How to Prep a Model for Molding

How to Prep a Model for Molding

  1. Sculpt clay to create a model, it may require internal structural support.  Be thoughtful about undercuts and the potential seam lines of the mold. (We mostly use Monster Clay; it is an oil based sculpting medium which is soft enough to constantly be changed, and can be melted and reused.)
  2. Add cone to create the pour hole. (You may super glue a plastic piece, like a yogurt cup, to provide rigidity. Additional clay may be needed to fill any gaps. 
  3. Smooth away tool marks with mineral spirits (isopropyl myristate, 99% isopropyl alcohol, naphtha, also work to differing degrees). Let dry for at least an hour before handling. Remove excess clay that becomes sticky from this process or allow for extended dry/curing time. You may repeat this process until desired finish is achieved.
  4. Add maker’s mark.
  5. Consider where the seams of the mold will be. Determine how many parts the mold will need to be to accommodate the details of the model. (This can be a difficult task and you might not get it right on the first try.) Use a permanent marker to draw the seam lines onto the model. Plan the order for making each part of the mold. 
    1. Things to consider, start with the most detailed piece to ensure it is not scuffed up throughout the process. 
  6. Review one more time for any small detail corrections. Once the model is fully prepped it is time to create a frame for the model/mold.
  7. Clean coddle boards. Scape off any plaster and wipe down with a wet towel to remove any plaster or clay from previous use. 
  8. Apply clear plastic tape to one side of a coddle board for mounting. Mockup the model and frame to determine where to mount the model on the board. At least 5/8 inch of plaster on all sides is usually ideal. 
  9. Once you have determined where the model needs to be mounted to allow for adequate plaster on all sides, use a permanent marker to draw/outline where the cone of the model will be mounted onto the taped coddle board. 
  10. Super glue model onto taped coddle board. 
  11. Use a large glass panel for the base. It may need supports (popsicle sticks) in the middle to keep it level and prevent bowing due to the weight of wet plaster.
  12. Mockup coddle boards with model on the glass panel, determine the best placement for all the pieces; the model should be centered on glass and all the boards should be stable. Then hot glue the coddle board with the model into place on the glass, stabilize by clamping it to another coddle board.
  13. Use wet pottery sculpting clay to support the model and build up a flat edge to the seam lines for part 1 of the mold. If the model is a simple shape that requires a single seam at the halfway point, it is a great time saver to pour alginate or plaster instead of building up clay. NOTE: It is better to have a small indent on the seam edge, (into wall) than to have a shallow slop down to the model because it is easily removed from the plaster. 
  14. Smooth the clay, try to get it as flat/level as possible. Use a brush and water to remove tool marks.
  15. Frame with coddle boards and seal all the edges with clay, again you want it as smooth as possible. 
  16. Now that the model and frame are fully prepped, it is time to do some math to figure out how much plaster is needed for part one of the mold. Measure the volume of the area the model is in (length x width x height) then subtract the volume of the model. To determine the volume of the model, ask yourself, about what percentage is the model taking up? It is best to underestimate to ensure there will be enough plaster to fully cover the model and create a 5/8-inch wall. 
    1. Example: The frame around my model is 5 inches by 5 inches and the depth from the top edge to the clay edge I built up is roughly 2.5 inches, so the total volume is 5 x 5 x 2.5 = 62.5in3. I think my model takes up about ¼ the area, so I will need to subtract 25%,  62.6 x 0.25 = 15.625. Total volume 62.5 – model volume 15.625 = 46.875in3. To make life easier, round up to 50in3 and we should have more than enough plaster to make part 1. 
  17. Consult the handy dandy Plaster Cheat Sheet to get the amount of plaster and water needed. Mix and pour plaster and wait one hour for plaster to set up. (Mixing and pouring plaster is its own project, if you need tips and trick on how to do that, check out our guide, How to Get Plastered)
  18. Remove coddle boards and clean edges of plaster. It is best if you are able to leave the model in part one of the mold. Do not separate them, if possible, this will make for smaller, better seams on the mold. 
    1. Sharpe edges are more likely to fracture, so best practice is to sand/scrap down the outer edges so it has a soft curve. Scrap the top edge so it is nice and flat, and hopefully level. 
    2. You might consider scraping/cleaning around the model to make it smooth or correct any imperfections from the clay buildup. Only do this if you are making another piece, do not alter pieces after the adjoining piece is made, it will cause issues with the piece fitting together properly.  
  19. Clean the model with water and brush, remove all the clay. If you want to make unmolding worlds easier, add small square/rectangle bits of clay to opposing sides of the mold, just large enough for a pair of pliers to fit in and help lever apart the pieces of the mold. 
    1. If the mold is 3(+) parts, repeat clay build up for seamlines.  
  20. Use a brush to apply Vaseline (or your preferred release agent) to part 1 of the plaster mold. 
  21. Assemble coddle boards around mold part 1 & model. Seal the bottom, outside edge of the coddle boards with clay.
  22. Plug the inner corners with Vaseline and brush the plaster mold with Vaseline brush to ensure any dry clay is removed and Vaseline has been applied evenly. (It should be thoroughly coated to ensure release, but not so thick that it has texture which would alter the joining of the mold pieces. It is an art to find the perfect balance.)
  23. Check the model for any small knicks or imperfections that may have occurred throughout this process and fix as needed. 
  24. Repeat math for part 2.
  25. Check Vaseline one more time.
  26. Mix and pour plaster. Wait one hour for the plaster to cure. 
  27. Do NOT separate part 1 and 2 from the model if making additional parts for the mold. Repeat steps 18-26. 
  28. Remove coddle boards. Clean edges. Carve the mold name, number and/or any other identifiers (I like to add a little doodle) into the sides of the plaster mold. 
  29. Use a pair of pliers to separate the mold pieces. If you skipped adding small bits of clay to make this part easy, tap the sides of the mold with your fist and/or soft mallet. It may be necessary to use a drill to make a small hole for pliers to go into. NOTE: If not enough release agent was used then the plaster will have bonded together and may not come apart without fracturing. 
  30. Clean inside edges if you desire, then strap/band the mold together and let dry for at least one week. Remember that the plaster is still very wet, so it is beneficial to have it elevated to allow circulation. Placing it in a room with a fan and heater is ideal for drying; and rotating the side it is laying on from time to time. 
  31. Enjoy!

PDF Resources: How to Mix Pottery Plaster,  Plaster Cheat Sheet, How to Prep a Model for Mold Making

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