9 molding and casting | Fab Lab Monterrey

MOLDING AND CASTING

When I read for the first time the book “Los rios profundos” of the peruvian writer José María Arguedas (1), long time ago ( I was 11 or 12 …) I was fascinated by the “zumabyllu winko”, that was a magic and powerful toy …”zumbayllu” is the spinning top, and “winko” is an irregular and magical object, that would have to be round; Thinking about this concept, some years ago, I developed a project to make jewels with an irregular rotation using profiles of people; The result, are objects whose shape cannot be generated or fabricated by hand; to make the jewels, until now I utilized the 3d printer and with resin, but the idea is to make them in metal, stone and others materials.

Based on this idea, for this activity I decided to experiment with this concept, because I am totally new using the process “molding and casting”, and I´m sure that I can learn something very usefull also for the project of the jewels.

The objects that I have tried to do are two Zumbayllu Winko, one using the profile of a person, and the other with the profiles of two people.

The final objects are not perfect … I made some errors in evaluating some details like the endmill that I used to finish the mold … or like the size of some parts that are too small … I’m planning to do another mold fixing the errors, but anyway I am really fascinated by the result of this activity: maybe because the spinning tops work very well … maybe because I used a transparent resin that generates a beautiful effect with the light … or maybe because is a toy …

Finally I have also a little surprise: during the precession, the spinning tops change from clockwise to counterclockwise, and due to the irregular shapes this change can be appreciated very well … is the gyroscopic effect or an optical illusion? As you can see in the following videos, my Zumbayllus are really Winko!

1. THE OBJECT

Testing shapes, and sizes

The first step was to build my object by revolution, putting the profile in different positions, and thinking of the diameter of the endmill that I decided to use ( 1/8 “), to define the size and the resolution.

Finally I selected the last one.

After to have built the first surface, I decided to do a little change to generate an object more interesting: the spinning top, by its nature, makes rotations with its movements … the shape of my object can be not exactly built by rotating on one axis; in this way, the shape of the spinning top is different when it is moving from when it is stopped.

Finally I built my spinning top with 3 profiles, but I used 6 making a small rotation of each one to have a best definition of the curves that I used as main shapes; I studied too the best position to divide the shape, thinking on the mold, and on the movements of the endmill.

Here we can see the differences between the first element ( left) built by rotation and the second one (right) generated with the 6 curves; I like the result because vertically the difference is very soft, but can be appreciated in top view.

After I tested this other shape, generated with two different profiles; the main idea, like the second one, is that the object is round only when it is moving, but here I added another concept that comes from a project that I developed and used for a jewels, generating objects whose shape comes from the merging of differents profiles; here I used two profiles, each one repeated two times;

Here the two spinning that I’m going to fabricate

2. STRATEGY

I am planning to make at first a positive mold with wax, then the negative with silicon and at last my final objects with resin. I will mill the first mold by dividing the pieces each one in two parts; I have to leave a pipe to cast the resin and another smaller for the air.

3. THE POSITIVE MOLD

The “box”

Milling:

To mill the mold, I used the Roland Monofab SRM-20, with two endmills: 1/8″ flat for the first step and 1/8″ rounded to finish, and wax; The machine spent totally 4 hours to finish the work.

Rough

Finish

The machine starts to finish the piece with high definition with steps of 0.2 mm

Captura de pantalla 2015-04-07 a las 1.52.14

After 1.5 hours, finally the mold is ready! the resolution is very good, but I’m worried about the thickness of the pipes and the final part of my objects … some parts seems smaller than that I though

5. THE NEGATIVE MOLD

To make the negative mold, I used silicon rubber P-85 with catalyst. For my piece I prepared 200 gr. of silicon, 2% of catalyst and a pigment that helps to see if the rubber is mixed well.

The material can generates bubbles, and this means holes in the mold, for this is better to cast with a wire as thin as possible and trying to put all the material in the same position.

We can eliminate the bubble moving the mold by hands for 2 minuts, but fortunately on the labs of our university we have a Vacuum Machine (Vac II) and I used it to be more sure to eliminate the bubbles; this is very fast … just a minut.

After 4 hours, the negative mold is ready. The resolution is very good, no bubbles, and is very esay to remove from the first mold.

6. CASTING

TEST 1 :

The first material that I used is Urethane rubber ( 50% Urethane and 50% catalyst) that works very fast: the max time to mix with the catalyst and cast into the mold is 3 minutes and the piece is totally dried in 15 minutes; This one was just a test, because in this short time is impossible to fill the mold with a tiny pipe. I filled just the deeper part of the mold to check the quality of the surface; The material is heated and expands within seconds! The quality of finish is very good and the shape is perfect without any deformation.

TEST2

The second material is Resin of Polyester ( 1% catalyst) that needs more time to dry: 15 minutes to prepare the mix and cast, and 40 minuts for the first solidification; the piece will be totally completed in 48 hours. At first the Resin appears yellow, but finally the piece will be transparent pink; we can add some pigment, but for this test I want to see the original color ( I am totally new with these materials …)

As I told before, the time of the first solidification is more or less 40 minuts, but to have a strong piece we have to wait totally 48 hours; I decided to leave the mold closed during all one night, because some of the parts are really tiny and I dont want to risk to have deformations

Now I can open my mold … is very easy to remove the piece; the silicon that I used is elastic and vey clean.

Finally my tops hare ready! The joint is too big, and to resolve this detail I have to use another endmill for the final border of the positive model … and some parts are very very tiny … but the surface of the piece is perfect.