resin casting tips for a beginner

[kornbeef]

Hi Guys.

I want to experiment with casting a few parts in resin to save making multiple scratch parts basically.

I know some of you are real good at it so want to pick your brains. It would help if someone was UK based who could point me in the direction of suppliers and brands of stuff to use.

This is something completely new to me so expect some dumb questions.

Like what do I need. (is lego an essential item Bo?) products, tools etc

what should I be aware of?

Basic H&S, I know to work ventilated etc.

Any guidance would be appreciated.

TY in advance

Keith

[uncletony]

Hey Keith,

I can't answer the best place to get supplies in UK, but no doubt someone else can.

As far as H&S goes, this stuff isn't terribly toxic, either the RTV mold rubber or the resin itself. Obviously you don't want to ingest it, and you want to avoid prolonged exposure to your skin, but ventilation that is sufficient for your other modeling tasks should be fine for this as well. You probably want to wear latex gloves and keep a roll of paper towels around to sop up any spills, but really, common sense and a tidy work area to start with should see you fine.

Legos required? Hardly. I like them because you can reuse them and they are modular. Using them makes it very easy to estimate mold volume and thus the amount of material you need to mix. But you can use almost anything to make mold boxes. MDF, styrene sheet, cardboard are some of many options.

One piece molds are super simple to make. Anything that doesn't have any undercuts is a candidate. I have been busy making oil tank caps lately with 1 piece molds.

2 piece molds are a little more complicated, but still easy enough. There are two basic methods that I know of:

1) suspend part in box and pour rubber to separation line. Wait for that to cure, coat with release, and pour 2nd half.

2) embed part in clay up to intended separation line. Pour rubber. Once cured, deconstruct box, remove clay but leave part embedded in RTV. Rebuild box, flip over, and pour 2nd half.

With either method you need to create some kind of registration so that the mold halves line up correctly when you are ready to pour parts. In method 1, the easy way is to carve little pyramids in the 1st half. In method 2, the easy way is to poke dimples into the clay surrounding the part.

Once you have the two halves of you mold, you need to create a pour spout and at least one vent. This is probably the area that you'll spend the most time perfecting. Basically your #1 enemy is bubbles in the resin. Ron and Lucasz etc have degassers and pressure pots that help them control bubbles. You will not .

I have slowly learned that bubbles like to form in sharp corners as well as in any high pockets in the mold. I've found that a disposable pipette (like I use for mixing airbrush paint) is great for injecting resin into detailed molds. I've also experimented with "painting" resin in the cracks and crevasses before assembling the mold and pouring.

Hope that is of some use. I encourage you to try this, I think you will discover that it is lots of fun, and is kind of liberating in a way, too.

Cheers

[uncletony]

PS:

Scott (albatros1234) shared this link a while ago, I found it very useful when starting:

http://aeroscale.kitmaker.net/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=469&page=1

[uncletony]

Supplies list:

Disposable plastic cups, clear-ish, 7oz / .2L
Popsicle sticks
Plasticene modeling clay
Latex or nitrile gloves (disposable)
Paper towels
Freezer paper or similar to cover work area

Materials to create mold boxes (Legos, MDF, or whatever)
Two part RTV silicone mold rubber-- 4 hour cure
Two part casting resin -- 20 min cure

RTV Rubber mold release (special product to prevent RTV from bonding to itself - only needed for 2 pc. molds)

[uncletony]

Workflow:

Obviously the starting place is creating your master. One thing to keep in mind is that RTV will release from nearly anything, so your master can be made from stuff that you wouldn't ordinarily use in a finished part. Stuff that is too heavy, or too soft, or fragile, or unpaintable, or unstable -- like electrical tape, something I used recently. Think about what you can do with wax, lead foil, clay, etc. RTV picks up virtually every detail too, so keep that in mind as well.

Oil tank master made from acrylic rod, electrical tape, styrene, blobs of epoxy and lead wire:




The second step is planning how the part will be cast, taking into account undercuts, where mold seams will be least visible, etc. for two part molds you want to think long and hard about how to vent the part so as not to trap any bubbles at the top. You can optionally create masters for the pour and vent sprues before creating the mold, or you can simply cut them directly into the cured RTV after your molds are cast.

The next step is to build the mold box. As mentioned earlier, I have identified two basic methods for two pc  molds. The first is to embed the part up to the intended mold seam in plasticene (box sides not shown here):


The second method is to suspend the part in the volume of the box:


For one part molds you merely need to secure the master to the bottom of the mold box so that it can't float. Plasticene is usually sufficient:


Now it's time to get pouring. One nice thing I forgot to mention about Lego -- it is very easy to estimate volume by simply counting studs and brick height of your box. A single Lego stud unit is 8x8x10mm (full height brick) or 8x8x3.3mm (plate height brick). By doing this simple math and dividing by 1000 you get ccs.

That said, I usually just make up the smallest amount of RTV I can reliably measure in the plastic beer cups, which ends up being about 2fl oz. I start by making a mark with a sharpie on two cups at the exact same height, about 1/2" from the bottom. I pour each part of the RTV in a separate cup, to the line. Pouring out in separate cups allows you to correct overpours etc without ruining the batch. The two parts are then poured into a third cup. Now you mix them up with a Popsicle stick, always scraping the sides, and meanwhile trying not to churn too many bubbles into the mix.

Once mixed sufficiently, you are ready to pour. If you are pouring a 1 pc mold or are NOT pouring the first half of an suspended two pc mold, all you do is slowly pour from a corner until your part is covered to the desired depth. When you are doing the first half of a suspended mold you have to be very careful that the mold is level and stop the pour as soon as you have reach the desired seam level. Not as hard as it sounds, mostly because the surface tension of RTV it is really pretty easy to achieve the intended seam.

Now you go do something else for at least 4 hours, longer is better.

edit: typo in cubic mm to cc conversion fixed

[uncletony]

(Workflow, Cont.)

If you were doing a 1pc. mold, simply break down the mold box and remove the master, and you are ready to start popping copies.

If it is a 2 pc mold, there is more work.

Embedded mold: break down the mold box and carefully remove the plasticene but leave the part embedded in the RTV. This is critical. Clean up any plasticene residue with isopropyl alcohol and a brush, tweezers toothpicks, whatever. Neatness counts here. now reassemble the box so that the RTV and master are upside down from their original orientation. If necessary, carefully cut pyramidal registration divots in the areas next to your master, avoiding the areas where the vents will be. Coat all the exposed RTV with rubber mold release.

Using Lego in the base of an embedded style mold, there is no need to create registration divots; the impression from the Lego studs serve this purpose:


Suspended mold: all you have to do is remove the suspension pins and cut the registration divots. Coat all exposed RTV with rubber mold release.

Now mix and pour the second half as described previously. Let it cure, overnight preferably.

Once cured, deconstruct the box and carefully pull apart the mold halves.

Now it is time to cut a pour spout and vents into the RTV. With experience you'll figure out how much you need to do here. I tend to start conservatively and add and expand if the first resin pour doesn't work so well.

These vents and sprues were carved into the RTV for this oil tank:

[uncletony]

(Workflow cont. 2)

With the RTV mold created, The hard -- and the time consuming-- part is done. Now you are ready to start casting resin.

I mix the resin using the same (but new & clean!) cups and sticks as used for mixing RTV. Again, mark cups, carefully pour, then combine into a third cup, stir and scrape. The pot life of resin is much shorter than RTV -- a couple minutes only, and you will feel it start to get warm.

The basic idea is that you pour it slowly into the open top of you 1pc mold or the pour hole of your two pc. For many castings this is all you need to know. However for trickier castings you may have to get creative to avoid bubbles and voids.

For instance, for these D.V landing gear legs I gave up on trying to pour them thru my sprues and ended up simply pouring the goop into the mold open face and then squashing it together and rubber banding it then. Worked pretty well -- some flash but quite easy to clean up:



I've also found a toothpick useful for coaxing bubbles out of tiny crevasses and for "prepainting" the mold halves before pouring. Finally, as mentioned, a disposable pipette is great for injecting resin into otherwise stubborn intricate molds.


There. Now you know everything I do. Get busy

[kornbeef]

Wow Bo pretty comprehensive for sure. Lots of help and really appreciated you took so much time. 

The lego thing was meant as a joke but I forgot the emoticon 

Heaps of thanks now to dig some suppliers up locally.

Keith

[Whiteknuckles]

Great tutorial Bo, very informative!

This needs to be a sticky! (Des?)

Andrew

[Nigel Jackson]

Hi Bo

Thank you for this tutorial Bo. It is so helpful and informative that I would now feel comfortable about having a go.  This is just the sort of thing that makes this forum so good.

Best wishes
Nigel

[uncletony]

The lego thing was meant as a joke but I forgot the emoticon 

Oh heh, sorry.

[Bluesfan]

Beautifully explained Bo, many thanks. I hadn't considered resin but you've got me wondering now

Mark

[uncletony]

At the risk of overkill, diagrams of the "embedded" and "suspended" methods. (my terminology)



Embedded
 




Suspended

[Des]

Thanks very much Bo for your Casting tutorial, I'm sure it will be very useful to many modelers, like me 

Des.

[Ernie]

Thanks Bo.  Very informative tutorial.

Cheers,
Ernie