Post by icemanmodeler on Oct 15, 2019 9:30:00 GMT -5
Hi everyone, new to the forum and to molding and casting, watched a lot of tutorials on this topic but my first mold has an issue, i think due to it's size or orientation. Been working on a year on a statue and now it's in the molding and casting stage and for a first test i used a small humerus bone that's part of the statue base. The object is 11 cm tall with a wide top of 1.5 cm and the small base where the key is of 4 mm.
The problem is i always get a void in the center part of the piece and only the lower half of the piece gets casted. The mold is a 1 piece mold and made a zigzag cut on both sides for registration and removing the piece, also added a vent at the bottom. I think if there is now way of saving this mold i should use this bone with a femur bone and make a 2 part mold maybe with a cleaner seam line, maybe change the orientation of this bone 180 degrees. What do u think? Ran out of ideas, used a lower viscosity resin too for this and a syringe to slowly add the resin without bubbles but to no avail. More images bellow. www.flickr.com/photos/184995342@N07/?
First off , for this application ditch the syringe.....you will do nothing more than keep fighting with this mold. The syringe is not going to prevent air bubbles but rather add to the work time. Try slowly pouring the resin into the mold as it appears that the resin was trying to cure...b.t.w which resin are you pouring? You shouldn't need to cut any vents as it appears that the lower part looks as it should....if there was any issues with trapped air it would be in the lower portion(s) and not towards the top.
To help remove any air that may be trapped in your mold, squeeze it as you are pouring resin into the top... in a sense burp the mold. I've got some 1 part molds that are aprox. the size you got and I have never had any trapped air issues with squeezing the mold as I pour.
Brian's right, the syringe won't help here but squeezing the mold will help. If you're getting bubbles at the top and not the bottom then it sounds like they were trying to escape like they're supposed to but the resin cured before they popped. I got this all the time when I was pouring long two piece molds with RC-3 which is a fast curing resin, you'll need to use one that has a long open time like one of the Clears (you will need to color or prime and paint it afterwards) or the Slow Set 7. Also another thing you can do is pour smaller amounts - don't mix up the entire amount needed to completely fill the mold and pour the resin in layers. Resin will stick to resin (like silicone does) and it will not hurt a thing to do layers, I do layers all the time. It may take longer doing layers or using the slower resins but you'll be happier with the end result.
Post by icemanmodeler on Oct 16, 2019 17:30:49 GMT -5
Hi guys, thanks for the response, well i tried pouring without syringes at first and got the same result, tried longer work time resin and that was also thicker so i presumed that was the problem so now i tried less viscous and less work time resin and got the same result. Someone suggested i should use the air vent as the pouring hole and also make it bigger to pour. I am using local resin and live in Europe so resin and sillicone brand names are different but it's basically all pourpose platinum based sillicone and polyurethane resin. Started to make a 2 part mold like in the photo above and so far just made half of the mold with the clay, will need to sculpt it more and try that to see how it works. I think i will invest in a cheap vacuum degass chamber as it was suggested especially on small pieces like this but will also try pouring the resin in stages to see how it goes, i did cut a bit off the center line where the 2 molds connect around that area but i'm not sure if thats the reason.
I pour small molds, some even very small, and have never needed to degas anything particularly the resin. In other words hold off on investing any more money in this project buying a vacuum chamber and lets see if we can resolve this issue as I think you would be even more disappointed when the vac chamber didn't fix anything. Sounds like your mixing the right ratio which is half the battle. Go ahead and try pouring in layers like notoes suggested....I'm curious to see how that turns out as I have never attempted doing that w/ resin, but am willing to learn.
Your dealing with a different mfg. than I I use (Alumilite) so the fallowing information may work for you as well or you may need to slightly change it....
I pour Alumilites AlumiRes RC-3 so the fallowing comes from my notes on this.....
Mix between 25 and 30 seconds ...I use a stop watch as every second counts from mixing to pouring.
I have aprox 3 minutes of work time......that's from the second I stop stirring to when the resin is getting too thick to flow correctly......this is where I think you are getting into trouble.
Pour down the side of the pour gate (opening) of the mold and not directly in the center to allow air to escape thru the pour gate as well as the vent.
Keep in mind the larger the pour (part) the quicker it will cure....a humerus will cure quicker than distal phalanx as an example, if both are in the same scale, even if both molds are poured at the exact same time due to the humerus being a larger bone (part) than the distal phalanx. Cooling the mold before pouring will slow down the curing process some.
Lightly dusting the mold (inside) with talcum powder (baby powder) will break the surface tension and reduce or even eliminate the possibility of air bubbles in the casted part
Post by icemanmodeler on Oct 17, 2019 6:58:19 GMT -5
Hi thanks for responding, i will go step by step with my process in case i missed something or made a mistake somewhere. The resin i'm using now with low viscosity is Multicast 1 and has work time of 3-4 min , demold time to 90 min, low viscosity and recommended for parts with a max thickness of 80 mm as per the data sheet. This resin has a 100 to 100 part mixture so the first step i use is to do a light coat of mold release on the open mold and leave it a couple of seconds to evaporate. I use and electric small scale to measure each part in it's own cup, for this i use 8 ml of part A and 8 ml of part B , then i combine both in a cup and stir for about 10-20 seconds. On my last attempt i poured a bit with the syringe but first i hold the syringe up and tapped it to remove some bubbles and then poured some in, squeezed the mold, moved it around, tapped it, added more resin, did the same a few times then used some hard boards with a bit of tension clamps holding the mold together. I notice large bubbles coming out of the pour hole and sometimes staying there, in one video they suggested i spray some mold release there to make to bubbles release easier, will have to try that. I should mention that once the cast turned out great and i think i accidentally poured the resin in the vent and then also poured in the regular pour hole so i'm not sure.
Another thing you can try is lowering the surrounding temperature - the closer to 70 degrees the work environment is the faster the resin will cure as resin uses heat to cure. You can also "chill" the resin to aid in prolonging the working time but it also will thicken the resin when you're trying to pour so I usually cool down my work area instead. Brian is right, degassing will probably not help as the problem is tied to the resin - I never degass either and I normally get good castings even with the tiniest details.
Post by icemanmodeler on Oct 23, 2019 16:35:43 GMT -5
Wish this forum had notification, the void happens in the middle part of the object which is the thinnest part so i can't pour the resin in layers, not sure i want to mess with altering the temperature just yet. Been busy with some commission work and now will be working more on this, prepared the 2 part mold to see how it will work and will also try pouring on the vent that i created to see if there is any change. 2 part molds are so time consuming to make so will use it only on complex parts.
Post by icemanmodeler on Oct 25, 2019 8:20:52 GMT -5
The next pieces are going to be complicated due to shape complexity and undercuts in the keys, can u give me some feedback if it's oriented correctly. I'm not sure if i should do a 1 part mold or 2 part mold for the torso, it has deep keys in the arms , the neck and in the hip area, think it's going to be tricky.
Post by icemanmodeler on Oct 25, 2019 12:02:57 GMT -5
Maybe on the arms of the torso one arm is on one part of the mold and the other arm is on the other part, to be able to remove the object without tearing the sillicone in that area when removing the part.
For notifications, there should be an area that will let you "follow the thread" or maybe under your Profile's Notifications tab - it's been a while so I forget how it's done as I'm a mod so I get a notification of all new posts.
As for a two part mold, yes it is a bit of a time eater but there is another way but it might also be a little work in it. I have a very popular mold design in my shop - it's of banana slugs and they're mostly rounded with the flat part underneath were you don't really notice it. It's a one piece mold and to pour the material (most times it's gummies as this is one of my food grade molds) in you have to "open up" the cavity while pouring and the process of opening up the cavity gives the material a way to release any air bubbles (in your case, the thing that's causing the voids) and ensures even coverage. When you demold, you will likely need to do a little sanding but it might be a better option for you.
In the case of the bone, you could add a thin dividing line underneath the bone would be the opening to pour the resin in from. Here's a very simplified picture of what I mean from the side and the front - the bone is the red, the line divider is the black, and the bottom of the mold box is blue.
The line divider prevents the mold material from going under the bone which makes the bone easier to remove from the mold. You will need to remove the line divider after you demold but it shouldn't be too big of a problem.
The head is positioned right with the vents and pour area for pouring that. The rest look like they're oriented correctly too. Another option for those is to make two 1 piece molds, pour each side separately, then glue them together after demolding.
Post by icemanmodeler on Oct 31, 2019 12:08:01 GMT -5
Hi thanks for the advice, will test it out on the next molds, a couple things have happened since i last posted so i will post them here. Success!!! I was recommended to pour resin on the vent hole instead of the pour hole and it casted nicely, only the top part where the tube is didn't fully cast but i can repair that part. I started the head mold and after a few failed casts i got a nice result, there are a few bubbles at the top but it can be fixed, didn't bother to add the vent like in the picture, not sure i need it. I discovered the void problem was the pour vent was too small and while pouring it was blocking the air and creating a void in the center, to combat this i used a large syringe on the head and it works great. The method i used is, after collecting the resin in a large syringe, rotate the syringe upward so the air goes to the top, tap it and remove a bit of resin to release the air, then pour a bit, rotate and flip the mold to get it into all the crevices, tap it, squeeze it, pour a bit more, repeat.
Ordered a degass chamber and it should arrive soon, will try it out to hopefully remove more of the bubbles.