Casting Plastic Parts

[elementcollector1]

I need some help getting started with casting plastic parts for some hobby props I'm making. Could anyone direct me to a nice walkthrough, or answer a few questions? Also, not sure if this is the right forum for this, so if there is one, please move it there.

Q's:

- How do I make a mold box?

-What basic procedure can I follow for casting plastic parts? (these are going to be fairly large, anywhere from 2 inches to 2 feet and a variety of shapes)

-What materials are recommended? (best mold, best plastic for casting)

 

All help is appreciated.

[CaptainPanic]

Let me start by not answering your question .

I think nowadays, the easiest thing is to get access to a 3D printer. No need for mold boxes, and no need to fool around with dangerous molten hot chemicals. Or if it should only look pretty, but doesn't need to be strong, maybe candle wax is good enough? Much safer and easier than most other options.

 

Anyway, back on topic: You're probably looking for resins, because those are at least liquid: polyurethanes or perhaps phenol-formaldehyde resins. Note: all those contain dangerous materials, and you probably need a well-ventilated place. There's a fire hazard and some materials can be toxic. Not something to fool around with then. The exact method to mix components, and to get them to react depends on which resin you're using. Google showed me that there exist kits that probably include all components (I googled for 'plastic casting').

 

The method used for most plastic parts which are cast is called "injection molding". It includes an extruder which does multiple things at once: it mixes the plastic pellets and melts them by shear force, it blends in optional colors and other materials. So, at the extruder outlet, the plastic is molten and blended, and it is at high pressure. This mix is then forced into the mold by the pressure. These molds are almost exclusively metal. And that's not something you will easily do at home.

 

I have no idea about materials for a mold box, but I guess that this depends on the plastics you're using.

[elementcollector1]

Let me start by not answering your question .

I think nowadays, the easiest thing is to get access to a 3D printer. No need for mold boxes, and no need to fool around with dangerous molten hot chemicals. Or if it should only look pretty, but doesn't need to be strong, maybe candle wax is good enough? Much safer and easier than most other options.

 

Anyway, back on topic: You're probably looking for resins, because those are at least liquid: polyurethanes or perhaps phenol-formaldehyde resins. Note: all those contain dangerous materials, and you probably need a well-ventilated place. There's a fire hazard and some materials can be toxic. Not something to fool around with then. The exact method to mix components, and to get them to react depends on which resin you're using. Google showed me that there exist kits that probably include all components (I googled for 'plastic casting').

 

The method used for most plastic parts which are cast is called "injection molding". It includes an extruder which does multiple things at once: it mixes the plastic pellets and melts them by shear force, it blends in optional colors and other materials. So, at the extruder outlet, the plastic is molten and blended, and it is at high pressure. This mix is then forced into the mold by the pressure. These molds are almost exclusively metal. And that's not something you will easily do at home.

 

I have no idea about materials for a mold box, but I guess that this depends on the plastics you're using.

 

It seems OOMOO is the best way to go. As for the plastic being casted, it's probably going to be ABS or something.

 

 

 

[Phi for All]

I need some help getting started with casting plastic parts for some hobby props I'm making.

Please define and describe "hobby props".

 

As for the plastic being casted, it's probably going to be ABS or something.

Why ABS? Is there a reason you need such a tough and costly plastic? Polystyrene is half the price and with the small parts it's got plenty of strength.

[elementcollector1]

See http://www.instructables.com/id/Hey-an-introduction/, and especially http://www.instructables.com/id/Building-a-Portal-Turret/. I need something that's able to stand heavy wear and tear.

[Xittenn]

Why not plexiglass?

[CaptainPanic]

So, elementcollector1, forgive me if this is rude, but why did you ask the questions when you seem to know the answers yourself better than anyone so far?

[Phi for All]

See http://www.instructa...n-introduction/, and especially http://www.instructa...-Portal-Turret/. I need something that's able to stand heavy wear and tear.

Fair enough. There are many good reasons to choose ABS over polystyrene, and durability is one of them.

 

You say you're casting hobby props, does this mean one-offs for your own use, or are you mass-producing these parts? Injection molding is the way to go with the kind of detail you're showing but production molds get pretty pricey. Short-run molds are cheaper. In either case, the first step is a 3D CAD model of your parts.

[ewmon]

For someone new to molding, consider polycaprolactone that melts at 150°F/65°C, meaning that you can prepare it in a double boiler (which provides a consistent 212°F/100°C) on a hotplate or stove. Your local arts & crafts store probably has information on a variety of molding materials and techniques.

[elementcollector1]

I do have some polycaprolactone (Friendly Plastic) but not nearly enough to supply my needs.

@CaptainPanic: I do?

The hobby props are probably going to be somewhat mass-produced (i.e. maybe 4-5 castings total), but I'm hoping to keep things cheap.

As for plexiglass, it seems like a maybe. MP is low enough, but where can I find a good source?

[Enthalpy]

Thermoplastics are not really possible to cast, because they remain very viscous. This leaves bubbles and voids in the mould, and you don't get the desired shape. They also react with air when warm. That's why they use to be injected at tremendous pressure, by very strong pumps, in very resistant moulds - expensive, and for bigger series.

 

It's much easier to cast alloys like aluminium-silicon or aluminium-magnesium-silicon (AA6000 series). Dry (oven for long) plaster works as a mould.

 

With plastics, the accessible technology are thermosetting ones, like epoxy, which you can cast in a mould, or rather over a form, and reinforce with varied materials like glass fibres.

[elementcollector1]

Thermoplastics are not really possible to cast, because they remain very viscous. This leaves bubbles and voids in the mould, and you don't get the desired shape. They also react with air when warm. That's why they use to be injected at tremendous pressure, by very strong pumps, in very resistant moulds - expensive, and for bigger series.

 

It's much easier to cast alloys like aluminium-silicon or aluminium-magnesium-silicon (AA6000 series). Dry (oven for long) plaster works as a mould.

 

With plastics, the accessible technology are thermosetting ones, like epoxy, which you can cast in a mould, or rather over a form, and reinforce with varied materials like glass fibres.

 

Right, but do those have low melting points? (i.e. blowtorch temp or lower)?

Plaster is actually a great idea now that I think about it.

I've heard of people melting Legos into desired shapes, and Legos are made of ABS plastic.