Production Scheduling for Thermoforming

Thermoforming production planners and operations managers can use Schantt to model multi-press forming lines, sequence-dependent tool changeovers, conveyor oven throughput, and per-class routings with stage skipping, then generate optimized schedules that respect every constraint. This guide walks through configuring a complete thermoforming scenario.

This guide follows a fictional composite company built from industry research on thermoforming; all names, parameters, and figures are illustrative.

Industry context

Thermoforming heats a plastic sheet to a pliable temperature, forms it over a mould using vacuum or pressure, cools the part, then trims, finishes, and packages it. The process serves three broad product families: thin-gauge packaging (hinged containers, trays, lids), thick-gauge industrial components (automotive panels, equipment housings), and medical device packaging (sterilisation trays, wound-care blisters). Each family differs in sheet gauge, cycle time, cavity count, cooling behaviour, and post-forming treatment — one plant must accommodate all three.

A typical job-shop thermoforming plant runs two shifts (06:00–22:00), Monday through Friday, with a conveyor oven feeding up to six forming presses. Cycle times vary by product: thin-gauge items run approximately 48 seconds per batch in an 8-cavity tool, thick-gauge parts run 90 seconds in a 2-cavity tool, and medical trays run 72 seconds. Between forming and trimming, a mandatory cooling dwell of approximately two hours prevents distortion. Tool changeovers at the forming presses range from 25 to 55 minutes, depending on the direction of the swap. The single conveyor oven (OV-1) supplies heated sheet at 720 sheets per hour for thin-gauge material, 360 per hour for thick-gauge, and 540 per hour for medical-grade sheet — rates that reflect the different heating requirements of each material thickness.

After forming and cooling, parts move to one of three trim presses (TP-1 through TP-3), each capable of a subset of product classes. Thick-gauge industrial parts then pass through a secondary drilling station (DR-1) before reaching packaging, while thin-gauge containers proceed directly from trimming to one of two packaging lines. Medical device trays, which require no trimming or drilling, skip those stages entirely and travel from the cooling dwell to a dedicated packaging line (PK-2). The two packaging lines (PK-1 and PK-2) handle throughput at 60 to 120 units per hour depending on the product class. The plant switches to a reduced single-shift pattern (06:00–14:00) during lower-demand summer months, and observes seven non-working days per year: New Year's Day, International Workers' Day, and a five-day year-end shutdown.

PreciseForm Thermoforming employs around 55 people at a 4,600 m² facility, producing three product classes across five production stages with six forming presses, scheduled by a two-person planning team.

Process overview

flowchart LR
    B["Oven Heating<br/>(OV-1, flow)"]
    C["Forming<br/>(FP-1..FP-6, batch)"]
    E["Trimming<br/>(TP-1..TP-3, batch)"]
    F["Secondary Operations<br/>(DR-1, batch)"]
    G["Packaging<br/>(PK-1..PK-2, flow)"]
    B --> C
    C --> E
    E --> F
    F --> G
    C -.->|"Medical trays<br/>skip trim+secondary"| G

Production flow at PreciseForm Thermoforming — sheet is heated in a conveyor oven, formed on one of six parallel presses, trimmed, and packaged. Medical device trays bypass trimming and secondary operations, routing directly from forming to packaging.

Scheduling challenges and how Schantt handles them

At PreciseForm, the schedule is driven by customer order demand — each week the planners receive a list of quantities to deliver across all three product classes, and the task is to sequence production so that everything ships on time while keeping the presses running. (Readers whose schedule is driven by a different input, such as a weekly production target or make-to-stock replenishment, can adapt the same approach.) Schantt schedules forward from a start date, minimising total production time across the whole plan (the makespan), and this guide assumes a practical horizon of one to four weeks. Two optimisation modes support different planning styles: Auto mode explores both job sequence and machine assignment simultaneously to find the fastest overall plan, while Semi-Auto mode lets the planner fix the job order and lets the system optimise machine assignments around that sequence.

What Schantt handles well

• Multi-press forming stages — model each forming press as a machine within the forming stage; Schantt assigns products to compatible presses in Auto and Semi-Auto modes
• Sequence-dependent tool changeovers — capture mould-change durations as directional changeover times per press, so the optimiser favours sequences that cluster similar products and reduce setup time
• Mixed batch-and-flow pipeline — model the conveyor oven as a flow stage (throughput per hour) and forming and trimming presses as batch stages (cycle per batch), with Schantt timing both physics types correctly
• Per-class routing with stage skipping — define product classes with different stage sequences, using bridging transfer times for skipped stages
• Calendar-aware scheduling — set shift patterns, exceptions, and downtimes so schedules respect actual working windows
• Inter-stage cooling dwell — model mandatory cooling time as a transfer time between forming and trimming, applying a fixed forward delay

How Schantt handles each challenge

1. Tool changeover waste on shared forming presses.

• Changeover times are asymmetric: switching from thin-gauge to thick-gauge takes 55 minutes on some presses, while the reverse takes 25 minutes. Planners who sequence jobs manually often overlook these directional differences, leaving time savings on the table.
• Schantt models each changeover as a directional pair per machine and per product class transition. In Auto mode, the optimiser explores job sequences that cluster same-class work and favour low-changeover transitions, reducing the total time spent on setup across the schedule.

2. Mandatory cooling dwell between forming and trimming.

• A planner must ensure that every formed batch is followed by exactly 120 minutes of cooling before the next stage can begin, with no shortcut. When schedules are built manually, this constraint is easy to miss or approximate, leading to downstream idle time or rushed parts.
• In Schantt, the cooling dwell is modelled as a transfer time between the forming and trimming stages — a fixed 120-minute forward delay that every product in the thin-gauge and thick-gauge classes must respect. The simulator chains each trimming operation to start only after its upstream forming operation finishes plus the cooling dwell, making the constraint automatic and auditable.

3. Divergent routings across product classes.

• Handling three different routes on a shared set of machines means some products bypass stages that others need, and the material handoff between stages differs for each path. A static routing model cannot capture this — adapting it per product family requires manual tracking.
• Schantt lets each product class define its own per-class routing as a sequence of stages. Medical trays use a three-stage route (heating, forming, packaging) with a bridging transfer time of 125 minutes from forming to packaging — the 120-minute cooling dwell plus 5 minutes handling — while thin-gauge and thick-gauge products route through the full sequence. Each class follows its own path automatically.

4. Balancing oven throughput against press demand.

• Estimating oven capacity versus press demand across three product classes at once is a multi-variable problem. Manual schedules often oversubscribe a press faster than the oven can feed it, or underutilise the oven by sequencing too many slow-forming products consecutively.
• Schantt models the oven as a flow stage with per-class throughput rates and each forming press as a batch stage with per-class cycle times. The simulator feeds each press from the oven at the correct rate for the product class being run, and when material runs out before the next batch arrives, it inserts a wait-material pause. The planner sees this as a gap between processing segments on the Gantt.

5. Calendar complexity across shifts, holidays, and maintenance.

• When planners track working hours, holidays, and maintenance windows separately — often in a spreadsheet or wall calendar — a change in one element (a new holiday or a postponed maintenance day) forces a full schedule revision.
• Schantt models two calendars (standard two-shift and reduced summer single-shift) with the ability to switch between them via a schedule calendar period. Calendar exceptions handle the seven non-working days, and machine downtimes capture the oven cleaning and press calibration events. The schedule automatically respects every availability window and outage, and updating a calendar or exception immediately propagates through all generated schedules.

What to model in Schantt

The table below lists the first-class entities a planner creates to represent a thermoforming plant in Schantt. Sub-configuration items such as per-class routings, changeover times, transfer times, calendar exceptions, and downtimes are configured on the detail pages of these entities and are covered in the step-by-step setup that follows.

Step-by-step setup

1. Create the stages in order. Define five stages at positions 1 through 6 (cooling is not a stage — it is modelled as a transfer time): Oven Heating as a flow stage, Forming as a batch stage, Trimming as a batch stage, Secondary Operations as a batch stage, and Packaging as a flow stage. On each stage's detail page, set the transfer times between consecutive stages:
- Heating to forming: 2 minutes
- Forming to trimming: 120 minutes (cooling dwell)
- Trimming to secondary: 5 minutes
- Secondary to packaging: 5 minutes
- Trimming to packaging (for thin-gauge route): 3 minutes
- Forming to packaging (bridge for medical trays skipping trim and secondary): 125 minutes

2. Add the machines to each stage. Assign one machine to Oven Heating (OV-1), six to Forming (FP-1 through FP-6), three to Trimming (TP-1 through TP-3), one to Secondary Operations (DR-1), and two to Packaging (PK-1, PK-2). Each machine inherits the working hours of the default calendar unless a machine-specific override is needed.

3. Create the product classes and define their routings. Set up three product classes and assign each its stage sequence on the Product Class detail page:
- Thin-gauge food packaging — Oven Heating, Forming, Trimming, Packaging
- Thick-gauge industrial parts — Oven Heating, Forming, Trimming, Secondary Operations, Packaging
- Medical device trays — Oven Heating, Forming, Packaging (skips Trimming and Secondary Operations)

For the medical-tray class, no partial-transfer setting is needed — the bridge transfer time of 125 minutes from Forming to Packaging already captures the cooling dwell plus handling.

4. Add one product per class. Create a representative product for each class. These are the items that appear on the schedule and the Gantt:
- Hinged deli container (thin-gauge food packaging class)
- Automotive trim panel (thick-gauge industrial parts class)
- Sterilisation tray (medical device trays class)

5. Configure machine capacity parameters and changeovers. On each machine's detail page, set the batch cycle time and batch size (for batch-stage machines) or throughput rate (for flow-stage machines) per product class. Key values:
- Oven throughput (per class): 720 sheets per hour for thin-gauge, 360 for thick-gauge, 540 for medical trays
- Forming cycle times (per class on FP-1): 48 seconds (thin-gauge), 90 seconds (thick-gauge), 72 seconds (medical trays)
- Other presses have compatible subsets — assign each press the classes it can run. FP-2 and FP-5 handle thin-gauge and medical trays; FP-3 handles thick-gauge and medical trays; FP-4 handles thin-gauge and thick-gauge; FP-6 handles thick-gauge only
- Trim cycle times: 4 seconds per batch (thin-gauge on TP-1 and TP-2), 15 seconds (thick-gauge on TP-1), 30 seconds (thick-gauge on TP-3)
- Secondary drilling: 12 seconds per batch on DR-1 (thick-gauge only)
- Packaging throughput: 120 units per hour on PK-1 for thin-gauge, 60 for thick-gauge; 60 units per hour on PK-2 for medical trays

Then set the sequence-dependent changeover times on each press. Record the directional duration for every pair of product classes the machine handles (for example, FP-1: thin-gauge to thick-gauge 55 minutes, thick-gauge to thin-gauge 25 minutes, thin-gauge to medical-tray 40 minutes, medical-tray to thin-gauge 25 minutes, thick-gauge to medical-tray 40 minutes, medical-tray to thick-gauge 55 minutes). At Trimming, set the changeover between thin-gauge and thick-gauge on TP-1 (20 minutes switching from thin-gauge to thick-gauge, 15 minutes for the reverse). At Oven Heating, set changeovers on OV-1 for each class pair (thin-to-thick 20 minutes, thick-to-thin 15 minutes, thin-to-medical 15 minutes, medical-to-thin 15 minutes, thick-to-medical 20 minutes, medical-to-thick 15 minutes — these represent zone-temperature stabilisation time when switching sheet gauges). At Packaging, set changeovers on PK-1 between thin-gauge and thick-gauge (20 minutes each direction for format adjustment).

6. Configure calendars, exceptions, and downtimes. Set the default calendar to the standard two-shift pattern (Monday–Friday, 06:00–22:00). Create a second calendar for the reduced summer single-shift (Monday–Friday, 06:00–14:00). Add seven calendar exceptions for non-working days: New Year's Day (1 January), International Workers' Day (1 May), and five consecutive year-end shutdown days (24–28 December). Finally, add two downtimes: the oven belt cleaning (15–16 July) and a factory-wide press calibration (10 August, afternoon shift).

For step-by-step instructions on configuring each of these in Schantt, see the Schantt documentation.

Common mistakes

1. Using a single changeover time instead of directional pairs. A press that handles thin-gauge and thick-gauge tools may take 55 minutes to swap one direction but only 25 minutes the other. A single blanket changeover ignores this asymmetry and produces a schedule that overestimates or underestimates the real setup time.
- Fix: Enter both directional durations for every product-class pair on each press. Schantt uses the directional value matching the actual transition, so the optimiser can favour the faster direction when sequencing jobs.

2. Modelling cooling as a separate stage with a machine. Adding a dedicated Cooling stage with a dummy machine or a calendar-based delay introduces an artificial queue that does not exist on the factory floor and makes the schedule harder to read.
- Fix: Model cooling as a transfer time (120 minutes) from the Forming stage to the Trimming stage. No machine, no additional stage — just a forward delay that every formed batch automatically observes.

3. Creating one product class for all products with a single routing. When thin-gauge, thick-gauge, and medical-tray products share one product class, they inherit the same route through every stage. There is no way to make medical trays skip trimming without creating a separate class.
- Fix: Define a product class per routing variant. Each class gets its own stage sequence, and the bridge transfer time for the skip route keeps the schedule connected.

4. Entering throughput instead of cycle time for a batch press. Batch-stage machines such as forming and trim presses run one cycle per batch, not a continuous hourly rate. Entering a throughput value for these machines causes Schantt to time them as a flow line, producing unrealistic durations.
- Fix: On the machine detail page, set the batch cycle time and batch size for each product class the press handles. Only flow stages (the oven and packaging lines) use throughput values.

5. Forgetting to set per-class changeovers on every press that runs multiple classes. A forming press that handles thin-gauge and medical-tray products needs a changeover between those two classes, even if a neighbouring press already has the same pair configured. Schantt treats each machine independently.
- Fix: For every machine shared by two or more product classes, check that the changeover time entry exists for each direction. Missing entries cause the algorithm to assume zero changeover time between those classes.

What a good schedule looks like

Before moving to structured scheduling, PreciseForm's planners built weekly plans on a shared spreadsheet, manually sequencing jobs across the six forming presses while tracking cooling dwells, changeover sequences, and holiday windows by eye. The result was a schedule that worked in principle but hid significant gaps in utilisation and timing that only became visible when a press operator reported waiting for heated sheet or a trim die was not ready.

Before (manual spreadsheet):
- Forming presses switched between product classes several times per week, accumulating 3 to 5 hours of avoidable changeover time across the press fleet
- The two-hour cooling dwell was approximated rather than tracked per batch, causing downstream trim presses to idle for 30 to 60 minutes while waiting for cooled parts
- Holiday and maintenance shutdowns were spotted only when the affected week's schedule was being built, forcing last-minute re-planning
- Total production time for a typical mixed-product week stretched beyond the available working hours, requiring overtime or partial deferrals

After (Schantt Auto mode):
- Changeover time is reduced by up to 20 percent across the fleet — the optimiser clusters same-class runs and favours faster-direction transitions, freeing 6 to 8 hours of productive press time per week
- Every batch respects the exact 120-minute cooling dwell; trim presses start precisely when material arrives, eliminating idle gaps caused by approximation
- Calendar exceptions and maintenance downtimes are pre-configured, so any schedule built after the shutdown week automatically accounts for the closure — no manual checking
- Total production time for a mixed-product week drops by 15 to 20 percent compared to the spreadsheet baseline, with all three product classes completing within standard working hours and no deferrals