Level sensors for solids detect the presence or absence of bulk material, such as plastic granules or pellets, at a specific point in hoppers, containers and big bags.

Let’s look at how they work and how Vismec integrates them into granule conveying, dosing and storage systems to prevent machine downtime.

Capacitive level sensors in Venturi loaders

In plastic granule conveying, running out of material in the hopper of an injection molding machine or extruder is one of the most common causes of unexpected machine downtime.

When the machine runs out of granules, the production cycle stops, and restarting often results in waste and additional process stabilization time. To prevent this, the most reliable conveying systems integrate level monitoring at the point of use.

This is the case with the Vismec Venturi loader, an independent compressed-air feeding unit designed for conveying masterbatch and additives in granule form, which can also be installed above a drying hopper.

Let’s see its main features.

• Integrated capacitive sensor: it monitors the material level and activates a visual alarm when granules are running low, allowing the operator to intervene before the machine runs out of material.
• No moving mechanical parts: the Venturi effect generates vacuum without motors, reducing maintenance costs and ensuring quiet operation.
• Defined application range: suitable for short distances and low-volume production, with flow rates of up to 70 kg/h and adjustable conveying air power through a flow regulator.

How a capacitive level sensor works

A capacitive level sensor measures capacitance, the ability to store an electrical charge, between the probe and its surroundings.

When the granules cover the probe, they replace the air within the sensing field. Because the material has a different dielectric constant than air, capacitance changes, and the sensor sends a signal to the control system indicating the presence or absence of material.

With no moving parts, capacitive sensors work well with bulk solids such as plastic granules. However, sensitivity must be calibrated according to the material, as different polymers may produce different responses.

The sensor has a minimal adjustment range of approximately 24 mm. With such a limited range, the sensor does not indicate maximum and minimum material levels, but only the presence or absence of the material itself.

Level sensors on volumetric dosers: dosing continuity

When dosing masterbatch and additives, a consistent material feed directly affects the quality of the finished part.

A volumetric doser, for example, dispenses an amount of material based on screw rotation. If the hopper empties, the screw continues to rotate but the amount dispensed decreases. This can result in defective or out-of-spec parts that may only be identified during post-production quality control.

Monitoring the material level in the doser hopper helps prevent this situation by maintaining the quantity of material required for consistent dosing throughout the production cycle.

Within the range of Vismec dosing systems, the VD1 volumetric doser can be equipped with a Venturi loader featuring an integrated capacitive level sensor.

This solution complements other features that support production continuity: adjustable material loading time, quick-release screws available in different sizes according to the required flow rate, and a design that is easy to clean during color and material changes.

The VD1 doser is suitable for any injection molding or extrusion machine.

Level sensors for storage containers: the Viper big bag system

In granule storage, level sensors for containers perform a partly different function. In addition to indicating when material is running low, they signal when granules reach minimum or maximum thresholds in containers, silos and big bags.

Minimum and maximum level sensors are generally installed in containers and silos, while in big bags, which progressively change shape as they empty, level detection must take the deformation of the bag into account.

Knowing the level of stored material makes it possible to plan replenishment and replace empty containers before conveying to the machines is interrupted. This is particularly important in big bag emptying systems, as the material must be extracted completely and consistently even as the level inside the bag drops.

Would you like to reduce machine downtime caused by running out of plastic material during your company’s production processes?

Vismec can help you identify the most suitable monitoring solutions and determine where to install them along the entire granule handling process, from storage to dosing. Contact us for expert advice and a customized quotation.