Weatherproofing Dome

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

Relevant Contacts

Assignees

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:

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)

Centroform

https://www.weetect.com/thermoforming-polycarbonate/

https://www.youtube.com/watch?v=P8a2KrEOzxU

Open

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.

Open

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:

1. 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

  • plastic world ontario

    • 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

  • amazon

    • 1x1m with 1mm thickness is $76 and ships from China

  • home depot

    • 36x48 inch with 0.093 inch (2.3622mm) for $99

2. 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

3. 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.