In order for me to know how much rubber I'll need, I'll need to calculate the rough volume of the box with the part in it.
Calculating volume is simple - it's the total of the length multiplied by the width, multiplied by the height:
L x W x H = Volume (Cubic Inches/mm)
As you can see in the bezel-box picture a few posts back, there's quite a bit of space around the part to fill with rubber.
In order to cover the part, the rubber will need to be about 1.5" deep, so I automatically have my Height.
I measured the volume of the box in sections, and added up the values to get the total volume.
Bottom section:
1.5" x 2" x 15" = 45 Cubic Inches
Left/Right sections:
1.5" x 3" x 8" = 36 Cubic Inches (x2 = 72 CI)
Top section:
1.5" x 4" x 15" = 90 Cubic Inches
The area just above the part will also need to be covered with rubber, but it'll only be 0.5" thick:
Covering part:
0.5" x 8" x 10" = 40 Cubic Inches
Total volume: 247 Cubic Inches
Okay, now I have the rough volume for (one half of) the mold.
However, that doesn't tell me how much rubber I'll need, because I'm missing some information - the density of the rubber.
Since I'm going to use the Polytek TinSil rubber, here's the physical properties sheet:
http://www.polytek.com/products/tinsil_popup.html
See anything that would solve my answer?
Yes, it's the Cubic Inches / Pound value, or how many cubic inches of rubber are contained in 1 pound: 25.3
247 Cubic Inches / 25.3 = 9.76 Pounds
I'm going to need about 9 pounds to fill this half of the mold.
Conveniently, the TinSil 70-25 comes in a 1 pound bucket for $30, a 9 pounder for $116 ($12/lb), and a whopping 44 pounds for $444 ($10/lb), and the prices include shipping (it's heavy!).
Will I need a full 9 pounds just to pour this half of the mold?
Probably not - I can reduce the volume needed by padding the empty spaces with 1/2" foamboard, reducing the total volume by about 45 Cubic Inches, reducing the pour to about 8 pounds.
The pour volume needed will be further reduced by filling in spaces necessary for the pouring and venting channels, etc.
The other half of the mold will probably be 15" x 15" x 0.5", so the cubic inches would be 112, and the pounds necessary about 4.5.
And since I'm going to need a 9-pound bucket, I'm going to have to order it, as I doubt my local hobby shop will have any in that size.
However, I can still pick up some useful molding material I'll need, so I'll go make a run.
Later on, I'll be making a mold of the inside of the WRX's radio mounting holes so I can make LCD mounting blocks that are an exact fit - more on that later.