PROJECT OVERVIEW:
– to become acquainted with how to mill 3-dimensional forms using a 3-axis milling machine
– to experiment with mold making and repeatable part casting
– to learn about composites
MATERIAL:
– extruded insulating foam
– Smooth-On products “Oomoo 30” & “Smooth Cast 300”
DESIGN CONCEPT:
I initially was going to cast hooks to hang on the wall that would have a countersunk hole ready to receive a standard dry-wall screw. Due to material and tooling restraints, I focused on a more straight forward geometric form as seen below. The form incorporates two conditions in one: hard, right angles and smooth, curved surfaces. The purpose of these two different surfaces is to test:
a. how the machine’s tooling mills these different surfaces
b. how the composite casts into these different surfaces
MOLD FABRICATION 01:
The first mold was milled with the Shopbot 3 axis mill. The multi-surfaced part was accurately milled with the following criteria:
a. one, finishing pass with a 1/4″ ball nosed end mill
b. 9000 RPM
c. pass speed @ 10″ per second
d. 1/16″ step over
MOLD CASTING 01:
After the part was milled, it was de-burred by sanding it gently with 320 grit sand paper. Once the desired surface smoothness had been achieved, gesso is applied to the mold to seal the open cells the mill exposed during processing.
After the gesso completely dried, I was ready to pour the mold casting compound. First, cover the mold with a releasing agent (ordinary hand soap). The mold making compound I used for this was Smooth-On Oomoo 25. This process was very straight forward as the mixing ratio is 1:1. FOLLOW THE MANUFACTURES INSTRUCTIONS CAREFULLY.
As seen in the above image, only one half of my mold was cast. This is because just this one side consumed all my Oomoo :( and there was no more left. So plan B, Utrecht.
MOLD FABRICATION 02:
The second mold was milled with the Techno 3 axis mill. Needless to say, the volume of this second mold was considerably smaller. Notice the rectangular registration marks due to the finishing pass being performed by a flat end mill, instead of a ball nosed end mill. The multi-surfaced part was not as accurately milled with the following criteria:
a. one, roughing pass with a 3/8″ ball nosed end mill
b. one, finishing pass with a 1/4″ flat end mill
c. 9000 RPM
d. pass speed @ 10″ per second
e. 1/16″ step over
MOLD CASTING 02:
By using the same steps and techniques outlined in the “mold casting 01” section, this mold cast was successful. Notice the differentiation of color seen on the top of the finished mold. This happened because of an inconsistency in the part of the compound NOT labeled to shake first. So the moral of the story is…when in doubt, shake it.
When milling the mold, avoid 90 degree inward corner conditions. I found that the molding compound splits at these connections as seen below.
PART CASTING:
Casting parts is very similar to casting the mold. First, you apply a mold release agent onto the mold. Make sure to spread the agent onto the outer walls of the mold as this is where the two part mold will be compressing together. The releasing agent, when compressed, will form a seal. Next, mix the casting material as per the manufacturers instructions and pour slowly, allowing air to properly flow out of the mold as the casting material flows in. After the material sets up, the part should easily peel away from the mold.
By carefully increasing or decreasing the amount of compound mixed, and applying different amounts of pressure to the clamp, the parts began to become consistent.
ERROR TO AVOID:
a. When measuring the components of these compounds to achieve a 1:1 ratio, make sure you are on the most level surface as possible.
b. Shake all parts A & B even if it is not formally specified.
c. When mixing both parts together to form compound, mix thoroughly to insure a good chemical reaction.
COMPLETE FABRICATION 