HG models were introduced in 1990. The original kits featured full snap-fit assembly, an articulated internal frame (for the first two kits, which provides better range of motion and is more poseable), and utilized the molding technique known as System Injection, wherein multiple colors would be cast on the same part. In 1999, the High Grade Universal Century Line was introduced, which collected mobile suits from the Universal Century Timeline. In 2010, the line was expanded to include mobile suits from Future Century, After Colony, After War, Correct Century, and Cosmic Era, and Gundam series that did not fit in those timeline (Like Gundam AGE or Iron Blooded Orphans) received their own HG lines. In 2015, HG Revive, a subline within the HGUC line was introduced, which gave older HGUC kits redesigns that adhered to modern HG standards in terms of detail and articulation. The High Grade line is not exclusive to Gundam, as other mecha series, such as Mazinger, Kyoukai Senki, and Evangelion receiving HG kits of their own. HG Amplified IMGN was a subline introduced in 2022, which redesigned smaller robots with more humanoid proportions. A vast majority of HGs use polycaps, but Bandai has started to abandon the technology with the release of Witch From Mercury kits for better stability.