only get crumbs in that market now.

This is far more common than should be reasonable.


JMW
 
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I have seen more than one national icon closed down because oif it.

A fictitious example.

If the Acme Buggy Whip company who was the manufacturer of the finest most cost effective buggy whipps ever had diversified into making carburettors, then fuel injectors, they might still be in business today.

Regards
Pat
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A real and valid design.

A potential product?

Well, only if I can show compelling unique technical and/or commercial advantage and to the right company which isn't the majors.

The way to recognise the commercial and technical advantages is to be able to cross compare the piston, disc and new meter design with an experienced eye. Then verify with some initial investment in proof of concept models for flow testing.

This is definitely not a "go it alone" product, not in this mature market.

Compelling, with the major companies, means holding a gun to their heads, they don't want to obsolete their existing product with no real gain in margins or market share and don't value modest gains in either commercial or technical performance.

Going it alone would also mean finding investors and, according to US studies, inventors end up with maybe a 4% share of their own companies and get edged out of the company by the new owners after a short while. Royalties pay better than that.

The large companies don't want to obsolete their existing products but one of the reputable smaller companies? They'd be desperate to obsolete the existing designs because this gives them a means to grow market share with decent margins.

But then have to counter natural scepticism.
Improbable that I should have discovered a previously unknown single element meter not in the literature even as a failed concept?
And that it has remained undiscovered despite the R&D invested by major companies over the last 130 years since the nutating disc meter hit the market?
How fortuitous that it should also perform better than the existing two.

Showing that it is a valid functional design is easy.
Showing why anyone should care is more difficult.
Maybe the technical advantages won't be as compelling as is necessary.
Can't say.



JMW
 
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There is, or was, in the UK water act of 1942 or some when, the concept that meters should provide an "equable" means of charging.
This meant that what was metered should represent what was used. It did not have to accurately measure what was used because trickle flows are a significant proportion of the flow in the indirect UK plumbing systems (a relic of the Napoleonic wars where each house had a cistern or header tank which filled from the mains with a float valve).
Hence if you measured the majority of the flow range it would be assumed that, on average, the trickle flow represented a rough proportion of the metered flow.
Water companies levy their charges to make a profit. Cost plus margin. SO they ensure that the metered readings times price per unit satisfy this need.
A further thought it that this is also an area of metered transactions where increased measurement accuracy does not necessarily equate to better transaction accuracy. Utility metering is in its own unique category distinct from other metered transactions like petrol fuel oils etc.
It's fine if a meter has better measurement accuracy but it had better not be too costly.

But yes, low flow is an issue.
I also wonder if the no post moulding machining plastics introduced more tolerance uncertainty, more slip flow and compromised low flow performance, the solution to which would be a smaller chamber with higher rotational speed at high flows and increased wear.


JMW
 
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The part that came to my mind as looking like your description (minus the slit) is a syringe. They are precise, high volume, low cost molded parts with nearly no draft. As others have the said the material choice and details of mold construction have a huge impact on the ability to mold without draft.

Machining of plastics to improve fit is difficult. It is usually preferred to spend more on the mold design and construction.

Elimination of any fitting of the parts would improve unit to unit repeatability (interchangeability) which might be more important than actual accuracy for your application.
 
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I can't discover whether a draft is included or not.
Nor what effect this has on choice of materials or costs.
Clearly the nutating disc meter is ideal for moulding as there is no need to design in any special drafts. But I can't determine if there is a difference in the choice of plastics between piston and disc meters.




JMW
 
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For a water meter you need good abrasion resistance in the presence of water and exceptional dimensional stability and documented suitability for use in contact with potable water. Some grades of Noryl or modified PPO meet all three exceptionally well.

The syringe case is PP and it will easily jump over undercuts. This can be helped by a poppet valve and compressed air in the needle end so the cartridge is expanded by the air and blown of the core by the air pressure. PP is not really suitable for an accurate water meter and Noryl will not readily jump over an undercut. It really is horses for courses with real regard to the overall balance of all properties and the application requirements.

Regards
Pat
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Well, according to my DFMA training today you definitely need draft so there you go. Pay no attention to the 2 or more industry relevant drips under pressure and listen to my instructor who, no offense, was a nice guy but didn't seem to have more than a surface knowledge about anything technical.

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Just took apart a Schlumberger water meter and it has lots of visible external surface drafts on the chamber where draft angles don't matter but no internal draft angles and none on the piston.
In the "mature" design there can be none.


JMW
 
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