Foam/Ply Composites

By Paul Williams.
Article posted 16th November 2007.

Introduction

Foam/ply composites are a quick and easy way of building a fast electric boat. The principle is simple: a core of styrofoam is hot-wire cut to shape then sheeted over with thin plywood to produce a very strong, lightweight structure that is more than capable of withstanding the stresses and strains of high-speed racing. Not only is foam/ply very strong and light, it is also very bouyant and can absorb a lot of energy in a collision.

This outrigger hydroplane has front and rear sponsons made from foam/ply composite.
The full build article is here.

The type of foam used in my designs is an insulation styrofoam used in the construction industry which can be either pink or blue in colour. There are several brands available under tradenames like "Floormate" and "Polyfoam" that can be purchased from a builders merchants or scrounged from a building site. A slightly more expensive source of suitable styrofoam is "Craftfoam" by Panel Systems. This appears to be a regular insulation foam packaged in smaller quantities for modellers. If you do the maths, the price per square meter is quite high.

Foam/ply outrigger hydroplane sponson build-up. Each sheeting piece doubles as a cutting template. The numbers show the order in which the sheeting pieces are applied, each one is sanded flush to remove undercut (see below).

Method

The method I use is quite simple, and is an effective method of rapid prototyping - see this page for a step-by-step demonstration. Each ply sheeting piece doubles as a cutting template. Taking an outrigger hydroplane sponson as an example, the first template glued to the foam core blank is the main sponson profile. The two core blanks, complete with profile templates, are drilled for the booms, then keyed together with brass tubes and the initial profile cut with the hot wire.

Cutting the anti-trip angle.

Each successive sheeting piece is glued in place and the next cut taken, sanding down each time to remove undercut. The only template which does not form part of the finished sponson is the one used to cut the anti-trip angle. This template is held in place with double-side tape. Before gluing each template/sheeting piece in place, sand the cutting edge smooth to make sure the hot wire does not snag.

What You Will Need

To make foam/ply composite boats, you'll need:

• Foam, as above
• A hot wire cutter ("hwc")
• Basic modeller's tools
• Glue, suitable for use on foam
• Plywood

Hot wire cutter.

Glue

With regard to glue, there are several options available:

• Odourless "foam safe" cyanoacrylate superglue. Not only will it bond in seconds, it is also totally waterproof. The downside is cost. For a model of any size, superglue will rapidly work out very expensive. Also, you are under a bit of pressure with superglue as it goes off (cures) very quickly, so it's not so good for larger areas.
• Epoxy. Very strong, but expensive, messy, smelly and heavy. Curing times vary from 5 minutes to 24 hours, and you will tend to waste a lot as you have to mix it in batches as needed. You will need some method of holding the foam and ply together as the glue goes off, for example masking tape.
• PVA "white glue". PVA is cheap and there are waterproof types available. The downside to PVA is the curing time and the bond strength, which can sometimes be a bit weak with styrofoam. As with epoxy, you will have to hold the foam and ply together as the glue dries.
• Copydex. This is a contact adhesive that is applied to the foam and the ply and allowed to semi-cure before the items are brought together. Not waterproof and low bond strength, but cheap.
• UHU-POR. This is a foam-safe contact adhesive that, like Copydex, is applied to both the foam and the ply. After 5-10 minutes, the two items are brought together and joined. POR is an extremely useful glue for foam/ply modelling. It's cheap, strong and waterproof.

My personal preference is a mix of UHU POR for large areas, with foam-safe cyano for small areas.

UHU POR contact adhesive for foam. UHU POR is the most cost effective method of fixing ply sheeting to large foam cores.

Plywood

There are three grades of plywood that we can use for foam/ply composites. 0.4mm (1/64th" or 64th) and 0.8mm (1/32" or 32nd) are both readily obtainable from most model shops. There is a third, intermediate grade available, 0.6mm (1/48th"), which is a perfect blend of the lightness of 64th and the strength of 32nd. The only drawback with the 0.6mm grade is price and availability. It is only available in the UK from Mantua Models, and is slightly more expensive per square meter than both 64th and 32nd.

• 64th is excellent for making small, lightweight models that you're not going to race. Avoid the temptation to use it for racing models and Hydro 1 and above, it simply isn't strong enough.
• 32nd can be used for any model. It's light and very strong.
• 0.6mm can be used in place of 32nd when weight is critical. Mini Hydro models benefit most. The choice of 32nd or 0.6mm is down to cost versus performance. For Hydro 1 and above, the weight saving from using 0.6mm in preference to 32nd is minimal.
• 1/16th" (1.5mm) can be used where you want extra strength, for example the inside faces of hydroplane sponsons.

Plywood is made by laminating several thin layers together under heat and pressure. Ply sheets usually have a residue of the release agent applied to the outer surfaces during manufacture that is best removed with fine grade abrasive paper to ensure a good glue joint.

Hot Wire Cutter

If you don't already have a hot wire cutter ("hwc") see this article, "Making a Hot Wire Cutter".

Tools.

The basic modeller's toolkit should comprise:

• Large craft knife ("Stanley" type). The best "Stanley" type knife I've found is by Irwin. It has a fixed blade and a slimmer handle than the retractable type.
• Engineer's square - for cutting and gluing at 90 degrees. I have two sizes, 3" and 6".
• One or more scalpels. Swann Morton make the best scalpel handles. A "10A" size blade suffices for most jobs.
• Selection of steel rules. I have steel rules in sizes 150mm, 300mm and 600mm. I also have a thick, bevelled-edge cutting rule from Axminster tools. Tip: to prevent your rules from sliding around, apply a strip of masking tape to the back.
• Razor plane. A razor plane is a miniature block plane mounting a razor blade used for quickly and precisely trimming wood to size before finishing with abrasive paper.
• Razor saw. The best razor saw I've had yet is a "Victor" brand, again from Axminster Tools.
• Mitre block. A mitre block is used for cutting balsa and hardwood strip to 45 and 90 degree angles.
• Abrasive paper. I mainly use 120 grit wet-or-dry. I cut the sheets into four equal pieces then secure to a wooden block with masking tape.
• Cutting Mat. Cutting mats prolong the life of your work bench and your cutting tools. Cutting through ply or balsa into a hard surface will very quickly ruin the blade on a knife or scalpel. If you can afford it, get an A1 or better A0 size.

Undercut

When making items from hot-wire cut foam and ply templates, you need to be aware of an effect I term undercut. Undercut is where the ply template of a freshly cut core ends up very slightly proud of the core surface. This is because the wire is cooled when in contact with the ply template, and therefore does not cut as wide a track through the foam near the template. Every cut you make in foam will have an undercut on the template - use your sanding block to flush off the template level with the foam core.

The wire will tend to cut a wider track away from the cooling effect of the template, resulting in an undercut - the ply template ends up very slightly proud of the foam core. Simply block sand the template flush.

Undercut removed with sanding block.

The amount of undercut left by the hot wire is only tiny, but it is enough to stand the ply sheeting off the core and ruin the item, so it pays to take the time and remove any undercut before sheeting.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

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Last modified: 08th July 2010 @ 09:05