how to make [almost] anything mas.863.12   work about
composites_week 9  

composite shells

The focus of this week was to continue to expand on the skills developed during the molding and casting week to create a mold that could then function as the plug for a fiber composite piece. When I begin to plan out a project that invest effort into a mold I generally think of part repeatability with the minimum number of components. This week in particular I thought of panels that could be used as art work or wall pieces much like the tiling patterns you might normally find on the floor.


Another software experiment I was working on in the background this week was to code some steering behaviors into vectors moving around in space.

Above is the final rhino model of the mold then exported as a .stl for partworks 3d. In the triangular tiling the moments where you read rhomboids are quite interesting so I made the part as a rhomboid that could later be separated into two triangular parts if needed. (A sort of dual part mold) In the spirit of spiral development and surface population I also created a mirror mold.

This week I used partworks 3d to set up a simple surface milling operation. The screen above asks you to orient the part in the world space related to the shopbot

You are then asked to locate the xy position, machining margins, and the depth of your part.

In the interest of speed being that my material was foam I opted to by pass the roughing pass for the finishing pass.

For a one pass finishing operation in foam I used a 1/4" ball end mill making sure there there was enough of the tool length exposed to reach the bottom of my part. Speeds and feed are noted in the image above. I went back and forth here with bit diameters and step-over amounts to keep the milling time reasonable. When milling I usually try to double the length of the estimated runtime because it is often in-accurate but in this case it estimated 52 minutes and took only about 15 minutes longer than estimated.

The preview of the tool-paths.

Since we are working with foam this week I opted to bypass the cutout pass as well to keep the foam parts from moving around at the end of the job. The tool offset from the part and the depth of my part allowed enough room to use a utility knife to trim the part out of the remainder of the stock.

The partworks 3d tool path simulation lets you have one last look at what the program should do.

At the shopbot I made sure the bit was ready to go and zeroed the z with the zero plate on top of my piece. Then zeroed the xy location and loaded the sbp file exported from partworks.

First passed in the blue foam.

The surface definition of the mold and its mirror nearly 90% complete.

Even with a low machine time and a low cost material like foam I had hoped to make a least a batch or two of parts eventually, so I made sure to take care to seal the foam mold before layup. I first sanded any really rough edges and then used the heat gun to fuse the surface of the foam back to a more continuos surface finish. To be doubly sure that the parts would release I covered the foam in gesso and let the parts dry overnight. The third measure to make sure the parts would de-mold was to apply two coats of mold release spray following the guidelines on the can.

While waiting for parts to dry I begin to organise and prepare all of the components of my vacuums bag setup. There are a lot of consumables for composites. I also knew that the layup time is completely dependent on the pot life and you do not want to flounder with a step or two here or there while you have resin ticking away in the cup as you are laying up the panels.

I cut and pre-taped the bag trying to leave extra material to account for the z height of the parts. I also knew in advance that since the parts were to be components I would need flanges on the parts to connect them. I knew this would mean I would get a lot of resin run-off of the bottom of my parts and I put 2 layers of bleeder fabric and a layer of peel plastic as the base for the parts.

I mixed resin in small batches sized to my parts mixed 1:1 (10 pumps of each per part). Mix throughly for a full minute or two. Don't be afraid to change gloves often and avoid the resin coming in contact with your skin.

I laid up my parts with three layers of fabric each taking care to dip and wring out excess resin from each sheet and then used a system of pulling, brushing, and kneading to get the fabric to pull down evenly over the part. You can see here that I ended up with a lot of extra material around the flange edges but when the fabric is wet it grows a bit as you pull it over the mold. I knew at this point that I could trim the excess after de-molding.

After getting everything as smooth as possible applying a top layer of peel plastic and 3 layers of bleeder I sealed up the putty tape seams and used a rigid plastic connection hose with a one-way valve. I would recommend to try to cover the end of your air tube with a bit of bleeder to protect excess resin from getting anywhere near your air system.

Here are the parts after being under vacuum for 6 hours to be safe.

I used the band saw to clean up the excess around the edges of the flanges and then used the belt sander to clean up and other mate edges. It originally seemed like I might not get the foam cores back our of the shell but while using the belt sander I found the the action provided enough friction based heat to allow the flanges to flex a little and I could work my flanges off of the mold little by little. Above are the minimally affected molds on the left and the resultant parts on the right.

Here are two intact and cleaned finished parts. The quality of the surface finish was not quite as nice as I had hoped, but there is still some potential to post process them.

In reviewing the parts with Kenny he suggested that I might not have had my parts under a full vacuum judging by the small bubbles on the inside of the finish surface as show above.

Above is a close up of the positive surface finish. The wrinkles were probably caused by the uneven peel plastic layer and not a full vacuum. I also discuseed with Kenny the options of sanding, sandblasting, and potentially finishing the parts for a final surface.
  -- jared laucks -- © 2006-2012 --