Measure the loss from mold and material changeovers

Mold and material changes cost every minute from the last good shot of one run to the first good shot of the next, purge and trial shots included, multiplied by how often each kind happens. A single stopwatch reading can’t price that. Log mold changes and material changes as separate reasons for two to four weeks, then total each type’s stopped minutes and run-up.

This guide is for plant managers running a molding fleet. It works one hypothetical week on a 250-ton press and ends with which change to attack first. The counting side of a mold change, re-entering the shot time and cavities, is in monitoring injection molding machines.

Mold change or material change?#

A mold change replaces the tool and its settings, while a material change keeps the tool and replaces what flows through it. Both stop the press, but different people do the work and different things make it slow.

Both are changeovers in the glossary sense: the clock runs to the first good part. What fills that time differs.

Log the two as separate reasons. Averaged together, three 95-minute mold changes and six 30-minute color changes give a 51.7-minute mean that describes neither, and the fix for one does nothing for the other.

Inside material changes, record the from and the to. A purge from black to natural runs longer than natural to black, and a resin swap that needs a dryer cycle runs longer than a colorant swap on the same resin. Keep color change and material change as one reason until your log shows they last differently.

Where does each changeover start and end?#

Each changeover starts at the last good shot of the old run and ends at the first good shot of the new one. The Lean Enterprise Institute measures changeover time the same way: from the last piece of the completed run to the first good piece after the change.^[1]

On a molder, name the event behind each boundary, so every crew times the same thing.

The restart is not the end. Between the restart and the first good shot sits the run-up: purge, trial shots and adjustments, with the screw and the hydraulics working and no good parts coming out.

A change can swap both the mold and the resin. Log it as one mold change, since the mold swap sets its length, and note the new resin. If the hung, heated mold then waits on resin still in the dryer, split the stop at the moment the press was ready and give the wait its own reason, such as Waiting on material. The minutes still count in the week’s changeover loss; the split sends the fix to whoever runs the dryers.

Crew handovers are a different loss with different fixes, so keep them out of this log. Shift changeover losses covers them.

Does purge scrap count as changeover loss?#

Yes. Purged resin and the trial shots made before the first good shot fall inside the changeover, so they are changeover loss, in minutes and in material. The stopped part costs availability; the run-up costs both run time and resin.

Record three numbers for every change:

• run-up minutes, from the restart to the first good shot sign-off;
• purge weight in kilograms, weighed off the floor or the purge bin;
• trial shots × cavities, logged as scrap with a changeover reason.

Log trial-shot parts under their own scrap reason, such as Purge or Startup, rather than a general reject code. Scrap from a changeover and scrap from a drifting process need different fixes.

On a hot-runner mold, purge in two steps: the injection unit first, then the hot runner, so contaminants in the barrel don’t carry into the manifold.^[4] Time the two steps as separate elements, so a long run-up shows which step to work on.

A material change also has lead work that never touches the press. Many resins must be dried before they reach the hopper. Covestro lists 2 to 3 hours at 120 °C in a dry-air dryer for one polycarbonate grade, with moisture no higher than 0.02%.^[2] A 20% glass-fiber grade of the same resin needs 4 to 6 hours.^[3]

Start the dryer at least the drying time before the change. Start it when the press stops, and the press waits hours for resin, a wait no faster purge can recover.

Work one week on a 250-ton press#

Price a week of changeovers two ways: by type, from counts and means, and from the logs, from stopped minutes plus run-up. The two totals must agree. When they don’t, a change is missing a timestamp.

The downtime log alone shows 6.0 hours. It misses 1.75 ÷ 7.75 = 22.6% of the loss, and color changes hide the larger share of it: 60 of the 105 run-up minutes.

Put the hours in the units your plant reviews. At 480 parts an hour, 7.75 hours is 7.75 × 480 = 3,720 parts the press didn’t make. At a hypothetical rate of $90 per machine hour, it’s 7.75 × 90 = $697.50 a week, before the purged resin.

Marking these changes as planned doesn’t take them out of the total. A scheduled changeover still lowers availability under the common OEE convention, as planned vs unplanned downtime explains.

Which change should you attack first?#

Attack the change type with the largest weekly total, run-up included, unless the schedule can remove changes for less effort. In the example week, mold changes lead with 4.75 hours against 3.0, even though there are half as many.

Rank by the weekly total, not by the longest single change. A 95-minute mold change looks like the obvious target, but six short color changes a week can outweigh it on another press.

You don’t need a faster color change if the planner can drop it. Running two black jobs back to back instead of black, natural, black removes one change and its purge, and costs nothing on the floor.

Track molding changeovers automatically#

IoTFlows puts a SenseAi on each molding machine, which detects every stop from vibration with no PLC connection. Create Mold Change and Material Change categories, as the molding setup guide describes, and operators classify each stop in a tap, with the from and the to in the note.

The Downtimes report ranks the two reasons by hours and trends them week by week. Its CSV export carries one row per stop, with category and note, for the per-type math above. Injection molding machine monitoring shows what each press reports.

Datwyler runs more than 20 injection molding machines in Ontario, California. Its results list “30% less material-changeover downtime, from targeting the material types that needed the longest setups”, within six months, as Datwyler’s story describes.

Start the log on one press#

1. Create the reasons

   Add Mold change, Material change and Waiting on material. Add Color change as its own reason only once your log shows it runs differently from a resin swap.

2. Post the boundaries at the press

   Write down the last good shot and the first good shot as the start and stop, and name who signs off the first good shot for each type.

3. Note the from and the to

   On every material change, record the outgoing and incoming resin or color, so you can rank the pairs later.

4. Time the run-up and weigh the purge

   Record the minutes from restart to sign-off, the purge weight, and the trial-shot parts as scrap with a changeover reason.

5. Split mixed stops

   Split any change that ran into a fault or a wait for dried resin, so each part keeps its own reason.

6. Total the week two ways

   After two to four weeks, compute each type’s weekly total by count × mean and from the logs. Chase any gap between the two before you trust either.

7. Pick the first target

   Use the signal table to choose the type and the first move, then compare the next four weeks with the same rules.