  Reaching high densities is a key step towards cold-collision experiments with
polyatomic molecules. We use a cryofuge to load up to 2$\times10^7$ CH$_3$F
molecules into a box-like electric trap, achieving densities up to
10$^7$/cm$^3$ at temperatures around 350 mK where the elastic dipolar
cross-section exceeds 7$\times$10$^{-12}$cm$^2$. We measure inelastic rate
constants below 4$\times$10$^{-8}$cm$^3$/s and control these by tuning a
homogeneous electric field that covers a large fraction of the trap volume.
Comparison to ab-initio calculations gives excellent agreement with dipolar
relaxation. Our techniques and findings are generic and immediately relevant
for other cold-molecule collision experiments.
