This dissertation presents novel mechanical designs and advances in both polymer layer multiplication and extensional rheology. In Part I, advances in polymer layer multiplication include the investigation of typical thermoplastic layer multiplication extrusion dies and the redesign of such by means of experimental and computational investigation. Results show a decrease in flow instabilities including viscous encapsulation and elastic instabilities. Similarly, an innovative layer multiplication system for highly filled and elastic rubber compounds is presented. Both of these novel systems for thermoplastic and thermosetting polymer systems are validated and understood by numerical methods using ANSYS Polyflow and CFD-Post. These advances in multilayer coextrusion allow for layering of new families of rheologically complex and as well mismatched materials, thus widening the processing capabilities at the Center for Layered Polymeric Systems. In Part II, advances in extensional rheology include a newly developed extensional rheometer for achieving high Henkystrains. The Meissner Extensional Rheometer Accessory or MERA is limited in strain only by sample rupture. Experimental validation is performed using two materials, a styrene-butadiene rubber (SBR) and a linear polystyrene (PS); the results are compared with those obtained using the well-known SER. Due to its design, it was possible to achieve homogeneous extensional flow up to unprecedented real Hencky strains in excess of 8. The second half of Part II covers a rheological understanding behind the mechanism of sharkskin, a melt fracture instability during extrusion. Solution-styrene-butadiene-styrene blended with various polybutadienes is studied. Experimental extrusion results show a behavior difference depending on the molecular architecture of the latter, polybutadiene. Extensional rheological techniques are used to investigate the behavior. Results show a higher sensitivity to stress relaxation after a step-strain rather than typical stress growth coefficients in extension.