Homemade wood slab mold with lexan plastic liner

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Aug 20, 2009
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Lexan (Polycarbonate) Lining on wood molds:

In the past, we have tried cutting up fun foam sheets, several vinyls, freezer paper, plastic cutting board covers, and whatever else we could find to make liners. They start out great, but after a few batches the fun foam wrinkles and stretches, the vinyls wrinkle and stick, the freezer paper is OK but time consuming, and the plastic cutting board material wrinkles and sticks. There is always a battle between the liner, the soap, and me.
My experience is that Lexan lined molds offer the least amount of prep-work to use (just spray with a little Pam), and the best results for the soap (smooth and no sticking). We have not used silicone or teflon lined molds, so cannot compare results with those. We believe they will work very well, but more expensive.

We do not want to come across as pushing Lexan. We have no vested interest. We do not sell molds (or soap) of any kind. We are just trying to share my experience. Lexan is similar to Plexiglas (Polymethyl methacrylate), but carbon based versus acrylic. They make airplane windshields out of this stuff. It is tough and slick.

We spray all surfaces lightly with Pam before we pour, add the batch, then let the saponification process happen. We then put it in the freezer for a couple of hours. When we take it out, we remove the molds wing nuts and dowel pins and the sides basically fall off, leaving very nice smooth soap. We make the molds as two L shapes sides and a base held on by pressure and two dowel pins (see pictures below). The only problem we had was the base on the slab molds. It just slide off the first the base the first few times we used it, but later batches tended to act like a large magnet with the bottom. It took lot of careful pressure and sliding to get the soap off the base. We solved this by going back to a Fun Foam liner just on the bases of the slab molds. The dividers hold it down flat. We have no idea why the sides and dividers and sides work so well, yet the base did not. Anyway, with the foam base everything works great. We now use our freezer paper to wrap steaks instead of lining soap molds.
Next time I will try not gluing the lexan base piece down, which might make it easier to slide if off the soap after unmolding.

Detailed Instructions for the mold:

There are some YouTube videos by Keith Brown that show you how to make the wood part. He makes a log mold with removable ends instead of my slab mold with two "L" shaped sides (see pictures below), but the idea is basically the same.

Part 1 - [ame="http://www.youtube.com/watch?v=yog0-s9OsDA"]http://www.youtube.com/watch?v=yog0-s9OsDA[/ame]
Part 2 - [ame="http://www.youtube.com/watch?v=fTuN1RLT8fY"]http://www.youtube.com/watch?v=fTuN1RLT8fY[/ame]
Part 3 - [ame="http://www.youtube.com/watch?v=04guyicRDi0"]http://www.youtube.com/watch?v=04guyicRDi0[/ame]

Keith's videos are mega-better than my words could ever be, but for the record here is what we did.
We do use good quality wood. Go to the hobby lumber section at Lowes or Home Depot to get the good stuff. It is very straight and free of knot holes. You can get walnut or oak if you want, but much more $. We think the high quality pine is just fine.

We bought the Lexan on eBay from Gene Wileson (PopDisplays), but there are many other sources. I've seen some sizes at Lowes on occasion.


We covered the sides and bottom of my two slab molds and a couple of log molds with the 1/32" (0.03”), and later made dividers for the two slabs and our 3" diameter round PVC molds from 1/8" Lexan.

For the slab mold

1. My base piece is a 1" by 10 9/16" by 10 1/16" high, which the sides wrap around. This gives the mold an inside dimension of 10 1/2" by 10", once the 1/32" Lexan is glued to all four sides. This provides for a 3 by 4 grid of 12 bars, all 3 1/2" by 2 1/2" poured 1" thick. This is perfect if you plan to cut the twelve bars yourself. If you use the 1/8” dividers, the bars are not exactly 3 1/2" by 2 1/2" because the 1/8" dividers takes up some room when pushed down into the soap. However, the bar ends up slightly thicker than 1" because the dividers displace the soap up slightly when pushed in. We did not think about this before we decided to make dividers (oops). The bottom line is the bar volume is the right amount of soap intended, as long as you calculate your batch for a 10” by 10 1/2” by 1” container (42 oz). If you want dividers, AND exactly 3 1/2” by 2 1/2” by 1” bars, you will need to adjust these measurements. The longer side will need 1/4” more (two 1/8” thick dividers), and the shorter side will need 3/8” more (three 1/8” dividers).

2. My sides are all 1" by 4" pine, which is actually 3/4" by 3 1/2" when you measure it. You will need two pieces 10 13/16" long, and two pieces 11 5/16" long. One of each length screwed together makes an "L" shaped side. The longer piece butts onto the shorter, so the deck screws go in from the shorter piece side. We did pre-drill these with a 1/16" drill bit to avoid splitting the wood. Keith uses nails for the permanent two corners, but we felt 2" deck screws were better. It is best to cut all the pieces first, then fit it together on a good flat surface when ready to start drilling and screwing.

3. The two “L” shaped pieces fasten together with one hanger bolt and wing nut at each opening corner. The bolt is called a 10-24 X 2 "hanger bolt". It has wood screw threads on one end, and machine screw threads (10-24) on the other.

a. You drill a pilot hole into the wood on the left (first picture) with a 9/64" bit, as straight as you can.

b. You take two wing nuts with a 10-24 thread and screw them onto the machine screw end, facing each other so you can lock then together against each other. This is just to give you something to hold on to while you screw the wood screw end into the hole you just drilled.

c. Once it is in to the top of the wood screw threads, stop and take off the two wing nuts.

d. Next you drill a receiving hole in the right piece of wood with about a 1/4" drill bit. This give you plenty of slop to squeeze and pull the sides tight around the base, and secure with a washer and wing nut when you assemble the mold.

4. Repeat these steps 1 through 4 for the opposite open corner.

5. The base is held in by pressure from the sides when assembled. However, we wanted to make sure the base did not fall out onto the floor with a fresh batch of soap in it. So we also drilled one 3/16" hole in the center of one side into the base (about an 1 1/2” deep), and the same to the opposite side to hold two 3/16" by 2” long dowel pins. With a dowel pin on two sides into the base, it will not fall out on you. It is best to drill this part when fully assembled, to make sure the dowel pin holes line up exactly.

6.The lid will need to be 11 9/16" by 12 1/16", the overall outside dimensions of the mold. We used two more hanger bolts and wing nuts on opposite corners (looking down onto the mold) to fasten the lid down. However, a bungee cord or masking tape might work just as well for that.

7.Once the wood mold is done and all together (except the lid of course), we started measuring, cutting, and gluing in the 1/32" thick Lexan lining.

8.We cut them all 2 1/2" high. Two of them will be 10" minus 1/32" or 9 31/32 long, and two are 10 1/2" minus 1/32" or 10 15/32" long. The 1/32" shortness accounts for the need of the thickness of one piece butting up against another.

9.The Lexan sheet has a thin plastic film covering on both sides. You can draw all your cut lines right onto this film with a thin tipped permanent marker. Then make sure you peal this off from both sides after cutting, and before you start gluing and assembling.

10. We glued the 1/32" sides and bottom pieces to the wood using a thin coat of Liquid Nails. We thought about Gorilla Glue, but feared it would expand too much.

For the dividers

1.You will need three pieces 2 1/2" by 10 1/2" of the 1/8" think Lexan, and two pieces 2 1/2" by 10".

2.You need three 1/8" slots cut into the 10" pieces at every 2 1/2" mark. Make the slots 1 5/16" long.

3.You will need to cut two 1/8" slots into the 10 1/2" pieces at every 3 1/2" mark, also 1 5/16" long.

4.We have a router table which helps. We just measured and drew the slot lines on the film coating, and then just eyeballed the lines on my router table and went for it. You do have to be careful when starting the blade into the slot, because the router bit will tend to want to pull the Lexan in the direction of the rotation. You also have to clean all the cuts when done with a fine file or razor knife to make sure they are reasonably smooth.

5.Then all the pieces slide (tightly) into each other to form the 3 by 4 egg carton grid. We were going to glue the divider pieces together, but they fit tight enough that it wasn't necessary.

Slab Mold Pieces

Slab Mold With Dividers In

Foam Liner To Fix Sticky Base Problem

We spray, pour in soap and colors, do our swirls, push in dividers. This has the added feature of pulling colors down into the edges of each bar.

A finished sample of Vanilla Cherry, with very nice smooth edges and color pulled down by dividers.
Thanks for the post. The instructions are very clear and your soap is lovely! :)

That is a fantastic mold and the soap is just beautiful . Thank you for sharing it with us .

What a great (and exhaustive) idea!! :D I wanna get some of those dividers too. Hubby makes my molds, but not having to cut my slab molds sounds awesome. Thanks for posting this! Pretty soap too!

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