Problems Roto-casting by hand

[rgk123]

Hello, I am new to mold making and casting, and working on my first project. But I am running into some issues I am not sure how to resolve.

I have a 2-part mold and using roto-casting to create a hollow cast (the object is too big to cast solid). I am manually spinning the mold myself.

I am using resin that is suitable for roto-casting, but so far my attempts have failed. I get a paper-thin shell and a large solid mass in the middle. Since I can't see the inside, I am not sure if I spinning too fast, or spinning too slow, or not spinning for long enough.

Any insights appreciated, thanks!

 

[Psicorp7]

It sounds like you are not spinning for long enough and too fast. Consistent spinning at a reasonable speed (one rotation on each axis every 1.5-2 seconds) will be crucial over a time that is roughly double the allotted cure time should be about right. If you are spinning it too fast the resin does not have time to grab onto the outside. The movement will also sometimes affect the cure time. Sometimes faster and sometimes slower depending on the brand.

Lastly, I like to do two pours. One half for a print coat and one for a build layer. That way it is even and not lumpy in spots that have deep indents like a nose. I assume you have an area you can pour into without reopening the mould? If needed you can drill a hole through the cured resin to access the inside of the print coat if needed. Good Luck!

 

[iloveyouiknow87]

Hello, I am new to mold making and casting, and working on my first project. But I am running into some issues I am not sure how to resolve.

I have a 2-part mold and using roto-casting to create a hollow cast (the object is too big to cast solid). I am manually spinning the mold myself.

I am using resin that is suitable for roto-casting, but so far my attempts have failed. I get a paper-thin shell and a large solid mass in the middle. Since I can't see the inside, I am not sure if I spinning too fast, or spinning too slow, or not spinning for long enough.

Any insights appreciated, thanks!

Hy, may I know how big it's the model, some pics will help, based on what you are saying my advice are the following :
Usually polyurethane have a time limit for roto casting, that depends on what kind of resin are you using, surely you are using polyurethane but there are several different types, with different workable timing.
Even if the producer advise you let's say for example 5 minutes for complete hardening, that doesn't mean you can spin the mold for 5 mins, you have to make various attempts and maybe leave the bottom of the mold open so that you can see how the resin works and get optimal results,
The other advice is on the amount of resin you are using, the more resin you use the less time you have available, as the resin rise in temperature a big amount of resin will accelerate this process, so maybe if you have a big mold, start with a thin coat, just to lock the shape and then procede with multiples coatings until you reach the desired thickness.
Again I strongly suggest you to figure it out first with the bottom of the mold open so that you can see how the resin works.

 

[rgk123]

Thanks for the advice, I do not have an opening, I am just pouring some resin into the mold, closing it up and then rotating it for 5 or so minutes.

The object is about 9" deep, 5" tall, and 6" wide.

I tried Smooth Cast 57-D (3 minute pot life) and Smooth Cast 65-D (2.5 minute pot life).

Based on your advice it sounds like I am pouring too much in at once, accelerating the cure time.

I will try cutting an opening into the mold so i can do multiple coatings. What do you recommend using as a plug to stop resin from getting out while spinning?

Thanks!

 

[joberg]

You could use a latex or rubber plug (don't forget to spray some mold release on that plug). It seems that your cure time is fast to use in a slush cast.
It's true that you should do small layers, one at the time and build on it. I use to put my mold inside a sphere(each half was secure with tape) and then rolling it from North to South and then East to West and repeating until cured.

 

[rgk123]

You could use a latex or rubber plug (don't forget to spray some mold release on that plug). It seems that your cure time is fast to use in a slush cast.
It's true that you should do small layers, one at the time and build on it. I use to put my mold inside a sphere(each half was secure with tape) and then rolling it from North to South and then East to West and repeating until cured.

Should I try another resin that takes longer to cure? Most resins I've seen on the market have a pot life of less than 5 minutes.... :/

 

[joberg]

Some resin, or other kind of products have a longer pot life; problems with these is that you really have to get a slush cast machine. I don't see you slush casting that mold, by hand, for a few hours.

 

[rgk123]

Sounds like what I need to do is remake or modify my 2 part mold to have an opening so I can pour in multiple coats, but if I have to keep cutting holes through each layer to pour in more, will I just have a weaker wall where the plug is located? or should I just locate the hole someplace I will be content with having an opening?

 

[ajaxjones]

Does it have to be hollow, could you use a polystyrene foam insert to make a void and pour the resin around that?

 

[animator]

I would suggest practice rotocasting in just half of the mold so you can see how the resin reacts to your rotocasting motion. It also gives you an opportunity to see how the resin slows and cures.

As other people have mentioned I have better luck building up multiple thin pours.

You might try Smooth-On Task 15. It has a longer working time, thin walls hold their shape really well and casts have returned to the correct shape when bumped or dented.

 

[rgk123]

Thanks for the advice! I'll try it out the half mold method to get a better idea of how the resin sets and take a look at Task 15.

 

[Duncanator]

There are also some resins that have a more progressive cure, which helps with "lumping" while roto-casting. Fast cure resins tend to be liquidy (is that a word) until they suddenly reach the tipping point when it suddenly goes BANG and a lump cures and spreads.
There are other resins that cure a little more progressively - gradually getting thicker. This allows the resin to continue building up on the mold walls more evenly with less lump formation.

Smooth-On's Task 15 is made with roto-casting in mind. BJB's 1630 A/B resin had a nicely progressive cure, as I recall - but it has been a few years since I have used that one.

 

[JayCVenlo]

really cool topic guys.
I am all new to this. i will be making my first mold this week.

I have one important question... how do you calculate the quantity of resin to be used for the object? Is there a formula?
Lets say i am casting a cylinder. 12cm diameter x 30cm height. And i want my piece to be between 3 and 5mm thick....

Any advise?