Working with Coroplast

Cutting Corrugated Plastic

Coroplast is an easy material to cut. Thinner sheets are easier but most thicknesses can be cut using a simple utility knife. Cutting along the flutes is very simple using an utility knife, and several commercial cutters used by sign makers are also available.

Cutting across the flutes requires a bit more care but can be easily done with a utility knife. It is easier to use a straight edge such as a ruler to guide the knife.

Commonly found 4 mm thick corrugated plastic sheets can be cut with sturdy scissors or with a utility knife. Several lighter passes with the knife is easier to control than one heavy pressure pass.

Drilling Corrugated Plastic

It is also possible to drill Coroplast. IRWIN Speedbor Bits have little spurs on the edge that help make clean holes, as do other bits. Hollow core paper drills make tidy holes in Coroplast but a regular drill works too. Unidrills can be used for the thinner boards. These are the drill bits that have little steps in them.

For small holes a heated needle or nail will easily melt through and make a tidy hole.

A forster bit makes a tidy hole with almost no burr.

Drill bits with small side spurs worked best particularly in larger sizes. All the small holes I tried were a bit ragged. I think I could melt a small hole if I needed a tidy edge.

The spade bit worked very well.
Thinner sheets can also easily be punched using hole punches. This is useful if you want to insert grommets.

Scoring and Folding Corrugated Plastic

If you want a very sharp edge to your fold then you can cut the outside surface of the Coroplast leaving only one layer to act as a hinge, don't cut too deep, and make a bend that way. This weakens the edge but you can get a sharp corner. If you are bending against the flutes you have to cut almost to the bottom side.

Cut corners can always be covered with tape to make them less scratchy and to strengthen it.

Most boat projects will require you to score the spot you want to fold and then bend it WITHOUT CUTTING the surfaces. When the Coroplast is scored it gets crushed on the score line and this makes it easier to fold in that spot. There are scoring tools out there but you can easily use a screen inserting tool sold to replace damaged window screens. You can also use a pizza cutting wheel that has been dulled using a grinder. (Be careful they can be nasty sharp.)

Softening the bend line with a hair dryer or heat gun will help make the score. Don't burn yourself and don't melt your Coroplast. polypropylene melts at about 170 Celcius.

Above is my experiment using a heat gun on the right and no heat on the left. The thickness is 4 mm. The heated side is a better fold. 4mm can be folded without scoring but is good if you are scoring against the flutes. Scoring is almost essential if you are using thicker corplast.

It helps to use a ruler or piece of wood or metal to guide your scoring tool. If your bend is to be a curve it is useful to mark it first using a thin magic marker.

On the left margin I have a link to a Youtube video about scoring.

After scoring your bendline it helps to use a piece of wood or metal to support one side of the bend while you bend the other side. The principle is the same as when you bend metal using a brake.

Taping Corrugated Plastic

Polypropylene is very resistant to glues and tapes. For this reason most of the manufacturers will treat the sheets either by heat or corona process to to modify the energy of the surface, not to burn off any fat, and make it easier to paint or accept glue. Instructions to treat the sheets usually suggest using a propane torch and gently passing a flame over the plastic. Be careful, the plastic an melt and the drops stick to your skin and burn, or the plastic will burn. You need only to just play the flame over the surface. You can check if your sheet has been treated by putting some water on it. If it beads very high and refuses to stick to the sheet then it has not been treated. If the water spreads on the board in a thin layer then it has been treated and will stick to tape better (not perfectly though and if you tape or glue a seam you should check it before use).

Before Gluing it sometimes helps to clean the surface with methylated spirits. This is also known as denatured alcohol.

Chose your tape carefully. 3M (Scotch) VHB or very high bond and Scotch make some tapes that will stick to Coroplast. Scotch® Tough Heavy Duty All Weather Duct Tape, 1.88" X 45 Yd, Gray/silver Scotch Tough Ducktape is recommended by Ken Simpson in his boat build. Black Gorilla is also recommended by some builders. If the tape has a structural role take great care. It might even be useful and safer to add a few small wire stitches under the tape as insurance. If the tape is decorative or used to finish an edge then it is not so critical since your safety will not be compromised.

Gluing Corrugated Plastic

For the same reasons corrugated plastic sheets such as Coroplast are hard to tape, they are also hard to glue and paint. They simply don't wet out and can't allow the glue to get a good grip. The heat or corona treatment helps, as does wiping with denatured alcohol, but it is not a sure process.

Many people have experimented with various glues. The most successful have been some specialized hot melt such as 3M sells, or some of the AC Super Glue. Loctite 135436 Clear 406 Prism Instant Adhesive, General-Purpose, Surface Insensitive, 20 g, 0.7 fl. oz. Bottle cyanoacrylate (superglue) will work. Loctite 406 glue

A third class of adhesives that have been used successfully are the polyurethane construction adhesives. SIKAFLEX-11FC has been used successfully in some applications.

After reading this page, Rick Webber, UK sent me the following email. Many thanks to Richard Webber for sharing.

Boatquickcare.com advertise their glue as being good for polypropylene It looks like a form of Cyanoacrylic glue.

Check out the result of my tests on different types of glue.

Screws, Nuts and Bolts on Coroplast

Rivets and nuts and bolts will hold well as long as you use washers to distribute the load. IF you do not it's likely that the screw head will tear out of the plastic as it stretches or tears. Because of the weight they are not usually the fastener of choice. Some nylon fasteners are light and can work well.

Lacing and Zip Ties on Corrugated Plastic

Lacing or using zip ties is a good way to ensure that a seam is solid. It is also a good way to back up a glue or tape joint.

Another advantage of using zip ties or lacing is that the seam can be taken apart to modify or put away the project.

If the major strength of your permanent seam will be lacing, then it is possible to tack the seam using contact cement or hot glue to help keep pieces in position until you have laced them.

It is good practice to make smooth holes to allow for lacing or zip ties. This can be done using a drill or by using a punch. Small holes can be melted.

Heat Welding

Because polypropylene is a thermoplastic it can be welded using heat. Professional shops use ultra sonic or hot air. I think this is a bit tricky for thin walled corrugated sheets though.

Corrugated plastic is emerging as an interesting boat building material. As we learn to use it we can develop its boat building potential. Go and play but be careful!

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