Introduction: Vacuum Formed Concrete Mold

By Concrete Exchange

In this Instructable we'll share our process of creating vacuum formed plastic molds for making concrete stepping stones. This involves:

  • Designing the Shape
  • Making the Positive (Sculpted Clay or Plaster, CNC Routed MDF, Etc.)
  • Creating a Rubber Mold (Castable Polyurethane)
  • Casting an Epoxy Positive (Aluminized Epoxy Resin suitable for Vacuum Forming)
  • Drilling Holes and Finishing
  • Vacuum Forming the Plastic

Step 1: Sculpting Clay

Tools and Supplies:

  • Plasticene (Roma Plastilina #2)
  • Clay Modeling Tools
  • Paper for Template
  • 3/4" Plywood
  • Spray Acrylic Lacquer

Design:

This 2” x 17” x 16” stepping stone is based on a leaf from the plant Monstera deliciosa, or the split-leaf philodendron.

Because the concrete needs to release from the mold, there can be no undercuts in the shape. For vacuum forming, it should have at least 2-6 degrees of draft angle (taper).

Sculpting in clay was a new part of the process for us. Our previous stepping stones were modeled in CAD and cut from MDF on a CNC router to create the positive from which the rubber mold was cast.

Modeling Clay:

We used Plasticine, an oil-based, modeling clay. It comes in different consistencies, from very soft to very hard. Unlike water-based clay, it will not dry out or shrink, and can be used repeatedly.

Template:

We sketched the general shape, scanned the sketch, and in Photoshop created and printed a full scale template. The full scale template was useful in determining the overall size of the stepping stone, and for laying out the plywood interior filler.

Plywood Filler:

To conserve modeling clay we used a plywood filler, constructed from two pieces of 3/4" plywood, cut to shape with a jigsaw, and screwed together for a total thickness of 1-1/2”. We cut the 3/4” plywood the same shape as the template, but approximately 3/4” smaller all the way around (1-1/2” overall).

Base Board:

Screw the plywood filler to a 3/4” base board. The base board should be smooth and flat, and measure about 2 ft. x 2 ft.

Build up Clay:

We cut the clay into slabs just over 1/2" thick. Scoring the clay and heating it slightly made it easy to press onto the plywood filler.

Sculpt:

Shaping the clay was accomplished using a few standard clay tools and smoothing by hand. Some areas were carved down, others were built up. It is important to maintain a consistent draft angle (mentioned above) all the way around the sides of the sculpted clay.

Acrylic Lacquer:

After shaping and smoothing, our sculpted clay positive is almost ready to use to make a rubber mold. To make it easy to remove the rubber mold from the clay, spray the clay with an Acrylic Lacquer. The lacquer gives it a glossy finish and allows an easy and damage-free release of the clay from the rubber mold. If you use a sulfur-based modeling clay and silicone mold rubber, a lacquer coating is necessary.

Step 2: Casting Rubber

Casting a rubber mold of the positive allows us to make multiple castings from a single mold. There are many mold rubbers to choose from; we used a polyurethane rubber from Polytek with a 60 shore hardness. This medium hard material is flexible enough to easily separate from the clay positive, but rigid enough to retain its shape for casting epoxy.

Tools and Supplies:

Form Walls:

The form walls should be 1/2" taller than the 2" positive or 2-1/2". Using curved form walls and double-stick carpet tape, attach the form walls to the base board. Maintain a 3/4” gap between the form walls and the clay positive. Use 100% silicone sealant to fill any large gaps in the form walls. The rubber is so thick it is not necessary to fill small gaps.

Before Pouring:

After the form walls are in place, and the sealant has cured, blow out any debris with compressed air and spray the form liberally with a mold release like Polytek's Pol-Ease.

Use a level to ensure the form is on a flat and level surface.

Calculate how much rubber to mix:

You can do a volume calculation for an exact amount, or just mix 14 pounds of rubber, which will be more than enough. It's better to have a little extra than not enough because the rubber must be mixed and poured in one batch.

Mix Rubber:

This is a two part rubber with a mixing ratio of 1:1 by weight. Put a 5 gallon bucket on a scale, zero the scale, and weigh out 7 lbs. of the thicker part. Zero the scale again. Pour in 7 lbs. of the thinner part. Using the 1/2” corded drill and mixing paddle, begin mixing at a slow speed, occasionally scraping the sides and bottom of the bucket. For the best results, after mixing for a few minutes, transfer the rubber into a second 5 gallon bucket. This will leave behind any unmixed rubber, which will not cure. Continue mixing.

Pour Rubber:

Choose where you are going to pour the rubber into the form. Pour either on top of the clay or onto the base board next to the form wall. Pouring to just one place minimizes the formation of air pockets. Let the rubber flow around the form, pouring slowly until it is filled. Smooth the rubber flat with a spatula as needed.

Rubber Storage:

Polyurethane rubber has a limited shelf life, even unmixed. To extend the shelf life of the parts, spray Poly Purge Dry Gas Blanket into each of the two containers and seal immediately.

Curing:

The rubber mold will cure in 24 hours. If it is still tacky after 24 hours it will never harden because the ratio was incorrect or the parts weren’t thoroughly mixed.

Release the Clay:

Separate the rubber mold from the clay positive. You now have a rubber mold for casting the Epoxy Tooling Compound.

Step 3: Casting Epoxy

Epoxy Tooling Compound:

This is a two-part aluminum-filled resin designed for vacuum forming. The aluminum content allows it to withstand high temperatures used in vacuum forming. It is not as durable as a solid metal part would be, but it is easier to work with and therefore well suited for medium scale vacuum forming.

Mixing Tools and Supplies:

  • 1/2" Corded Drill
  • Heavy Duty Mixing Paddle
  • (2) 5-Gallon Buckets
  • Quart Mixing Container
  • Kitchen Scale
  • Bathroom Scale
  • 1" Thick Insulating Foam Knockout
  • Clear Packing Tape
  • Chip Brush
  • Pol-Ease 2300 Mold Release
  • Trowel

Clean the Mold:

The rubber mold must be dry and free of dust and debris.After cleaning apply Pol-Ease Mold Release.

Place the rubber mold on a flat, level surface.

Prepare the Foam Knockout

Cut a piece of 1” thick insulating foam the same shape as the rubber mold, but approximately 1-1/2” smaller all the way around (3” overall). Apply clear packing tape to the edges and bottom of the foam knockout for ease of removal.

Calculate how much Epoxy to Mix:

One quart of this epoxy weighs about 2 lbs. 6 oz. (the grey container in the first photo). You will need 13 -15 lbs. of epoxy to fill the rubber mold. (You can confirm this by filling the rubber mold with water and measuring how many quarts it holds.) It is unnecessary to mix extra epoxy because although some of the epoxy will stick to the sides of the bucket, space will be taken by the 1" thick foam knockout.

Mixing Ratio:

For large vacuum forming applications like this one, Polytek recommends using the PolyPoxy 1030 Resin with the PolyCure 1230 Hardener. The mix ratio is by weight, 100 Parts Resin : 15 Parts Hardener. Refer to Page 60 of the Polytek Catalog for more information.

Weigh the Hardener:

The hardener (amber) can be weighed out on a kitchen scale in a mixing container. Zero the scale with the empty container on it. This is where things get messy, so put on the rubber gloves.

Weigh the Resin:

The resin (grey) is extremely thick and difficult to measure, let alone mix. On a cold day you can turn the bucket upside down and the stuff will barely move. Keep it in a warm place to soften. Put an empty 5 gallon bucket on a bathroom scale. Zero the scale. Use a trowel to scoop the resin into the bucket.

Add Hardener to Resin:

Pour the hardener into the bucket of resin. Use a spatula or a mixing stick to get every bit of hardener.

Mix:

This stuff is so thick it will break the spot welds on cheap jiffy paddles and twist them up like spaghetti. Use only a powerful 1/2" corded drill with a strong mixing paddle. You will need a second person to hold the bucket while you mix. Scrape the sides and bottom of the bucket with a trowel, then mix again. Repeat this until everything is mixed. Then pour the epoxy mixture into a second 5 gallon bucket and continue mixing. This extra step will separate out unmixed epoxy and ensure successful hardening.

Brush and Pour:

Begin by pouring enough epoxy into the rubber mold to cover the bottom to a depth of 1/4" - 1/2". Brush the epoxy into the mold, especially around detailed parts. This will help eliminate small air bubbles. Next place the foam knockout in the mold. Continue filling the mold. When completely full, place a weighted board across the mold to hold the knockout in place.

Clean Up:

Use water pressure and acetone to clean the epoxy from tools and buckets. Any epoxy that ended up on your clothes is pretty much permanent.

Cure and Mold Release:

The Epoxy Tooling Compound will cure in 24 hours. You can either remove it from the rubber mold as soon as it cures, or after sanding the back (see next step). After 24 hours if you can dig your fingernail into the epoxy, it is too soft to use and will never harden, probably because of unmixed material, or an incorrect ratio of resin to hardener.

Step 4: Preparing Mold for Vacuum Forming

Sand the Back Flat:

Leave the epoxy casting in the rubber mold while sanding, to protect it. If you do remove it from the mold before sanding, place it on a piece of foam so it doesn't get scratched. Sand the back of the epoxy casting with an orbital sander until it is flat.

  • Orbital Sander (60 Grit)


Drill Holes:

Holes are necessary throughout the casting for vacuum forming, but especially at the lowest and most detailed points. The holes should be very small (#60), or they will show up as dots in the finished mold. Ideally holes should be drilled at least every 3", but it depends on the level of detail in the casting. For an example of a mold that needed many more holes, take a look at the Alhambra stepping stone mold.

Use a #60 bit in a drill press. (The small size of the drill bit necessitates the use of a drill press). The epoxy has a tendency to heat up while drilling, which can close up the hole. Drill two or more times to clean out the hole. (Note: If you didn’t use the 1” foam knockout you will need a longer #60 Aircraft Extension Bit in order to penetrate 2” of epoxy).

  • #60 Drill Bits
  • Drill Press

Finishing:

Fill any holes larger than 1/16" with Bondo so they won’t read through to the plastic. Sand rough areas until smooth, up to 400 grit. Using finer grits is unnecessary because the plastic stops picking up detail beyond 400 grit.

  • 220, 320, 400 Sandpaper
  • Sanding Block
  • Bondo Filler

Remove Foam Knockout:

Remove the foam knockout with a small pry bar. If you covered the bottom and sides of the knockout with clear packing tape it should come out easily, Scrape out any remaining bits of foam.

Step 5: Vacuum Forming

Plastic Cut Size:

For a mold that's going to be filled with concrete, we use 3/16" thick ABS (black) or 1/8" thick Polystyrene (white). Cut the plastic to 26" x 26", which is the largest size that fits on the Formech 660 at the Tech Shop.

Heat Time:

Heating time varies depending on the thickness and type of plastic used. For 3/16" ABS and 1/8" Polystyrene (white) set all heating elements on the Formech 660 machine to Level 4. Heat for 160+ seconds. Consult the Formech Vacuum Forming PDF for different heating times for different types of plastic and for troubleshooting.

Trim the Plastic:

Using a band saw, trim away the excess plastic, leaving a 1" border around the edge. Sand the edge with sandpaper.

Learn More:

One of the best resources for learning more about the vacuum forming process is the Vacuum Forming Guide (a free PDF) by Formech. There is a lot of good information about making molds for vacuum forming.

Step 6: Pouring Concrete

Casting Concrete:
For more detailed instructions on casting a concrete stepping stone, check out the How-To Make Concrete Stepping Stones Instructable.

Thank you for taking the time to read this Instructable. For more information on mold making and concrete casting visit the Concrete Exchange and explore our concrete design gallery and how-to video library.