Car Body Mold & Polymer Clay

Return to Sitemap
2012-08-14 Rev.


BACKGROUND - Down through the years, I have thought it would be interesting to have a "car" that had front wheel drive and various back ends, dividing behind the driver - short for city driving, longer for group trips, minivan, pickup or even camper.  Since this would be awkward to store, etc., I considered a commercial scheme of renting the specialize rear ends. Besides the rather critical point of attaching the two parts under current side collision standards, another problem needing solution was locating the gas tank(s) and having the ability to drive (move) the front end disconnected.
Obviously a fantasy in a full sized vehicle, I had considered several times making a model in metal, perhaps motorized with radio control. 
Then as I began working in in polymer clay making hollow parts with slab construction and answering online questions, I saw an opportunity and challenge in making a mold for polymer and possibly making a car body in the plastic. This is a report on the results.
With the project in the back of my mind, I came across a display of car models or reasonable size at my CVS drug store for only $5.99, buying this one (Audi A8) for the configuration of the front end. My actual original thought was to cut this apart, dealing with the chassis somehow as I took the model apart, but then doing a polymer clay version intruded its thoughts.
This is the model disassembled into its lower running  gear (pull the toy back and let go and a spring drives it forward), the inner shell that provides "seats" and "luggage" and conceals the model's works, and the body shell.
The model has clear plastic windows except the doors and the front doors open.
The body was prepared for making the mold by filling the inside of the door windows and the wheel wells with aluminum foil supported by plasticine clay.
The mold detail is done with latex rubber molding compound with the result shown here folded open before showing some of the steps in the process. Normally this would not happen until a plaster backing is made (below).  The mold shown was built up over several days by painting a moderately thick layer of the creamy white latex over the model and letting it set to the amber color shown. The process could have been sped up with a small fan.
In the far, the emulsion of the latex in water is kept from setting by ammonia in the water.
The latex mold holds detail marvelously but is so soft that except in unusual conditions support must be provided to put material in the mold without distorting it.
  Although not a very good picture, this shot shows the transparency of the mold material and the flange extending it out onto to the foil.
  But the purpose of the image is to show the first step in making a plaster backing that does not trap the model in the plaster.  A ridge of plasticine clay has been placed with one side aligned with the centerline of the car.
Heavy duty aluminum foil is folded up to make a basin to control the plaster of Paris which is made moderately thick.  The foil could have been doubled to prevent the sag visible at the bottom or a flat container could have been pushed up against the outside.
Once the plaster has set, the plasticine is pulled back and removed, leaving an edge for the centerline.  The plaster is sprayed with a separator - I use spray vegetable cooking fat.
  A second batch of plaster is added.  Ideally, both sides will be leveled so when the plaster is upside down it sits level on its own, but this can be a minor consideration.
  When set and partly dried to increase strength, the bottom foil and the two sides are pulled free to continue drying.  Latex rubber turns white when damp as from the plaster and it shows in this picture where the right half of the model was covered about 24 hours after the left and thus is damper at the time of separation.
Placed back in the plaster backing which was tied together, white original Sculpey (softer than Sculpey III) was rolled out about 1/8" thick and cut to approximate shape and pressed into the bottom and sides of the rubber mold.  The aluminum foil shown was added to provide support during baking.  Before attempting to remove the rubber mold, the whole thing was put in the refrigerator for several hours to make the polymer clay as stiff as possible to reduce damage.
The rubber bent backwards off the front and back, releasing the stiffened clay.
The result after baking with no trimming or sanding. As mentioned above, the door windows and wheel wells were lined before the rubber mold was painted on.
Below are pictures of the baked product and a plaster copy made in the mold - the plaster shows more detail of lines because of its liquid nature and find grain.
The plaster copy from the mold, directly below and a repeat of the polymer clay image above for side-by-side.

Contact Mike Firth