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jmw (Industrial) (OP)
7 Feb 12 12:28
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
www.ViscoAnalyser.com

 

jmw (Industrial) (OP)
7 Feb 12 12:40
Let me add this which I found on a web site which is pretty much my understanding of what is best but not what the consequences are from not observing this recommendation:

Quote:

Design Considerations

Part design should include draft features (angled surfaces) to facilitate removal from the mold. Depending surface length draft angles down to half a degree are reasonable. Typical draft angles should be about 1 to 2 degrees for part surfaces not exceeding 5 inches.
Dimensional tolerance specification will govern the part cost and manufacturability. If you have a small region of the part that needs higher tolerances, say the location of a critical feature used for alignment. DO NOT specify tight tolerance, instead design and plan for post molding processes such as machining using "assembly intent" fixturing.
This nicely reflects the issues I have.
The whole purpose of the shift to plastics is to eliminate any form of post moulding machining.
Tight tolerances are essential.
The part must be capable of being simply assembled to another part within which it must move with minimal working clearances. The other part will also be moulded without any final machining and it too has internal and external surfaces that are critical.

JMW
www.ViscoAnalyser.com

 

MacGyverS2000 (Electrical)
7 Feb 12 12:41
I'm no expert, but it seems to me that whenever the terms "precise tolerance" and "mass produced" are spoken in the same sentence, it should be followed by the term "arm and a leg".

While it's possible one such machine exists, I cannot image a tight-tolerance mold of a long tube without some amount of draft.  I can imagine an expanding mold, but that removes either "high-tolerance" or "mass-produced with the same mold" as options.

I would think post-process machining is the best option here, like drilling out the tube, or in the case of the slit running the length, a properly shaped router bit.

Dan - Owner
http://www.Hi-TecDesigns.com

KENAT (Mechanical)
7 Feb 12 13:18
You don't say what you mean by 'tight tolerance' but there are places that do molding for medical applications etc. that claim some pretty tight tolerances.

If you're outside of the typical tolerance range then you probably need to find a 'partner' to develop it with.  Generally I'd expect draft to be required, though depending on process there is sometimes some limited leeway.

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MikeHalloran (Mechanical)
7 Feb 12 13:44
A slot through the wall for the entire length removes much of the rigidity from the part, so ID and OD become theoretical concepts, not so much measurable features.

E.g., the OD is meaningful only while the ID is supported by a precise mandrel, and conversely.

The slot also makes it possible to wrap the part from sheet, which will have a more uniform thickness over its unwrapped area than would a molded cylinder.

As for draft, and other issues, the best thing you can do is make a nice drawing of what you actually want, and spend a couple of hours marking up a print with a good molder.  Bring a lot of red pencils/markers.


 

Mike Halloran
Pembroke Pines, FL, USA

jmw (Industrial) (OP)
7 Feb 12 14:00
The real question relates to the consequences of including or not including a draft angle.

How significantly does this affect the quality of the final component or the tolerances achievable against target values? Costs?
Choice of and cost of materials and fillers?

Can we make this a comparative response?

This is a real world application.

The original design was in the late 19th century using cast and machined brass/ bronze for component 1 and moulded and machined silvonite/ebonite (mineral filled hard rubbers) for component 2 (the primary component here) with a lot of final craft skills to fit them together.
In the 1960's the switch to plastics enabled a major manufacturing change based on no machining and no craft skills. It meant simple assembly using unskilled labour. It also meant they were able to do away with virtually all inspection and use the final test and some innovative methods to fine tune component design.

The trouble is I suspect that it was manufacturing who wanted to revolutionise the product using plastics but could only go so far battling against the product design engineers who probably were by now the guardians of the sacred form (i.e. the product design was "mature" but matured a generation or two before they were born).

Some of the original 19th Century design compromises necessitated by the available materials and manufacturing methods are still evident in the 21st century - which plastics would have resolved but didn't, so hence I am assuming the product design engineers fought to protect the original design and manufacturing did what the could (it was enough to sweep the field of most of their competitors).
But it is also why I suspect there were no drafting angles allowed which would compromise the presumed purity/perfection of the original design.

Unless you eliminate all machining and craft skills, plastic only delivers some possible material cost advantages.... virtually nil.
So the lack of ability to hit some tight tolerances in some aspects of the design would explain why they have changed about the only thing the design engineers would allow - the size.

Dimensions?
think of a cylinder about 5cm across, height 2-2.5 cms with a wall thickness of around 3mm fitting inside another previously machined component.

A competing technology has plenty of opportunity for drafting angles though it's design is also subject to the same problems of design purity (it is a late 19th century design) but is less good in some performance aspects and better in others.

But its this use of the drafting angle that I need to get my head around.


I'm trying to assess if this would be cheaper and deliver better tolerances in the finished component.  

JMW
www.ViscoAnalyser.com

 

MikeHalloran (Mechanical)
7 Feb 12 14:45
Absent many unrevealed details that may contraindicate...

I forgot to mention that the axial slit, if molded in, coincidentally reduces the circumferential stiffness enough that it should be possible to extract the part from the mold despite zero draft.  

In fact, in a slotted tube type part, it should be possible to mold threads or annular features of limited depth in either ID or OD (not both) with zero draft on the major portions of either.  ... because the part can be temporarily deformed while still hot, to get it out of the mold, and cooled on/in a fixture to get back its design shape.

 

Mike Halloran
Pembroke Pines, FL, USA

MacGyverS2000 (Electrical)
7 Feb 12 14:45

Quote (MikeHalloran):

The slot also makes it possible to wrap the part from sheet
Duuuhhhhh... forest for the trees, Dan, forest for the trees :(
 

Dan - Owner
http://www.Hi-TecDesigns.com

jmw (Industrial) (OP)
7 Feb 12 17:42
Yes, sorry. Absent many details. The design is a bit more complicated in that there is a web across the centre of the tube which also has a chunk missing where the slot is but it stops you making from sheet or deforming to remove.
OK, let's see if I can draw up the part with no drafting angles.

JMW
www.ViscoAnalyser.com

 

jmw (Industrial) (OP)
7 Feb 12 18:17
jmw (Industrial) (OP)
7 Feb 12 18:25
MikeHalloran (Mechanical)
7 Feb 12 18:59
Don't put any draft on the drawing unless the molder demands it.
It shouldn't be necessary.  ... depending on the resin.

Do specify where ejector pin marks are not acceptable.

It may distort from perfectly round as it cools.
... depending a lot on the resin.

 

Mike Halloran
Pembroke Pines, FL, USA

patprimmer (Publican)
7 Feb 12 19:06
The second drawing helped a lot.

As the slot is full depth it reduces the strength of the wall of the cylinder, and in the absence of restraint from a cavity, the cores should be easy to extract as the part springs a bit. Kinda like putting piston rings on an engine. The centre diaphragm complicates it a bit but in my opinion it is still possible from that point of view.

Could you mould it with the axis of the cylinder across the parting line instead of along the line of draw and with hydraulic side cores to extract the cores after the mould is just cracked open.

I believe you should be able to extract the cores with no draft in that design if there is no constraint from the outside. The equivalent to the piston ring gap in the design should make it flexible enough from that perspective.

A lot will also depend on exact materials used and your definition of precision and on how much time you are prepared to invest into fine tuning the process to maintain precision.

Several possible not yet mentioned hick ups I can see are:-

1) There most likely will be a sink in the outer surface where the diaphragm meets the cylinder.

2) There will be a tendency to pull into a reversed draft shape due to higher shrinkage in the area where the diaphragm meets the cylinder inside wall. Details of part shape and mould cooling will be critical.

It looks like part of a water meter. If so, continuous exposure to water will influence material choice.   

Regards
Pat
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jmw (Industrial) (OP)
8 Feb 12 13:06
Perceptive Pat.
It is part of a rotary piston meter.
The competing component is the nutating disc.
This is what I am attempting to establish, if one meter is more expensive/difficult to manufacture than the other.
Here is a sketch of the disc. The edge of the disc is theoretically part of the surface of a sphere but possibly, for ease of moulding is whatever profile suits best.
 
So what I am trying to ascertain is to what extent the different geometry of these components affects the difficulty of producing finished moulded components with no machining and as good a tolerance and finished dimensions as possible.
And is there a cost benefit?
The disc has pretty well no restrictions on drafts. Even the notch, which appears here as flat faced is not flat faced but bevelled....
 

JMW
www.ViscoAnalyser.com

 

jmw (Industrial) (OP)
8 Feb 12 13:10
Pud (Mechanical)
8 Feb 12 14:02
Looks all fairly trivial* to me...unless I've missed something!

My choice of material: nice, very low shrinkage, great in cold water, mPPO. (e.g. Noryl.)

For close fitting parts it is usual to leave metal on the tool, run it and adjust to get desired fit.

I see it as being an open and shut tool, and for ~25mm draw length, draft can be zero.

*Trivial as in low complexity.

Cheers

Harry

www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

jmw (Industrial) (OP)
8 Feb 12 14:10
Thanks Pud,
So as I understand you, both the piston and disc are equally easy to make and I don't need a draft angle on the piston despite the straight parallel internal and external geometry (the disc has no need for special draft angles)?
Equal costs?
Equal manufacturing complexity?
Equal control of finished product tolerances?
No special materials?
(I must check what materials are actually used.)
We are talking about hundreds of thousands annually and ultimately every home will have one.  

JMW
www.ViscoAnalyser.com

 

Pud (Mechanical)
8 Feb 12 14:29
As depicted, nothing untoward - the ball section is not a good design for plastics, but not too difficult.

imho, the whole assembly could probably usefully be redesigned for modern materials.

I've no idea regarding costs - (I have really, but not for here!), moulders need "real" pictures to get that!!

H

 

www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

patprimmer (Publican)
8 Feb 12 16:41
If it's a water meter I would expect PPO to be the material as it is already used extensively and successfully for many years and has excellent hydrolysis resistance and dimensional stability in contact with water.

Pud

I was a bit concerned with it sticking in the fixed half if it was straight open and shut.

Regards
Pat
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jmw (Industrial) (OP)
8 Feb 12 18:01
As I said, the original designs were developed to suit 19th century production and materials.
The change to plastic came in the 1960's.

Reading around it seems the Xtel XK (a PPS alloy) is the coming material for compatibility with potable water.
This site -for some reason I couldn't get into the Chevron Petroleum site - seems to have some good moulding guides.

 

JMW
www.ViscoAnalyser.com

 

jmw (Industrial) (OP)
8 Feb 12 18:03
But it does talk about wall draft again.....

JMW
www.ViscoAnalyser.com

 

patprimmer (Publican)
8 Feb 12 19:04
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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Pud (Mechanical)
9 Feb 12 8:51

Quote:

with it sticking in the fixed half if it was straight open and shut

Pat, with all those core pins for the small holes on the moving half, I would be pretty sure of it coming off the fixed half.

Cheers

H

www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

jmw (Industrial) (OP)
9 Feb 12 9:03
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
www.ViscoAnalyser.com

 

patprimmer (Publican)
9 Feb 12 9:07
Not a lot if the scale up is in proportion.

Regards
Pat
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jmw (Industrial) (OP)
9 Feb 12 10:14
Pud:

Quote:

imho, the whole assembly could probably usefully be redesigned for modern materials.
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
www.ViscoAnalyser.com

 

patprimmer (Publican)
9 Feb 12 17:09
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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jmw (Industrial) (OP)
9 Feb 12 17:41
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
www.ViscoAnalyser.com

 

patprimmer (Publican)
9 Feb 12 17:53
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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Pud (Mechanical)
9 Feb 12 19:31
Buggy Whips! now there's a thought...

jmw: is this a possible product or an academic exercise?

I hate typing, me ...

H

www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

jmw (Industrial) (OP)
10 Feb 12 9:46
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
www.ViscoAnalyser.com

 

patprimmer (Publican)
10 Feb 12 19:29
I have found with meters anything that improves low flow accuracy is popular as meters typically read low in that region and suppliers do not like undercharging.

Regards
Pat
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jmw (Industrial) (OP)
11 Feb 12 7:13
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
www.ViscoAnalyser.com

 

HDS (Mechanical)
14 Feb 12 7:38
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.  
jmw (Industrial) (OP)
14 Feb 12 8:21
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
www.ViscoAnalyser.com

 

patprimmer (Publican)
14 Feb 12 17:55
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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Pud (Mechanical)
15 Feb 12 8:17
Pat, whatever happened to syndiotactic PS? iirc, that was/is excellent in water too.

H

www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

patprimmer (Publican)
15 Feb 12 8:29
Sorry Harry. I'm not familiar with that one. Styrene was never my strong point.

Regards
Pat
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KENAT (Mechanical)
15 Feb 12 21:57
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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jmw (Industrial) (OP)
16 Feb 12 7:54
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
www.ViscoAnalyser.com

 

Pud (Mechanical)
16 Feb 12 12:27

Quote:

listen to my instructor

Kenat: ask him how screw-on milk bottle tops are made. (No, they're not unscrewed either. Takes too much time)  

www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

MacGyverS2000 (Electrical)
16 Feb 12 18:53
Pud,

If I'm not careful, a wrong tug on the "safety strap" on those milk caps show just how easily they can be removed without unscrewing winky smile

Dan - Owner
http://www.Hi-TecDesigns.com

patprimmer (Publican)
16 Feb 12 19:05
Yes, lots of caps including milk bottle and aerosol can caps are jumped off the core. Unscrewing moulds are somewhat bulkier and more expensive than one that jumps the core over the undercut as well.

Notice the change from no draft to actual undercuts.

This is somewhat design and material sensitive though. PC does not jump of undercuts very well at all, but PP is quite good at it.

A ramp angle on the undercut helps a lot.

Of course the core has to be out far enough so there is no external constraint on the moulding.

Undercuts in cavities are a lot harder than undercuts on cores.

Mouldings that jump over undercuts tend to spray themselves all over the factory floor and unreachable regions of the machine.

 

Regards
Pat
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bobslo (Mechanical)
18 Feb 12 15:24
Perhaps a suggestion for new generation design:
If you use two different materials  with different shrinkage and with NO ADHESION you could mold as follows:
- first shot piston (lower shrinkage material)
- second shot disc (with piston as insert). Material for disc should have higher shrinkage.
Such process exclude assembly process tolerances.

Perhaps this sounds a little-bit futuristic but (some) new throttle bodies with vane inside are done in this way.
   
Richar24 (Industrial)
12 Mar 12 2:26
Just a few quick observations.
1. Parts with a "C" cross section are very difficult to mold and hold tight tolerances. The issue is that depending on where the gate is located the slot will either open up or close down. If the baffel is molded in place, the bore will be a different diameter (probably smaller at the open ends). Inserting the baffels as a second operation will avoid this issue.

2 Draft is always better, Without draft on the core, you may have to retract the core out of the cavity with the mold closed (This increases your mold cost). The standing piece of steel that produces the slot will require either a slide or draft to allow for a shut off. These options increase the cost of the mold .

3. If you dont need the slot on the side the full length of the part it may simplfy the mold.

4. You can mold the baffel seperately and adhesive them in place or sonic weld them in if this is an option.
A single baffel plate can be molded in place, and the second baffel plate inserted ito the bore and fixed in place.

5 A really cheap mold can be made that has the core, cavity and slot all cut in the solid, but will likely prove difficult to mold. This will be a decision you will have to make based on your part volumes and mold costs and part cost. Their are a number of ways this part can be produced

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