Weatherproofing Dome
Big Project | Project | Project Manager |
---|---|---|
Pegasus 2 | Waterproof Dome | @Smile Khatri |
Task Description
Design and develop a manufacturing plan to create a top cover for Pegasus 2. The cover must protect all avionics from rain and other external conditions.
Context:
In W24 term, the waterproofing plan for the drone was to stretch a large piece of latex over the top and wrap it around the X-frame. The issue with this solution is that it reduces the aerodynamic efficiency of Pegasus because the latex is not rigid. It is also a challenge to determine the exact shape and size of latex required to stretch over the top perfectly due to two GPS antennas on the top.
Constraints
Constraints | Written By | Append Date |
---|---|---|
Must be rigid | @Smile Khatri | 2024-06-01 |
Must not block GPS and ELRS signals | @Smile Khatri | 2024-06-01 |
Light-weight (<150 grams) | @Smile Khatri | 2024-06-01 |
Relevant Contacts
Subteam | Contact | Contact Description |
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Assignees
Assignee | Asana Task | Date |
---|---|---|
@Divit Gawri @Pranav Bommireddipalli @Rui Zhu | 2024-06-06 |
Task Progression/Updates
Author: @Divit Gawri 2024/06/07
Initial Research
Factors to consider when selecting material:
Ease of manufacturability (moulds, special equipment needed, time, can the mould be reused, etc)
Restriction to design (what sort of bends can be made, typical thickness, adhesives/mounting)
Weight to Strenght Ratio
GPS/ELRS wave interference
https://www.weetect.com/thermoforming-polycarbonate/
Next Steps:
research costs for manufacturing + fabric material for each option (Pranav)
take measurements of drone for CAD + Mounting (Rui)
DIY Thermoforming option (if MS cannot share thermoformed)
Look into the weight of different materials (Divit)
Verify materials don’t interfere with GPS/ELRS Signals (Divit)
MONDAY JUNE 24TH
Material Selection Analysis:
Material | Strength to Weight | Interference with electronics | DFM | Thickness Options |
---|---|---|---|---|
Thermoformed Plastic (PC) |
| No |
| Thin is 0.25mm to 2mm Thick is 2mm to 12.7 mm |
Fibre Glass |
| No | better strenght to weight ratios more simple design required
|
|
ABS - balanced properties - 0.048mpa , coloured, seems thicker?
PP - 0.044 mpa, weaker tensile and lower density, but excellent impact resistance, chemical resistance (fire?), and flexibility
Polycarbonate - 0.0542 mpa, transparent, high impact resistance, good stiffness. impact is 600 j/m
Hips - 0.03 but cheaper and well balanced
Fiber glass- 0.278 mpa, density of material is usually double, but 10x tensile strenght.
impact is 100j/m2
Should be a bit flexible/not brittle, and how about failure characteristics?
priority of impact resistance? (find good value)
how about for tensile, should support weight of drone, force off flight and heavy rain
fiberglass can delaminate due to UV rays. PC will decoulour
Fiberglass will be damaged slightly by modifications like drilling, PC will not
Fiber glass is 4x better in tensile (stretched material)
PC is 6x better in impact strength, much more ductile
PC has higher insulator properties (bad, increases heat staying in drone?)
Fiberglass is more scratch resistant
Summary: As PC has better impact strength to weight ratios, it should be chosen. The tensile strength expected is low as there is just rain and flight forces involved. These should be well below to tensile strength required to break PC. However, impact strength is better in PC, resulting in better performance in collisions.
Additionally PC is expected to be lower weight as there is less excess resin. Furthermore, better tolerances and quality are expected with PC, if the thermoforming process is successful as the videos illustrate.
Some downsides of PC are that it may be a to good insulator, resulting in heat accumulating the dome. Also it may be prone the scratches.
Manufacturing Analysis:
Thermoforming:
mould must fit in machine dimensions (w/l/h)
needs plastic sheet
plastic should be heating uniformly and start to melt when ready
The method supports more complex designs relative to fibreglass
Mould Design should have holes for vacuum
Needs vacumn for the machine (any type works)
https://centroform.com/vacuum-forming
https://www.weetect.com/thermoforming-polycarbonate/
https://www.youtube.com/watch?v=P8a2KrEOzxU
Fiberglass:
Similar to carbon fibre manufacturing
Regular layup with 3D printed mould
Need to determine resin ratios and curing process if this is selected
Decision: Both options are feasible, choose the material that works best for the product.
Design Analysis:
manufacturability
strength/cracks/fea - bending from stresses of air and wind
mounting points
serviceability/latching?
Next steps/timeline:
mould design and finishing of 3d printed moulds
material thickness and sourcing - should be durable, meet strength requirements (FEA), and compatible with machine
vacuum pump
design optimization
June/26/2024 Meeting
Concerns:
Weight: at 2mm thickness, the dome weighs about 400g (material is polycarbonate)
Solutions:
Make it thinner (1mm)
Make the M9N antennae stick out of the dome
Use PET or a different material which can be made thinner and lighter, but will be weaker. Also a different method of manufacturing may need to be used.
Author: @Smile Khatri 2024/07/02
The dome was misaligned when I opened the assembly so I re-mated it.
Thoughts:
I like the use of surface modelling
Recommend looking at PC options online, delivery could take awhile
If we add cutouts for the GPS modules then we can go much lower on the height, which saves more weight and is much easier to manufacture
to prevent water from entering through the dome, a janky solution would be to wrap plastic around the rod of the antenna to leave no openings
rest of the components are much lower than the GPS
last thing is manufacturability. I recommend reading design guides for thermoforming like this one Thermoforming Design Guide | Profile Plastics
make sure that the formed PC can be removed from the mold. This means we need to start looking at mold design
main thing here is draft angles
here, the green colored sections mean that the draft angle is above 3 degrees. But the yellow indicates that draft angle in that region is less than 3. Draft angle may vary based on male mold vs female mold
the arrow represents the direction of pull. This is the direction that you will pull your part off the mold
if you have a negative draft angle or angle near 0, it will be impossible or extremely difficult to remove your part after it has formed
so from the quick analysis above… if our draft angle threshold is 3 degrees, then we need to adjust our spline
Author: Divit/Pranav/Rui 2024/07/05
Next Steps:
Source Polycarbonate (1mm)
ensure it is large enough to cover piece (surface area)
Current design is 261.1357 inch^2
thickness of 1mm
compare various options with price, reviews, quality
3 day shipping, $25; located in toronto
Clear polycarbonate sheet:
does this pose problems with UV?
available in 0.03'', 0.06'', and 3mm (0.762mm, 1.5mm, 3mm)
available in 12x12'', 24x24'', 48x48''
0.03'' thickness: 24x24'' is $13, and 24x48'' is $26
0.06'' thickness: 24x24'' is $19 and 24x48'' is $37
1x1m with 1mm thickness is $76 and ships from China
36x48 inch with 0.093 inch (2.3622mm) for $99
DFM
Fix design of dome to meet 3 degree draft angle constraint
Read design guide and resources to verify design is good for thermoforming
verify dimensions of dome fit within thermoformer specifications
Mold design
Research mold designs and thermoforming
Male vs Female Mold
Holes layout for vacuum
Any coatings/primer to improve surface finish
Current foam block is 96x22x4 - concerned it is not enough height
Notes:
17x17x3.96 Base/Wight/Height dimensions of dome
Find more resources for thermoforming
Centroform EZFORM LV1827 110v tabletop vacuum forming machine - https://www.robotshop.com/products/centroform-ezform-tabletop-vacuum-forming-machine
Forming area of 18 x 27 inches
Design Guide Key Points
draw ratio calculation to determine inital thickness
predrying pc?
July/21/2024 - Mold Design and Draft Angle Updates
Dome:
Draft Angle has been updated to approx 3.75 degrees
No Interference with antennas
Very close to some 3D printed mounts - However I think it will clear when mounting in person. If an issue, mounts will have to be moved (can do)
Mould:
Opted for Male mould due to better surface finish, simplicity, and reduced material consumptions
2 Variants, 1 with no base, other with base
Variant 1:
Will not include wood base
as a result there will be a slight pattern in the base of the octagon of the dome
this pattern is similar to as seen below and just impacts appearance
May drill holes into mold to improve vacuum near edges (tbd)
I suggest giving 1st attempt with this mold, and if need base, can make one for mold
In the manual the method used is based of variant 2
Variant 2:
Includes wood base that mold is mounted on top of (adhesives or bolts)
Wood results in no pattern on base
However, I am concerned the part will not form as well as there is additional layer the air must escape from
Circular pattern of holes should be drilled in the wood base to improve the vacuum
With a base the mold is only 1 inch to the edge of the forming area
Variant 3:
Instead of wood base, use foam as material
Will result in consistent texture finish between base and dome
Requires more material and increases mold manufacturing time significantly
Therefore I don't think this is a feasible option
Plan on using a laser cut stencil from wood to cut the base into the octagonal shape after the dome is made with its square base.
Process:
Install PC to former
Heat up to abt 180C - should sag significantly
Turn on vaccum
Lower PC around mold quickly
Once cooled to 90C, turn off vacuum and lift part
Additional Challenges
Largest challenge is determining if the vacuum is strong enough pull the PC around the male mould. if possible, it would be nice to drill holes into the mould to aid the vacuum in suction of the PC near the edges. I can better determine if this is needed once examining the machine and how the vacuum system works.
Increasing the draft angle will also help with vacuum forming. To do this the 3D printed mounts mentioned earlier will have to be moved.
Mold material tbd - is the foam high enough density? what is its melting point. If these are poorer then 3D printing, 3D printed moulds should be used
Finalize PC purchase list (verify dimensions when looking at machine)
Notes:
Thin washers under the mould to be placed for better airflow
Mold should be smaller then base plate by at least .5-2 inches on each side
Recommended thickness is 3mm of plastic. However 1.5 is lower weight so I think its better
Should sag 2-3 inches
It is possible to drill hole from top to bottom
https://egr.vcu.edu/media/engineering/documents/mne/EZFORM1827.pdf
Author: Smile Khatri 2024/07/22
CAD review
Is it possible to add slightly more volume inside the cover? The cover is too close to some of the mounts. Some mounts can move but lets try to minimize how many need to move. Lmk if there is anything blocking this!
Another thing is the actual part will probably shrink since we’re heating it up and cooling the PC again, so we want a good amount
Another thing is, we’ll probably add some cutouts for the bolts connected to the arm-clamping blocks. It would be preferred to have a rectangular cutouts, so we don’t have to bolt the arm clamping blocks after putting on the cover
Author: Divit Gawri 2024/08/16
Dome Design Overview:
File Location: Comp2025 > Weatherproofing Dome
Results:
Draft at just under 6 degrees
Comes at 330g in solidworks for 1.5 mm and 220g for 1mm. Should apply density factor to reflect plastic sheet weight when calculating expected weight. The latex cover is 256g for comparison.
Bolt cutouts for arm-clamping blocks to be drilled with a drill bit slightly larger (1 size up) then bolt heads. Drilling would be much easier and accurate then doing rectangular cutouts. However, If rectangular cutouts are still preferred, I can create some in the CAD.
Clearance to Mounts (3.5mm):
If shrinkage is greater than 3.5mm then the mounts will have to be moved
Clearance to GPS (10mm):
If shrinkage is greater than 1 cm vertically, it would result in failure.
I believe 1cm should be enough, but couldn’t find any sources, and am not fully confident.
If we elect to add more clearance, more weight will be added.
Potential Design Improvements:
Don't really like the radius of the part that connects to the base as an less steep decent would look more better and improves stress concentrations. Also reduces mass. However, doing so will cause interference with the electronic mounts, so I don't think its worthwhile to improve on this.
Thickness:
1.5 mm thickness material based on research/draw ratio calculations.
Material:
ABS - much cheaper and provides more trials. Should be used.
PC - Was selected initially because of slightly better strength properties. However, due to price, shouldn’t be used unless ABS is poor quality.
Mold Design:
Location: COMP2025 >Weatherproofing Dome > Mold Development > Mold Assembly
Mould to be created with CNC and foam within the bay
Dimensions of mould are: 16.7 x 16.7 x 3.425
kept the mold simple, just use a Dremel and stencil to trim the corners where the part will be a square.
mould will sit on thin washers to allow more area for the vacuum to provide suction
Mold should be good to fit in thermoforming base plate (18x27) but DOUBLE CHECK
Potential changes:
Add holes to mould with drill and 3-4 inch long drill bit
Add a coating to protect mould
Remove base altogether and let thermoformer baseplate be base.
Manufacturing Process:
Install material to former
Heat up to abt 180C - should sag a few inches
Turn on vacuum
Lower material around mold quickly
Once cooled to 90C, turn off vacuum and lift part
Use Dremel/stencil to cut base to shape
Apply Velcro and test
Timeline/To Do:
Locate Thermoformer and Validate Dimensions for mold (ASAP)
Provide mold design for manufacturing (ASAP and discuss lead time to CNC the mould w/machine shop)
Order Material (once mold design is started to be CNC) (typically 2-4 business days shipping)
Finalize locations to drill holes in mold for airflow improvements (After CNCing started and before the mold is done CNCing)
Setup Thermoformer with vacuum (After CNCing started and before the mold is done CNCing)
Start Trials (once the mold is manufactured)
Timeline depends on how quickly the mold can be made, I will be onsite until Aug 27 at the very latest. Although hope to get done by Aug 25.
Purchase list (order after CNC started)
Weight factors: ABS from Amazon is 2.5 times what SolidWorks estimates. Polycarbonate from plastic world is 1.1 times what SolidWorks estimates.
Size needed: 22x31 per trial
Plastic worlds:
Clear polycarbonate sheet 24x48: 37 + Tax + 73 ship = 124 CAD 2 days
2 sheets of 24x48: 74 + 50 = 140
3 sheets of 24x48: 111 +tax + 50 ship = 182 CAD 2 days
Potential Failures, Prevention, and Impact:
Too much shrinkage - Clearances to combat this. However, if not enough will likely have to redo mold.
Mould air pressure - if the material does not compress to the mould well, more holes can be drilled to the mould. If issue persists then a stronger vacuum should be used.
Mold melting - The melting temperature of the foam being used should be validated (find specification of foam).
Poor quality - Likely cause would be material, would require a new sheet of thermoformable material.
Author: Divit Gawri 2024/09/18
Dome Thermoforming Guide
Tasks to do in preperation for Thermoforming:
Foam Material Melting Point: Validate that the foam mould will not melt at thermoforming temperatures (max 180C). If it melts near this point it is still ok as the max temp for the PC is 180C, and it likely will cool down or not heat up the mould to such a high temp.
Test can be done with maybe a fire and heat gun or finding the foam specifications?
Manufacture Mold: Provide the mould CAD to the machinist to manufacture the mould. Also, inquire if they can produce the drilled holes for the vacuum suction.
Materials: Will need PC sheets, and foam to make the mould (already have).
Best option found for PC was by Nathan on discord under the purchase request thread for the waterproofing dome. https://www.aircraftspruce.ca/catalog/mepages/2024_leadedge.php
Cut PC: Cut the PC to the dimensions of the thermoformer with a bandsaw.
Moulding Procedure:
Install material to former
Heat up to about 180C - should sag a few inches
Turn on vacuum
Lower material around mould quickly
Once cooled to 90C, turn off the vacuum and lift the moulded part
Use Dremel/stencil to cut the base to shape of drone
Apply Velcro and test
Full Guide: https://egr.vcu.edu/media/engineering/documents/mne/EZFORM1827.pdf
Before forming the actual dome I would suggest doing a trial with a small piece of extra PC (sheet is bigger than required) and dummy mould (3d printed or any 3d shaped material) and see how the vacuum/process performs. Can also try a miniature dome mould 3d printed, and see how that goes.