DIY Plastic molding...possible?

[RickM]

Looking around at the various past and present projects on this board I noticed that very little is done with plastic. Sure, metal and fiberglass lend themselves better to a home shop but perhaps there's technology unknown to me.

On my Carrera - 930 conversion the extent of plactic work I encountered was light. I reconfigured and "plastic welded" the windshield washer bottle for the new front end. Beyond that what is there?

So, I'd appreciate hearing from youz guys on any DIY techniques or experience on blow, vacuum, injection or other molding work you've done.

(Not counting CNC machining)

[cstreit]

I made a plastic ashtray in high-school shop many moons ago.

Ill advised project...

[RickM]

Heh heh

You mean the Acrylic in the oven ashtray?

[Series900]

12 years ago, I came up with replacement door pockets as a saleable idea, we hemmed and hawed, finally made a deal with a local manufacturer in thermoforming. ready to plunk down the cash and ricks started advertising fiberglass ones. Bummer, we had performance produts on the hook too. Loook on ebay , you can purchase injection molding equiptment for 40k

[Harlan Chinn]

Solid resin casting is possible at home for such things as figurines and any solid object. Vacuum formed plastic is possible on a small scale but limited to the size of your vaccum frame and vacuum. People use vaccum forms to make clear lexan bodies for R/C and slotcars. Injection molding requires elaborate machines for which precision molds are made and the molten plastic is injected. The tooling for these molds make this an extremely costly endeavor for small production runs.
I have done some resin casting of vehicles and loads for model railroading. The technique of heating plexiglas and laying over a master form is called slump molding which is what most of us did at one time in shop class.

[RickM]

Thanks for the info....keep it coming.

Damon, wouldn't thermoforming the pockets be faster and cheaper than constructing from fiberglass?

[Matt_'77_2.7]

The most viable short production room plastic items are usually done in polyurethane cast in silicone molds. The silicone is fairly expensive, but far, far less than have an aluminum or steel tool made. There are probably several sites on the web that can give you more specifics. The rapid prototyping industry for production runs under 100 is mostly based on this procedure. If you have more questions about it, pm me and I can be more specific.

Matt

[john_colasante]

I have done some molds and casting but it's been a few years. There is something very significant about using a mold material like silicone. Your parts can have detail in them that is not possible with more traditional injection molding where the mold material is machined steel. I forgot what this is called, something like back-relief. In other words, you can wiggle a part out of a silicone mold because the silicone is flexible. With a steel mold the part must be able to drop out. I'm probably not being clear, sorry.

[RickM]

This makes sense (I think). Hard molds are limited in their abilty to allow heavy undercuts. I have cast into silicone using lead, tin/antimony and pewter. Aluminum is easier and cheaper than most people think..... http://www.foundry101.com/

Is there any type of plastic sheet that can be heated to a soft state (Like a wet noodle) and then layed over a postive or negative mold where it would harden? Perhaps not to a rigid state but semi-fexible.

[85eurocarrera]

Not sure but I believe Matt and John are talking about RTV silicone molds (Room Temp. Vulcinization). Then use 2 part urethane resin to cast multiple parts, up to 25 per mold depending on geometry of part. You can pull undercuts but not to large. 2 part urethanes come in all different durrometers for hard or soft parts such as grommets and bushings.

I have cast a couple of tail light covers on a Pontiac Le Mans for a friend with a clear urethane. We used the real part for the pattern. Also can overmold stuff like strain reliefs ect..

Vac formed headlight covers for the track but thats about it for the P-car.

Joe

[BGCarrera32]

Not that I am the authority, but 85eurocarrera is correct on RTV. It is generally advisable to avoid undercuts in part design anyway, but when needed in an injection mold pickouts and cams are done to accomplish this every day. For some extremely boring but useful video, get Ben Ridge's "Reproduce Almost Anything- Basic Silicone Mold Making" available from Eastwood http://www.eastwoodcompany.com

Keep in mind that when you start using decent resins like those from GE or Vantico, you can spend $ pretty quick.

[RickM]

Funny, my experience in using RTV Silicone has been very condusive to allowing undercuts in my work. But this is with metal. I've been able to get 150-200 uses out of most molds....using tin. However this is dependent on the type of silicone one is using. There are many.

I don't know much about high production methods so I can't speak to them.

[john_colasante]

I have used two kinds of silicone molds. One was poured two part RTV and remains soft and flexible (and fragile). The other was baked vulcanized and was much tougher. Both allow reasonable undercuts. The more undercuts the more wear on the mold.