All "Scrap" monsters gain 200 ATK and DEF. When a face-up "Scrap" monster(s) on the field is destroyed by a card effect and sent to the Graveyard (except during the Damage Step): You can Special Summon 1 "Scrap" monster from your Deck. You can only use this effect of "Scrap Factory" once per turn.
Relies on other card effects to destroy your Scrap monsters.
Most Scrap monsters have an effect that lets you gain some advantage when they are destroyed via card effect, and many of them are able to destroy themselves or other monsters you control. So you can use Scrap Factory along with these effects to gain additional advantage.