Giuliano

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).

Attached Images

    

Giuliano

I finished applying the posterboard to the back of the bezel.

Instead of filling the holes with clay, I just cut posterboard bits to size, and glued them to the back with plain white glue, which should pop off fairly easily when I need it to.

Now the pieces of posterboard are flush with the rear of the bezel openings, and should provide for a nice sharp mold line.


I've got large sheets of 1/2" thick posterboard ready to form the molding box, which I'll go over in the next post.

Attached Images

  

imprezive one

This is really interesting, I own a 2002 wrx and this would be very helpful to me. Eagerly awaiting the next post. Heh heh.

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killerfishes

Its not only cool that your doing it, Its cool your making a walk thrue.
Thanks.

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Giuliano

Thanks!

I'm doing it partly because I want to get a clean build out of it, and partly so other Impreza/WRX people have another option for displays - the 8" 4:3 LCD's, with the better-supported standard resolutions of 640x480 or 800x600, etc.

And if I can sell a few copies of the finished product and cover my costs, even better.

Giuliano

In order to make the first half of the mold, you'll need a box to pour the liquid rubber into, which will also hold the part.

In this case, the part is a bit big by itself, so the box needs to be big, with plenty of room around the part for resin pouring channels and vents.

I'll get into channels and vents in the next few posts.


This box is made out of 1/2" posterboard, easily obtainable from your local crafts store (Michael's, etc), and is 16" square by 4" deep.

The box was glued together just with normal white glue, but you could use wooden toothpicks inserted at the joints to strengthen the box.

I didn't in this case, as a glued 4-corner box is usually strong enough to hold the liquid rubber as it cures.

Oh, and if you're going to be cutting posterboard by hand, I highly reccomend picking up the Xacto Board Cutter tool - it's $15, but highly worth it - you can use it to cut clean 90 and 45 degree cuts.


The first picture shows the empty finished box.

I cut holes in the bottom to fit the pegs on the back of the bezel (I removed the white plastic clips for this).

Having holes in the bottom is not normally a problem, as the edges of the bezel will be sealed off with modeling clay, and will be trimmed to match the shape of the bezel.


The second picture shows the bezel in the box, ready to be edged with clay.

The closer the part is to the bottom of the box, the less rubber you will need to pour to cover it.


You may notice that this box does not have a top - you won't need one.

When this first half of the mold has cured, the resulting piece will be flipped over, and will sit on the bottom of the box, face up.

The part will then be placed back into it's mold, and the other half of the mold will be poured on top of the back side, forming a complete 2-part mold.

I'll be scouring my local hobby shops tomorrow for the silicone RTV rubber I'll need, but being as I may need quite a lot, I may have to mail-order a small bucket's worth... and that'll take about a week.

Attached Images

  

Seraph

Let me know what kind of liquid rubber you are going to use and what kind of plastic you are going to use to mold the final product. I have a slightly different project (Subaru SVX Related) that I am working on that I am stuck because of this.



L

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Giuliano

My initial cast may depend on what rubber I can find at my local stores, but as for the kind of rubber, it's silicone RTV rubber.

RTV = Room Temperature Vulcanizing, where it cures at room temperature (as opposed to in an oven).

I generally prefer the silicone rubbers and polyurethane resins from PolyTek:

http://www.polytek.com/

Silicone RTV rubber:
http://www.polytek.com/products/silicone_mold.html

I generally prefer the TinSil 70-25 series rubber for most applications:
http://www.polytek.com/products/tinsil70.html

Silicone rubbers, as opposed to polyurethane rubbers, are generally non-stick, so you don't usually need to coat the mold with a release agent prior to casting.


As for the polyurethane resin (plastic), I like PolyTek's Poly 15 series:
http://www.polytek.com/products/poly15.html

The Poly 15-6 is good for general part prototypes, and the 15-3 is contains a filler material, good for final part runs.


If I can't find what I need at my local stores today, I'll probably end up ordering a 8-9 pound bucket of rubber, and a 4-5 pound bucket of resin.

Giuliano

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.

frankz

this thread says, "Sticky me!"

Giuliano

And I'd say, Thanks, but wait until I'm done with the thread.

There's more to come...

Giuliano

Here's a few pictures of how the LCD I'm planning on using might look when situated in my WRX's dash:

http://www.mp3car.com/vbulletin/show...02&postcount=8

Giuliano

Since the LCD takes up so much space, I'm going to want to mount it securely to the dash instead of the bezel, so in order to do that, I'm going to have to make some brackets.

As you can see in the pictures, the red lines indicate where the brackets would need to fill to support the LCD.

Instead of trying to bend metal into shape, I'm going to make an exact-fit part, using 2-part molding putty (pic attached) to capture the exact shape and size of the area.

To use the putty, you take equal parts of the blue and purple putty, knead them together, and push the putty into place. The now-activated putty will then cure in about 5 minutes, and you can then pull it out with an exact impression.


But you can't make a bracket out of rubber, right?

No - instead, what you'll have is a positive mold, or what the actual part would look like, but made out of rubber.

In order to duplicate that rubber blank, you would then have to mix more silicone molding rubber, and pour it on top of the rubber blank.


But what happens when you pour rubber on top of rubber? It fuses together! Uh-oh!

The solution to avoiding that is to coat the rubber blank in what's called a mold release - normally this is something like talcum powder, but there are other things you can use with silicone rubber as a mold release:

• Talcum Powder
• Lacquer Primer Spray Paint

The added benefit of using the Lacquer Primer is that it doesn't stick to the rubber - so if you spray the inside of your finished mold and let it dry, when you pour your resin, it cures inside the mold against the primer, and bonds to the primer for a pre-primed part out of the mold, ready for painting.

Attached Images

    

Giuliano

I made two right-angle pieces of foamboard, and used them to form the molding putty up against the upper radio mount points on the WRX console.

The picture shows the two rubberized moldings - the part on the left is actually the piece for the right side of the opening, and the part on the right is the one for the left side of the opening.

The sides of the foamboard that face "up" are in the center.


As you can see, the left piece (the "right" piece) turned out fairly well.. I may re-mold the other piece.

The pieces will be trimmed and re-shaped to fit better, and may be shortened to properly support the LCD in the right position.

Attached Images

 

mp3oplecarrier

Could you explain why the box has to be so much larger than the original part?

It looks like there will be a lot of space around the area that needs casting, but I'm sure there must be a reason for that, is it just strength of the mould?

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