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.