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What is the Difference Between Stack Molds vs. Tandem Molds?

When tandem molds entered the scene nearly two decades ago, they provided a new twist on stack molds. They are similar but different. So, what are the differences between stack molds and tandem molds? We provide an overview here.

Stack molds have been around since the 1960s. They employ two or more molds with the same dimensions and superficial area stacked next to each other. So, without changing the molding press or tonnage, stack molds boost productivity and reduce part costs by increasing the number of parts without increasing in press size or clamp force.

Tandem molds are a variation of a stack mold, having alternately filled layers. While one layer is filling and molding parts in its cavity (or cavities), the other layer is ejecting its part(s). This design allows for different parts and designs to be run in each layer, using different shot sizes or plastic volume in each half. Tandem molds are more often used with family parts for assembly and longer cycle time components.

Key Differences

The key difference between stack and tandem molds lies in the material injection process. Unlike stack molding, where injection occurs concurrently on multiple molding surfaces, the melt in tandem molds happens in alternating surfaces at once. While stack molds increase productivity by doubling, tripling, or quadrupling part production, tandem molding optimizes processing costs by using the cooling time of one molding surface to injection sequence of the other molding surface.

Other differences in the two molds include:

  • Stack molding technology is purely mechanical racks and pinions.
  • Tandem molds are electronically controlled with magnetic locks.
  • Stack molds open via dual parting lines and eject all molded parts in one single cycle.
  • Tandem molds eject only molded parts in one parting line at a time.

Key Benefits

Stack molds deliver these advantages:

  • Reduce the need for multiple molds and multiple presses.
  • Enable all parts in all layers to be molded in one shot.
  • Allow molded parts to have different geometries, sizes, and fill rates.
  • Increase mold throughput.
  • Speed up molding cycle times.
  • Deliver immediate and exponential cost savings.

Tandem molds deliver these benefits:

  • Produce more parts and volume with small shot sizes.
  • Allow family components to be produced simultaneously from a single mold.
  • Support different fill rates.
  • Offer a wide variety of solutions via thermal gating.
  • Are compatible with many injection molding machines.
  • Help solve width and length limitations of the injection molding press.
  • Potentially double the output of the available injection molding equipment and labor.

Use Case Examples

Stack Mold Example:

A customer wanted to increase throughput based on marketplace demand for a plastic roof ridge vent. Rather than running two molds on different presses, we engineered a stack tool that doubled molding capacity on a single press.

Tandem Mold Example:

A customer’s part was taking too long to cool, thus wasting valuable press time and money. We engineered a tandem mold to run two parts in sequence from the same press and from the same mold.

A1 Tooling’s strengths are engineering and manufacturing of complex, multi-shot, multi-action, stack and tandem molds. Bring us your project and we’ll help evaluate whether a stack or tandem mold is best for you.


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