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Injection molding was created to make parts faster and more accurate. This introduction changed manufacturing forever and allowed engineers and designers to innovate like never before (like 3D printing has done today). While single-cavity molds are used regularly for prototyping and even higher volume applications, multi-cavity molds were created to help meet increased production needs and reduce part costs. 

In the early 1990s, a new technology called ‘stack molding’ allowed plastics manufacturers to double production output while reducing product costs and keeping customers happy through quality and quantity – a rare trait in most businesses. This article aims to deliver some insight into just how this is possible.  

Typically, manufacturers favor stack molds for producing small to medium-sized parts, like bottle caps or syringe bottles (hint: we can do this with large parts now, too; see below) because they offer a simple design and substantial production volume capabilities. When manufacturing smaller parts like bottle caps or syringe bodies, stack molding machines are sufficient to produce two, four, six, etc. parts simultaneously.   

This is advantageous for purchasing, engineering, & supply chain departments because the ability to mold multiple items at once reduces the lead time & cost of the final ordered product. The molds will cost more because of the need to create multiple cavities, but the price per part decreases because the machine time is shared across more molded parts.   

On some occasions, this stacked capability also allows manufacturers to produce multi-component assemblies or a whole family of parts – either option being in the buyers’ favor. More often than not, though, stacked molds produce replicas of the same part.  

Evolution of molding, using pallets as an example

What are stack molds?

Note – See our FAQs for more information  

Injection stack molds are innovative tools used in plastic part manufacturing, stacked horizontally on a single injection molding machine. Each of these mold cavities or mold faces can produce an individual part or, if you have multiple cavities, the same number of parts as your cavitation is built to. The process begins with the simultaneous injection of plastic resin into all cavities at once. Meticulous designers create these molds with core and cavity sets that seamlessly align when stacked, ensuring the production of high-precision parts.    

Once the injection is complete and the plastic has cooled and solidified, the stack mold separates, facilitating the straightforward ejection of the completed parts. As needed, you can repeat this efficient process, with the mold closing again to produce additional parts in the next cycle. 

Our business is your plastic

But – what if you require large plastic parts (i.e., crates, vaults, large housings, or pallets) and want multiples produced? – are injection stack molds for you?

The answer is a resounding YES! We have machines big enough to mold plastic parts – like pallets – two (or more) at a time. Instead of one cavity (one part per cycle), we get two or more per molding cycle.

Stack molding works with small or large parts, if large enough machines are available. This simultaneously delivering both shortened lead times and more affordable, cost-effective parts. You will find this a tangible benefit for your product offerings and your bottom line.


Curious about how stack molding can benefit your business?

Contact us today for more details and resources on this efficient, cost-effective plastic molding method.

Contact us about stack molding options

Macay Hunter

Macay Hunter

An expert in plastics manufacturing, Macay works with engineers and supply chain managers during the planning, design and implementation stages of their molded plastic productions.