Research on Foams for RAFT Project

In an effort to “kick-start” progress the RAFT project for the Marine Robotics Team, I spent a chunk of time today researching foams. Last weekend, Georgia & Jaya did some initial research on pourable foam. Given that the ‘mystery’ foam we have in N52 turned out to be a high-density, expensive piece that was donated years ago (and that the dust created when cutting it is a carcinogen) I’ve decided that we will use a pour-foam similar to the foam Brooks & Rob used when making the Hover Group rafts.

I want to use polyurethane expanding foam –  it is commonly used in marine applications and US Composites claims that “once fully cured this foam can be laminated over with any type of polyester, epoxy or vinyl ester resin without melting.” (1)

I’ve divided the task of creating the hulls for the raft into sub-tasks (each sub-task requires separate research / thought)

  • Deciding on Final Dimensions
  • Deciding on a Foam
  • Purchasing Materials
  • Creating a Mold
  • Preparing the Mold
  • Pouring the Foam + Curing
  • Removing Foam from the Mold

Dimensions

Pontoon

  • Length: 30″
  • Width: 5″
  • Height: 4″

Center 

  • Diameter: 17.5″
  • Height: 6″
For purchasing the foams, density & price should be taken into consideration.

Density

From US Composites 
  • 2lb / cubic foot: recommended for void filling in nonstructural applications, is approximately 95-98% closed cell which resists absorbing water, however continuous water submersion can eventually lead to loss of buoyancy over a period of years. We recommend this product strictly for flotation applications.
  • 3 lb/cubic foot: can be used as a lightweight foam for carving or sculpting as well as some limited casting applications
  • 4 lb/cubic foot: suited well for most flotation applications that require more support than the 2LB offers. This foam is recommended to be used when supporting gas tanks and some deck areas. Choose this density if you feel that the foam will be required to withstand minor to moderate loads.
  • 8 lb/cubic foot: extremely hard and rigid, like that of a soft wood, uses include casting, carving, sculpting, sign making as well as areas where an extremely durable and rigid foam is needed for support.
From these descriptions, it sounds like 4 lb/cubic foot is best suited for our application. US Composites / other companies selling polyurethane use the notation “#4” to indicate that the particular polyurethane mix expands to a density of 4 lb/cubic foot.
*In SolidWorks, the material properties of polyurethane (rigid) corresponds to #8 foam.

Working out the math with the #4 foam and see if our current dimensions would yield a workable raft .. .

Pontoon 

  • Mass = 1.57 kg
  • Volume = 0.0098 m^3

Middle 

  • Mass = 1.894 kg
  • Volume = 0.0236 m^3
Total
  • Mass = 3.464 kg
  • Volume = 0.0432 m^3
Other weight on raft
  • Main support truss (each beam weighs approx 500 grams x 3) = 1.5 kg
  • Motors (each bilge pump cartridge weighs approx 500 grams x 4) = 2 kg
  • Motor supports (estimate 1.5 kg each x 2) = 3 kg
  • Electronics – rough estimate 8 kg
  • Accessories (sensor cage) – rough estimate 7 kg
Total weight: 25kg (or 55lbs)
Density of water: fresh – 1000 kg/m^3 or saltwater – 1025 kg/m^3
Floats by displacing … 0.024 m^3
Using these dimensions, half of the foam hulls will be above the waterline.
My intention was for at least 10% of the foam hulls to be above the waterline. In the scenario I described above, an additional 10kg would result in 20% of the foam hulls to be above the waterline. An additional 15kg would result in 10% of the foam hulls to be above the waterline. Given that we want to keep the raft as a multi-purpose surface craft, this is a nice range to work in. I can’t imagine when we would have a payload that weighed more than 15kg.

Price

The total volume of the two pontoons and the center hull = 0.0432 m^3 or 1.53 cubic feet. Comparing with US Composites and AeroMarine, it is cheaper to use US Composites. The foam is sold in a 4 lb kit (1 cubic foot) or the 16 lb kit (4 cubic feet). We’ll purchase the 16 lb kit @ $67.00

Molds

The way I see it, we have two options. Build a wooden mold and coat the wood in shellac or use a plastic mold.  We have plenty of wood on hand so I decided to explore the plastic mold route / compare the price of the plastic to the price of the shellac for the wood mold.

  • From the ContainerStore I found a 22.75 gal “Tote with Lid Clear” for $17, Dimensions: Length – 31-1/2″,  Width – 17-3/8″, Height –  13″ h

I imagine making three hulls at once, like …

  • From US Plastics I found the perfect mold for the center foam – a durable 5 gallon tub for $11, Dimensions: Diameter – 18″, Height – 7″

So we’ll be going with the plastic molds. These two molds will work perfectly and it will eliminate the need to make wooden molds/forms.

** IMPORTANT: From US Composites, the foam can be poured in layers for large jobs. But for best adhesion you should wait at least 15 to 20 minutes before pouring over a previous batch.

Misc Supplies / Tips for Working with Polyuerthane 

Guidelines From US Composites 

  • This two-part liquid, expanding rigid urethane foam is a closed cell, pourable foam, which will resist the absorption of water.
  • Once fully cured this foam can be laminated over with any type of polyester, epoxy or vinyl ester resin without melting.
  • All expansion rates and times given are temperature critical. Temperatures below 75 degrees F will lower the expansion rate therefore requiring more foam. Ideal working temperature is 75 to 80 degrees F or above.
  • Accurate measuring of these products is extremely critical. US Composites sells measuring containers 
  • Working time before foaming: approx. 45 seconds
  • Time before full expansion: approx. 5 minutes
Examining the different resins sold by US Composites I found 435 Standard Polyester Layup Resin (Marine Grade Resin). But then I found that US Composites sells both resins and gelcoats. After a quick google of gelcoats, I found the wikipedia article which explained that 
Gelcoat is a material used to provide a high-quality finish on the visible surface of a fibre-reinforced composite material. The most common gelcoats are based on epoxy or unsaturated polyester resin chemistry.
I think a Gelcoat is more appropriate as a finish on the rafts than a resin. US Composites sells pigments to add to their neutral gelcoat (1 ounce of pigment for every quart of gelcoat). I like having the raft be a bright, visible color.
US Composites recommends thinning the gelcoat for better pentration of porous surfaces using Styrene (and thinning at 10% to 15% by volume). 1 pint of Styrene would be more than enough for 1 quart of Gelcoat

BOM as of 4/21/2012

Links

  1. US Composites  http://www.uscomposites.com/foam.html
  2. AeroMarine http://www.aeromarineproducts.com/boat-foam.htm
  3. “Working with Two Part Expandable Urethane Foam” http://bertram31.com/proj/tips/foam.htm
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