The Steel Supply Company provides shims to a variety of construction trades. Solid or Slotted, Plastic or Steel. It should be noted that by solid we mean non-slotted, or what is sometimes called "Plate Shim." The plastic version of these plate shims is still solid throughout. This is differentiated from some plastic shims on the market that are actually hollow in the core. Shims that are not solid cannot deliver the compressive strength required for most construction projects. The plastic shims shown in Fig. 1 are all solid throughout. Note the slotted plastic shim on the left of the image. It's actual dimensions are 4" x 6" x 1/2" thick, solid throughout, and is compression rated to support 90,000 lbs. before permanent deflection occurs. Based on its surface area, that is approximately 4,500 psi.
Steel Manufacturing Blog: Keeping it Steel
When shims are required to be angled, wedge shaped, or anything other than flat squares and rectangles, specific steps have to be taken to offset the difficulty these angles create.
For the construction trade, plastic shims are an essential tool but care must be taken to assure they are rugged enough for the intended use. Before even considering the compressive strength of a plastic shim you must first ensure that is it should be solid with uniform plastic throughout. Lower cost hollow plastic shims are manufactured but do not deliver the strength or durability required for most construction projects.