Running Molding Around the Clock Without Overbooking

Plastics multi shift scheduling is the problem of taking one long molding run and spreading it across shifts, days and machines without ever committing a press twice. EDGEBIC by User Solutions treats a shift as a bucket of hours per machine, fills those buckets in a ranked order, and records exactly which machine consumed which hours on which date. The result is a schedule a night-shift supervisor can act on and a planner can defend.

For the mechanism without the plastics framing, see multi-shift scheduling explained. For this sector's wider constraint picture, see plastics and rubber production scheduling, and for the cross-industry view, how different industries use EDGEBIC.

Why molding breaks single-shift thinking

Molding runs are long. A 40,000-shot order does not fit in one shift and was never meant to. Three consequences follow, and most spreadsheet schedules handle none of them.

A run has to be describable across shifts. If your system can only say "this job runs Tuesday", the night shift has no idea what is theirs.

A run has to survive a partial first shift. Tooling changes and material arrivals mean jobs rarely start at the top of a shift, and rounding a 10:00 start up to the next shift throws away six machine hours.

And a run has to respect machine identity. Two jobs can share a work center's capacity when it has four presses. They cannot share the same press at the same time. A schedule that only tracks work center totals will happily book both.

How capacity is built before anything is scheduled

Before placing work, the engine builds a bucket for every combination of work center, date and shift in the horizon. Each bucket's size comes from one formula:

net hours = shift gross - break - downtime - partial closures
raw       = net hours x machines
capacity  = raw x (utilization / 100)

A press cell with three machines on an 8-hour shift at 80 percent utilization offers 19.2 hours in that bucket. Not 8, and not 24. Utilization is a deliberate planning choice, and holding back 20 percent for the variability molding always has is a legitimate one. What matters is that the number is derived rather than typed, so changing the shift pattern or adding a machine updates every bucket at once. Calendar details, closures and overrides are covered in shifts and calendars explained.

How one run fills those buckets

The engine finds every bucket with available capacity from the job's earliest start forward, ranks them, then walks the list taking the smaller of what remains and what the bucket offers.

Ranking is not arbitrary. Buckets that can start immediately outrank buckets that cannot, larger available capacity outranks smaller, and earlier shifts on a day outrank later ones, so a day shift fills before the night shift on the same date. Dates still lead: the engine does not skip Monday night to reach Tuesday day.

The simple case

One press, one instance, day shift 08:00 to 16:00, a run needing 12 hours starting Monday.

The operation starts Monday 08:00, ends Tuesday 12:00, and carries a per-day breakdown of 8 and 4. That breakdown is what makes the plan usable on the floor: Monday's crew owns eight hours of it, Tuesday's crew owns four.

The realistic case

Two presses, day shift 08:00 to 16:00 and night shift 16:00 to 00:00, a run needing 28 hours that cannot start before Monday 10:00.

Monday day nominally holds 16 hours (8 hours times 2 machines). The 10:00 earliest start leaves six hours per machine, so the usable figure is 12 hours. The engine books those 12, load balanced at six hours on each press, and 16 hours remain. Monday night offers a full 16 across both machines and absorbs the rest exactly.

Mon day   10:00 - 16:00   Press A 6 h, Press B 6 h   (12 h)
Mon night 16:00 - 00:00   Press A 8 h, Press B 8 h   (16 h)
Finish: Monday 24:00. Breakdown recorded as 12, 16.

Two things to notice. The late start cost six machine hours, not a whole shift. And the job finished on the same calendar day it started, because the night shift was treated as capacity rather than as an afterthought.

Why the ranking is what it is

The three ranking rules exist for reasons a molding planner will recognize.

Immediate availability outranks everything because a bucket you can start in now is worth more than a larger bucket tomorrow. Larger available capacity comes next because filling one big bucket beats scattering a run across four small ones, which is both easier to supervise and fewer changeovers. And earlier shifts on a day outrank later ones so a day shift fills before the night shift on the same date, which matches how most plants staff and supervise.

The ranking is a tie-break within reachable dates, not a license to jump forward. The engine does not skip Monday night to reach Tuesday day just because Tuesday scores higher: dates lead, scores order what is available on and after them.

Understanding this saves an argument later. When a run lands partly on the night shift, it is because the day shift was full or the job could not start early enough, not because the engine preferred the night.

The weekend case

One press, day shift Monday to Friday only, a 20-hour run starting Friday at noon. Friday offers the four hours to 16:00. Saturday and Sunday have no shift, so no bucket exists and nothing is allocated. Monday and Tuesday absorb the remaining 16. Elapsed calendar time reflects the closure while recorded hours reflect the work, and nobody is left explaining a plan that shows Saturday production.

Three ways machines get chosen

Once a bucket is selected, the engine has to decide which physical machine takes the hours. Three modes exist, and picking the right one per work center matters more in plastics than in most sectors.

The one-per-day case is worth a worked look because molding plants nearly always have equipment that needs it. Three dryers, one shift, three jobs of six hours each arriving the same Monday. The first job takes dryer one from 08:00 to 14:00. The second finds dryer one used and takes dryer two. The third takes dryer three. All finish at 14:00, on separate machines, and no fourth job can slip onto a dryer that day. Pooled capacity would have been 24 hours and would have allowed exactly the kind of sharing the process forbids.

Load balancing is the opposite instinct and is right for presses running interchangeable work: splitting hours across machines finishes the order sooner. The difference between the two is a single flag on the work center, and machine instances explained covers the underlying model.

Why this is finite capacity in practice

Every allocation writes back to the bucket it came from, so the next job sees reduced availability. That feedback loop is what makes the schedule finite rather than a wish list, and it is the difference discussed in finite versus infinite capacity scheduling.

The practical test is simple. Load one more job than your plant can physically run this week. An infinite-capacity plan will place it inside the week and report no problem. A finite-capacity plan will push it into next week and show you which resource ran out. The second answer is less pleasant and considerably more useful.

Configuration checklist for a 24-hour molding plant

1. Define every shift you actually run, including the night shift and any weekend pattern, with real start, end and break times per day of week.
2. Set machine counts honestly. Four presses is four instances, and taking one down for a rebuild is a per-day capacity override on the affected dates rather than a count edit.
3. Choose utilization deliberately and write down why. Do not model the same buffer twice by also shortening the shift.
4. Set the one-per-day flag on dryers, ovens and any equipment that must be dedicated for a batch. Leave it off for interchangeable presses.
5. Get maintenance out of the schedulable pool. Shift-change routines and PM are hours the presses will not be running, and there is no downtime record to create in the current release. Net a standing weekly loss into that work center's shift hours, and enter dated maintenance as a per-day capacity override with a reason, so those hours stop being invisible.
6. Read the per-day breakdown on a long job after the first run. If a shift's hours look wrong, the bucket that produced them is where to look.

Reading the per-day breakdown

The most under-used output of a multi-shift run is the per-day hour breakdown attached to each operation. It is the bridge between the planner's Gantt bar and the supervisor's shift.

A 28-hour run showing 12 and 16 tells the Monday day crew they own 12 hours of it across two presses and the Monday night crew owns the remaining 16. Neither has to interpret a bar that spans two shifts. When the day crew logs 10 hours instead of 12, the shortfall is attached to a specific date rather than smeared across the job, and the reschedule moves the remainder rather than the whole operation.

It is also the fastest diagnostic you have. If a day's figure looks wrong, the bucket that produced it is where to look: the shift definition, a plant holiday, a per-day override, or the machine count. Every one of those is visible, and none of them requires reading the schedule to work out.

Where it sits with the rest

Multi-shift allocation is the layer that turns everything else into a dated plan. It consumes the changeover times produced by mold changeover sequencing as part of each operation's required hours, and it evaluates whichever machine the pool logic in press pool routing selected. Get all three right and the night shift stops improvising.

Bring your shift pattern, machine list and one long-running job to a demo of plastic manufacturing scheduling software, and we will show you the run spread across the shifts that will really carry it.