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Choosing a Cutter

From The HSMG Journal January 2006

By Linda Stevens of Upland Soap

Sooner or later, every soapmaker who wants to start making a profit has to get serious about cutting all those bars of soap. Cutting equipment can run the gamut from simple and cheap, to elaborate and expensive. Most soapmakers start out with the cheapest and simplest possible equipment — a kitchen knife or something similar. Unfortunately, problems immediately arise for all those who can’t cut anything straight with a knife. Cutting by knife takes a lot of time, makes ugly bars if not perfectly straight, and creates a lot of waste.

However, a new entrepeneur with a start-up soapmaking enterprise probably doesn’t have the capital to run out and buy thousands of dollars worth of automated cutting equipment. For most, it’s a gradual increase in the quality and quantity of cutting expertise and equipment. I get a lot of inquiries from soapmakers who are ready to start spending money on professional cutting equipment and usually they start out on the wrong foot when asking about cutters and molds to fit their cutters. “My finished bar is 4 ounces, what size cutter do I need and what mold will fit my bar size?”

Unfortunately, it’s impossible to figure out what they need from just that little bit of information. Since the mold and cutter deal with WET soap and the finished DRY soap ends up with the desired size and weight, you have to work backwards.

What size and weight is the finished DRY bar? Since freshly cut bars will shrink as they dry, how much WET soap makes that DRY bar? What size mold and cutter are needed to mold and cut that WET bar so it will end up with the desired DRY bar? And how do you check it?

Standardized Recipes

When you make soap and then let it dry, it will shrink. The shrinkage is caused by the evaporation of water from the soap. How much water you start with in your water-to-lye ratio will determine the weight and size difference between a WET bar (just made) and a DRY bar (water evaporated out). How much smaller the dry bar gets is called the Shrinkage Rate. [For the purposes of the examples in this article, a water-to-lye ratio of 2.1 was used (the lye weight x 2.1 = the water weight).]

If you standardize your recipes so every recipe has the same water-to-lye ratio, it will ultimately make your job much easier. Any recipe that has the same water-to-lye rate (and subsequently the same Shrinkage Rate) will work with your chosen mold and cutter system. It’s important to note that if you make a recipe with a different shrinkage rate you could conceivably end up with a slightly different size finished bar. That might not be a problem if you a dealing in small quantities, but if you want all your soaps to be exactly the same size (and fit the boxes you purchased or want to purchase), then standardizing is the way to go.

Shrinkage Rates

First thing is to determine the anticipated Shrinkage Rate for your BASIC soap recipe BY WEIGHT. (For this test, don’t use fluffy fillers like chopped oatmeal in your test recipe ). A bit of math is all that is required once you have the right preliminary information. The Shrinkage Rate by Weight is a very handy piece of info for later on.

Step-by-step, here’s what you do:

  1. Determine the total weight of all your oils, additives, EO’s/FO’s and lye (let’s say the total is 95.98 ounces). This is your dry soap weight (remember, it’s only the WATER that evaporates).
  2. Add the water weight. (let’s say it’s 25.70 ounces of water). Total wet soap weight is 121.68 ounces.
  3. Divide the dry soap weight by the wet soap weight: 95.98 ounces divided by 121.68 ounces = .79 (or 79%). That means every batch and every bar is about 79% soap and unsaponifiables and 21% water.
    The amount of water that will evaporate out is your Shrinkage Rate BY WEIGHT. In this case, the shrinkage rate is 21% if our bar is 100% dry and all our weighing and measuring is 100% accurate.

If you weigh a cut bar of wet soap and compare it to the weight of a completely dry bar, the difference should just about equal your calculated Shrinkage Rate. But wait, you need more data, so next you’ll have to figure out what the shrinkage rate is in dimensions.

Bar Dimensions

Your bar is going to shrink in weight and ALSO in size. You must document the Shrinkage Rate (by size) in order to know exactly what size bar you will end up with.

Get one of your best cut, VERY dry bars that is the shape and size you want for your final bar. If you have boxes, make sure it fits in the box you have, with not too much wiggle room.

Beware! If you box up your bars before they are completely dry, you can get a false reading. If you have any question about how dry your bars really are, put a bar you “think” ‘is dry in your box, then set if off to the side for about a month and then weigh and measure it again. You might be surprised at how much more shrinkage there is.

Once you have your completely dry, perfectly sized DRY bar, you need to measure it to get the cubic inches. Multiply the height x width x length (in inches) to get the cubic inches. For our example, let’s suppose that the dimensions are 3” x 2” x 1.125” = 6.75 cubic inches.

From your next batch of soap, cut a FRESH wet bar the same dimensions you originally cut the dry bar that you like. [Note: Cover this batch with plastic (even if you don’t normally do so) because for accuracy you want to keep all the water weight in the soap while it’s sitting in the mold.] After you cut your fresh bar, weigh it. The weight should about equal the finished dry bar wight divided by the percentage of soap in the batch. In our case, that would be a 4 oz bar, divided by 79% soap = 5.06 oz WET bar.

Now measure it carefully to determine the WET dimensions. As with the dry bar, you need the cubic inches (height x width x length). For our example, let’s suppose that the dimensions are 3.125” x 2.125” x 1.25” = 8.30 cubic inches.

It’s important to work quickly because you don’t want the bar to dry before you have finished weighing and measuring it. Then set it off to dry. Once your sample bar has fully dried (check the weight to be sure), measure it again. It should be the same dimensions as your previously measured dry bar. Compare the size to your original dry sample, and you will know the amount of shrinkage by dimension.

Calculating the Shrinkage Rate (by Size)

Divide the DRY size (6.75 cubic inches) by the WET size (8.30 cubic inches) and you know how much of your bar is left after it shrunk down (in this example, 81%). Since you started with 100%, you know your bar has shrunk 19% (100% - 81%). Notice how close Shrinkage Rate (by Size) is to the percentage of water in your base recipe.

Calculating the Average Shrinkage Rate

Since few of us can do measurement that are 100% accurate, we can overcome any small errors by averaging. If you average the total of water (21%) and shrinkage (19%), you get 20%. That means that on average there is 20% water and 80% soap in your wet bar, and it will shrink approximately 20% in dimension.

This AVERAGE shrinkage rate can be used to calculate all your remaining data.

Cubic Inches of 1 oz and 1 lb of WET soap

In dealing with your soap, you need to know much room it takes up. You already know the cubic inches of one bar of WET soap, and know how much that bar weighs.

To find out how many cubic inches one ounce of wet soap is, just divide the cubic inches of your bar by the number of ounces in the bar, then multiply by 16.

NOTE: It’s easier to measure a larger block of soap. To double-check the cubic inches of 1 lb of soap, pour exactly 1 pound of your WET soap into a 1 quart milk carton (cut the top off the carton), and cover it with plastic wrap to keep any water from evaporating. When the soap is set up, carefully unmold it and measure the dimensions. Multiply the height x length x width and you’ll get the cubic inches of one pound of your wet soap.

Having done our homework for our example, we know that our wet soap is about 20% water and 80% soap, and requires about 1.63 cubic inches of space per ounce of wet soap. That’s 26.08 cubic inches per pound of wet soap. (If you have done a 1 pound test which is more accurate, use the figures from your one pound test.) If you do all your measuring and math very carefully, you will know how much of YOUR wet soap any mold will hold as long as you know the cubic inches of the mold. This is good information to double check your mold size figures later on.

Remember the numbers used here are examples. Your shrinkage will be higher if you use more water, or lower if you use less water. It’s important to do all your measuring and calculating with YOUR basic recipe.

Now that you know the AVERAGE Shrinkage Rate for your recipe, you can easily find out how much WET soap you need, to end up with your desired DRY bar. A little more math: The desired DRY bar finished weight divided by AVERAGE percentage of soap in a WET bar equals the necessary WET weight.

What??? Here’s an example: Say you want a finished DRY bar of 4 oz. Using the example recipe (above), divide 4 oz by the percentage of soap (80% or .80) in a WET recipe: 4 ounces divided by .80 = 5 ounces. So you need 5.00 ounces of wet soap to end up with one fully dry bar that weighs 4 ounces. If pouring a mold for 20 soaps, you’ll need to pour at least 100 ounces of wet soap in that 20 bar mold. (You’ll need to add some more ounces if you’re shaving the top layer of ash or shaving wrinkled sides).

Want to know how dry your bar really is after sitting on the rack for a few weeks? Now that you know the wet weight and the eventual dry weight, just put it on the scale and you can easily see how much water is still left in the bar. Any weight over your dry ounce weight is water to evaporate.

Figuring the Mold Size and the Right Cutter

Now that you know the wet weight and wet dimensions of your perfect bar of soap, it’s just a matter of extrapolating the dimensions of one wet bar up to several bars to determine the size mold you ultimately need, and the right cutting equipment for your mold.

You want a mold that is big enough to be profitable. Labor costs money and YOUR labor ain’t free. However, a word of caution: Know what your physical lifting limit is. Your maximum SAFE weight-lift limits will ultimately determine the maximum amount of soap and number of bars you will get out of each mold.

Now it’s time to move on to deciding what type of mold and cutting system you want to buy. There are several types of mold and cutting systems to consider.

Flat slab mold and wire bar grid cutter.

A flat slab mold is shallow and large. A bar grid system cutting system cuts the slab into finished bars. It takes a bit of elbow grease to cut a slab of soap; the more wires the grid has, the harder you have to push to cut your slab into individual soaps. A grid cutter is limited in how long and wide the frame of the cutter can be before the wires are too long and start to bend, incompletely cutting the soaps in the middle.

The folks who make grid cutters will tell you the maximum length and width of the cutter wire frame to achieve good results every time. Also remember that your own strength is another limiting factor. Cutting a 12 bar grid with thin wires is easy, just push down. A 20+ bar grid requires thicker wires and is much harder to push down and it may need leverage enhancements built into the cutter by the manufacturer.

Whatever size grid cutter you look at, ask the manufacturer how many pounds of pressure you will have to manually exert to cut the number of bars/size of grid you are wanting to buy. If it’s too much for your strength level, choose something else. Some manufacturers make a manual grid cutting system that can be upgraded later to an electric, motor driven or pneumatic system. Always plan ahead when buying entry level cutters, in the event that you want to upgrade if business picks up.

If you decide to go with a bar grid cutting system, tell the cutter manufacturer the dimensions of your wet bar and they will make a grid cutter to suit your needs. They will also recommend the size mold to work perfectly with their cutter, if you don’t already have molds that will work. You don’t have to use their molds, but you will need to know what size they recommend.

Some molds require lining and then the cutter will need to have 4 extra wires to shave away the wrinkled sides of the slab, for nice even edges on each bar of soap. Some molds don’t require lining, which saves on wasted soap shaving and the cutter doesn’t need the extra wires.

If you are using embossed slab molds, then you have to use a grid cutter and you may want to send one of your grid molds, so the manufacturers has an exact replica of the soap to be cut. You can also check with the manufacturer of your embossed slabs to find out what cutting system(s) they recommend, and check on the web for price comparisons.

Log Molds and Log Cutter

The next option is to use log molds with a wire cutter similar to a bread loaf cutter, which cuts an entire log (or most of it) into individual soaps all at once. The most important information at this point is how THICK your wet bars must be. Most log cutter manufacturers will make a log cutter with wires exactly spaced to your specifications.

Log Cutter manufacturers have limits as to how long their cutter will be, so check to make sure you don’t get a cutter that is too short for your log, unless you are willing to cut a longer log into two shorter logs that fit in the cutter. Log cutters are very efficient, easy to use, and make soap cutting a whole lot easier with no mistakes. A log cutter with wires spaced at 1.25 inches apart will cut a 15 inch log into 12 equal bars, all at once. Very efficient compared to the single bar cutter we all used when we first started making soap!

Some manufacturers will make a log cutter to cut more than one log at once. Just remember, the problem of longer wires bending is a limitation, and the amount of strength required to push those wires down through a thicker amount of soap. Cutting two logs instead of one is a lot more efficient, if the cutter is still easy to use. Manual cutters are usually limited to one or two logs at once, and are much cheaper than screw-drive or electric/pneumatic cutters.

By the way, this is a point where all our math comes in handy! (See the Do The Math! section below.) We already know how much our little 4 ounce dry bar weighs when wet (5 ounces) and what size it is (8.30 cubic inches). We know that a 15 inch log that is 3.125 wide x 2.125 deep will hold 99.60 cubic inches of soap. Since our bars are 8.30 cubic inches, we know that mold will hold exactly 12 bars of our soap (99.6 divided by 8.30). Since its 15” long, when unmolded and cut 1.25” thick, it will cut 12 bars.

Now we know what size log mold we need, what size log cutter we need, and how much wet soap to make, in order to get we will end up with 12 each 4 ounce dry bars of exactly the right dimensions.... Whoa!

Block Mold with Log Splitter and Cutter

Maybe you want to get even more efficient and make blocks of soap, to be split into logs. There are several ways to address splitting a larger block into two or more logs. But beware of the weight factor when dreaming of how much labor you can save on a block cutting system.

Because you now know how much one of your bars weighs when WET, you can extrapolate that out to find out how much a freshly poured block of soap is going to weigh. One hundred 4 ounce dry bars, using our previous calculation of 5 ounces WET, will weigh 500 oz (or 31.25 pounds) PLUS the weight of the mold. If you aren’t OK with picking up that much weight, then don’t order a block mold and cutter system for such a big job.

But let’s say you’re OK to move around 20-30 pounds, and you’re ready to work with a block mold. First you need to know how long each log must be. Let’s use our example of a WET bar that is cut at 1.25 inches thick, and we want to cut 12 soaps from each log, so we need a 15 inch long log. Let’s say you want to make the same 4 ounce bars we have been discussing, which are 3.125 inches long and 2.125 inches wide WHEN WET, and you’d like to make 4 logs at once, totalling 48 finished bars.

If the logs are laid side by side, 4 logs would equal 8.5 (4 x 2.125) inches wide by 3.125 inches tall. Or, going the other way, 12.5 inches (4x3.125) wide by 2.125 inches tall. Pick one way or the other to lay out your logs, so that you can settle on a final width dimension for your 4-log block.

It’s a good idea at this point to check on the wet weight of 4 logs. A 4-log block with our chosen bar dimensions and wet weight will weigh 240 ounces, or 15 pounds (48 bars x 5ounces). Assuming that the mold weighs about 10 pounds, the mold when full will weigh about 25 pounds. If you’re OK with handling this amount of weight, then it’s on to the next step.

Once you have unmolded your 15 pound block of soap, you need a block-splitter to cut the block into 4 equal logs, so you can cut the logs into bars on your handy log cutter. A manual sliding block splitter (which uses your muscle power) can cut one, two, three, or four logs out of one block, just push the block through the wire(s). All you have to do is tell the block splitter manufacturer how many logs you want, the dimensions of each log, and the uncut block dimensions.

In our example, we are using a block mold 15 inches long x 8.5 inches wide x 3.125 inches tall to be cut into 4 equal logs, 15 inches long. It will cost more to make a block splitter to cut all four logs at once, because it requires more parts and labor. However, it will save a lot of time in your shop if you can cut the entire block all at once because it takes more of your labor (time is money!) to cut a single log at a time.

You have to decide how much you are willing to spend right now on your new block splitter, verses how much of your own labor you are going use or save, over the next few years. At this point we are talking about MANUAL block splitters, but there are also screw-drive, electric, and pneumatic block splitters available which are extremely efficient, and of course will cost more.

Mechanical Cutters and Splitters

A screw-drive splitter has a wire grid and pushes the wires through the soap block by use of a wheel or crank that turns a long screw moving the grid up or down, backwards or forwards, depending on the design. Manual screw-drive equipment is not as effortless as electric or pneumatic equipment, but you can still cut a large block into multiple logs with minimal physical effort. As long as you can move a large block of soap onto the cutter and turn the handle or crank to move the wires, you can cut several (12 or more) logs out of a larger block of soap. You will still need to use your log cutter to cut each log into bars.

If your production is at 4,000 or more bars a month and growing, you are definitely ready to look at electric and/or pneumatic cutters. They are the creme de la creme of soap cutters and they require only the energy to get your soap block or log onto the cutting plate; the rest is as easy as pushing a button. Pneumatic cutters use air compressors (hence pneumatic) instead of muscle power to do all the hard work. They are fast, efficient, and a dream to use.

The pneumatic cutters I have seen include built-in safety equipment to ensure that your fingers are well out of the way when the wires start moving down into the soap. When spending the money for a really efficient electric/pneumatic cutter, you must consider the amount of labor (not to mention wear and tear on your body) you are saving over a long period of time.

It’s cheaper to buy a grid cutter that cuts all your bars at once, but you have to purchase and pour more molds to cut 1000 bars of soap.

Alternatively, if you buy both a block splitter and a log cutter, it will cost more for the cutting equipment (2 separate cutters), but you need to buy fewer molds to make the same 1000 bars of soap. Electric and pneumatic cutters cost in the thousands, but will save hundreds of hours of labor over the course of a year. If you are trying to decide whether or not it’s cost effective to buy an expensive automatic cutter, just figure out how much it will cost to HIRE a soap cutter, by the hour, including payroll taxes and insurance. Then take that hourly labor figure and extrapolate it out for the next year or two. An automatic cutter should cut your labor in half, at least, to be cost effective.


Of course we all would like to run out and buy an effortless cutter, tomorrow! But, not having won the lottery lately, we have to move up in increments. Hopefully this article has helped you decide how to move up to bigger and better production.


Do the Math!

Do the Math 2

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