Need Help!- Making molds for a Pedal Car

[Di4l]

Hello all,

I'll try to be as brief as I can. I am currently working on a pedal car project for my kid. I am really tired of seeing on the stores pedal cars that pretend to look like real cars but have no similarities to the pretended thing, so I have decided to build one that looks like the real ones. There will obviously be some differences due to size constraints (no roof, small windshield, etc...).

The chosen car model is a 1970 Ford Mustang fastback (1st image). I am building on my home made CNC milling machine the different body parts out of Expanded Polystyrene from which to make the mold negatives. (The second and third images shows a crappy picture of the left door, left front fender, left rear fender and hood).

I am now starting to think about the molds. My original idea was to make them out of fiber glass, but I am now not so sure and I am more inclined towards the silicon/rubber molds. the main reason is their flexibility so that I need not worry so much about the demolding process, as they can be "flexed out" off corners and bents. I've benn also suggested the posibility of making the molds out of plaster. But I beleive they are too rigid, fragile, heavy and can be damaged easily.

The final dimensions of the body will be 1300 mm (~51 inches) in length, 720 mm (~28 inches) wide and about 310 mm (~12 inches) height. The bigger part is the hood, which is about a third in length of the car: 430 mm (~17 inches) and 500 mm (~19 inches) wide.

I've been reading a little bit about making silicon molds, so the rough general process I am aware of.

Now the questions:

1. What kind of material do you think is the best for making the molds? fiber glass? silicon/rubber? other?
2. Which is cheapest?
3. How many parts can be made (roughly) out of each kind of mold?
4. Expanded Polysytirene is not perfectly smooth, it always has some tiny holes so, before making the mold, I need first to apply some sort of filler and then smooth it out with very fine sand paper. What filler do you recomend?
5. Once the molds are made, I was planning to make the final parts out fiber glass. Is there any restrictions to the material of the molds? I mean, can parts be made of fiber glass using say, silicon molds?
6. Do you recomend a different material for the final parts rather than fiber glass?

The reason for making molds is that there are already several friends that are interested on making a copy of my project for their kids IF mine turns out good, or reasonable good. And I am not planning to mill each time the body parts.

Any help, suggestions, directions will be apreciated. Thank you very much,

Di4l

[txcncman]

You are correct about plaster. Bad choice. Silicone is ok. Rigid foam is ok. Fiberglass over wood frame I think might be best.

The final finish layer is just auto body filler to hand sand to smooth. Coat with mold release before putting fiberglass.

In boat building, we spray gelcoat on first before fiberglass for color and smooth finish on part.

Be sure to embed some aluminum attachment points to be able to secure to the frame.

[Di4l]

Thank you for your suggestions and quick response txcncman.

I've been told that fiberglass molds tend to deform over time. How true is this?

In my project. there are some parts that have a "lightly hard exit" (I dunno if this is the correct expression, english is not my mother's language) when demolding. It certainly would be impossible to demold if it were a rigid kind. Fiberglass is somewhat flexible/deformable. Do you think it may be a big problem? (My guess is that you'd need to inspect the mother part first to answer this question).

About the frame to support the final body. My idea is to make the frame from metal. Thin (~1 mm (~0.04 inches) thick) iron squared-section tubes. Aluminium would be best as it is lighter, but it is harder (impposible for me) to weld. Since in the end the project is going to turn into a "battey powered kid's car", frame weight is a smaller issue (to a limit).

Once I have all the mother parts made, I will then design the frame, take precise measurements so I know where the body will be supported. Thanks for the reminder about aluminum attachemnts.

Thank you very much,

Di4l

[txcncman]

The stability of the wood frame under your fiberglass mold would be a large factor in the mold deforming over time. How much deforming would be acceptable for an application such as this is up to you. I would think 3mm to 6mm would be acceptable deformation for this project. I would think fiberglass molds over wood frames could be made to be at least this stable. What do you think?

When making complex shape molds it is important to consider "draft angle". Look up "draft angle" and mold making on the internet to get more ideas on this.

The fiberglass panels you create on your molds will be somewhat flexible. This will aid in removing the finished panels from your molds. In boat building, the main problem encountered when removing the fiberglass products from the molds was encountered when not enough mold release agent was applied to the mold before "laying up" the fiberglass. In some of these cases, both the mold and the fiberglass product were damaged during the release process. We would repair such damage by sanding the damaged areas and repairing with automotive body filler and then re-sanding smooth again.

[Di4l]

I think you are right, and 3mm to 6mm deformation is very acceptable for this project. I believa that, in addition, the frame will reposition the final part and most of the deformation will be corrected.

Thank you very much indeed for all your help. I will go for the fiberglass over wood frame.

If anyone is still interested on the end result of this project I will keep this thread updated or, if not considered the correct forum for that, I'd open a new one on the right place.

Thank you very much again,

Di4l