It's possible to dab without a carb cap, but experts wouldn't recommend it. The purpose of a carb cap is to trap and secure the vapors and prevent them from melting too rapidly. More importantly, it's the necessary tool that prevents the loss of vapors in the dabbing process. The vapors will drift off into the air if a dab nail isn't secure after the introduction of the concentrate. Using a carb cap is also critical in helping airflow and controlling the heat levels without burning the concentrate. Vaporizing at lower temperatures would be practically impossible for devoted dabbers if not for the carb cap.