This article will go over how to make a 3d molded backpack frame sheet from Kydex (a thermoplastic). This project is a little more involved than some other types of DIY gear projects, but with some basic tools and materials this is something you should be able to do at home.

A frame sheet is a piece of foam or plastic that is inserted into a backpack to give the pack structure and help transfer the weight of the pack to the hip belt. A molded frame sheet has several advantages over a flat plastic frame sheet. It is more rigid, which means it can handle heavier loads. It is also more resistant to “barrelling” or bulging into your back, which can happen with frameless packs or non-molded frame sheets. It can also be shaped to fit your back for more comfort.

A molded frame sheet weighs the same as a flat frame sheet made from the same material, and yet you gain the advantages listed above.

Step 1

The first step is to determine what curve you want for the frame sheet. There are a couple of ways to do this. If you have a pack with aluminum stays that fits you well and is comfortable then the easiest way is to remove the stay from the pack and trace it onto a large sheet of paper.

The curve should be about 1” longer than the back panel of the pack you are making the frame sheet for. Be sure to keep track of which end is the bottom. The side of the curve that goes against your back should face up.

Step 2

The next step is to build the mold that will be used for forming the kydex. The mold is made from a series of ribs cut to the curve that you have determined, and covered in a flexible skin. A 3/8” thick material such as wiggle wood or Kerfcore is recommended for this.

To determine the shape of ribs, draw a second curve that is offset below the original curve by the thickness of the skin material. Draw a straight line about 1” below the lowest part of the curve. Connect the ends. This creates the shape of the ribs.

You will also need a top bar. The top bar is used to help mold a channel down the center of the frame sheet. This channel contributes to the rigidity of the frame sheet.

To determine the shape of the top bar draw a third curve that is offset .125” above the curve of the rib. Draw a straight line about 1” above the highest point of that curve. Connect the ends. This creates the shape of the top bar.

Step 3

The next step is to cut out the parts. The ribs should be cut out of .75” thick material. The top bar should be cut from 1/2” thick material. Most any wood material such as plywood, MDF, or particle board will work. First cut out the rib and top bar drawings and use them to transfer the shape onto the wood. You will need to cut out 5 ribs, and 1 top bar using a jigsaw or bandsaw. Once you have all 5 ribs and 1 top bar cut out you will want to use a sander to sand the curves so they match the original pattern. It can be helpful to clamp the 5 ribs together and sand them all at once so that they are all the same shape.

After you have all 5 ribs cut and sanded you will need to cut the 2 end pieces that link the ribs together. These parts should be the same height as the end of the ribs and about 2 inches longer than the width of the frame sheet for. Generally 14 -15 inches should be plenty.

Step 4

Now you are ready to assemble the mold. Glue and nail or screw the 2 end pieces to the ends of 2 of the ribs forming a rectangle. Nail a 3rd rib centered in rectangle and attach a 4th and 5th rib to either side of the center one. If using screws it is a good idea to drill pilot holes to avoid splitting the ribs.

Once the frame has been assembled, you are ready to add the skin. The size is determined by measuring from the outside of the frame to the outside edge of the center rib. This is the width of the skin. The length is determined by measuring along the curve of the rib. You will need to cut 2 parts of the skin material this size.

Attach the skins to the frame with glue and brad nails. The skin should be flush to the sides of the frame. The goal is to leave a .75” wide gap between the 2 skins down the center of the mold. This creates a channel that helps to add rigidity to the frame sheet. Start attaching the skin at one end and work your way to the other making sure the skin follows the curve of the ribs. Once skin is attached, use sandpaper to round over the inside edges of the skin.

Step 5

The next step is to cut the kydex into the shape of the frame sheet. kydex is a thermo formable plastic that is used to make sheaths for knives and holsters for handguns. It can be found at leather working supply stores. Online, it can often be found on Amazon. I recommend using the .08” thickness. Other thicknesses may work but I have not tried them. kydex can be cut with a bandsaw, or jigsaw, but I have found I get the best results scoring it with a utility knife and snapping it. Just be sure to get a good deep score, before attempting to snap it.

Determine the shape of the frame sheet based on the backpack it will be used with. Often backpacks will have a sleeve that retains a foam pad. This pad could be removed and used as a guide for sizing the frame sheet. I prefer the frame sheet to be the full width of the pack at the top and taper as in goes down towards the hipbelt. If you are unsure of what shape to cut the kydex cut a sample from cardboard and test how it fits in the backpack. If you are making a pack from a Stitchback pattern , the pattern will include a full size template for the frame sheet.

You may want to drill holes in the frame sheet for additional weights savings. A hole saw works well for this. Avoid drilling the holes down the center of the frame sheet as they could interfere with the channel that will be molded there.

After the shape has been cut out, use sandpaper to remove any burrs and sharp corners and soften the edges. You don’t want any sharp edges that could act as abrasion points once its inside the backpack.

Step 6

Finally you are ready to mold the frame sheet. The kydex will need to be heated until it is soft and flexible. People who work with small pieces of kydex for knife sheaths and the like, often use a toaster oven for heating the plastic. Obviously a frame sheet is too large to heat in a toaster oven, and although it could be heated in your kitchen oven, the fumes that are produced when kydex is heated are quite toxic so this is not advised. I have found that I can heat the kydex with a heat lamp and a heat gun.

I heat the kydex while it is sitting on the mold. I have found that this takes some of the guess work out of determining when the kydex is hot enough, because as soon as it reaches the point where it is flexible it will begin to sag down into the mold. Of course this requires some caution as you are using heat around a wood mold, so use common sense and stop if the wood starts to smoke (I’ve never had this happen). It is a good idea to do this in an area with proper ventilation and use respiratory protection.

Here is the process I have found to work well for heating and forming the kydex.

1- Lay the kydex on the mold. Be sure it is oriented correctly with the bottom of the kydex at the bottom of the mold.

2- Position a heat lamp 8-10” above the kydex. Use a heat gun to heat the edges and areas of the frame sheet that aren’t heated by the lamp. Don’t let the heat gun stay in any spot for too long. If a shiny spot starts to develop on the kydex that means it has gotten too hot. Reposition the lamp so it is further away, or avoid that spot with the heat gun.

3- When the kydex gets hot enough, it will start to sag into the curves of the mold. You will be able to see that some areas are not hot enough as they won’t lie flat against the mold. Heat these areas with the gun or by repositioning the light until the kydex follows the curves of the mold. You can use a gloved hand or a block of wood to help press the kydex against the mold.

4- Once the Kydex is laying flat agains the mold you are ready to form the center channel. Use the heat gun to heat the center of the frame sheet from top to bottom. You want this section to be quite flexible. You can test the flexibility by pressing it with your finger. You want the kydex to be able to press into the groove in the center of the mold. Once the kydex is hot enough, take the presser bar and making sure it is oriented correctly press it into the Kydex. Use clamps on either end to push the top bar into the groove in the mold. The clamps don’t need to be very tight, just enough to help coax the kydex down into the groove.

Clamping the top bar in place, forming the center channel.

5- Clamping the top bar in place might cause some areas of the frame sheet to lift off of the mold. This can especially be the case right next to the top bar. Fortunately kydex is pretty forgiving and these areas can be reheated and formed. At this point I will generally turn off the heat lamp and move it out of the way.

Leaving the top bar clamped in place I will heat any trouble areas with the heat gun and press them back flat against the mold. It can be helpful to use a block of wood to press the kydex down and hold it in place until it has cooled enough that it doesn’t spring back. Once all of the trouble areas have been taken care of, leave the frame sheet to cool with the presser bar still clamped in place until it is completely cool to the touch.

6- Once it has cooled remove the clamps and take the frame sheet off the mold. You now have a custom molded frame sheet that can be inserted into your backpack.