Plastics and Rubber: Silos, Additives and Traceability

Technical guide by Arvin, METRAVON Instruments · Reviewed September 2026

Short answer

Use separate measurement functions for bulk resin inventory, high and low alarms, minor-additive dosing and production-batch records. Plastic pellets are often suitable for non-contact continuous level measurement, while carbon black, mineral fillers and fine additives require closer review of dust, adhesion, static charge, hazardous-area requirements and feeder behavior. Do not size one weighing system to cover both large main-material batches and very small additives unless the required resolution is demonstrated.

Material behavior changes the design

Silo inventory measurement

For non-contact radar, confirm silo height, diameter, filling point, internal structures, dust, temperature, nozzle and expected surface profile. Pneumatic filling can create a strong material stream and dense dust. Mount away from the inlet and dust-extraction flow, and verify performance during filling rather than only when the silo is quiet.

Level-to-mass conversion requires a vessel strapping table and material-specific bulk density. Pellets, regrind and aerated powders may have different densities even when they feed the same process. Preserve the original level data and document every conversion parameter.

Point-level and flow protection

Use high-level alarms to prevent overfilling and low-level or flow checks to protect conveying and dosing equipment. Select the sensing principle from particle size, minimum density, adhesion and static behavior. Protect probes from the direct filling stream without creating a shield that traps material.

At hopper outlets, a covered probe does not prove material is flowing. Regrind and cohesive additives can bridge or rat-hole. Where blockage has a significant production consequence, combine the level switch with feeder feedback or another suitable flow indication.

Minor-additive weighing

Check the smallest target against total scale capacity, installed division, vibration and feeder overshoot. If a minor ingredient is only a small fraction of the main scale range, use a dedicated small-capacity scale or a validated premix process. A higher-resolution display alone does not create usable measurement resolution.

Match the feeder to powder or pellet behavior. Control coarse and fine feed, material in flight, stabilization and tolerance handling. Validate repeated batches at the smallest target and after realistic changes in hopper head, moisture and refill condition.

Static charge, dust and grounding

• Follow the plant's bonding, grounding and electrostatic-control design for vessels, conveying lines and instruments.
• Route and shield signal cables according to the electrical design; software filtering cannot correct unsafe grounding.
• Confirm dust classification, ignition risk, extraction and occupational-exposure controls for carbon black and fine powders.
• Verify equipment certificates against the actual hazardous area rather than a general product description.
• Keep maintenance and cleaning work within approved isolation and personal-protection procedures.

Changeover, cleaning and contamination

Color, grade or formulation changeover requires more than a recipe selection. Define the emptying, purging or cleaning sequence and the acceptable residual. Provide access to product-contact surfaces and prevent dead zones in hoppers, flexible connections and feeders.

Record the previous and next material, cleaning confirmation, raw-material lot, recipe version, actual weights, manual additions and release decision. When contamination matters, the weighing and data system must retain the exception rather than silently treating a manually corrected batch as normal.

System integration

Before connecting to a PLC or MES, define equipment and batch identifiers, units, decimal precision, recipe ownership, target and actual fields, alarm codes, timestamps and offline behavior. Keep dosing control local. If records are uploaded after an outage, use stable batch keys so retransmission does not duplicate production.

Commissioning and acceptance

1. Verify level readings with different materials and during pneumatic or mechanical filling.
2. Test high, low, blockage and device-fault signals through the complete control loop.
3. Calibrate each weighing range and run repeated minimum, normal and maximum batches.
4. Measure cycle time, batch error, overshoot, residue and out-of-tolerance frequency.
5. Perform a representative material or color change and verify cleaning and traceability records.
6. Test power and network interruption without losing control state or duplicating batch data.

Information required for selection

Provide resin, rubber, regrind, filler or additive name; particle form, bulk density, moisture, adhesion and static behavior; silo and hopper drawings; conveying method; minimum dose and tolerance; cleaning and changeover rules; dust and hazardous-area classification; batch-record fields; and PLC, MES or platform interfaces.

Frequently asked questions

Will static charge affect measurement?

It can affect equipment, signals and dust safety. Apply the site's bonding, grounding and shielding requirements; do not rely on damping or software filtering as a substitute.

Why use a separate scale for additives?

The additive target may be too small relative to main-scale capacity, division and feeder overshoot. A dedicated range can improve useful resolution and controllability.

How should carbon-black buildup be managed?

Reduce direct impact, provide safe inspection and cleaning access, and evaluate an appropriate antenna or purge arrangement. Set maintenance from observed buildup and signal trends.

What is needed for batch traceability?

Keep raw-material lot, recipe version, batch identity, targets, actuals, deviations, cleaning status, alarms and manual interventions with controlled timestamps.

Map material flow from storage to the process

Document every silo, day bin, vacuum receiver, loss-in-weight feeder and mixer connection. Identify which measurement controls replenishment, which protects against overfill and which confirms consumption. Resin pellets, regrind, fillers, carbon black, oils and minor additives behave differently; one default sensor or feeder setting will not cover all duties. Define expected rates, minimum operating stock and allowable production interruption for each stream.

Vacuum conveying creates rapid level changes, dust and electrostatic charging. The instrument must tolerate pressure cycles and should not face the inlet. Ground conductive parts according to the plant design. When a receiver cycles frequently, choose response and damping that detect useful states without generating chatter from every conveying pulse.

Inventory conversion and material identity

Radar measures distance to a local surface. Convert it to volume with the actual vessel geometry and to mass with a representative bulk density. Pellets and regrind can have markedly different density and angle of repose, so the material identity must select the correct calculation. Retain level, volume, density and calculated mass as separate data fields.

For costly additives, reconcile stock movements against weighed receipts and batch consumption. Investigate consistent bias, unexplained losses and step changes after refilling. Do not hide discrepancies by repeatedly editing density. Verify whether buildup, mixed material, an incorrect lot assignment or conveying loss explains the balance.

Minor-ingredient accuracy and feeder control

Determine absolute tolerance at the smallest batch. If the required addition is too small for the main scale, use a dedicated minor-ingredient system or preblend. Feeder performance depends on particle shape, aeration and head pressure. Establish coarse and fine rates with real material and compensate in-flight product without allowing uncontrolled self-learning.

Interlock weighing against open discharge paths and mechanical contact. Confirm stability before recording a weight, but do not use so much filtering that cycle time becomes excessive. Record target, actual addition, tolerance, lot and correction. A corrected batch should retain the original deviation for quality review.

Contamination control and changeovers

Colour, additive and allergen-like cross-contamination can make a small retained quantity important. Review dead legs, probe guards, flexible connectors and hopper corners. Define sequencing from light to dark or compatible formulations, purge quantity and verification method. Instrument surfaces and seals must tolerate the cleaning chemicals and temperatures actually used.

Barcode or lot confirmation should prevent the wrong material from being connected to a destination. Where automatic routing is used, verify valve feedback and destination permissives before transfer. Store routing events with batch records so a quality investigation can reconstruct the actual path.

Commissioning and ongoing capability

Test high and low level under conveying conditions, not only by touching the sensor. Verify radar at several levels and capture echoes during filling. Run batch trials for the smallest and largest additions, assess repeatability and measure cycle time. Simulate empty feeder, blocked transfer, wrong material confirmation, sensor fault and network loss.

Monitor batch deviation, feeder duration, scale zero, radar signal quality and inventory reconciliation. Use control charts or defined action limits to distinguish random variation from gradual wear. Revalidate after material grade, feeder, software, vessel or conveying changes. Preserve the approved configuration and recipe history.

Information required for selection

Provide vessel drawings, every material and grade, bulk-density range, temperature, conveying method, pressure cycle, inlet position, required level and weight outputs, smallest and largest batch, tolerance and production rate. Include hazardous-area classification, cleaning method, contact-material restrictions, control interfaces, traceability fields and photographs. Identify the business consequence of overfill, shortage, wrong material and dosing error. These details determine whether the solution needs continuous radar, independent switches, weighing, feeder control or a coordinated combination.

Engineering note: Compatibility, combustible-dust controls and occupational-health measures for carbon black or unknown additives require project-specific review.