Last updated: 2026-09-01 · 9 min read

What Is a Stack Mold — and When Does It Double Your Output Without a Bigger Machine?

A stack mold is a multi-parting-line injection mold that uses two (or more) cavity planes stacked behind each other along the mold-opening axis. When the mold opens, both parting lines separate simultaneously — the front cavities open between the stationary platen and the floating intermediate plate, and the rear cavities open between the floating plate and the moving platen.
The result is that one machine shot produces parts from both cavity planes, effectively doubling the output per cycle without increasing the clamp tonnage requirement. Stack molds are used for high-volume, flat or shallow-draw parts (closures, containers, lids, caps, thin-wall packaging) where the projected area of a single cavity plane would otherwise consume most of the available platen area.

Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. See also related guide.

Key Facts

  • A stack mold doubles (or triples) the number of cavities per shot without increasing the projected area on the platen. A 32-cavity stack mold (16 + 16) occupies the same platen footprint as a 16-cavity standard mold. The clamp tonnage requirement is based on the projected area of one cavity plane (not the sum of both), because the injection pressure acts on one plane at a time. This means a machine that would clamp a 16-cavity standard mold can run a 32-cavity stack mold on the same tonnage.
  • The floating intermediate plate (also called the shuttle plate or the floating A plate) is the key mechanical element. It is not attached to either platen — it floats between the tie bars and is guided by leader pins and bushings. During mold opening, the toggle mechanism or a mechanical linkage pulls the intermediate plate away from the stationary half and toward the moving half, separating both parting lines in a single, synchronized motion. The opening stroke is approximately twice the mold thickness (because both parting lines open).
  • Stack molds cost 40–70 % more than a standard single-plane mold with the same total cavity count, because the floating plate, the dual ejection system, the extended leader pins, and the synchronized opening mechanism add complexity. However, the stack mold produces twice as many parts per cycle on the same machine, so the per-part cost is lower at volumes above approximately 500,000 parts. Below this volume, the additional tooling cost is not recovered.
  • Cycle time in a stack mold is governed by the thickest wall in either cavity plane — because both planes are served by the same injection unit and the same cooling time. If the two planes produce identical parts, the cycle time is the same as a standard mold. If the two planes produce different parts (a family stack), the cycle time is set by the slower-cooling part.
  • Ejection in a stack mold requires two ejection systems — one for each parting line. The front cavities eject from the floating plate (toward the stationary side), and the rear cavities eject from the moving plate (toward the operator side). The ejection sequence can be simultaneous (both planes eject at the same time) or sequential (one plane ejects first, then the other), depending on the part geometry and the robot or conveyor arrangement.

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    Stack Mold vs Standard Mold

    Factor Standard Mold (Single Plane) Stack Mold (Dual Plane)
    Parting lines 1 2 (or 3)
    Cavities per shot N 2N (or 3N)
    Platen footprint N cavities N cavities (same)
    Clamp tonnage based on N cavities based on N cavities (same)
    Mold height standard ~2x standard
    Opening stroke 1x part height + runner ~2x (both planes open)
    Tooling cost baseline +40–70 %
    Cycle time cooling of N parts same (identical parts)
    Per-part cost at 1M+ baseline ~30–45 % lower

    How the Stack Mold Opens and Closes

    1

    Injection phase — both parting lines closed

    The injection unit fills both cavity planes simultaneously through a common sprue or hot-runner manifold. The melt flows through the stationary platen, into the first parting line (front cavities), and through cross-over channels in the floating plate into the second parting line (rear cavities). The clamp holds both parting lines closed under the full tonnage of the machine.

    2

    Cooling phase — mold closed, parts solidify

    Both cavity planes cool simultaneously. The cooling channels in the floating plate serve both the rear face of the front cavities and the front face of the rear cavities. The cooling time is governed by the thickest wall in either plane. If both planes produce identical parts, the cooling time is the same as a standard mold.

    3

    Mold opening — both parting lines separate

    The moving platen retracts. A mechanical linkage (toggle, gear-and-rack, or harmonic puller) connects the moving platen to the floating plate. As the moving platen retracts, the linkage first pulls the floating plate away from the stationary half (opening the front parting line), then continues to retract with the floating plate (opening the rear parting line). The total opening stroke is approximately twice the mold thickness. Both parting lines are now open, exposing the parts in both cavity planes.

    4

    Ejection — parts drop or are picked from both planes

    Ejector pins in the floating plate push the front-cavity parts toward the stationary side (or toward a robot pick-up zone between the two open parting lines). Ejector pins in the moving plate push the rear-cavity parts toward the operator side. The ejection can be simultaneous (both planes eject at once, and a robot or conveyor collects parts from both sides) or sequential (the front plane ejects first, the robot picks, then the rear plane ejects). The mold closes and the next cycle begins.

    When a Stack Mold Is the Right Choice

  • High-volume flat or shallow-draw parts — closures, lids, caps, containers, thin-wall packaging, medical syringe components, and disposable cutlery. These parts have low draw depth (less than 30 mm), uniform wall thickness, and short cooling times.
    The stack mold doubles the output without requiring a larger machine or more clamp tonnage.
  • Platen area is the bottleneck — when the part’s projected area is large relative to the available platen, a standard mold may only fit 8 or 12 cavities. A stack mold fits 16 or 24 cavities in the same platen footprint by using a second cavity plane behind the first.
    This is the most common reason to specify a stack mold.
  • Annual volume above 500,000 parts — the 40–70 % additional tooling cost is recovered when the per-part savings (from doubled output per cycle) accumulate over enough parts.
    Below 500,000 parts, the standard mold with a longer production run is usually more economical because the tooling cost is lower and the production time difference is absorbed by the schedule.
  • Machine availability is constrained — if the molding facility has limited machine capacity and adding a second machine is not feasible (floor space, capital, or lead-time constraints), a stack mold on the existing machine doubles the throughput without a capital equipment investment.
  • Parts are identical in both planes — stack molds are most efficient when both cavity planes produce the same part with the same wall thickness, same resin, and same cooling time.
    If the two planes produce different parts (a family stack), the cycle time is governed by the slower-cooling part, and the efficiency gain is reduced.

    • Design Constraints and Risks

    Constraint Impact Mitigation
    Mold height doubled requires machine with sufficient daylight opening verify max mold height + opening stroke
    Floating plate alignment misalignment causes flash, uneven walls precision leader pins, taper locks
    Crossover channels melt path through floating plate adds pressure drop hot-runner crossover or generous runner diameter
    Dual ejection system maintenance complexity, two sets of ejector pins standardize ejector sizes, scheduled PM
    Machine tie-bar spacing floating plate must fit between tie bars verify tie-bar clearance for floating plate

    Cost Breakeven Analysis

    The breakeven volume for a stack mold is the point where the per-part savings (from doubled output) exceed the additional tooling cost.
    The calculation is straightforward:

    Standard mold tooling: $40,000 (16 cavities)
    Stack mold tooling: $60,000 (16 + 16 = 32 cavities, +50 %)
    Standard per-part cost: $0.12 (machine hour / 16 cavities)
    Stack per-part cost: $0.06 (machine hour / 32 cavities)
    Per-part savings: $0.06
    Breakeven: ($60,000 – $40,000) / $0.06 = 333,333 parts

    Above approximately 333,000 parts, the stack mold is cheaper per part. Below that volume, the standard mold is cheaper because the lower tooling cost is not offset by the per-part savings. In practice, the breakeven ranges from 300,000 to 700,000 parts depending on the part complexity, the resin cost, and the machine hour rate. — Arthur, Tooling Engineer at aCatMould

    Frequently Asked Questions

    Can a stack mold produce two different parts (a family stack)?

    Yes. A family stack mold produces part A in the front plane and part B in the rear plane. Both parts are molded in the same shot, using the same resin. The cycle time is governed by the part with the longer cooling time. Family stacks are used when two mating parts (e.g., a lid and a base) are produced in equal quantities and assembled together.
    The risk is that if one part has a higher scrap rate than the other, the production ratio is disrupted and inventory of the higher-yield part accumulates.

    Does a stack mold require a special injection molding machine?

    Not a special machine, but the machine must have sufficient daylight opening (the distance between the platens when the mold is fully open) to accommodate the stack mold’s total height plus the double opening stroke. A stack mold is approximately twice as tall as a standard mold, and the opening stroke is approximately twice the mold thickness. The machine must also have sufficient tie-bar spacing to allow the floating plate to travel between the tie bars.
    Most modern toggle-clamp machines can accommodate stack molds within their tonnage range, but the maximum mold height and opening stroke must be verified before specifying a stack mold.

    What is a triple-stack mold?

    A triple-stack mold has three cavity planes (three parting lines) and two floating plates. It triples the output per shot compared to a standard mold. Triple stacks are rare — they are used only for extremely high-volume, thin-wall parts (disposable cutlery, closure caps) where the annual volume exceeds 5 million parts and the machine has sufficient daylight opening to accommodate the tripled mold height.
    The mechanical complexity (two floating plates, three ejection systems, synchronized opening of three parting lines) makes the tooling cost approximately 100–150 % more than a standard mold with the same single-plane cavity count.

    How does maintenance differ for a stack mold?

    A stack mold has approximately twice the number of components — two sets of cavities, two ejection systems, a floating plate with leader pins and bushings, and the mechanical linkage for synchronized opening. Preventive maintenance intervals are the same as a standard mold (cleaning, lubrication, wear inspection per shot count), but each maintenance event takes longer because there are more components to service.
    The floating-plate alignment is the most critical maintenance item — if the leader pins wear and the floating plate shifts, both parting lines are affected simultaneously, producing flash on parts from both planes.

    Can an existing standard mold be converted to a stack mold?

    In principle, yes — an existing cavity set can be duplicated and mounted in a stack-mold frame with a floating plate and a second parting line. In practice, the conversion requires a new mold base (the stack-mold frame is fundamentally different from a standard two-plate or three-plate base), new cooling channels in the floating plate, a crossover runner system or hot-runner manifold, and a dual ejection system.
    The cost of the conversion is typically 70–90 % of a new stack mold, so the conversion is only economical if the existing cavity inserts are in good condition and the production volume justifies the investment.

    What is the maximum cavity count for a stack mold?

    The maximum cavity count depends on the platen size and the part’s projected area. For small parts (closures, caps, 10–20 mm diameter), a stack mold can accommodate 64 + 64 = 128 cavities on a standard machine platen. For larger parts (containers, 100 mm diameter), the maximum may be 8 + 8 = 16 cavities. The limiting factor is the platen area available for one cavity plane — the stack mold doubles that number, but cannot exceed the platen’s physical dimensions.

    Key Takeaways

    1. A stack mold doubles (or triples) the output per cycle without increasing the platen footprint or the clamp tonnage requirement. It is the most effective way to increase throughput on an existing machine when platen area is the bottleneck.
    2. The tooling cost is 40–70 % higher than a standard mold with the same single-plane cavity count. The breakeven volume — typically 300,000–700,000 parts — is the point where the per-part savings from doubled output offset the additional tooling investment.
    3. Stack molds are best suited for high-volume, flat or shallow-draw parts (closures, lids, containers, thin-wall packaging) produced in identical cavity planes. Family stack molds (different parts in each plane) are possible but introduce cycle-time and inventory-balancing risks.
    4. The machine must have sufficient daylight opening and tie-bar spacing to accommodate the doubled mold height and the double opening stroke. Machine compatibility must be verified before specifying a stack mold.
    5. Maintenance is more complex (two sets of cavities, two ejection systems, floating-plate alignment), but the preventive-maintenance intervals are the same. The floating-plate alignment is the single most critical maintenance item.

    A production planner who evaluates a stack mold against the annual volume, the machine availability, and the per-part cost target avoids two common errors: specifying a stack mold for a program that will not the breakeven volume (the tooling surcharge is never recovered), and specifying a standard mold for a program that runs 2 million parts per year on a machine that could double its output with a stack mold.
    The breakeven calculation is simple — the only variable that requires engineering judgment is whether the machine can accommodate the stack mold’s height and opening stroke.

    Sources
    Plastics Technology — stack mold technology
    MoldMaking Technology — multi-parting-line molds
    Beaumont Technologies — mold filling analysis

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet