Use Automation to Monitor Inventory in Resin Silos
Imagine your job didn’t involve climbing silos, guesswork or inventory shortages. Instead, each day workers stayed safe and inventory was convenient, accurate, and reliable. That’s what level sensors and software can do for your plant—easily and affordably.
Measurement Starts with the Sensor
In most cases, the most important job of a level sensor in a resin silo or hopper is to ensure adequate resin is flowing through the system to avoid process interruptions and reduce cycle times.
Level sensors are easily installed or retrofit on most any resin silo. Level sensors measure distances with high levels of accuracy, generally within +/-0.2". The types of sensors best suited for plastics are immune to the noise and dust that occur when filling the silos. They report inventory levels in real-time and are highly reliable, with a long service life and minimal maintenance.
Sensor Technologies to Measure Pellets, Powders or Flakes
A dust-penetrating non-contact radar is a go-to sensor for noncontact continuous level measurement. The advent of narrow-beam 80 GHz technology ensures reliable operation in dust and fast update rates. However, there are other types of sensor technology that may be better suited to the application or to the budget.
A 3D scanner that can measure multiple points on the material surface can account for surface variations of the resin in the silo, as well as conditions such as cone up or down, to account for irregularities when calculating volume.
Bob-style or weight-and-cable sensors work much like a tape measure—but automated. They take measurements at pre-determined time intervals such as once an hour, several times a day, or on demand. They have been utilized in plastic applications for more than two decades and are budget-friendly, mechanical devices with a long service life.
A battery-powered laser level sensor can be used if conduit and power are not readily available. This type of sensor can save significantly on wiring costs and hiring electricians. Laser technology performs best in low-dust applications typical of many pellet silos.
Measuring Liquid Polymers or Additive Tanks
Non-contact radar level sensors also can be used in liquid applications at plastics plants. Liquid resins and plasticizers are easily and continuously measured, even if condensation or turbulence are present. An 80 GHz level sensor can accommodate liquid tanks commonly 8' to 12' in diameter and 20' to 30' tall.
It is important to understand that level sensors measure the distance to the material from the sensor at the top of the silo. If the expectation is an accurate weight instead, a strain gauge or load cell may be a better option.
Simplicity Lies in the System
Thanks to sophisticated software that does the math for you, the distance to the resin can be converted to an estimate of mass or volume. The accuracy of the estimate is dependent upon entering the correct silo dimensions into the software and the evenness or irregularity of the piling or topography of the resin in the silo.
Processing and purchasing personnel can rely on the software for timely alerts and trend reporting. They need to see their silo inventories in multiple areas in the plant as well as when they are off site. Readings in the software program also are used by purchasing managers to monitor inventory, place orders in a timely manner and know when to schedule deliveries.
User-Friendly Software Platforms
As far as software is concerned, users need to determine if the software should be installed on a local area network (LAN) to access from a PC or a cloud-based software-as- a-service (SaaS) and accessed from the internet on a phone, tablet or PC. Both are easy to work with and highly affordable.
A software program installed on your local network is a one-time software purchase per plant location. The software accommodates up to 120 silos. Cloud-based programs typically are subscriptions based upon the number of silos you are monitoring. The cost can be several dollars per month per silo, plus a cellular program if one is required.
Cloud-based programs do not require an IT expert to install or manage the program. Updates to the software are provided by the host company, which also will provide the necessary training and ongoing support.
Both LAN and SaaS programs offer similar features. Both have easy-to-use, graphical interfaces. Bins can be sorted by type and status. An administrator can control who has access to the program and what information is available to users. Historical reports can be quickly downloaded for specific time periods.
Perhaps the most loved feature of inventory management software is automated alerts. Sent via text or email, these alerts help plants ensure that silos or hoppers don’t run empty and interfere with production. When used in tandem with purchasing, orders can be placed in a timely manner and deliveries scheduled so as not to interfere with production.
Preventing Overfills and Hopper Outages
A point-level indicator alerts when resins reach a certain point in the silo or hopper. Indicators are commonly used to prevent overfilling resin silos and ensure that hoppers have adequate material to feed into the molding process. They do not measure continuously, but instead indicate when material reaches a certain point.
Rotaries are the most tried and true level control. They operate by rotating a paddle in the material. When the bulk material reaches the level of the paddle, the paddle stops turning and sends an alert via a relay. Conversely, it can alert to a low level in a hopper if the resin falls to a level below the paddle and begins turning. These devices are offered with paddles for very light to heavy resins. Rotaries are inexpensive, last a long time, and are offered in fail-safe models if supplying material consistently is critical to a process.
The vibrating rod is another point-level sensor commonly used in plastics. It works by vibrating when uncovered; when the vibration stops upon being covered by resin, it sends an alert. It has adjustable sensitivity settings that can be adjusted for pellets or powders. A vibrating rod is not recommended if static electricity is present because it can cause the material to cling to the blade and create a false signal.
For large, active hoppers that need continuous tracking, a non-contact radar can track levels in real time and update in just seconds.
Customize a System to Your Operation
As inventory systems are very flexible, they are used in most any size of operation. An important budgetary consideration is scalability. Some plants want to add level sensors to silos incrementally or choose to selectively monitor only silos that are very active.
Vendor-managed inventory can be a valuable strategy for molders. Providing resin suppliers access to inventory enables them to be proactive in ensuring supplies are adequate and deliveries are scheduled to prevent shortages.
Different materials may require another type of sensor technology. For example, dust can impact the short-term performance of some technologies that have trouble seeing though dust while the silo is being filled.
It is easy to have a variety of sensor types at a single location, each suited to the size of vessel and the type of resin in it. There are sensors compatible with analog or 4-20 mA outputs, Modbus, or HART that can be applied as needed for
the job to be done.
For the most part, equipment used by compounders, extruders, injection molders, rotational molders and refiners is very similar. What you are molding doesn’t matter so much as how active the vessel or hopper is.