  We established a production method of a good millimeter-wave absorber by
using a 3D-printed mold. The mold has a periodic pyramid shape, and an
absorptive material is filled into the mold. This shape reduces the surface
reflection. The 3D-printed mold is made from a transparent material in the
millimeter-wave range. Therefore, unmolding is not necessary. A significant
benefit of this production method is easy prototyping with various shapes and
various absorptive materials. We produced a test model and used a two-component
epoxy encapsulant as the absorptive material. The test model achieved a low
reflectance: $\sim 1\%$ at 100 GHz. The absorber is sometimes maintained at a
low temperature condition for cases in which superconducting detectors are
used. Therefore, cryogenic performance is required in terms of a mechanical
strength for the thermal cycles, an adhesive strength, and a sufficient thermal
conductivity. We confirmed the test-model strength by immersing the model into
a liquid-nitrogen bath.

Instrumentation and Methods for Astrophysics;Applied Physics