This feature becomes available after you have beaten a couple of levels in Chapter 2. In Survival mode loot crates are awarded by progression and its contents will depend on how deep into the mode you managed to go before you were killed. You will receive more weapons parts the deeper you go. If you die in Survival mode you have the chance to revive once by spending two tickets or start over again from the beginning for one ticket. Take advantage of Survival mode whenever you have tickets so you can earn weapon parts.