Loraine Clarke Fablab 2013 Week 11

Composites (Page 1 of 2)

Week 11 Overview

This week Fab Academy covered making composites using a mold. Materials, the various mold options, different processes involved in casting the final object and safety were covered.

The assignment for the week was to
- design and make a 3D mold (~ft2)-
- and produce a fiber composite part in it

Link to this weeks academy information page.

Making a Kneeling Stool

The coffee table in my sitting room is very low so I'd like to make small kneeling stools to use at the table. These are based on meditation seats which get to you kneel on the ground and the stool supports the weight of your body. An example of these meditation stool is shown here on the image to the right.

The plan is to create the positive form in low density foam of the stool and then use the vacuum bagging technique to create the stool.

Step 1: Making the 3D model

I made the 3D model of the stool in solidworks. The stool is roughly 150mm in height which means I needed to use 3 sheets of foam. So I made the 3D model of the stool in 3 parts.
The 3 piece are all individual parts. I saved these as individual STL files for machining individually.

Download the STL files for the 3D model here

Step 2: Milling the Low Density Foam on the CNC machine

The CNC I used to mill the foam is a Denford MRC40.

I didn't trust the double sided tape to hold down the foam securely on the bed of the machine so I made a second bed and screwed the foam to this bed. Then I secured this to the bed of the machine with double sided tape for putting down carpets, the really strong stuff. You can see an image of this shown on the right. As I wanted to cut right down through the full depth of the material 50mm I used a 1/4 inch ball nose bit with a 40mm long flute. As there was not steep angles on the model using a 40mm flute to cut a depth of 50mm worked but caution would be needed for steeper angles. A second reason to have a second bed. If I am cutting the whole way through the material I need to be sure that if the bit starts to cut the top of the bed it should be the second bed instead of the machine bed.

The software I used to communicate with the Denford MRC40 machine is called 'Virtual Reality CNC Milling’ and 'QuickCAM pro'.

I've created detailed a step by step pdf that shows the setting up the machine physically, importing the STL 3D model and creating the tool paths to mill the piece.
Link to the pdf for 'Using the Denford CNC for milling

Download the FNC files for milling

milling foam pieces on a CNC machine

Some images of milling the 3 pieces of foam for the meditation stool

Step 3: Preparing the piece for casting

The next step is preparing the foam to cast the composite onto. I used the vacuum bag technique which means the exterior of the resin and material that touches the vacuum bag will be the actually exterior of the final composite piece.

To prepare the model I attached the 3 pieces of foam together using industrial double sided carpet tape. Next I sanded down the foam to give it a better finish and make sure all 3 pieces didn't have any undercuts that would make it hard to take out the foam after casting.

We got some advice on how to prep the mould before casting. 2 options are:
1) coat the mold in epoxy resin. Then apply one coat of the release agent
2) cover the mould in parcel tape. Then apply one coat of the release agent

I opted for number 1 and coated the mold in epoxy resin. Then 5 layers of wax.

Next step was to cover the model in Meguiars Mirror Glaze #8 as my release agent. I was very liberal in applying the layers of this. I applied 5 layers waiting 15 minutes between each application of the Glaze. Each coat needs to be rubbed on the mold and then after 15-30 minutes buffed off before the next layer of wax. For new molds it actually recommends applying 3 to 5 coats. The epoxy resin seemed to raise the pieces in the foam making the surface very rough. If I was doing this again I'd sand the surface again after applying the epoxy. This would make the surface much smoother before applying the mirror glaze and the final composite would be much smoother.

Step 4: Test piece before casting the real composite

Looking at other people's projects it seems like the cling film and polyehtuane works well together instead of release agent but it didn't seem to work well for people with epoxy resin.

A link to some useful information for vacuum bag technique . I pretty much used this as a core guide along with looking at other people's projects and advice too.

Layers of Material for the Composite

First layer is burlap material
second layer of carbon fibre (unnecessary but there was some material going to waste in the lab so I decided to use it)
Final layer of material was the aesthetic one. Since it was going to be a seat for the sitting room I decided to use a material decorated with a pattern
Next was a breather layer which I used Burlap.
Then layer over a layer of cling film with holes in it to let the air through
Push the resin right into the material for each layer of material

I took some inspiration from here . Where they didn't coat the outer layer of material in epoxy. Decided to to do the same with my stool to try and get a nice material finish on the outside. The hope was that the stool might end up with a less plastic looking outside and look more like fabric.

I also took some inspiration from here . Where they used baking paper instead of cling film as a perforated film layer with holes in it. Most people used cling film for this layer. She also used Vaseline between the mold and the material. Although the resulting surface finish isn't perfect I thought I'd try it so I could take the male foam from the mold. Fingers crossed it works

Next I made a tester piece with the following layers :

Vaseline - on the model
Burlap soaked in Epoxy resin(Layer 1 of final composite)
Burlap soaked in Epoxy resin(Layer 2 of final composite)
Cotton Fabric not soaked in Epoxy resin to let the other epoxy soak into it(Layer 3 of final composite)
Grease Proof Paper with holes in it to let the air through
Burlap with no epoxy to act as a Breather Material letting the epoxy soak out
Vacuum-Bag

The process for this is shown in the below images.

stages of tester composite piece

The test piece worked so well. It came away from the wood without any problem so the Vaseline worked well. The greaseproof paper also came away from the epoxy with no problem. Other people had lots of problems with the clingfilm getting stuck to the epoxy.