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.