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The Physics and Mechanics of Stack Molds
Clamp force has to hold the mold shut against injection pressure acting on the projected area of the parts and runner. On a single face mold, doubling the cavities doubles that area, so it needs about twice the clamp and a bigger platen. In a stack mold the two parting lines sit in series: the same clamp force passes through the first level, the center section and the second level, so it holds both. Clamp tonnage stays close to what one level needs, while the parts per cycle double.
How Stack Molds Double Throughput
- Clamp in series: both levels share one clamp force. As a rule of thumb, plan about 10 to 15% more clamp than a single level of the same cavities, not double.
- Melt through the center: a heated sprue bar or transfer nozzle carries melt from the machine nozzle through the first level into the hot runner in the center section, which feeds both levels.
- Synchronized opening: a rack and pinion drive keeps the center section exactly halfway between the platens, so both parting lines open the same amount at the same time for ejection or robot take out.
- The trade offs: the press needs about twice the opening stroke of one level, more shut height, enough shot size for both levels, and a platen rated for a heavy mold. TK’s stack molds weigh from about 7 to 19 tons.
High-Cavitation Molds Up to 128 Cavities
Closures, test tubes, syringes and medical disposables run in the millions, so every cavity has to make the same part, every shot. TK builds molds from 1 to 256 cavities, including a 128 cavity flip top cap mold for 1.15 g caps and a 64 cavity hemostatic clip mold that runs a 9.6 s cycle on a 230 ton press with hot valve gates direct onto the part. TK finished twelve 128 cavity molds for one program in 6 months.
Interchangeable Cavities Without Hand Fitting
Molding inserts are machined to a tolerance of 0.002 mm, so cores, cavities and stack inserts are interchangeable and assemble without hand fitting or matched sets. A worn or damaged insert can be swapped for a spare and the mold goes back to work, and TK supplies spare parts and tool repair service. For molds built to North American press standards, see our export molds for U.S. presses.
Naturally Balanced Melt Flow Distribution
Manifold Rheology
If one cavity fills first, it packs harder and the last one may short shot, so part weight and dimensions spread across the mold. TK lays out manifolds with natural balance: the same flow length, channel size and number of turns from the sprue to every drop. The layout is checked in Moldflow and proven with short shot studies at trial. On one 32 cavity mold, TK held injection balance to less than 5% across the cavities.
Most of TK’s packaging stack molds use full hot runner valve gate hot halves, and on two 32+32 programs one hot half is shared between the base mold and the lid mold. Hot runners come from the supplier your plant specifies. See our DFM and Moldflow page for the simulation work.
Conformal Cooling in Multi-Cavity Tooling
On a high cavitation mold, every second counts many times over. Take a 64 cavity mold running 6,000 hours a year at 85% efficiency: cutting the cycle from 10 s to 9 s adds about 13 million parts a year from the same mold and press.
TK uses BeCu (Moldmax HH) core inserts and core pins where heat builds up, and 3D printed conformal cooling inserts where drilled lines cannot follow the part. The inserts are printed on a DMP 350 in 1.2709 maraging steel and aged to 50 to 56 HRC. Conformal cooling can cut the cycle by up to 35%; on a 12+12 two shot flip top cap mold it cut the cycle 22%, from about 21 s to 17 s. More on our conformal cooling page.
Stack and High Cavitation Molds Built by TK
A selection of stack and high cavitation molds from TK’s packaging and medical programs. Press tonnage and cycle times are from TK’s project records.
| Part | Cavities | Resin, Weight | Press | Cycle | Notes |
|---|---|---|---|---|---|
| Lid, ø69 x 45 x 9 mm | 48+48 | PP, 3.8 g | 1,000 T | 12 s | Full hot valve gate nozzles, servo ejection |
| Flip top cap, ø51 x 17 mm | 48+48 | PP, 4.8 g | 700 T | 12 s (est.) | 3 versions in one mold |
| Shampoo flip top cap | 32+32 | PP, 9.8 g | 600 T | 14 s | U.S. program, servo ejection and in mold closing |
| Container base, ø73 x 20 mm | 32+32 | PP, 8.2 g | 500 T | 9.6 s | U.S. program, 16 weeks kickoff to shipment |
| Container lid, ø73 x 7 mm | 32+32 | PP, 5.8 g | 500 T | 10 s | Shares its hot half with the base mold |
| Flip top cap, ø55 x 37 mm | 24+24 | PP, 12.8 g | 550 T | 12 s | Full hot side gate |
| Thin wall lid, 105 x 128 mm | 6+6 | 0.55 mm wall | 500 T | 7 s | U.S. program |
| Toothpaste flip top cap | 128 | PP, 1.15 g | Not listed | Not listed | Single face high cavitation |
| Hemostatic clip (medical) | 64 | PP, 1.1 g | 230 T | 9.6 s | Hot valve gate direct onto the part |
Capacity for Large Launches
Stack and high cavitation programs often launch several molds at once. TK delivered eight 8+8 cavity housing stack molds as one package, running a 12 s cycle per tool, and has launched 12 molds together on single programs. With capacity for more than 2,000 molds a year, TK can finish a project of more than 1,000 cavities in a short time.
Interactive Stack Mold ROI Calculator
Compare three ways to make the same part: a single face mold, a single face mold with twice the cavities, and a stack mold. The calculator works out parts per hour, clamp tonnage, opening stroke and how many presses each option needs for your annual volume. Open “Show the math” to see every step.
Show the math
Clamp uses common rule of thumb factors by resin and assumes a hot runner. The stack mold clamp adds 10% over one level as a rule of thumb; thin wall parts and long flow lengths need more, and real values are confirmed with Moldflow. No prices are shown because mold and press costs vary by program; Brandon quotes each option. Default numbers are an example only.
Technical Sourcing FAQ: High-Cavitation & Stack Molds
Evaluating High-Cavitation or Stack Tooling for Your Next Program?
Send your part files, annual volumes and press specs to Brandon Henderson, Technical Sales Support Manager for the Americas. He will review cavitation, stack versus single face, press fit and lead time with TK’s engineers.