Stack vs. cube: Making the optimum choice

Cube molds are suitable for multicomponent or multimaterial molding in which two components (or two materials) are molded simultaneously. They are also suitable for the inclusion of any number of secondary operations such as label placement (IML), clear-coating, overmolding and/or insert-molded components. A cube mold can also provide the advantage of molding products with a thick wall section. “The key to the success and cost effectiveness of a rotating cube mold is whether these operations can be done in-cycle,” explained Seidelman.

However, without a thorough evaluation of all facets of the manufacturing process, choosing a cube mold may mean sacrificing cycle time and productivity. The overall cycle time required for cube molds should be the first consideration, Seidelman explained. “Can the other operations (i.e. an in-mold label placement, an assembly operation, a clear coat process) be done within that initial cycle time or will you have to extend the overall cycle to accommodate additional operations? Unless you have a molding cycle that is long enough to allow the additional operations within that cycle, you may be losing productivity.”
   
Determining whether to build a rotating cube mold and perform secondary operations in cycle or to build a multi-face stack mold – and perform the required secondary operations such as label placement and assembly, downstream – requires evaluation of the related costs of each type of mold and production method.

While it may be very cost-effective to build a cube mold and include secondary operations as a part of the molding cycle, you will need to consider issues such as part handling: will you need a robot on the mold and a conveyor to take the parts from the mold to the next step in the process such as placing a label onto the part – which also may require a robot if you need greater accuracy than manual placement may achieve – or assembling two parts manually? Additional handling of parts is an opportunity for parts to become damaged. To minimize part handling, improve productivity and quality, robotics may be required, which adds to the cost of the manufactured parts.

That is why it is critical to determine up-front whether or not you will gain greater manufacturing cost savings using a fast-cycling, highly efficient single-face multicavity mold, a multi-face stack mold, or whether greater cost savings can be achieved by using a cube mold.

Some OEMs are beginning to rethink their decision to use cube molds. They are finding improved economies by changing their manufacturing back to double-face stack molds, thereby increasing productivity and throughput, and performing any secondary operations away from the press.

“Obviously, a cube mold isn’t suitable in all applications,” cautioned Seidelman, “so a careful evaluation and comparison of all costs associated with the entire the manufacturing cell must be done. Do not limit your decision to just the cost of the mold. Be sure to include costs related to the specialty molding machine that will be required, equipment to complete the secondary operations, robotics, productivity, scrap (increase/reduction), and other considerations that will impact the cost of producing parts in a high-volume production environment. This will help you determine the optimum type of mold to purchase and realize an optimum return on investment.”