The following tutorial will walk you through the steps to develop and the fabricate your own fiberglass shell chair.
The process will assume that you have developed your own CAD file for a chair but will take you through the process of making a form, pouring fiberglass on top of this form and finishing the fiberglass once it has set.
Step 1: 1. Preparing the Model for CNC
1.1 The 3D model must be turned into a form which the fiberglass can be poured onto. To do this I flipped my shell shape so that the seat faced down and the edges (as best as possible) laid flat on the x plane. I then made this a solid form (filling in the seat cavity). The result is shown in the illustration.
1.2 This model must then be sliced into sections. I sliced my model every 1.8 inches, the thickness of the 4ft x 8ft polystyrene sheets I used. Other material can be used but just remember the thinner your material is the more of it you will need to build up thickness and the more time you'll need to cut all those sections out.
1.3 These slices can the be laid out to fit within the 4 ft x 8 ft boundary of each sheet of material (multiple sheets may be required based on the size of your chair).
Step 2: 2. Material Estimation
Laying out these sections onto 4ft x 8ft sheets can help you understand how many sheets of material you will need to make your form work. I then export the file as a .STL file.
Once my tooling.stl file was sliced and laid out to fit within an area of 4 ft x 8 ft sheet. I was easily able to see that I needed 5 sheets of 1.8" thick 4 ft x 8 ft polystyrene.
Step 3: 3. Developing Tooling
Next you'll need to import this file to a CAM software. Here I'm using RhinoCAM. If you attempting to cut Non-dense foam download the 'Feeds & Speeds' attachment included. This file has some recommended settings for cutting various materials which will work with most 4ft x 8ft mills.
Be sure to add tabs in your CAM software (this is an option in most tools) as this will keep the pieces in place while they are being cut.
Step 4: 4. Machining the Foam
4.1 Make sure the CNC is up and running (consult your shop supervisor to be sure everything is in proper working order).
4.2 Place the polystyrene on the CNC bed.
4.3 Be sure to use some double sided tape between the material and the bed of the mill to prevent the foam from moving around while the file is being cut.
4.4 If you're CNC has a vacuum bed you'll want to turn the suction on now.
4.5 Next you'll want to load the first cut file (you will need to cut one sheet of material at a time).
4.6 Repeat until all slices have been cut out. If your cam software doesn't label your slices you may want to take a marker and just label each piece so that it's easier to remember what's what later on.
Step 5: 5. Section Assembly
Now that everything is cut out, you'll want to collect all of the layers of machined foam and place them in the intended order. This can be tricky if you have a lot of sections but if you marked the pieces as mentioned in the previous steps, the numbers will help guide you.
I chose to connect my sections using 3" drywall screws and fasteners but you can also use 3M Polystyrene Foam Insulation 78 Spray Adhesive. In my experience this can eat away at polystyrene. You can sometimes limit this effect by spraying the adhesive from farther away. Either method should prove strong enough for our purposes.
Sections should be added one at a time to ensure that pieces do not shift while glue is drying or screws are added.
Step 6: 6. Finishing the Formwork
Tool and Materials Needed:
- White Plaster Dry Mix
- Minwax Finish Wax
- Paint Brush
Now you're going to want to coat the formwork with plaster. This will seal the foam and enable smooth removal of the fiberglass components later on.
6.1 Lay even coats of plaster on tooling, allowing plaster to thoroughly dry between coats.
6.2 Ensure the coats thoroughly and evenly covered in plaster, brush in spots if you missed any.
6.3 After all necessary plaster is applied and has been given time to dry. Apply single coat of Minwax Finish Wax
Step 7: 7. Apply Release Agent
Tool and Materials Needed:
PVA Release Agent (Caution, this substance is toxic and should be used with care)
The following step will help ensure that the fiberglass chair shell will release from the formwork.
7.1 Brush formwork with one layer of lacquer and allow to fully dry.
7.2 Brush formwork with three layers of PVA, being sure to allow each layer full dry before applying the next.
Step 8: 8. Introduction to Wet Lay-up
Technique: wet lay-up 'pre-preg':
"[Fiber glass] is made up of mat of woven reinforcement with resin impregnated between the fibers. The quantity of resin is precise so that each layer of the lamination can provide optimum performance. This reduces weight and increases the strength of the product" (Via Manufacturing Processes for Design Professionals by Rob Thompson)
Required tools and materials:
Synthetic fiberglass veil
Vinyl Foam: divinycell 5 lb. density
White plaster dry mix
PVA release agent
Step 9: 9. Wet Lay-up: Veil
Begin by covering the formwork with synthetic fiberglass veil.
Both the top and bottom layers will be covered in the Synthetic Fiberglass Veil in order to minimize the need for sanding in post production.
Step 10: 10. Wet Lay-up: Resin
Mix epoxy resin and hardener (ratios and mixing time can usually be found on packaging).
Pour the mixture onto the veil covered formwork. Use fiberglass roller to evenly spread the epoxy resin. The fiberglass should now be evenly saturated with epoxy resin.
Step 11: 11. Wet Lay-up: Strand Fiberglass Mat
11.1 Lay sheets of 1.5 oz. chopped strand mat fiberglass on top of your formwork and use the rollers to lay them down.
11.2 Apply epoxy resin.
Step 12: 12. Wet Lay-up: Repeat
Repeat these steps until you reach your desired shell thickness.
Step 13: 13. Wet Lay-up: Finishing
Finish by covering the formwork with synthetic fiberglass veil in order to minimize the need for sanding in post production.
Step 14: 14. Allow Resin to Cure
It is important to allow resin to cure for the total Amount of time described on the package.
Step 15: 15. Remove Fiberglass From Formwork
After the Fiberglass has cured you should be able to pop the shell off your formwork. Some sanding may be required to smooth out the edges and surfaces of the shell.
At this point you will have completed the fabrication of your shell. If your design does not have integrated legs you may attach a set of off the shelf legs, or build your own.