Production Scheduling for Silicone Sealants

Silicone sealant production blends batch mixing with continuous filling and passive curing — a hybrid flowshop that planning teams typically manage on spreadsheets despite the complex interplay of directional changeovers, multi-machine stages, and cure delays. This guide shows how to model that pipeline in Schantt, configure the key parameters, and produce a schedule that respects the real constraints of a silicone sealant plant.

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

Industry context

Silicone sealants are single-part, moisture-cure adhesives produced in a two-stage pipeline: batch mixing of base polymer (polydimethylsiloxane, or PDMS), fillers, and additives, followed by continuous dispensing into cartridges or sausage packs. The material then cures passively by ambient moisture, a process that takes hours or days depending on the formulation. This guide covers single-part (RTV-1) silicones, which account for the majority of construction and industrial sealant production; two-part (RTV-2) systems, which involve convergent mixing of separate components, follow a different topology and are not addressed here.

Three main product classes dominate the silicone sealant market: Acetoxy Construction (standard cure, opaque, the highest-volume class), Neutral Construction (low-odour, flexible, used in applications where cure by-products must be minimised), and UV-Cure Specialty (instant-cure, used in precision assembly and glazing where throughput is critical). Each class shares the same two production stages but differs in mixing eligibility, changeover behaviour, and cure duration — differences that make a one-size-fits-all schedule unworkable.

A typical mid-size facility spans roughly 3,500 m² and produces around 3,000 tonnes per year across approximately 15 active SKUs. Production runs Monday through Friday in two 8-hour shifts from 06:00 to 22:00, with four annual non-working days and scheduled maintenance windows. The mixing stage uses two planetary mixers (1,200 kg batch size, 150-minute cycle) and one sigma-blade kneader (600 kg batch, 210-minute cycle). The dispensing and filling stage runs two cartridge fillers and one sausage-pack filler, with line speeds of 700 units per hour for cartridges and 500 units per hour for sausage packs. Cure times vary by class: 24 hours for Acetoxy Construction, 48 hours for Neutral Construction, and 5 minutes for UV-Cure Specialty — a span that ranges from near-instant to two full days, demanding per-class scheduling treatment.

Novaseal runs roughly 65 people at a 3,500 m² facility, making 3 product classes across 2 production stages, scheduled by a 2-person planning team.

Process overview

flowchart LR
  MX["Mixing<br/>(Batch — 3 parallel mixers)"]
  DF["Dispensing and Filling<br/>(Flow — 3 parallel fillers)"]
  MX -->|"180 min QC hold"| DF

The two-stage silicone sealant production pipeline: batch mixing feeds continuous dispensing and filling, with a 180-minute QC-hold transfer time bridging the stages. Cure delays are modelled as per-class transfer times after the filling stage.

All three product classes (Acetoxy Construction, Neutral Construction, UV-Cure Specialty) route through both stages — no stage skipping. UV-Cure Specialty's near-instant cure (5 minutes) is included for routing completeness and does not require a rack hold.

Scheduling challenges and how Schantt handles them

Silicone sealant production is driven by a make-to-stock, forecast-driven demand pattern — the planning team works from a monthly or weekly sales forecast and schedules production runs to maintain inventory levels across roughly 15 SKUs. (If your plant schedules against firm customer orders rather than forecasts, the same modelling approach applies; your job list simply comes from an order backlog instead of a forecast.) The scheduler's objective is to minimise total production time across all jobs in the plan, scheduling forward from a chosen start date over a practical horizon of two to four weeks. Within that horizon the planner must sequence mixing batches so that the right material reaches each filler at the right time, while respecting colour-changeover penalties and keeping cure delays from pushing packaging into the following week.

Schantt offers two optimisation modes. Auto mode accepts a list of products and quantities without prescribing a sequence — the algorithm decides job order, machine assignments, and timing to minimise total production time. Semi-Auto mode lets the planner fix the production sequence while the algorithm optimises machine assignments within that order. This guide describes the scenario in Auto mode; planners who need to preserve a specific sequence (for example, a customer-mandated run order) can switch to Semi-Auto with the same model.

What Schantt handles well

• Mixed batch-and-flow pipelines — Schantt models batch mixing (fixed-cycle) and continuous filling (throughput-rate) in the same ordered route, with material handoffs and wait-material pauses when a downstream stage outruns upstream supply.
• Multi-machine stages with shared and dedicated resources — Planetary mixers, sigma-blade kneader, cartridge fillers, and sausage-pack filler each modelled as parallel machines with per-class eligibility.
• Sequence-dependent changeovers with directional asymmetry — Directional changeover matrices per machine and product-class pair cover light-to-dark (20 minutes) versus dark-to-light (45 minutes) colour transitions and chemistry-crossovers (30 minutes each direction).
• Per-class routing with differentiated cure times — Three product classes (Acetoxy, Neutral, UV-Cure) share the same core route but carry distinct cure durations via per-class transfer time entries.
• Inter-stage material hold as transfer time — Ambient moisture-cure (24–48 hours) modelled as a fixed forward delay between filling and final packaging, elapsing in wall-clock time with no machine consumed.
• Shift-aware calendar modelling — Two-shift weekday pattern (Monday–Friday, 06:00–22:00), calendar exceptions for holidays, machine-specific downtimes for maintenance, and a year-end shutdown all affect timing and render as Gantt overlays.

How Schantt handles each challenge

1. Directional changeovers on shared mixing equipment.

• The two planetary mixers serve both Acetoxy Construction and Neutral Construction classes. Changing from a light-coloured batch to a dark one takes 20 minutes, but the reverse — dark to light — requires 45 minutes of cleanout. Chemistry crossovers between Acetoxy and Neutral add 30 minutes in either direction. A manual planner applying a single average changeover loses the opportunity to sequence intelligently and spends an estimated 2–3 hours of non-productive cleanout per 8-job sequence.

• Schantt models changeovers as a directional, per-machine matrix. Each from-class-to-class transition carries its own duration, so the algorithm can favour a light-to-dark sequence over dark-to-light where it shortens total production time. Chemistry crossovers are entered alongside colour transitions on the same machine, and the scheduler factors every transition's actual time into each job's start. The result is a sequence that clusters similar classes and respects directional time asymmetry without manual guesswork.

2. Batch-to-flow coordination and filler starvation.

• A single 1,200 kg planetary batch feeds around 4 hours of cartridge filling at 700 units per hour. If the next batch is delayed — because the mixer is occupied with a different class, undergoing a long dark-to-light cleanout, or waiting for a QC hold to elapse — the filler runs dry and idles until the next batch arrives. Manual coordination across batch and flow stages is a common source of hidden downtime.

• Schantt models the material handoff between the batch mixing stage and the flow filling stage explicitly. When the downstream filler has consumed all available material from upstream, the simulation emits a wait-material pause on the filler's operation, visible on the schedule and Gantt as a separated segment labelled with its cause. The algorithm minimises total production time by sequencing mixing jobs so that filler supply arrives before starvation occurs, and the planner can see exactly where gaps appear.

3. Dedicated UV-Cure lane with differentiated cure timing.

• UV-Cure Specialty runs exclusively on the sigma-blade kneader (600 kg batches, 210-minute cycles) and fills on the sausage-pack filler at 500 units per hour. Its near-instant 5-minute cure stands in stark contrast to the 24–48 hour ambient moisture cure of the other two classes. Modelling three different cure delays on the same route — without a separate curing stage — requires per-class timing that a spreadsheet cannot maintain cleanly.

• Each product class carries its own cure transfer time — Acetoxy Construction at 1,440 minutes (24 hours), Neutral Construction at 2,880 minutes (48 hours), UV-Cure Specialty at 5 minutes — configured as a fixed forward delay after the filling stage. The scheduled delay elapses in wall-clock time (continuous, not constrained to working shifts), so a Neutral Construction job that finishes filling on a Friday afternoon will show completion for packaging on Sunday afternoon. No separate cure stage or rack-tracking is needed.

4. Shift-constrained production with calendar exceptions and maintenance.

• The plant operates Monday to Friday, 06:00 to 22:00, with two overlapping 8-hour shifts. Four annual non-working days, two half-day maintenance windows (one per mixer), and a year-end shutdown further limit available production time. A 48-hour cure delay that must elapse in wall-clock time should not be compressed by non-working gaps, while machine processing must respect shift boundaries. Reconciling these overlapping time models manually is error-prone.

• Schantt's calendar model distinguishes working time (used for machine processing) from wall-clock time (used for transfer delays). Cure transfer times are set to elapse continuously, so a 48-hour delay spans the full calendar regardless of weekends or holidays. Machine processing — mixing cycles, filling runs, and changeovers — advances by working time only, clamping to the next shift start when an operation would overlap a non-working gap. Annual holidays and maintenance downtimes block scheduling automatically and render as shaded Gantt overlays with reason labels.

What to model in Schantt

The following table lists the five first-class entities a planner creates to represent a silicone sealant facility in Schantt. All counts are bare integers drawn from this guide's dataset. Sub-configuration items such as changeover entries, per-class routings, transfer times, calendar exceptions, and machine downtimes are configured on the detail pages of these entities — they are described in the step-by-step setup below but not enumerated here.

Step-by-step setup

1. Create the two stages in order. Define Mixing as a batch stage at position 1 and Dispensing and Filling as a flow stage at position 2. On the Mixing stage detail page, add a transfer time of 180 minutes from Mixing to Dispensing and Filling to represent the QC hold buffer between stages. On the Dispensing and Filling detail page, add the cure transfer times to the downstream packaging step — these are not a physical stage but a scheduled delay applied per product class after filling.

Cure transfer times:
- Acetoxy Construction — 1,440 minutes (wall-clock)
- Neutral Construction — 2,880 minutes (wall-clock)
- UV-Cure Specialty — 5 minutes (wall-clock)

2. Add the six machines to their stages. On the Mixing stage, create Planetary Mixer A, Planetary Mixer B, and Sigma-blade Kneader. On the Dispensing and Filling stage, create Cartridge Filler 1, Cartridge Filler 2, and Sausage-pack Filler.

3. Create the three product classes and define each class's routing. Acetoxy Construction and Neutral Construction both route through Mixing then Dispensing and Filling, with access to the planetary mixers and cartridge fillers. UV-Cure Specialty routes through both stages but is restricted to the sigma-blade kneader at mixing and the sausage-pack filler at dispensing. Partial transfer is not enabled for any class — the full batch must complete before the downstream stage begins.

4. Add one representative product per class. Create Novaseal AC-100 under Acetoxy Construction, Novaseal NC-200 under Neutral Construction, and Novaseal UV-10 under UV-Cure Specialty. Each product inherits its class's routing and machine eligibility. In practice a facility may have five or more SKUs per product class; adding the full SKU catalogue is a straightforward extension once the class-level model is validated against these representative products.

5. Configure machine capacity parameters and changeovers. On each machine's detail page, set the batch or flow parameters, then enter the directional changeover times between classes.

Mixing batch parameters (cycle duration / batch size):
- Planetary Mixer A — 150 min / 1,200 kg (Acetoxy and Neutral)
- Planetary Mixer B — 150 min / 1,200 kg (Acetoxy and Neutral)
- Sigma-blade Kneader — 210 min / 600 kg (UV-Cure only)

Filling flow throughput:
- Cartridge Filler 1 — 700 units/h (Acetoxy only)
- Cartridge Filler 2 — 700 units/h (Acetoxy and Neutral)
- Sausage-pack Filler — 500 units/h (UV-Cure only)

Changeovers on planetary mixers (both machines identical):
- Acetoxy to Neutral — 30 minutes (chemistry crossover)
- Neutral to Acetoxy — 30 minutes (chemistry crossover)
- Light to dark (Acetoxy light to Neutral dark) — 20 minutes
- Dark to light (Neutral dark to Acetoxy light) — 45 minutes

Changeovers on Cartridge Filler 2:
- Acetoxy to Neutral — 10 minutes (inter-class)
- Neutral to Acetoxy — 10 minutes (inter-class)

6. Configure the calendar, exceptions, and downtimes. Set the default calendar to Monday–Friday, 06:00–22:00. Add four calendar exceptions (New Year's Day, International Workers' Day, and two year-end shutdown days). Add Planetary Mixer A's H1 maintenance window (June 22, 08:00–12:00), Planetary Mixer B's H2 maintenance window (November 30, 08:00–12:00), and the year-end plant shutdown (December 24 to January 2).

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

Common mistakes

1. Entering colour-directional changeovers but forgetting chemistry-crossover entries on the same machine. Planetary mixers handle both Acetoxy Construction and Neutral Construction. A colour transition from an Acetoxy batch to a Neutral batch may also be a chemistry crossover, and if only the colour-directional time is entered while the chemistry-crossover time is omitted, the algorithm applies the wrong duration. Fix: For every machine shared by two or more product classes, enter both the colour-directional and the chemistry-crossover changeover pairs explicitly.

2. Using one blanket changeover time per machine instead of per-pair directional entries. A single changeover value applied to all transitions ignores the 20-minute versus 45-minute asymmetry between light-to-dark and dark-to-light. The algorithm then treats both directions equally, losing the sequencing advantage that favours light-to-dark runs. Fix: Enter a separate changeover row for each directed from-class-to-to-class pair, with its own duration.

3. Omitting the QC-hold transfer time between mixing and filling. Without the 180-minute transfer time from Mixing to Dispensing and Filling, the schedule assumes material moves instantly between stages. The filling stage then starts before the QC hold has elapsed, producing start times that are earlier than the floor can achieve. Fix: Add a transfer time of 180 minutes from the Mixing stage to the Dispensing and Filling stage on the Mixing stage detail page.

4. Setting cure transfer times to working-time instead of wall-clock. If a cure delay of 1,440 minutes is configured to respect working time, a job that finishes filling on Friday at 20:00 will complete its cure in working minutes stretched across several calendar days, producing a packaging start that is unrealistically compressed. Fix: Ensure cure transfer times are configured as wall-clock (continuous) delays, not constrained to the shift calendar.

5. Forgetting to restrict machine eligibility per product class. If UV-Cure Specialty has access to all three mixers by default, the algorithm may assign it to a planetary mixer whose batch size (1,200 kg) and cycle time (150 minutes) do not match the UV-Cure formulation's sigma-blade process. The same risk applies at the filling stage: without eligibility restrictions, a filler that lacks the correct nozzle or pump configuration for a given class may be assigned unscheduled work. Fix: On the product class routing page, set machine eligibility so that UV-Cure Specialty routes only to the sigma-blade kneader for mixing and only to the sausage-pack filler for dispensing. Similarly restrict Neutral Construction from the UV-Cure-only filler to prevent incompatible assignments.

What a good schedule looks like

Before adopting Schantt, this scenario's schedule is built manually on a spreadsheet. The planner re-solves the light-to-dark sequence every time the demand forecast changes, applying a rule-of-thumb changeover average that overestimates some transitions and underestimates others. The result is a schedule that contains hidden filler starvation gaps and inconsistent cure timing, requiring frequent rework when actuals diverge from the plan.

Before (manual spreadsheet):
- Light-to-dark sequence re-solved from scratch each time demand changes — no persistent optimisation
- Average changeover times used instead of directional values, producing optimistic timing for dark-to-light transitions
- Filler starvation gaps discovered only after a job runs short on material
- Cure delays tracked manually per batch, leading to occasional packaging starts before cure completion

After (Schantt Auto mode):
- The algorithm sequences jobs to favour light-to-dark transitions and clusters chemistry-compatible runs, reducing total changeover time compared to a manual average-based sequence
- Material handoffs between mixing and filling are modelled explicitly — filler starvation gaps are visible on the Gantt and minimised by upstream sequencing
- Cure transfer times apply automatically per product class, elapsing in wall-clock time without manual tracking
- Calendar exceptions and maintenance downtimes are factored into timing from the start, so the schedule never schedules work into a holiday or outage

The planner loads the product list — typically a week's worth of mixing batches and their corresponding filling quantities — selects Auto mode, and reviews the optimised Gantt. Where a specific production order is required (for example, a customer-mandated Neutral Construction run that must precede an Acetoxy run), the planner switches to Semi-Auto mode for that job list, fixing the sequence while retaining machine-assignment optimisation. The time saved on changeover sequencing and starvation-gap detection is redeployed to higher-value planning decisions — tuning batch sizes, evaluating shift extensions, or responding to forecast changes with a single re-schedule rather than a full manual rebuild.