A method of reducing surface imperfections in an injection molded part , comprising: injecting a first thermoplastic polymer into a first cavity ; characterized in that forming a ribbed structure comprising ribs in the first cavity, wherein each rib in the ribbed structure includes a microstructure on an outer portion of a rib ; and overmolding a layer formed in a second cavity by injecting a second thermoplastic polymer into a second cavity at an interface between the layer and the ribbed structure, wherein the interface is disposed on an outer portion of a rib including the microstructure , wherein the microstructure of the ribbed structure remains unfilled with the first thermoplastic polymer or the second thermoplastic polymer ; or injecting a first thermoplastic polymer into a first cavity ; forming a layer in the first cavity , wherein the layer includes a microstructure ; and overmolding a ribbed structure comprising ribs formed in a second cavity by injecting a second thermoplastic polymer into the second cavity , wherein the overmolding occurs at an interface between the layer and the ribbed structure , wherein the interface is disposed on an outer portion of the layer including the microstructure , wherein the microstructure of the layer remains unfilled with the first thermoplastic polymer or the second thermoplastic polymer ; forming a microchannel at the interface between the layer and the microstructure or at the interface between the ribbed structure and microstructure ; injecting a gas into the microchannel ; and enlarging the microchannel with the gas forming an open channel to reduce the amount of shrinkage experienced by the part . injecting a first thermoplastic polymer into a first cavity ; characterized in that forming a ribbed structure comprising ribs in the first cavity, wherein each rib in the ribbed structure includes a microstructure on an outer portion of a rib ; and overmolding a layer formed in a second cavity by injecting a second thermoplastic polymer into a second cavity at an interface between the layer and the ribbed structure, wherein the interface is disposed on an outer portion of a rib including the microstructure , wherein the microstructure of the ribbed structure remains unfilled with the first thermoplastic polymer or the second thermoplastic polymer ; or injecting a first thermoplastic polymer into a first cavity ; forming a layer in the first cavity , wherein the layer includes a microstructure ; and overmolding a ribbed structure comprising ribs formed in a second cavity by injecting a second thermoplastic polymer into the second cavity , wherein the overmolding occurs at an interface between the layer and the ribbed structure , wherein the interface is disposed on an outer portion of the layer including the microstructure , wherein the microstructure of the layer remains unfilled with the first thermoplastic polymer or the second thermoplastic polymer ; forming a microchannel at the interface between the layer and the microstructure or at the interface between the ribbed structure and microstructure ; injecting a gas into the microchannel ; and enlarging the microchannel with the gas forming an open channel to reduce the amount of shrinkage experienced by the part . <EOS>