How to: Duplicating a radio bezel.
In previous threads, I've mentioned that I'm planning on putting a 8.4" 4:3 LCD panel in my 2003 WRX's dash, behind the radio bezel.
The LCD is just the right width for the opening, but it's about 2cm too tall, so I'd normally have to modify the bezel.
Instead, I'm going to make a copy of the bezel using casting techniques and silicone rubber with polyurethane plastic, and will then modify the copy to fit the LCD.
In this thread, I'm going to detail how you go about casting a duplicate part, using a 2-part mold.
I've attached a few images to this post, and I'll go over them now.
This image shows how the LCD panel would stick out behind the bezel, and is shown for reference.
In order to cast this part using two halves of one mold, I'm going to have to make a copy of one side, then the other.
I'm going to start with the front side, since that's the more important side to me.
In order to mold the front side, I'm need to fill in the holes so the rubber doesn't pour through.
In this picture, I've cut out pieces of sticky-back foamboard to apply to the back of the bezel to cover the holes. The top piece is actually two pieces stuck together, and actually fits inside the opening, reducing the depth of the opening by 5mm.
Applied to the back:
Viewed from the front:
There are some minor gaps in between the plastic and the foamboard, and they will be filled in with some modeling putty.
The next few posts in this thread will show the making of the box to hold the part, and the mixing and pouring of the rubber (tomorrow).
Calculating how much rubber I'll need..
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.
1.5" x 2" x 15" = 45 Cubic Inches
1.5" x 3" x 8" = 36 Cubic Inches (x2 = 72 CI)
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:
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:
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.