Is Draft Angle a Must? 5

[rubyschmerling]

I’m working on a design for a hand pump with an o-ring (OD 0.504” dash-12) as a seal in the piston. The cylinder bore would be constant diameter (ID 0.500”). Bore length is 5-1/2”.
Injected material is Polypropylene (PP).
Some mold maker said the cylinder part couldn’t be ejected successfully without a draft angle.
However, even a very small draft angle of 0.2 degree is not feasible for the o-ring to seal. I tried to stay away from custom seal. The o-ring was tested successfully in a prototype.

I could not find collapsible core for 0.500”

Here are some options I received from mold maker but I’m not sure which one is the best to follow:
1. Change material to have less shrinkage than PP (PP shrinkage is 4-5%) material has to be FDA approved.
2. Use polished core and spray lubricant every 2-3 cycles, than cool part on fixtures.
3. Manufacturer the part in different method than injection molding (maybe use a ready made pipe)
4. Custom design the core (3 long parts sandwich like) and to pull the centerpiece. This would allow easier part ejection.
Thanks in advance for any tips.

 

[patprimmer]

PP homopolymer should shrink about 2 to 2.5%. Over 4% means something is seriously wrong with your moulding process or design.

I don't know if it will work, but if you want to attempt to eject of a zero draft core, you will need several things:-

1) The core will need to be well polished ONLY IN DIRECTION OF DRAW.

2) You will need a tube ejector.

3) You will need a poppet valve to allow air to blow the moulding off the core.

4) you will need to pull the core completely clear of the cavity before attempting to strip the moulding from the core.

Other points that will help:-

a) Use a low shrinkage grade.

b) Use an elastomer modified or high impact grade that will stretch more easily to blow off the core.

c) Use a grade that has extra mould release lubricant. To much causes a build up of deposit on the mould, but this may be a necessary evil to obtain the mould release level you require.



Regards

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[TheTick]

Make the bore undersize and machine it.

 

[MikeHalloran]

For modest quantities, I'd use an aluminum mold, split on the bore diameter, on a manually operated molding machine, with a piece of chrome/polished shafting as an insert, and press the shaft out after the part is removed from the mold. Figure on throwing the shafts away after N parts.

Machining a bore that long and getting a decent finish in PP would be a challenge.

You might be able to mold the part with a tapered bore, mold split normal to the bore axis, then push in a hot piece of shafting, and press it out after the part cools. Tricky process.

Collapsible core wouldn't work, even if you could get one; the witness marks from its seams would scratch the o-ring.

This is bizarre, but might work: Set up the core pin with a low speed motor, and spin it during ejection.







Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

I agree with Mike. PP will be very difficult to machine.

You would need to hold it inside a bore in split blocks so you could hold it in place to machine then split the blocks to remove the part.

Another way to hold softer plastics for machining is to freeze them in liquid nitrogen. It makes them very rigid and it is cheaper than it sounds. A reamer would be my tool of choice to remove the taper in the bore.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[spider007]

Can you split the cylinder in half and mold them seperatly, then weld them together in a fixture?

 

[bvanhiel]

Find out how syringes are made. It may be a matter of not using an o-ring, but using a more flexible seal.

-b

 

[Pud]

Well,

imho, given the polished core, poppet valve to relase vacuum as part is ejecting (unless there is an open end)there will no problem - but...

There will need to be sufficient wall thickness to prevent buckling as it is stripped off the core with the stripper plate, and the tool should be kept fairly hot (40-50 deg C).

You may find the problem will be getting a moulding machine with sufficient opening stroke (daylight) to mould and eject the part as it going to be quite a small shot weight and hence ideally a small tonnage press. Might be better to have vertical hydraulic core....

Syringes, btw are made from multicavity tooling (lots) - mainly from PP. Some of the newer metallocene catalysed PP's have quite high flexural modulus and may be better regarding stiffness for your application.

Cheers

Harry

 

[bvanhiel]

Pud,

What kind of draft/coring is on the multi-cavity syringe tools? I think that is what ruby needs to know.

It's either undrafted, and that gives you an example that can be emulated. Or it is drafted, in which case you'll want to copy the seal rather than use an O-ring.

-b

 

[Pud]

Hi All,

They are undrafted, polished to mirror finish.


Cheers


Harry

 

[rubyschmerling]

Many thanks to all of you
Ruby

 

[TheTick]

an idea I would like to try someday...

Put an undercut at the top end of the cylinder so that as the part is drawn off the core the walls are flexed away from the core.

 

[ewh]

I know that this is a late response, but wouldn't splitting the mandrel like this work?
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[MikeHalloran]

No, for two reasons:

- The mandrel is still full round, and straight, when assembled. A collapsible core, if you could build one in that size, would pull away from the part, radially, in segments, before axial separation was attempted. The diagonal slash cut does not allow the mandrel to be 'shrunk' before the die opens.

- In both cases, i.e. slash cut and collapsible mandrel, the flash left by the mandrel seams would cut an o-ring, guaranteed.



Mike Halloran
Pembroke Pines, FL, USA

 

[ewh]

Yep, I forgot about the flash. Thanks, Mike!

 

[mk2000]

Can you consider an insert in the mould.

Regards