Plastics Thermoforming Tool Design: Plug Vs Cavity Molds

One of the most fundamental decisions when designing a mold for the plastics thermoforming process is whether to use a plug (male) mold or a cavity (female) mold design. This critical decision is often brushed over in the literature yet has the greatest single impact on part wall thickness and wall thickness variation. Other considerations when deciding on this fundamental design concept include location of thinnest cross section, material shrink rates, a defect known as "webbing", and draw ratio determination. Given access to even the most basic thermoforming equipment, students can design a wide variety of molds to test and evaluate the difference between plug and cavity mold designs for similar part geometry. This paper will discuss such an activity. Specific Learning Objectives include: 1) The student will understand the relationship between mold design, material deformation and part wall thickness. 2) The student will understand the effect of materials shrink rates and part release from the mold. 3) The student will be able to explain why parts produced with plug molds have less wall thickness variation than those produced with a cavity mold. 4) The student will be able to calculate linear draw ratios on thermoformed parts and explain the relationship of these ratios to wall thickness variation.