In-mold gas injection utilizes injection pins, which are strategically placed within the mold. They can be placed in the mold cavity or in the cold runner system. For multiple gas channels that are separated, multiple gas pins can be installed. The most common gas-assist injection method is to partially fill the mold to between 80 and 95% and then inject gas into the cavity. Melt prefills as low as 50 to 60% have been achieved. The disadvantage to this method is that sometimes a switchover or hesitation mark appears on the surface when the melt shot is stopped and the gas begins. In order to eliminate this surface appearance defect, the mold must be filled completely. The overflow process and the blow back process were created to allow the gas to displace the melt outside of the primary mold cavity after the mold is completely filled with the polymer melt. The overflow process utilizes a secondary cavity where a slug of material is allowed to accumulate. The blow back or counterflow technique pushes material back into the injection unit or into a secondary cavity, see figures below.