Large Expanding Foam Prop - Monster Hunter Greatsword

About: What was I doing again?

This is my first attempt at a large prop. I ended up using materials that I was comfortable with and learned several placed where I could improve the design for next time. The goal was to create a 1:1 scale version of a Montser Hunter greatsword. Overall I am happy with the design. Now if I could only figure out how to make the armor.

Step 1: Source Material and Planning

The images show the "Jawblade" greatsword that I am going to make. The problem with props of this size is weight. The ones next to the wall were made from a poured latex in a mold. They also are about ~5 feet total in length. The full source material scales to about 10-11 feet when drawn. I would need a much lighter material and stronger internal support to hold the weight without flexing.

I chose GreatStuf expanding insulation foam for the organic shape I was
recreating. Simpler shapes should use foam board since it is quicker to build with and even lighter.

Either foam would need a protective layer. Most used paper mache as the hardening layer. I attempted this, but ultimately modified this step to better suit my build.

The basic idea was outlined in another build I found on the internet, but I cannot find the page again. The last picture showed their basic plan of steps in that build. The prop was also only about 6 feet in total length so a wooden dowel worked well for the core structure and handle.

The most important thing to remember when doing any build, costume, or prop is this phrase, "Fix it in post." Do your best to plan ahead and get the desired effect, but do not allow a mistake or imperfect feature to keep you from moving on to the next step. All things can be fixed with forced perception or painting techniques.

Step 2: Cardboard Outline for Sculpting

This step should be done with any build, regardless of material. Paper can be used as the first sketch layer, but it can be done on the cardboard center as well. Unfold a large box and begin sketching the desired shape and general features.

A cardboard center gives an outline to follow with the 3d shape as foam is added. This piece will define the overall size of the prop, but it is not seen at all in the finished product. Also note that since the prop is built on top of this layer, it will only get bigger. Mark the centers at both ends to help with aligning the sketch over a large distance.

I wanted the blade itself to be as large or larger than a man so I made the sketch roughly 6 feet. This would mean that the handle should be 3-4 feet giving an overall length of ~10 feet which will fit my scale.

Add features like the hilt and blade edge to get an idea where these will be as you sculpt.

Sketch thin detail pieces (like the dragon teeth) so that they are in line with the cardboard corrugation. This will be used in the next step to reinforce them.

When you are satisfied with the sketch, cut it out from the cardboard to use as a template while layering foam.

Step 3: Reinforcing the Cardboard Center - Prepping for Expanding Foam

One thing I noticed about the finer details (dragon's teeth) was that they could flex under the weight of the uncured foam. I reinforced them with wire from landscape marking flags. These inserted into the corrugation of the cardboard. This gives them more resistance to flexing as the foam is added.

I chose standard 10 foot length, 1 inch diameter, schedule 40 PVC electrical conduit for the handle and support. This grade has more ductile strength than normal PVC allowing it to flex under stress without cracking. The schedule 40 electrical conduit I used has molded connection points at the end. This connection will stick out from the hilt to insert the rest of the PVC into the support to use as a handle. The support was cut to have the molded joint just below the hilt and the opposite end before the edge of the sword. You want to make sure that it will not be in the way when you are carving the foam at edge.

Holes (1/4") were drilled into the PVC so that foam could be expanded into the support. One at the hilt end was used for a wire to hang the blade during future steps.

Zip ties and duct tape were used to secure the template to the frame. A thin 1/4" strip of wood was used on the opposite side of the cardboard so that the zip ties would not pull through the cardboard when under stress.

There is a hole in the center of the blade which posed a challenge for providing structure. In the end I ran the PVC straight, but separated the wood to both sides. Everything here will be painted flat black to hide it during pictures. (Fix it in post)

Step 4: First Foam Side


I needed about 8-10 cans of expanding foam for this sword. I cannot be sure of how much will be needed, so always buy more and return the unused portions. Once started, you cannot pause adding foam to one side. The second side cannot be done until this side dries.

NOTE: Do not touch or damage the outer skin of the foam as it expands. If you do, then it will release the expanding gases and ruin the reaction.

Start by creating long lines of foam. Slowly move out into the details, but the correct speed must be judged based on how quickly the foam is expanding. The goal here is to create enough foam to be sculpted back down to the desired form.

NOTE: Remember that the handle needs to be the center of the sword and not the cardboard since it is attached to the side. Make the layer with the PVC thicker than the opposite layer to make the handle in the center the prop.

Let this layer dry to the point where it will not slide when touched. The foam turns yellow when exposed to sunlight for extended periods, but this does not hurt the foam at all.

Step 5: Second Side of Foam

To layer the other side while not crushing the first layer of foam that is still drying underneath the formed skin, a metal wire was tied to a hole in the PVC at the end of the hilt.

The next layer was added while hanging, but the foam had a tendency to slide to the end. Not significant, but it got worse as more foam was added. Either way, make sure a sufficient layer is added for carving later.

It was allowed to dry hanging before placing in the garage overnight.

Step 6: Carving

This step makes a lot of mess. I do not recommend using a saw as it will make foam dust. Hot wire could be used, but I went with large sharp knives. A large curved blade made most of the cuts. An old steak knife made the more intricate cuts around the teeth. This removes the foam in large slices/sections that are easier to collect in the trash. Waiting another day to have the foam dry more is also helpful. The outer skin that forms tends to be sticky and slows the cutting.

Sketch the rough design repeatedly with a marker as foam is removed to keep a memory of the shape. A little forgiveness for yourself is needed as you carve the foam. I am no sculptor and this took several hours as I was being careful. You can add more foam if a big mistake is made, but it is best to carve a little at a time.

One issue with the expanding foam is that not all the lines will form together. Some push others upward and leave large voids inside the sculpt as the outer layers are removed. This will be fixed in the next step. For now just get near the final shape.

Step 7: Fixing the Voids

This step is really easy. The foam has been roughly sculpted to the final size, but the voids need to be filled. Simply spray more foam inside the structure and let dry. Cut off the tumors that form outside the next day. Reshape the foam as needed.

I redrew the features again so I could determine if I was happy with the final dimensions for everything.

The surface will still largely be pocketed with smaller craters, but the next step fixes that.

Step 8: Skinning the Beast - Cheater's Paper Mache

I do not have any experience with paper mache or finishing materials before this so I began to improvise. I attempted paper mache with recycled paper, but the pieces did not want to adhere to the foam very well. The exposed foam cells also soaked up the glue solution out of the pieces. I decided to go a different route.

I decided to use masking/painter's tape as the paper layer with a glue applied to the outside. I purchased tape in 1 inch and 3 inch rolls. This went very well as the tape would stick to the foam without needing glue. 1 inch strips were used in the detail areas and complex curves while 3 inch strips covered the larger areas. This is where all the open cells and bumpy surfaces starts to become smoothed over. The old knife came in handy again as I trimmed the tape to fit in between the teeth.

Once all the tape was applied to the foam sculpt, it was time to seal it with glue. I purchased 1 gallon of waterproof woodworking glue and used about half of the jug. Lay some plastic below the workspace to catch any glue as it runs off. Use bristle brushes and paint the glue over the entire surface and into all crevices. Allow it at least 12 hours to dry before handling. The glue also pooled in some areas and took more time to dry. I had to do each side separately on a table to prevent drips. Do NOT wipe the excess glue while working. Let it dry and trim off with a sharp knife or with sanding.

If desired, sand the glue by hand to achieve the needed texture. I wanted the surface more natural like bone, so only minimal sanding was done in areas where the glue was very thick.

This procedure protected the foam from water absorption, but also made a very flexible outer coating. I have taken this to several places without damaging the sword after being handled a lot in transport.

Step 9: Painting

I am also no artist. I used basic model painting techniques like dry brushing to get the finished product. The first picture shows the paints I used.

I hung the blade up and started with a gray primer before adding satin black to the hole and hilt. The rest of the blade was base coated in satin Ivory spray paint.

Matte black paint was painted into the hole and spaces between the teeth. This helped to separate them more since the foam expansion made them closer together.

All other colors were dry brushed in layers of lighter colors as I saw fit to match the game model.

I made the blade edge and teeth lighter colors to distinguish them from the main body.

Black and silver paint were mixed and painted over the hilt section.

The entire blade was then covered in Matte clear sealant spray to protect the paint.

I apologize that I do not have more pictures of this step during the painting as I did it all the evening before a convention.

Step 10: Handle - Reinforcement and Finishing

The excess from the original 10 foot PVC pipe that was used inside the blade will now make up the rest of the sword. It can be trimmed for a smaller prop, but I needed the 3 feet of leverage to hold the blade comfortably and match the source material design.

I trimmed the excess to about 38 inches which gave me plenty of leverage for lifting the sword. However, the PVC joint alone would flex under the weight. I could have glued or screwed the handle into the joint, but I wanted a little more support from the handle. It also needed to be easily removable for transport in my car. Two 6 foot sections are much easier to transport than a 10 foot piece.

The extra skeletal support came from some galvanized steel conduit that was inserted into the handle. As long as the steel conduit is less than the total length of the PVC used, there will be no problems. I had about 6 feet of conduit, so there is 38 inches hidden in the full length of the handle, and the rest is going into the blade as extra support while elevated.

Insert the steel pipe into the handle and secure with self tapping sheet metal screws. Screws were added at even intervals along the handle until the end was reached. I added a PVC cap and more screws as a decorative pommel.

The entire handle was primed in dark grey and wrapped with brown fabric. Electrical tape was used to secure the ends of the fabric rolls. I had tied leather strips to hide the tape, but those quickly untied themselves during the convention.

Securing the handle to the blade is done by the use of 2 decorative Chicago screw assemblies. These work very well and are easy to remove for disassembly by hand. Insert the handle with steel into the blade and drill through the pipe that can be seen in the hole of the blade. This is where the 2 pieces will be joined for a stronger connection.

NOTE: Make sure to drill the holes through the PVC in the blade and the steel conduit at the same time. Also be sure to insert the steel and handle into the blade fully before drilling. Otherwise the holes will not line up for assembly and you will be a very sad panda.

Once done, bolt the handle to the blade and go hunt some monsters!

Halloween Props Contest 2016

Second Prize in the
Halloween Props Contest 2016



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    13 Discussions


    2 years ago

    So if I REALLY wanted to make say a sword? How would I go about making a mold for it? Cause I got this awesome sword that I designed but didn't make because I didn't think I would be able to make it as good as I wanted it to be. Also I still wanted it to be used for hitting my siblings so strength is a thing too.

    1 reply

    Reply 2 years ago

    1) I have no experience with molds, hence why I used canned foam.

    2) The strength of this prop is only for display, light handling, and transport. It cannot be used for an impact.

    If you want to mold a sword then look for instructions for LARP props made from foam latex.


    2 years ago

    Great stuff is a polyurethane foam (ya all prolly know this), but you can buy the unmixed components to mix and pour for large castings. This might be more uniform than the spray can, as well as having the ability to choose foam density specifically for your application.

    1 reply

    Reply 2 years ago

    I know the compounds can be ordered and poured, but that would have required a mold. Either type of mold (block shape or final pour mold) would require sculpting experience. I also have more experience with the canned version than the poured mix. This was more of an experiment to see how I can build it with easy to use materials in a short amount of time.

    The biggest benefit to the great stuff foam is that ability to extrude more material to build the shape instead of trimming away.


    2 years ago

    Dude this is epic, great work.

    I've been playing around and looking at using the expanding foam cast idea, I've seen others have some quite impressive results but seriously wondering if the expanding foam is strong enough, was your foam quite strong when finished drying/setting?

    2 replies

    Reply 2 years ago

    If the foam is allowed to expand in a mold, than it is much more durable. The outer layer forms like a foam latex would. The inner foam structure is not as flexible, almost like standard foam board insulation. Spraying into a mold is difficult as the lines will cause uneven expansion throughout the structure. The outer skins also do not mesh together. They press into each other as the foam inside expands. Also if there is not enough room to allow proper expansion, then the interior foam cavities will collapse. If the skin is punctured while expanding due to an edge or corner, then the gases escape and the foam collapses.

    However, used in an open air expansion (maybe 1 half of a mold at a time) it could work. Excess would need to be trimmed and an outer protective material is needed to sand/paint/finish the model.


    I recommend using the foam insulation board if possible, but this design needed a more natural shape. The spray expanding foam is also a great alternative to pouring latex or foam rubber on a budget.


    2 years ago

    did you have some time constraints in letting the foam dry? I don't get why it was sliding, did you let it cure all the way before putting on another layer?

    1 reply

    Reply 2 years ago

    The sliding occurred when I was attempting the second side while it was hanging. I was hoping the foam would form fast enough to not be an issue, but the sliding was still minimal. I also was rushed to finish this part while I still had daylight outside.


    2 years ago

    Nice job! I can't wait to see what you make next. :)

    1 reply

    Reply 2 years ago

    Thank you. I am very slow at building the large props, but I try to document them as best as I can.