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Mold Locked Design..Multiple Side Action/Collapsible Core?
2

Mold Locked Design..Multiple Side Action/Collapsible Core?

Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
I've got a design that is currently Mold locked.  It consists of two mating parts.  The issue is with the Female part.  I know by opening the bottom, it can be made and by eliminating that undercut.

I'm looking for some suggestions on how to make this design work.  The design can change, but the intent is a "twist-fit" connection that must be removable, multiple times.  Must be a Robust connection.




Thanks in advance.
Chad
  

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

You could make it as is.  You'll need lifters on the female part and 4 slides on the male.

As you said, you could eliminate the need for lifters on the female part by opening up the bottom.

You could also eliminate the need for slides on the male part by blowing out the undersides of the cam paths.

-b

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

I see no real problems

You need the parting line of the core on the female part to be level with the flat side of the engagement dogs. This means a core on both halves.

You absolutely need side cores or an unscrewing mechanism. Side cores will be a lot easier.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
This is good news to me!  Thanks for the quick responses so far.

I can visualize how the male part will be made.

bvanhiel, I could blow out the underside of the cams, but I would lose the aesthetic.  I wish I could do that...Unless there is another approach to have  same Diameters on both sides.

but...I am having trouble visualizing the female part still.. I am not familiar "lifters"

patprimmer or bvanhiel, could you try to explain the mold actions for the female part.

Thanks

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Any half competent moulder or toolmaker should be able to show you examples.

It simply opens in line of draw.

It will need some draft from both ends toward the parting line of the core.

The cores will need draw polishing.

You will need an undercut on the moving half core to ensure it stays on the moving half during mould opening.

You will need a stripper plate to get it of the moving half core after the mould is opened.

I would think a sprue running down the centre of the core with say 4 sub gates into the inside bore of the female part half way between each locking peg or dog.

The drawing shows no radii at any corners. They will be necessary to avoid breakage t the stress risers.

The slots will need 4 side cores, or some side edges of the slot will need to be laid back to line of draw of the side cores if you want to get away with only 2 side cores.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

You do not say which material, so I am not considering special requirements foe specific materials.

Any reputable raw materials manufacturer will have technical bulletins that show how this works, and any basic mould design book will show it.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

A lifter is a piece of the tool that is driven by the ejector system, but moves at an angle to the line of draw.  It enables shallow undercuts on the insides of parts.

Here's a link that describes a lifter:
http://www.copper.org/applications/industrial/PDF_files/Sept_99.pdf

-b

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

I don't understand why you made the pins half-round.  That gives up half the strength, and the edges can cut into the mating part during some parts of the mate cycle.

Speaking of strength, a coarse, possibly multiple lead, Acme or buttress thread would be _much_ stronger, and could still have a locking feature when fully engaged, e.g. with a bump on the male thread and a dull hook in the female thread.

What you've got is a topologically inverted adaptation of a BNC connector; but those are _always_ made of metal; the design is not robust in plastic.  Consider instead adapting the design of MS twist-lock round shell multipin connectors, traditionally made in zinc, but the geometry would be robust in plastic too.

Mike Halloran
Pembroke Pines, FL, USA

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Fillets would be added to the original design, those little dogs would be fully filleted, they were quickly truncated at the top for clearance, but there is more room to round the tops rather than flat.

Thanks, for the tips, still a little confused about how the female part is made...I guess I need to actually see some tooling that does the same/similar thing.

Anyways, as per Mike's suggestion, Here is a rough design with a 4 lead acme twist-lock.  Wasn't sure about your dull hook and male bump..I visualized it in reverse bump on the female and notch in the female at the bottom of the threads... see below:







Thanks,
Chad

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

ha ha ha....now your getting into really though stuff....from the tool suplier point of view.

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Seems like my first design is simpler. I can increase size to reduce stresses..But my concern is durability of the plastic after X amount of cycles.  I am looking to get a RP model made this week..test it for functionality.  I'm thinking about FDM..any other suggestions?

Also, does anyone think I should reverse the male and female, put the Cams on the inside, that way the dogs are on the male part.  Due to mounting conditions, the female part is not removable, that way if a dog does break, the male part can be replaced.

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

If the cam is on the inside you will need a complex unscrewing core.

Leave it how it is for reasons of tooling costs.

Improve the strength of the dogs.

Use a material that will take the load. GF nylon maybe, depending on the load and other environmental factors.

If you need a glass filled material, warpage or distortion will be an issue due to differential shrinkage along and across the flow.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Yeah, that's the general idea.  The bumps probably don't need to be that obvious.  If they're a little more subtle, you may be able to just unscrew the core out of the female part, or rather unscrew the core to eject the part.  Of course you need to stop the the threads instead of tapering them to a knife edge.

Mike Halloran
Pembroke Pines, FL, USA

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
heh..two conflicting posts about unscrewing dies...

In terms of material..I think something with higher abrasion resistance and also higher corrosion resistance due to sea water exposure.


RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
in addition: I am concerned about nylon's moisture absorption.

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

What precision do you require, what is the environment, how variable is the environment.

Many screw together parts that are used in boat fittings are made of nylon.

Swelling can be an issue, but not to the degree that some suppliers of other materials might infer.

All materials have their strengths and weaknesses and we need to know all the details to work out the best overall compromise.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
I would require close fit for this application.  The restraint feature is pretty key to how the unit functions.  Operating temp is like 25degF to 95degF and will be fully submerged in sea water  ..nothing too extreme

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Depending on load, fatigue , Acetal might do, or PET or PP

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
UPDATE:

I got a polyjet Rapid Prototype of the parts.  I went with a variation of the first design and it turns out the peg diameters are too large, hence making the snap way too difficult.  Also, dimensional tolerances of the RP weren't what I expected, so a little sanding was required.  btw, what dimensional tolerances can I expect from an Injection Mold?

I think one solution is decreasing the pegs' diameters or further investigating the screw option.  I think what I initially drew was too large of the thread.

Any recommendations on the thread size/dimensions?  OD of the male thread is appx 1" - 1.25"

Thanks!

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

The answers to your questions are very dependant on material selection and final design details. There are many factors that inter react.

The rapid prototype will not reliably predict the feel or force required to jump over undercuts when assembling or disassembling.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Right now, nylon, ABS, or acetal are my options.

Unfortunately, I am looking for a rapid prototype that will give me the same mechanical feel.

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Not gonna happen; the microstructure is too different, even if you RP with the same material.  You can get closer to the real feel with an aluminum mold, for about the same amount of machining time it would take to make one prototype from billet.

BUT, and I don't mean to put too sharp a point on it, you shouldn't be designing by 'feel' here.  You should instead be evaluating the 'feel' of objects that seem about right, and reverse engineering them.  Simple beam models in Excel are good enough for this.  The idea is to find out what friction forces and spring rates and such produce the 'feel' that you think is right, and then use that information to _engineer_ the feel into your product.

Mike Halloran
Pembroke Pines, FL, USA

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Mike,

Point taken and on that note, I don't know many..or any products with similar material and effect to analyze.  I'm having trouble finding that "MS twist-lock round shell multipin connectors"

Thanks

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Most MS connectors, and BNC connectors, work the same way, and it's _not_ how you first think.  

If you did have an MS connector there, and you engaged the shell with no insert, you'd find that it twisted easily, but it wouldn't 'lock', unless you were trying to separate it.  That's because the bumps on the thread don't 'spring' against the form in the mating thread, they spring against axial compression of either the insert (that holds the electrical pins), or a rubber gasket somewhere.

Similarly, topologically speaking, if you look _real_ close at a BNC connector, you'll find a wave spring concealed deep in the grooved half, that tries to separate the connector.  That's what you feel when the pins go over-center as you lock the connector.

Mike Halloran
Pembroke Pines, FL, USA

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Mike/All:

The name for the connector is a "Bayonet"  stumbled on that in my research.  I've taken your advice and I feel that the wave spring is the best approach.  

This leads me back to a plastic design question..I'd like to seat the spring in the female bore, which would lead me to put the engagement dogs on the male part and the grooves on the female part, for a clear insertion of the spring.  (I prefer this for other reasons as well)  So it's really a manufacturability question for injection molding the blind grooves in the female part.  

Any opinions?

Thanks,
Chad

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Any thoughts?


Thanks,
Chad

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

I once rang my barber for a haircut, but he said I would have to come to his shop.

You need to post more details.

Most things are possible if you are willing to experiment and pay the price re tooling cost and cycle times.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Here is what I am talking about...  The internal blind Undercuts grooves.  Do-able for a decent price?  How would it be done? Collapsible core or something?



One thing I have been thinking about is possibly breaking the part into 2 or 4 pieces, but I would really hate to do that.  Or cutting the grooves through the whole wall and exposing them to you can have a side action to create the grooves. But Then I need to add a wall back to it.  I don't like that solution either.

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

You could mould with the groves all the way through, but the OD to small, then shrink fit or weld a sleeve over it to cover the groves and get the correct OD.

You could have collapsing internal cores, but I think by the time you finished talking to a good Swiss toolmaker, you might need some resuscitation.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
If I use two parts, I think the sleeve would need to be a cup so the grooved part can seat and it would need an anti rotation feature in both parts.  I'm concerned about the parts coming apart.  Can welding be automated or easily done keeping quality in mind??  I've only done manual plastic welding...

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Some feature, e.g. a flange, prevents the male part from pushing all the way through the female part even if you remove the radial pins from the male part.

For that reason also, the grooves in the female part don't need a 'bottom'.  I.e., they can be formed by axial fins from the far/bottom end.

Mike Halloran
Pembroke Pines, FL, USA

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Mike,

That's correct about the flange.  I can relieve the bottom of the grooves, but I cannot punch a hole all the way through(if that's what you're saying).  I need the blind bore, as a wave spring is seated at the bottom of the bore and the inner wall creates a pilot for the spring.  Unless somehow, the part is punched through and a second part is molded to cap the bottom..?  the key is that the two parts are well fixed and the joint between the two is waterproof.

Not Sure what axial fins are?

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Yeah, punch the bottom of the grooves all the way through...

and use a rubber disc instead of a wave spring.

Mike Halloran
Pembroke Pines, FL, USA

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
That definitely simplifies the mold, but that leaves me with 4 holes in the bottom to fill.  

Maybe a small plate to glue to the bottom?  Some other part with an o-ring to insert on the bottom?  Or two-shot molded with soft seal/ring? And I can put and cylindrical/circumferential groove on the bottom? hmmm

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

You can get quite good results by ultrasonic welding thermoplastics. It is a widely used process and can reliably produce strong waterproof joints. For instance, the old in line fuel filters for cars with carburettors were made that way.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Is there any technique to designing for ultrasonic welding?  Do most injection molders have ultrasonic welding equipment?

Is it a costly process?  I'm assuming it is cheaper than the over complicated mold for the original design.

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Maybe I'm a little late to the party here, and I have not read thru all the posts, but something just struck me looking at the image at the top of this thread - the image of the 'cup' with the external female bayonet mount undercuts. My idea might get in the way of any aeshtetic you need for the part, but I'll leave that judgement up to you.

In this thread here has been talk about multiple slides and/or moving cores to relieve the undercuts of the 'L' shaped bayonet feature. It looks like (upon quickly glancing thru the posts) that it has been assumed thus far that the orientation of your original part as shown at the beginning of this thread wants to be molded such that the parting line of the mold is parallel to the top or bottom of the part, and then to add additional tooling to achieve the (4) necessary undercuts on the outside of the part as shown in your original sketch.

One alternative that will affect how the part looks on the outside of the 'cup' yet still retain bayonet funtionality might be to lay the 'cup' down in the mold such that ONLY ONE SIDE ACTION would be required to produce the inner round core of your part. As for the four 'bayonet' 'L' shaped features, two of them could be moulded without side actions as they could be located such that the natural draw of the mold would produce their geometry, and then the geometry of the other two female bayonet features that lie across the parting line could be modified so as to releive their undercuts. Even though material is releived from the two features that lie across the parting line, they could quite possibly still retain the functionality of the bayonet mount at those two locations. (Another words, the locking feature that is molded across the parting line would no longer be 'L' shaped as you originally depicted, but would end up sort of as a nearly flat surface in the area that receives the pin, and as a nearly flat radial groove in the area in which the pin is allowed to traverse.

Examining the concept a little deeper...If you've followed me thus far, and if I'm seeing your original sketch correctly, the two 'L' shaped bayonet pin tracks that are molded away from the parting line could retain what appears to be a pin-detent feature. The two detent features that would be molded at the parting line of the mold might be lost, but maybe once you up-end the 'cup' 90 degrees, you might find a way to include a detent at the parting line (by making one half of each groove smaller and offset slightly from the groove that is molded in the plate opposite the grooves parting line) if you truly needed four detent features.

I currently don't have any modeling program at hand, otherwise I'd produce the concept for you.

As a side note, on the blue model that appears above this post, you show four external wings...they would not mold well as skinny as you have them shown...I would recommend that you fatten them up to about 2/3 the thickness of your general wall thickness of the part.      

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

In my opinion there will be little or no problem with the external wings other than maybe they will shrink a bit less and therefore warp a little, depending on material selection and moulding conditions. It certainly won't be a problem filling them if the gates are in the correct place.

The one side core design cannot be applied to the current design as the undercuts are on the core, not the cavity.

On the original design, it is possible with 2 side cores if done right. They will be short stroke and not require a lot of force to move them.

By laying the main axis of the moulding 90 deg to line of draw, being accross the parting line, a big core requiring a long stroke with a high force to open would be required. I expect it would cost more than 2 smaller side cores.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Mousetrap,

you've got me slightly lost in your post.  Currently the blue design is the direction I am going.  The radial fins are there to prevent the part from spinning after installation..geometry is open to change for those..possibly unnecessary. Drawing would be helpful I think in this case.



I think the two part sonically welded assembly is looking more viable at this point, unless the blue one above can be made easily/responsibly.  This discussion has been very informative to this point and I thank everyone for their input.  I'll work up some more drawings and post them soon.

Thanks,
Chad

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?


You might investigate the lost metal technique to produce your internal bayonet forms:

http://www.coretechsys.com/metal.html

as an example...


Cheers


Harry

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Branson is the leading supplier of ultrasonic welders, but there are others who I cannot recall at this time.

There are some minor design considerations to designing a part to be ultrasonically welded, but the equipment suppliers can supply this, as can the plastics raw materials suppliers.

A moderate proportion of injection moulders have in house ultrasonic welding. Some companies also do welding only on a contract basis. The equipment is not prohibitively expensive, and purchase may be justified on a large project.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Based on previous discussion, One of the cores would look something like this:


and the resulting part would then look like:


I think I like this approach.  Its seems reasonable and then I need to cap/seal the bottom off with another part...

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

A soft circular gasket nestled in the cup shown should seal off the full circular end face of the part not shown.  Four lugs on the gasket rim could retain it.  No need to seal off the slots left by the core; they're outside the pressure boundary.





Mike Halloran
Pembroke Pines, FL, USA

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
Unless I am not fully understanding your suggestion..If there is a gasket in the cup(shown)..there is direct contact with water in the cup(shown).  Since the outside diameter and bottom surface are fully captured, the slots left from the core are exposed to a capturing surface which is permeable.  The back side can be sealed with a welded on disk.  I need a positive seal from the remaining slots.  I'll post another of the underside.

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
So I'm at the point where I've tried to create uniform wall thickness by adding pockets in the part on the underside...making the bottom core slightly more detailed.  

I think I'll still need the external features requiring some multiple side action.  So right now I visualize the mold in 4 parts basically.  the two parts along the cylindrical axis and two parts for the side actions for the radial/wing features.

Thanks everyone for tips and great ideas so far!
 

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

Question:  I don't see in any one of the postings that ask the question of what is your volume?  Is this a low volume requirement or high volume? That will determine just how the tool will be built, or if a tool should be built. If this is low volume, you may consider machining from bar stock of which there are suppliers that can take your CAD file and program their equipment within aprox. 20-30 minutes and be makeing your parts.

Regards,

mtpi.us

RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

The OP comes across as intelligent, competent and professional and he mentions tooling cost several times which indicates he is aware of their magnitude and is therefore able to asses viability of the project.

Regards

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RE: Mold Locked Design..Multiple Side Action/Collapsible Core?

(OP)
mtpi,

For prototyping and testing purposes, machining might be feasible, but I don't think the geometry and nature of the design lends to efficient machining.  Maybe if the parts are machined in pieces for easy machining/setup will make that method more effective.  I am currently I am at the phase where the design is fairly locked down, and design changes can be adjusted with mold inserts on the cores/cavities.  When it does go to production, tooling will be set up already and quantities will be high enough to justify the tooling.

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