Chalk Extrusion Machine Product

Overview

Chalk extrusion machines produce school chalk sticks and industrial marking chalk by continuously mixing calcium carbonate powder with water and binder into a slurry, extruding the paste through precision die holes into cylindrical sticks, cutting to length, and drying. These machines combine wet-mix chemistry, positive-displacement pumping, precision extrusion, and controlled drying to achieve 300–600 chalk sticks per minute with consistent diameter, length, and hardness. The process is fundamentally different from dry-pressing; extrusion allows for rapid production and excellent stick integrity, though it requires careful management of slurry viscosity, drying kinetics, and product hardening to avoid warping or cracking.

How It Works

Slurry Preparation and Mixing

The process begins at the Slurry Mixing Vessel vessel, a double-jacketed stainless steel tank (50–200 L capacity). Calcium carbonate powder (precipitated or ground limestone) is charged into the tank along with water and an organic binder (typically gum arabic, carboxymethyl cellulose, or synthetic polymer). A Mixer Drive Motor (variable-speed, 10–50 rpm) drives a High-Shear Mixer Blade (dual-stage turbine blade), generating high shear forces that homogenize the powder and water into a uniform paste. The RTD or Thermocouple Probe monitors slurry temperature; mild exothermic mixing heat is managed by the jacket, which can be cooled with chilled water if needed. A Pressure Sensor relief valve prevents over-pressurization.

Typical slurry composition is 70–80% calcium carbonate, 15–25% water, and 2–5% binder. The viscosity is 500–2000 cP—highly shear-thinning paste that flows smoothly under pressure but resists sagging when stationary.

Slurry Feeding and Pressurization

The Slurry Feed Pump (gear pump or progressive-cavity screw pump) draws slurry from the mixer tank via suction and meters it to the extrusion head at controlled volumetric flow rate. A Pressure Sensor downstream monitors back-pressure; a relief valve prevents system pressure from exceeding 50 bar. The Encoder senses pump displacement or motor speed, providing flow-rate feedback to the controller for synchronization.

Extrusion and Stick Formation

At the extrusion head, the Extrusion Head Assembly assembly forces slurry through a Extrusion Die Plate under 20–50 bar pressure. The die plate is hardened tool steel with 6–12 precision-drilled cylindrical orifices (6–12 mm diameter, 50–100 mm depth). As slurry exits the orifices, it forms continuous cylindrical sticks. A Die Jacket Heater maintains die temperature at 25–35 °C; warmer dies reduce slurry viscosity (faster flow, thinner walls), while cooler dies increase viscosity (slower flow, thicker sticks).

Immediately below the die plate, freshly extruded chalk sticks fall or are guided onto the discharge conveyor.

Stick Cutting to Length

The Length Cutting Station station cuts the continuous chalk strands into individual sticks. A Cutting Blade Actuator (pneumatic or motor-driven linear actuator) drives a Cutting Blade with 1–5 Hz cutting frequency. A Cut Length Stop precision fence controls stick length to ±2 mm tolerance (typically 70–100 mm). The Encoder counts blade strokes, confirming consistent cut rate and detecting blade wear or stalling.

Drying and Moisture Removal

The most critical process step is the Drying Conveyor System conveyor system. Freshly cut chalk contains 15–25% moisture and is soft; it must be dried to 2–4% moisture over 4–8 hours to achieve final hardness and prevent warping. The Drying Conveyor Belt (stainless steel mesh) transports chalk at 2–10 m/hour through a heated drying chamber maintained at 40–60 °C.

Two Heating Element banks and a Blower Motor circulation fan distribute uniform heat across the chalk bed. Two RTD or Thermocouple Probe probes (inlet and outlet air) feed back to the Control and Synchronization PLC, which adjusts heater output via PID control to maintain setpoint. Under-drying (moisture >5%) leaves chalk brittle and prone to mold; over-drying (moisture <1%) embrittles chalk and may cause cracking due to differential shrinkage stress.

Cooling and Hardening

After exiting the drying chamber, chalk proceeds to the Cooling and Hardening Zone zone—a Cooling and Vibration Table vibration table at room temperature. Here chalk cools gradually, hardens further, and equilibrates its internal moisture distribution. Vibration helps settle chalk and prevent sticking. A Defect Detection Sorter (optical or mechanical sensor) inspects each stick, rejecting cracked, warped, or undersized pieces by diverting them to a scrap chute.

Discharge and Packaging

Good chalk is transferred to the Discharge and Collection system—a low-friction discharge belt conveyor directing chalk into collection bins or directly into box-filling equipment. A Collection Bin or Chute chute routes chalk into boxes for immediate secondary packaging or into bulk storage for later wrapping and cartoning.

Process Synchronization

All operations are coordinated by the Control and Synchronization PLC:

• Mixer speed: Maintained at constant rpm to ensure consistent slurry viscosity and binder dispersion
• Pump flow rate: Adjusted via VFD to match extrusion demand and maintain die pressure at setpoint
• Cutter frequency: Pulsed to deliver pre-set stick length based on conveyor speed
• Drying temperature: PID-controlled via heater modulation to maintain ±3 °C accuracy
• Conveyor speed: Driven by VFD and synchronized to cutter frequency

Encoder feedback from pump, conveyor, and blade enables closed-loop control. If flow rate drops (cutter blade dull or extrusion line blockage), pressure rises and the controller alerts the operator.

Engineering Considerations

Slurry Viscosity Management: Viscosity is highly temperature- and shear-dependent. A 5 °C change in die temperature can shift extrusion rate by 10–20%. Precise temperature control and feedback are essential for consistent stick diameter.

Binder Chemistry: Binder choice affects chalk hardness, water solubility, and dust production. Gum arabic produces soft chalk; synthetic binders (carboxymethyl cellulose) yield harder chalk. Over-binder (>5%) causes sticky extrusion; under-binder (<2%) produces friable, weak sticks.

Drying Kinetics: Chalk drying follows a parabolic (diffusion-limited) curve. Initial moisture loss is rapid (first 1–2 hours), then slows. Uneven drying causes warping; the center of a thick stick dries slower than edges, creating internal stress.

Die Clogging: Calcium carbonate precipitation or binder agglomeration can block die orifices. Regular flushing with water and periodic die disassembly (every 8–12 hours) prevent blockages.

Stick Cracking: Over-drying, rapid cooling, or high internal stress from uneven drying cause longitudinal cracks. Moisture content target of 2–4% and slow cooling (24-hour ambient equilibration after drying) minimize defects.

Production Metrics

A typical machine with 8 extrusion orifices and 2-second cycle time produces 240 chalk sticks per minute (14,400/hour). At 5 grams per stick, this equals 72 kg/hour or 4 tons/day. Power consumption is 7–15 kW, with drying heater accounting for 60% of demand. Extrusion die wear requires replacement every 500,000–2,000,000 sticks depending on wear rate and slurry abrasiveness. Slurry batch life is typically 8–16 hours; after which binder breakdown and moisture creep reduce extrusion quality.