What Is Cycle Time?
Cycle time is how long it takes to make one part. Mold closes, plastic goes in, part cools, mold opens, part comes out. Start the clock when the clamp closes. Stop it when the clamp closes again for the next shot.
Cooling is where the time goes
Most guys I work with obsess over the cooling portion. Makes sense. On a typical job the part just sits there in the mold waiting to get hard enough to push out. You can speed up injection. You can speed up the clamp. But you still gotta wait for the plastic to cool.
I ran a medical housing last year. Thirty-two second cycle. Customer wanted twenty-eight. We tried bumping the water flow. Got maybe half a second. Dropped mold temp ten degrees. Another second. Still stuck at thirty. The wall section near the gate was just too thick. No amount of fiddling with the process was gonna fix a part design problem.
Specialty work slows you down
Specialty molds are a different animal. Tight tolerances eat up cycle time. You push too fast and the part warps after ejection. Or you get sink marks. Or the dimensions drift outside spec by the afternoon when the shop heats up. So you back off. Run it slower. Customers hate hearing that.
Conformal cooling is not magic
Conformal cooling gets talked about a lot now. Printed inserts with channels that follow the part shape. Works great in some spots. I put one in a core that was running hot on a two-cavity tool. Dropped that cavity's cooling time by eight seconds. But the insert cost eleven thousand dollars and took six weeks to get. The ROI math only worked because we were running three shifts on that mold for two years straight.
Water temperature trips people up
Water matters more than people think. I've seen guys run fifty degree water when they should be running seventy. Or the opposite. They crank the chiller down because colder is better, right? Not always. Too cold and you get condensation on the mold surface. Parts stick. Or you frost up the channels and lose flow entirely.
Robots and ejectors add up
The other thing nobody talks about is the robot. If your picker takes four seconds to get in and out, that's four seconds sitting there with the mold open. I watched a shop cut six seconds off their cycle just by repositioning the robot and shortening the stroke.
Ejection setup matters too. Seen plenty of molds where the ejector plates move slow because somebody set the hydraulic pressure too low. Or the return springs are worn out and the plate creeps back instead of snapping. These things add up.
Multi-cavity balancing
On multi-cavity tools the slowest cavity sets your cycle. Doesn't matter if seven cavities cool in eighteen seconds if that eighth one needs twenty-two. You're running at twenty-two. Balancing cavity to cavity cooling is grunt work but it pays off.
Estimates vs reality
I don't trust cycle time estimates on new molds. Tooling engineers guess based on wall thickness charts and material data sheets. Reality never matches. First shots off a new tool usually run thirty percent longer than the quote said. Then you spend two weeks dialing it in. Sometimes you get close to target. Sometimes you call the customer and explain why their three million parts per year estimate is now two point four million.
Watch the trend
The shops making money on tight margins track this stuff religiously. Cycle time trending up over six months means something is degrading. Scale in the waterlines. Worn leader pins slowing the clamp. Heater bands dying on the hot runner. You catch it early or you catch it when the customer calls about short shots.