Making a vertical silicon mold

**dai sensei** · 12th February 2013, 09:35 PM

I have been asked to show how I make my vertical silicon molds, rather than using PVC conduit. I make them from dowels, usually 3/4" or 1", then in either 4/6/9 styles. I also make a well at the top of the mold, as for WW/Banksia etc cast under pressure, you need extra resin. Others just make the dowels longer, but if the finished surface of the cast is below the top, it makes the casts hard to remove from the mold.

I use a mixture of dowel from the hardware for tubes, and ply or melamine for the sides/base. I also use the blue softer and cheaper silicon, hence the sizes of the walls are based on this silicon. I also use a silicon dye to ensure the silicon catalyst is mixed in properly.

I start my joining the dowel to the ply well. You could use conduit, then hot melt glue it to the ply. Any timber components should be sealed with varnish or similar first.

Making a vertical silicon mold-silicon-mold1-jpg

You can make as many of these as you like for the cast you want. You could also make the resin well part over multiple dowels, just remember if the well extends over other tubes, that is the minimum number you must cast at the same time. This thread: https://www.woodworkforums.com/f204/m...-mould-108003/ , shows a couple of molds I made earlier from other silicons, and the well extended over all the tubes. I found I did not always have enough resin to cast all at once, hence make a well over each tube individually now.

Making a vertical silicon mold-silicon-mold2-jpg

Next I build walls around the tubes connecting to the base and each other.

Making a vertical silicon mold-silicon-mold3-jpg

Once you have it all together, fill it with water. then empty (quickly as it will probably leak, but the silicon is very viscus and won't) into a container to measure how much silicon it will take. The let it dry again before using. Just before casting the silicon, spray the mold with a debonder, the good stuff or I find that spray cooking olive oil works fine. Just make sure you use heaps and drain off excess.

Wait until silicon is set, then dismantle the box sides and remove base. The dowels can be removed by pushing from the base, as you would removing the final cast product.

If you are casting banksia or similar, where you need space around them for the resin, make a timber plate with small dowels lining up with each tube. This can be placed on top after pouring in the resin and weighted down, this will keep your banksia etc from floating. I coat my little dowels also with silicon so they don't stick to the cast.

After your cast is set, remove any top weighted plate, then post cure in oven (100C for 15mins) and leave to cool, or leave in the sunshine/warm room. This helps with any sticky residue on the cast making them easier to remove. Careful when pushing from the bottom to ensure you don't push through the base of the mold.

Hope this helps

Cheers

**Dorno** · 12th February 2013, 09:49 PM

Thankyou kind Sir that is Certainly what I call a phone call that I am glad I made LOL

thanks again Ian

**vanngo5d** · 21st February 2013, 02:08 PM

Nicely done. Easy to follow I've been looking at making one for kitless but a little taller so it's the same blank used for the entire pen.

Really like the idea of the well at the top so when using pressure you will have a little extra resin .

Don

**Dorno** · 21st February 2013, 04:50 PM

I have been making this mold for several days now but after completing it I have come to the conclusion I will need around 2kg of silicone ? That is really going to cost a bit so may have to downsize the mold a little to make it cost efficient ?

Cheers Ian

**salty72** · 22nd February 2013, 07:29 AM

Dorno,
why dont you insert another set of 4 'pipes' upside down and in between the other 9

the mold currently has a base with 9 pipes attached to the bottom plate - what if you were to invert another 4 and attach them to the top plate

this will reduce the amount of silicon by nearly 50% - you won't be able to use both ends at the same time... but if you make one set 40mm and the other 25mm, this will ensure you will also save resin by using the best size diameter for each project.

**dai sensei** · 22nd February 2013, 11:30 AM

With that soft silicon, just ensure the wall thickness is 15mm thick, but apart from that you can fill the remaining spaces with anything including as Salty suggests.

**salty72** · 22nd February 2013, 12:17 PM

Originally Posted by dai sensei

With that soft silicon, just ensure the wall thickness is 15mm thick, but apart from that you can fill the remaining spaces with anything including as Salty suggests.

Neil which soft silicon are you referring to... bananSkin or ultraSil

With ultraSil I have no problems with wall thickness around 8mm even on the 30mm long funnel I made - As you can see from the TECH data below BananaSkin is very soft and will stretch ALOT whilst the UltraSil is harder and less flexable/stretchy

TECH details for BananaSkin

Hardness, Shore A 12Tensile Strength [mPa] 3Elongation at Break [%] 600Tear Strength [KN/m] 18

Linear Shrinkage [%] 0.1

TECH details for UltraSil

Hardness, Shore A Approx 25
Tensile Strength [mPa] UNK
Elongation at Break [%] 300-400
Tear Strength [KN/m] UNK
Linear Shrinkage [%] 0.1

**Dorno** · 22nd February 2013, 05:44 PM

Well guys here is the offending mold

so as you can see it has no chance of falling apart in a hurry

it was only made with this timber because it was free and plentyfull.
The mold has come up great and although you can't see them it has the filling pot on top my only problem is when I measured how much silicone I will need came up with about 2kg which works out to be expensive but well worth it in the end giving me a chance to do many types of casts including the banksia pods I have. I am not sure yet wether to reduce the size of the mold or reduce the bank balance

but as usual I am interested in your thoughts.
IMG_6703.jpg IMG_6705.jpg IMG_6708.jpg

Cheers Ian

**dai sensei** · 22nd February 2013, 08:54 PM

Hmm, your spacing might explain the excess silicon

**Dorno** · 22nd February 2013, 09:56 PM

please tell me more

don't forget I aint real bright when it comes to this crap

I thought I had measurements right so please explain

This is second attempt so will have to get it right soon

Maybe

Cheers Ian

**Dorno** · 22nd February 2013, 10:26 PM

Hey Neil and Mark this is basically what my mold looks like if you can understand my ordinary drawing but hope its ok

please note that this is just rough drawing but I hope you get the idea of what I have done ?

Cheers Ian

**dai sensei** · 23rd February 2013, 08:36 AM

Your plan looks right, just the execution looks out. All the spaces between the dowels and from the dowels to the form should be 15mm (except where you used differing dowels). From your photo, if the dowels are 30mm, the gaps would apear to vary from 15 to 50mm.

Given you used 30 and 25mm dowels, your box box form should be internal 3x30+4x15=150 by 2x30+25+4x15=145

**Big Shed** · 23rd February 2013, 09:26 AM

Ian, 2kg of silicone doesn't seem over the top for the sort of mould you are making.

Just to put it in perspective I have 2 of Fred Wissens' vertical moulds, each weigh around 1.4kg, one makes 6 x3/4" the other 4x3/4 + 2x7/8, they cost me $50 + (a heap of) freight each.

Given that 2kg of silicone on Ebay goes for $80 inc freight that doesn't seem over the top for such a hefty mould.

Filling it up with PR will cost a bit too

I agree with Neil, your photo(s) doesn't seem to match your plan dimensions.

Just as an aside, the blanks are a bit of pain to get out of this type of mould, not helped by the fact that the interior of the cavities aren't all that smooth, the blank have quite a few sharp "pips" on them.
Fred's advise was to blow a bit of compressed air beside the blank, that works, but make sure you hold your hand over the blank! They can fly up a fair bit

**Dorno** · 23rd February 2013, 01:12 PM

Well Guys I am going to pull the dam thing apart again and start from scratch and see what I can come up with

I am determined to get this to work although still a little unsure whats going wrong

I used double sided woodworkers tape to stick the dowel to the square block then put a screw up through the base plate and the 40mm block and into the dowel to hold it all in place (Is this necessary) and this is where the problem arises in holding the dowel straight.
Do you think I can get by using just the woodworkers double sided tape ?
Neil I realize that some of the dowels in the photo do not got vertical (Straight up and down ) and they move off to one side but is there any harm with that ? it would not reduce the amount of silicone needed would it ? I will start again and post some pics as I go along and hope that I can nip any problems before they happen

if all else fails I may have to be not so greey and make either a small mold or make a couple to cover it

all options are on the cards

I look forward to solving my problem somehow

Cheers Ian

**dai sensei** · 24th February 2013, 08:15 AM

Your dowels not being perfectly vertical will not make any difference. My guess would be you didn't predrill holes in dowel and bottoms were not perfectly cut square, hence the minor slopes, but not worth worrying about. The thing that looks wrong however is the size of the sides, the gaps all look too big. I would just fix them, as it does make a big difference on the silicon volume and simple fix, then go ahead.

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