PU jounce bumper cycle time and mold stations are usually calculated in the wrong order. Most teams start with the mixer’s shot output, then size the mold carrier around that number. The line runs, and every week it still misses target because the cure time at the mold determines how many carriers you actually need. Before you lock a carousel or platen layout, you need to calculate the filled shot weight, segment the real demold-to-demold time, and let the longest serial operation set the station count. This method uses part volume, molded density, scrap, and line availability to avoid buying too many or too few molds.
Start with the Shot Weight the Mold Actually Demands
The input that drives station count is not the machine’s maximum output. It is the filled shot weight the cavity requires at the target molded density. For a microcellular PU jounce bumper, calculate the shot weight from the part volume and the molded density, then add a trim allowance. If the part CAD volume is 480 cm³ and the molded density target is 0.55 g/cm³, the net cavity weight is 264 g. With 4 percent trim and venting loss, the shot weight is about 275 g.
Filled shot weight matters because it sets the output the machine must deliver during the fill window and also changes the heat load the mold carries. A part that is 6 percent overfilled may skin faster but demold hotter, and the cure time can stretch. That is why I base the station calculation on the filled cavity weight, not on the pump curve alone. The machine needs to deliver 275 g within the fill segment. If the metering system holds ratio deviation tight, the main constraint is still the mold’s ability to reach green strength before demold.
Shot weight errors rarely show up as a bad first shot; they show up as density drift and spring rate shifts after the mold reaches steady temperature. <PU Shoe Sole Workshop Math: From Precise Weight Calculation to Cup Test Truths [Part 2: Practical Processing]> covers how precise cup weights and fill calculations expose these shifts before they become a full batch problem.
Break Cycle Time into the Segments That Decide Station Count
PU jounce bumper cycle time is not one machine parameter. It is the sum of serial segments between one demolding and the next demolding of the same mold. The mixing head fill is usually the shortest segment. For a high-volume line, the cure hold and demold preparation consume most of the time.
| Segment | What happens | Example time |
|---|---|---|
| Mold closing and fill | clamp closes, 275 g shot at 60 g/s, cream and vent close | 15 s |
| Cure hold | closed-mold time until green strength allows demold | 240 s |
| Open and demold | unclamp, eject, remove part | 25 s |
| Mold preparation | release spray and insert loading | 15 s |
| Lost transfer | waiting for conveyor index or operator | 6 s |
Add the segments and the example lands at 301 seconds, or about five minutes per cycle. Any one of these segments can grow. The common mistake is to leave out lost transfer time and then wonder why the line runs below nameplate. For station counting, use the same sequence for the longest curing grade you will run, not the fastest grade from the trial.
Let Cure Time Set the Required Number of Mold Stations
Once you have a realistic demold-to-demold time, the station calculation is arithmetic. Convert the target gross output from good parts per hour into parts per hour after scrap, multiply by the cycle time in hours, and divide by cavities per mold.
N_raw = Q_gross × t_cycle / (3600 × C)
Target gross output includes scrap. If the plant must ship 600 good parts per hour and scrap runs at 5 percent, gross output is 600 divided by 0.95, or 632 parts per hour. With a two-cavity mold and a 301-second cycle, the raw station count is 26.4. Round that to 27 stations.
This is where most jounce bumper lines go wrong. Teams calculate the fill time and assume the injection machine sets the pace. A low-pressure shot can fill a 275 g cavity in four seconds, but if the part needs 240 seconds of cure, faster fill only increases idle time. The cure window remains serial per mold. More squeezing capacity cannot remove cure time. Only more mold stations or faster cure chemistry can change the output.
For microcellular elastomer parts like these, a low-pressure metering system with stable output matters more than raw shot speed. <The Ultimate Guide to Low-Pressure Foaming Machines for PU Elastomers> covers how output stability and mixing head design affect filled parts and why maximum throughput is not the best selection target.
Size the Carousel Index and Workstations Around the Real Bottleneck
For a mold carrier or carousel line, the station count translates into an index time. Divide the total cycle time by the number of stations. In the 27-station example, the index period is 301 seconds divided by 27, or about 11 seconds. In practice you round to a reliable index interval, such as 12 seconds, and then divide the curing time by that period to see how many cure stations the line actually needs.
With 240 seconds of cure at a 12-second index, 20 stations hold closed molds. Add one fill station, one demold station, and one mold preparation station. That gives 23 service and cure positions. The remaining four positions cover transfer buffers, operator delays, or a longer cure formulation.
This is the output that tells you whether the proposed carousel fits the floor plan. A team that asks for a 12-station carousel for this part has already designed a line that falls short of the 600 good parts per hour target. The correction is not to raise the pump speed. It is to add mold carriers or move to a two-cavity layout if the first calculation was single cavity.
If your program involves two or three durometer grades with different cure times, it is worth confirming the station count against the slowest grade before you freeze the line layout. Send the part volume and target output to [email protected] and the layout can be checked before the platen spacing is fixed.
Validate the Calculation with a Scrap and Utilization Check
Raw mold stations are not enough. The number that matters is the installed station count after scrap and line availability. If the 27 raw stations assumed no downtime and no quality loss, the real line will miss the target. Using the same example, 600 good parts per hour, 5 percent scrap, 88 percent availability, and two cavities per mold gives 30 installed stations, not 27.
N_installed = Q_good × t_cycle / (3600 × C × (1 - scrap) × availability)
Run that calculation with real downtime data. If the line has 30 mold stations but availability is only 80 percent, the output drops to roughly 533 good parts per hour under the same cycle time. That gap is not caused by the mixing machine. It is caused by maintenance, material changeovers, or operator relief.
I usually recommend locking the cycle time only after a trial block of five to ten shots at the target mold temperature. Watch the demold release force and the heat at the thick section. If the part needs more cooling because the skin tears or the dimple remains soft, the cure segment has not ended. The station count must reflect that measured time, not the optimistic time from the first acceptable part.
Availability erodes capacity faster on short-index lines than on long-cure batch presses. <Continuous PU Foam Production Line Maintenance: How to Prevent Costly Downtime and Defects> covers how small stoppages accumulate into hours of lost production on continuous PU equipment, which applies the same way to an indexed mold carrier line.
Get the Line Balance Checked Before the Mold Count Is Fixed
The most expensive moment to find a bad station count is after the carousel has been assembled. Teams often buy a machine around one throughput figure, add molds around another, and discover the two numbers do not share the same cycle measurement.
If you are deciding between a 20-station and a 30-station line, get the arithmetic checked against your slowest cure grade and your real scrap and uptime history. A line-balance calculation only needs the part volume or shot weight, the cavity count, the required good output per hour, and the expected demold time. The result tells you whether the bottleneck is cure, fill, demold, or material delivery.
Send those four numbers to Haifeng Polyurethane Machinery at [email protected] or WhatsApp 86 13566296633 and the mold station calculation can be checked before you approve the platen layout.
Common Questions About Cycle Time and Mold Stations
What is the fastest way to estimate mold stations if full cycle data is missing?
Use a conservative demold-to-demold time of five minutes and apply the station formula. For a single-cavity mold at 120 parts per hour, five minutes is 0.083 hours, so the raw count is ten stations. Add scrap and availability, and the installed count becomes roughly twelve. This gives a layout estimate quickly, but it should be replaced by measured cure time before purchase.
Should I base cycle time on the mixing machine’s shot output?
No. Shot output affects only the fill segment. A 275 g cavity filled at 60 g/s takes less than five seconds, while cure may need 240 seconds. If you use the mixer’s shot capacity as the cycle basis, you will undercount mold carriers and build a line that idles at the pour station. Use the mold’s demold-to-demold time instead.
How does cavity count change the mold station calculation?
For a fixed target output and cycle time, cavity count divides the station count almost linearly. If a single-cavity mold requires 60 stations for a given output, a two-cavity mold requires about 30 stations for the same net parts per hour. The reduction is not exactly half because larger multi-cavity molds can add demold handling time and more complex fill balancing.
Why do two lines with the same nominal throughput need different mold counts?
In one jounce bumper line I reviewed, 240 seconds of cure and a 12-second index limited output to 600 parts per hour until the cure station count was increased. Another line with the same nominal machine throughput ran the same output with fewer stations because its shorter demold time allowed a faster index. Cure time, demold handling, and mold preparation are the real variables. Share the part volume, target output, and slowest cure grade with [email protected] and the station count can be confirmed before layout.
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