What are the differences in chemical resistance between molded and extruded plastics?
The chemical resistance of plastics is significantly influenced by their manufacturing processes, particularly between molded and extruded products. Molded plastics, created through processes like injection molding, typically exhibit more uniform molecular orientation and density due to high-pressure formation in closed molds. This generally results in better overall chemical resistance as the compact structure provides fewer pathways for chemical penetration. Extruded plastics, formed by pushing material through dies, often develop molecular alignment primarily in the extrusion direction. This anisotropic structure can create variations in chemical resistance, with potentially weaker resistance perpendicular to the extrusion direction where molecular bonds are less dense.
The thermal history during manufacturing also plays a crucial role. Molded plastics experience rapid cooling under pressure, which can create a more crystalline or organized structure in semi-crystalline polymers, enhancing chemical resistance. Extruded plastics typically undergo slower cooling rates, potentially resulting in different crystalline formations that might affect their chemical stability. Additionally, molded products usually have lower internal stresses when properly processed, reducing susceptibility to stress cracking when exposed to chemicals. Extruded materials may retain more internal stress along the extrusion direction, creating potential failure points in chemical environments.
Surface characteristics differ substantially between the two processes. Molded plastics often have smoother, more consistent surfaces that provide better initial defense against chemical attack. Extruded surfaces may show more microscopic variations that could potentially allow earlier chemical penetration. Manufacturers must consider these differences when selecting appropriate manufacturing methods for applications requiring specific chemical resistance properties.
