This is very much an "about the journey" project.
A version of the SMD tape magazines that have built in DIN rails mounts
This is very much an "about the journey" project.
In order to get a part to more easily come out of a mold it's important to add a draft angle to the walls. Straight parts drag against the walls as they're ejecting from the mold.
Whereas parts with a draft angle separate immediately.
Silicone molds are very forgiving when it comes to draft angles. The material is stretchy and you can get away with both vertical surfaces and even some undercuts on your parts. From what I've been reading, other molds are not so forgiving and adding draft angles is required. Even with the silicone molds adding draft angles is going to help the part come out of the mold.
It turns out Fusion360 has a tool to add draft angles to your models.
So far so good. Where I previously ran into trouble was that adding a draft angle to the DIN rail clip made the magazine grip the rail badly. The draft tool is supposed to allow for deselecting certain faces but I found that interface to be too quirky to work with on my model and I added draft angles during the extrusion steps.
I'm about to revise my design based on what I learned from a test casting, so I thought I'd try out a few ideas on how to work around the issues I hit with the Draft tool.
I'm making changes to fix a few specific problems.
I wasn't very careful when pouring my test mold and I wound up with some problems with some tall narrow areas. For the main tape passage I'm just going to have to be careful not to trap an air bubble, but some of the other small openings don't have to be designed this way.
Also, I'm missing the flat front and larger top nubbin from the original design as that gave additional room for labeling.
So, getting started!
First thing I'm doing is adding a bunch of parameters. InteriorDiameter, TapeWidth, and TapeHeight reflect the tape dimensions and will be adjusted to produce multiple outputs.
Some of the other parameters (RailSqueeze, DINSlotWidth/Height) relate to fitting printed parts together and I prefer adjusting them in the parameters dialog. DINWidth is there as a constant so I can type DINWidth-RailSqueeze as a dimension.
LowDraft and HighDraft are values that are going to be used to set draft angles. 5 and 8 are absurdly high angles, but it makes it easy to see when I've flipped angles backwards. I'll change those to something more like 1.5 and 3 degrees for the final product.
I'm going to create the drawing for the DIN rail connector except that i'm going to leave out the actual critical surface for the rail connection. If I wait until the last step then I can have a lot more control over what (if any) draft angles are used.
The next bit is the big circle that holds the tape. The exterior of the circle is constrained to be tangent to the DIN rail. This leaves the circle unconstrained in one axis (up/down in the picture).
To constrain the second axis I construct a horizontal line that's offset from the other part of the DIN connector and make the circle tangent to that line. I've found this to be a very reliable method of constraining parametric geometry and Fusion360 won't randomly reverse dimensions or flip them around to odd angles.
Then it's a quick job to build some squares in the corners of the design and extrude once with the floor thickness + tape thickness.
From now on try to only use extrude with distance of To Object. Connect things together.
That seems to keep Fusion360 from getting confused when the parameters change. That becomes doubly important later if fillets are added and Fusion tries to figure out how to maintain the features.
Now check to make sure the model is stable over a reasonable span (in this case 50mm to 120mm).
Keep going but:
* Don't apply a fillet between the interior and exterior of the geometry
* Don't add the passage for the film
Now apply the draft tool. The hard corner between the interior and exterior makes fusion360 do the right thing
Make sure to...Read more »
My semi-rigid resin should be arriving soon! In the mean time I thought I'd have some fun and try casting some hot melt. As expected the result was a bendy version of the part. This isn't particularly useful for this project, but it might be a good approach for other sorts of parts.
I wonder if there's something that can be mixed into hot melt to make a more structurally sound composite material. I've done some searching for
On the bright(ish) side, I discovered that my test mold has a significant issue where an air bubble bridged an important part, so I'll be able to re-make the mold before I start using the resin.
So, what's this going to cost?
For the cast material I want something that's going to be semi-rigid so that it doesn't break when I clip it to a DIN rail. I'm thinking about using MD 50 from BJB. I haven't used it before, but the spec sheet sounds pretty good.
How much do I need?
Conveniently, Fusion360 knows how big solids are and has that info under the Properties menu option
My 80mm internal diameter magazine has a volume of 2e4 mm^3
I was brought up badly and so my brain needs that in different units:
2*10^4 mm^4 = 1.22 in^3 = 0.68 fluid oz (us)
The MD-50 spec sheet says the gallon kit gives 301 cubic inches of material. Fudging the volume up to 1.25: 300 / 1.25 = 240. Now, I have no plans to make 240 of these things, so per-part cost estimates here are going to be a mental exercise.
The gallon kit costs $114. Assume taxes and shipping push that to $130. $130/240 = $0.54. If I screw up a bunch $130/200 = $0.65
So far so good
According to Cura my 3d print of the same part consumes 21g of filament.
If I pay $30 for 2kg of filament: ($30/2000g) *21g = $0.315
I'm trying to tell myself comparing PLA to resin is like comparing plywood to oak.
Now for the mold.
Previously I've used a couple different silicone types from smooth-on. That works but it's a bit on the expensive side.
How much do I need?
I need at least one mold, but in order for this to make any sense I either need a bunch of molds or I need a mold that has a very low cycle time. MD-50 takes 4 hours before it comes out of the mold, so it'll have to be multiple molds.
Starting with one mold.
Adding 12mm of thickness outside my magazine I wind up with a mold that looks like this:
It has a volume of 3.546e6 mm^3 = 21.6 in^3 = 12oz
I can get a gallon of silicone for about $130 and I can get about 10 molds out of that kit, so $13/mold
Let's pretend I'm going to scale up and make all 240 magazines.
$130 + 130 = 260
260 / 240 = $1.08 per unit
If I could cycle the molds 3 times in a day that would allow for a production rate of 30 per day.
That's starting to sound promising, but there's a lot of room for improvement.
I printed some SMD cartridges and I'm generally really happy with the design, but I noticed a few things I thought could be a bit better.
The first is that the path that the film exits through is quite wide. Normally that's probably not a problem, but I'm a bit clumsy and prone to knocking things over, so I'd prefer a design where a part couldn't slip out through that gap.
So I put together a quick design in Fusion360 with a narrower film gap and tried putting my roll of WS2812B tape into the magazine and it didn't quite fit, so I'll want to print a bigger version.
How much bigger? It would be good to get a number before getting started
Well, how much tape of a given thickness can be held by a given diameter of magazine?
The tape follows a path that resembles an Archimedean Spiral
In polar coordinates the spiral is described by
where a is a constant offset from the center and b controls how much the radius increases per radian traveled.
We're starting at the middle, so a is zero.
The radius increases one tape thickness for each full circle. Taking a given tape thickness t and substituting into the above equation gives:
The tape I'm using is between 2mm and 2.5mm thick. Substituting those values give b of 0.318 and 3.98
For a given radius and thickness theta can be found
Subbing in 30mm for the radius and our b values that gives thetas of 94.25 and 75.4. That's the end of where we need to integrate. The start should be some number greater than or equal to zero. A tape can't be wound down to a point, so I'll use 2pi as the lower bound of the range.
Taking those numbers and plugging them into into Wolfram Alpha gives
So, somewhere between 1.1 and 1.4 meters.
How close is that result to reality?
Only thing to do is to mark where the tape starts to go into the magazine
1.37 meters! That's in the expected range.
Using that to refine my model suggests that a 40mm radius magazine would hold 2.4m of tape and a 50mm radius would hold 3.8m of WS2812B tape.
 - It would've been a good idea, but I just doubled the diameter and started modeling a prototype in Fusion360. I'm going back and writing up notes because I got curious.