The volume of an injection-molded part is 15.2 cm3. The volume of the sprue and runner is 8.5 cm3. The starting plastic is polypropylene; its specific gravity is listed in Table 8.1. The price of the plastic = $1.98/kg. A two-plate mold with one cavity is used. Determine the material price per piece if the sprue and runner (a) can be ground and mixed with the starting plastic so that material utilization is 100% and (b) cannot be reground because virgin plastic must be used.
Solve the previous problem, except that the two-plate mold has two cavities instead of one. The volume of the sprue and runner is increased to 9.6 cm3 to accommodate two mold cavities. Determine the material price per piece if the sprue and runner (c) can be ground and mixed with the starting plastic so that material utilization is 100% and (d) cannot be reground because virgin plastic must be used. (e) How would the use of a three-plate mold affect the answers in (c) and (d)? (f) How would the use of a hot-runner mold affect the answers in (c) and (d)?