ABS or 30% Glass Filled Nylon feature size
ABS or 30% Glass Filled Nylon feature size
(OP)
I'm working on a design that requires a ratcheting mechanism to be molded into the parts.
Two parts slide with respect to each other. Each of the parts would have triangular teeth molded on facing surfaces. The parts would click one step at a time as they are slid with respect to each other.
The ratchet is also one way. You can slide (click-click-click) one way but have to press a button to disengage the teeth to be able to slide them the other way.
I would like to make the steps as small as possible. I find myself wondering about what the minimum reasonable feature size might be from a moldability standpoint for something like this. Right now I have them specified as right triangles 0.050in by 0.050in. The pitch or step is also 0.050in.
Strength isn't a huge issue as the assembly probably won't see more than about 5.0lbs applied to the teeth. The ratchet will have about six teeth engaged at all times.
Are there any issues to consider if one were to use ABS vs. 30% GFN.
Any recommendations on a forum that might be more intensely dedicated to plastics engineering and design issues such as this one?
Thanks.
Two parts slide with respect to each other. Each of the parts would have triangular teeth molded on facing surfaces. The parts would click one step at a time as they are slid with respect to each other.
The ratchet is also one way. You can slide (click-click-click) one way but have to press a button to disengage the teeth to be able to slide them the other way.
I would like to make the steps as small as possible. I find myself wondering about what the minimum reasonable feature size might be from a moldability standpoint for something like this. Right now I have them specified as right triangles 0.050in by 0.050in. The pitch or step is also 0.050in.
Strength isn't a huge issue as the assembly probably won't see more than about 5.0lbs applied to the teeth. The ratchet will have about six teeth engaged at all times.
Are there any issues to consider if one were to use ABS vs. 30% GFN.
Any recommendations on a forum that might be more intensely dedicated to plastics engineering and design issues such as this one?
Thanks.






RE: ABS or 30% Glass Filled Nylon feature size
The 45 degree pressure angle might be a tad high, but the tooth size seems about right.
I've seen teeth about that size in a medical torque limiting screwdriver, in ABS I think. The teeth were arrayed like holesaw teeth, on the end faces of annular sleeves maybe 10mm OD and 5mm ID. The assembly included a small stainless steel spring to push the teeth together. That device was set up so you could apply essentially unlimited torque to undo the application screws, but the spring, working through the pressure angle of the ratchet teeth, would limit how tight you could make the application screws. The application screws held the electrode wires into a pacemaker, and the screwdriver was set up so a gorilla doctor couldn't overtighten the screws and break something or make the screws impossible to back out; all he had to do was tighten until the ratchet clicked. It only had to work twice, then it was supposed to be thrown away.
I don't think I'd try to make teeth much smaller. For a ratchet and pawl, I'm visualizing a way that maybe you could use two sets of pawl teeth on two pawls to halve the resolution, by offsetting one pawl by half a pitch. ... but I haven't drawn such a thing; it may or may not be possible to actually do.
Mike Halloran
Pembroke Pines, FL, USA
RE: ABS or 30% Glass Filled Nylon feature size
Also the glass fibre orientation and distribution to the tips of the teeth might be an issue. Cable ties use no glass fibre.
Other good gear tooth materials are acetal and polyesters.
Regards
Pat
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RE: ABS or 30% Glass Filled Nylon feature size
Meh, I suppose someone did the study and those were the numbers that fell out...
I work for a medical device company, and it's staggering the sheer amount of plastic that goes into a landfill for the sake of single use products.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: ABS or 30% Glass Filled Nylon feature size
The biggest design challenge to the plastics was that pacemakers are (or were then) predeployed by consignment to surgeons, who were known to store them on the package shelf of a car, possibly for months, so they would always have one ready to implant when it was needed. Car interiors are not a benign environment for plastics, what with high and low temperature cycles and a fair amount of ultraviolet radiation.
Mike Halloran
Pembroke Pines, FL, USA
RE: ABS or 30% Glass Filled Nylon feature size
I've attached a few screen shots of a mock-up in order to clarify the question further. For reference, the green part is about 2.5 by 2.9 inches.
You can see the proposed engagement of the teeth on the back of the green cover plate with those on the locking mechanism. Again, the idea is that they would be able to move in one direction but not the other unless the button is pressed.
The comment citing cable ties is an interesting one. My problem is that I don't think that I can mold this out of plain nylon. I need ABS or GFN for structural reasons. I least, that's why I think I need them.
Mike, you mentioned that a 45 degree pressure angle might be too much. Now that you've seen the images does your comment still hold? What would be a more appropriate pressure angle?
Also, the mock-up has the back side of the tooth at 90 degrees to the plate. I would have to tilt this somewhat in order to have a little bit of draft. I was thinking about keeping it to a minimum, like 0.5 degrees. I don't think that would affect the locking effect too badly.
This is not a medical device, BTW.
RE: ABS or 30% Glass Filled Nylon feature size
The problem with polymers in thin sections is flow length vs. thickness. Your item will not be a problem. Limiting feature size will be polymer molecule size.
I would suggest that unfilled nylon or acetal might be better for warpage than GF, and ABS, (although the latter being "sort of ok"), as its around the same price but processes slower, so part cost would be the same for improved properties.
H
www.tynevalleyplastics.co.uk
Why be happy when you can be normal?
RE: ABS or 30% Glass Filled Nylon feature size
I guess that part of the problem there is that you have to make absolutely certain that the part will not deform in use because a small movement could disengage a tooth that is only 0.025in high.
Thanks!
RE: ABS or 30% Glass Filled Nylon feature size
Cheers
H
www.tynevalleyplastics.co.uk
Why be happy when you can be normal?
RE: ABS or 30% Glass Filled Nylon feature size
I see some problems with the spring/hinge, by the way.
When the ratchet rides up and clicks, the teeth rotate around some point in the hinge, so they don't all have the same depth of engagement as they are riding up on mating teeth. IOW, all the force to flex the hinge comes from the teeth nearest the hinge, only. The others are just along for the ride. You need to take a side view section in both conditions in order to appreciate what's going on.
I don't think combining the hinge and the spring is necessarily a good idea anyway. A detailed force analysis may lead you in a different direction. Perhaps a completely separate, e.g. steel, spring, would be needed. Perhaps you could mold the spring as a longish cantilever bar that's twisted up over an adjacent hook at assembly to get a preload. Or perhaps you could support the moving tooth plate with four parallel cantilevers leading from its corners to the undeflected plate.
Something like this:
+===========+ +=============+
+---------------------+
| |
| || || || || || |
| |
+---------------------+
+===========+ +============+
So the ratchet plate could remain parallel to its initial plane as the teeth rode up and over their mates, all more or less in unison.
Mike Halloran
Pembroke Pines, FL, USA
RE: ABS or 30% Glass Filled Nylon feature size
RE: ABS or 30% Glass Filled Nylon feature size
Or, let my try tgml:
+===========+ +=============+
+---------------------+
| |
| || || || || || |
| |
+---------------------+
+===========+ +============+
Yeah, that's better.
Mike Halloran
Pembroke Pines, FL, USA
RE: ABS or 30% Glass Filled Nylon feature size
Nylon is the best for conformance to intricate mould detail and will flow a short distance into an 0.001" gap. This is one of it's problems at times as the mould requires very high precision at parting lines and pin holes to avoid flash. I have seen flash on a pin that is so fine that it is invisible to the naked eye, but it forms a suction cup that causes the moulding to stick quite tightly to the pin. From memory, nylons typically flow about 300:1 re length to thickness, but that of course largely depends on the specific grade and conditions.
Oh and as mentioned and recognised, you hinge design as such really does suck. It needs to be a lot longer at least. At best it would be a third part wit a matching wedge or shafts into angled slots and sliding action.
Regards
Pat
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RE: ABS or 30% Glass Filled Nylon feature size
Again, the model I uploaded is one in a series of thinking tools, if you will, that I am quickly throwing together in Solidworks to eventually converge on geometry that will be integrated into an actual design starting from a fresh clean file.
I am taking a look at cable ties as an interesting example of a simple way to do this that seems to work very well.
RE: ABS or 30% Glass Filled Nylon feature size
http://up
RE: ABS or 30% Glass Filled Nylon feature size
Do look at cable tie ratchets. Of the ones with plastic molded pawls, note particularly that the pawl cantilever is quite short, and generally deforms inelastically (takes a set) once the pawl teeth have ridden over just a few of the tail teeth. That's probably not what you want. You need a longer cantilever, which will probably need to be thicker in order to generate any ratcheting force.
Basically, you need to select a material, guess at a geometry, and run a cantilever beam calculation, balancing the force and the deflection while keeping the stress below what corresponds to an elastic limit for plastics. You don't need FEA; hand calcs will do, but putting them in a spreadsheet will make it easier to screw around with the parameters, a lot, before you bother to put the geometry into CAD.
Mike Halloran
Pembroke Pines, FL, USA
RE: ABS or 30% Glass Filled Nylon feature size
I'm wondering if something like this can be 3D printed for verification. Maybe the newest machines can resolve features this small?
RE: ABS or 30% Glass Filled Nylon feature size
We still have no idea how many use cycles, load, temperature, food/biological contact, UV exposure, dishwasher exposure, sterilisation, freezing etc apply here. Each can have a significant impact on material choice.
Regards
Pat
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RE: ABS or 30% Glass Filled Nylon feature size
The material choices are limited, so the mechanical properties will be different in addition to being nonisotropic because of the laminar deposition, so the forces won't be right, and the Z-axis resolution is still too coarse to get the surface finish, hence the 'feel', right.
Mike Halloran
Pembroke Pines, FL, USA
RE: ABS or 30% Glass Filled Nylon feature size
RE: ABS or 30% Glass Filled Nylon feature size
An epoxy mould in a bolster might make a better prototype or a simple aluminium mould with no cooling ejection etc to just get a few shots out at minimal costs.
Regards
Pat
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