Hi there. I have a rubber boot I'm trying to cast for an automotive application. I have 3D printed a reproduction of the original. My plan is to pour a silicone mold using High Strength 3, and then cast the boot using Flex 60. I am on my second iteration of this. The first time I was able to successfully create the mold, but upon pouring the casting the walls of the boot always had thin spots and voids in them in some places. Casting material would never reach every part of the mold. The part's walls are thin--they were about 1.3 mm but on my second iteration I have doubled that to about 2.6 mm in the hopes that that will help with the pouring problem. I am looking for any advice on how to orient the part in the mold and where to place the fill and vent sprue(s) to make sure I get good flow to every part of the casting. Here are some pictures of the part I am reproducing.
...The HS-3 is not firm enough for a mold in this application, thus the thin spots...the voids could be due to air not being vented correctly. Use a denser RTV Rubber Silicone such as HS-2 (denser/firmer) or HS-1 (densest/firmest)......I would try HS-2 due to the undercuts.
You could also use HS-1 for even more support - it's the stiffest silicone. Also you could mix up small amounts and do more than one pour at a time. Resin is like silicone - it will stick to itself and Flex 60 is still considered a resin type. What I would do is mix a small amount, pour, then either add some "vibration", use a stick or stirrer of some sort to "stir" the bubbles up, or hand rotate the mold to push the bubbles out the way they went in. And try to pour down the side of the mold when you pour - sometimes this helps to prevent bubbles getting in a bit. Now this is all stuff you can try to do before having to add vents, if they don't work for you then you'll need to add the vents.
Thanks to both of you for your advice. Here is a picture of the first mold I made out of HS3.
And this shows the fill and two vent sprues, which are located on the left half of the above-pictured mold so that the pour is going into the flat base of the boot and flowing down into the top of the part. Fill on the right, two vents on the left.
I am up for trying HS2 or even HS1 but the silicone has to be very strong and flexible enough to pop out of the strong undercuts. I'm not worried about removing the Flex 60 parts from the mold but I am worried about being able to pull the mold off my 3D printed reproduction blank, which is rigid plastic. You can see those undercuts here:
Any thoughts on whether HS1 would hold up to pulling the blank out of that part of the mold without ripping?
It also sounds as though you both may be advocating for more of a squish mold rather than having any sprues at all. Am I correct in reading that into your responses?
Your 2 part mold looks great...stick with the 2 part mold. Your design is a little bit complicated for a simple squish mold. You would have a very difficult if not impossible time trying to get that HS-1 to rip....in other words, the plastic 3D printed reproduction blank would break before the HS-1 would rip/tear.
Brian's right - the HS stands for "high strength" and it lives up to its name. I use HS-3 a lot and have used HS-2 a bit and both are very hard to rip, I normally go straight for my scissors when I need to separate it and even then I have to be sure to use my heavy duty ones or I'll have a hard time with it. With that kind of mold, you should have no problem with getting the print out and later getting the casting out. Just remember, the HS series is meant to be stretched and flexed - it can handle some very rough treatment while still giving you good support (you should see some of the rough handling I've put them through). And with that solid of a mold shape, you shouldn't have any problem with thin areas collapsing.
I bought five pounds of High Strength 1 and poured my mold in two pieces similar to the one pictured above. I allowed it to cure for about a week, but unfortunately the mold did tear after I separated the two halves of the mold. It tore when I was removing the rigid blank from the mold. The area where it tore is a severe undercut where there is just a thin strip of silicone. Here are a couple of pictures that show where the mold tore:
I'm kind of back to the drawing board, so any additional advice would be appreciated.
When I used the HS3 the first time around the silicone stretched and came out of the mold without tearing. Maybe I was just lucky.
Is there a way I could pour most of the mold out of HS1 and then use HS3 just for the part that fills around this undercut area so maybe it would have enough flexibility to come out without tearing? Will HS1 and HS3 bond together and make a seamless mold?
Should I re-think my mold set-up, maybe in three pieces?
I'm kind of at a loss here. This will be my third time pouring this mold and will almost certainly be my last attempt at making this project work.
I didn't realize how much/large of an under cut that was, nor did I realize just how hard the HS-1 cures to as I have never poured HS-1 ...defiantly a silicone to avoid for severe undercuts as there isn't the "give" like with HS-2 and HS-3 .... if nothing else ,you have taught us all a lesson on the higher durometer silicone and shore hardness.
To answer you question, YES, HS-2 or HS-3 will bond with the HS-1 so long as there is no rubber to rubber mold release, petroleum jelly, etc. on the surface. Just f.y.i the HS-2 has the highest tear strength of any of the Alumilite mold making rubbers.....don't confuse hardness with tear strength ....think tear strength as elasticity ....you can pull a rubber band a ways from its original size before it breaks, you can only pull an automotive tire a little bit before it starts to tear.
Your mold is good, so no need to make it a 3 piece
I've used both HS-2 and HS-3 poured against each other and can verify that they bond to each other very well you should have no problem with either of them bonding to HS-1 like Brian said. Both of those do stretch without tearing very well, I got to where I grabbed the scissors first when I had to separate pieces that I should have put a seam in but didn't. Let us know how things turn out, anything you can share could help someone else.
Actually I use neither to pour my molds, I just pour from a high area (about head high with the mold box at waist high) and pour in a slow, thin stream. The mold materials have a nice long open time so for the most part it's easy to be able to take your time pouring.