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June 26, 2024

How to Batch Materials from Bulk Bags

If you’re considering switching from manual 50lb bag handling to automatic bulk bag unloading in your production process, knowing what type of batching is best for your operation is critical for success. In this blog, we break down the different types of batching methods and compatible materials to help find the right solution for your operation.

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Two Types of Batching Systems

There are two different types of batching methods. Each has its pros and cons, but deciding which is best for an operation usually comes down to cost, speed, traceability, and accuracy. Before we dive into the specifics on how each method works, here’s a quick comparison chart you can refer to. Out of the two, weight-based is a more common choice due to its traceability and accuracy. 


Volumetric vs. Weight-Based Batching 

Volumetric Batching Weight-Based Batching
Less complex More accurate
Easier to maintain More traceability
Lower cost More control
Best for non-hygroscopic materials such as plastic pellets and ball bearings.  Works on all materials including salt, sugar, flour and other hygroscopic materials
Commonly used in: 

Brewing & Distilling

Commonly used in:

Food Production


Battery Manufacturing


What is Volumetric Batching?

Volumetric batching uses a relatively simple system to measure and dispense material from bulk bags into a production line. As the name would suggest, volumetric batching uses volume rather than weight to measure the material it dispenses. 


How Volumetric Batching Works

These systems work by first catching and weighing a sample over a known time (a 5-gallon bucket filled for 30 seconds, then weighed). Once that time/weight rate is known, a simple formula can be derived to create a batch simply by running the metering device for the required time to complete the batch. For instance, let’s assume a screw feeder delivers 1 pound per second, and the batch requirement is 100 pounds. We then know we need to run the system for 100 seconds to hit our desired batch. These systems often use simple timers or programmable logic controllers (PLCs) with pounds-to-time ratios pre-programmed. 


Limitations of Volumetric Systems

The upsides to these systems are low cost and simplicity. The downside is the lack of traceability. Because these systems are so low-tech, there’s no positive record that the batches are completed. If the bag is emptied before the batch is done, and it isn’t replaced, then the batch total will be light. Another problem that can occur is if there is a feeder malfunction. The system is time-based, so it can’t detect how much (if any) material is actually making it into the batch. Low-level alarms can help counter these situations but add to the complexity and cost of the system, which may make weight-based systems more appealing.


Material Compatibility

The types of materials that you can reliably dispense using a volumetric measurement are non-hygroscopic materials such as plastic pellets and ball bearings — anything that doesn’t absorb moisture. Whereas, hygroscopic materials, such as salt, flour, and sugar, are very absorbent and sensitive to humidity. The density and material handling characteristics of hygroscopic materials change dramatically depending on the humidity which can throw off your measurements. This could cause problems seasonally if you operate in a region that gets extremely humid in the summer. It can also cause problems if you source your material from a humid region. 

Here’s an example: Let’s say you source sugar from South America but operate in Michigan — the sugar is likely to absorb moisture from the environment before it’s shipped. As it moves into lower and lower humidity areas it slowly dries out. The longer it sits on your production floor, the dryer it gets, and the more its density differs from when you first got it. 

That doesn’t mean you can’t run hygroscopic materials on volumetric systems, but it would require regular calibration of the machine and awareness that the flow rate could change very quickly if your material changes humidity at any point.


Common Applications

With 1% variance (at best), this system isn’t known to be incredibly accurate. Some common industries where precise measurements and traceability are not as critical, and where volumetric batching is commonly used, are agricultural feed production and the initial stages of brewing and distilling. 

Figure 1. FormPak volumetric screw feeder. Equipment shown here can be modified to fit in height-restricted environments.


What is Weight-Based Batching? 

Weight-based batching is a method that uses scales to measure materials. In general, this type of batching tends to have higher accuracy than volumetric, and because it doesn’t base measurements off of flow rate, it can handle virtually any dry bulk material — making it the go-to choice for most industries.

There are three types of weight-based methods: loss-in-weight, gain-in-weight, and hybrid (systems that use both loss and gain measurements). Here’s a quick comparison chart as an overview of the two methods. 


Gain-in-Weight vs. Loss-in-Weight vs. Hybrid Weight-Based Batching


Gain-in-Weight Loss-in-Weight Hybrid Weight-Based
Single scale  One scale per ingredient One scale per ingredient + One checker
Measures ingredients one at a time (high accuracy) Measures all ingredients at once (high speed) Measures all ingredients individually, then checks gross weight of all ingredients (high speed and high accuracy)
A good choice if:

  • You’re not mixing many ingredients or if production speed isn’t a major issue
A good choice if:

  • You are mixing more than ten ingredients
  • Speed if critical
A good choice if:

  • You need maximum speed, accuracy, and traceability


Loss-in-Weight (LIW) Batching Explained

Loss-in-weight batching is by far the most common way of dispensing a known amount of material from bulk bags because it offers high speed batching. The weighing process is performed at each ingredient unloader, which means weighing out multiple ingredients can happen concurrently. 


How a LIW System Works

These systems use 10,000 lb scales on each bulk bag unloader and can incorporate nearly any type of conveying/feeding device. The two most common are valves (slide gate or rotary valve) and feeders (screw, vibratory, inclined, etc.) metered systems. 


  • Step 1: The operator enters in the desired batch to the scale controller
  • Step 2: The scale tares to zero.
  • Step 3: The metering device actuates bulk mode (Fast mode in the case of feeders or conveyors, fully open in the case of a valve).
  • Step 4: As it nears the desired amount, the device shifts to slow mode (feeder) or dribble mode (in the case of a valve).
  • Step 5: Then the flow is stopped at the target weight. 

Figure 2. FormPak fork-loaded frame with LIW flexible screw and hoist & trolley frame with LIW rotary valve


What is Gain-in-Weight (GIW) Batching?

While not as popular as loss-in-weight systems, gain-in-weight systems can be more accurate and verifiable, and some are even certified by the NTEP (National Type Evaluation Program) – an independent testing and certifying organization for scale accuracy and calibration.

The major difference with a GIW system is that you have a single scale downstream where you weigh all of your ingredients (instead of having one scale per ingredient on a loss-in-weight system). That also means that only one ingredient can be weighed at a time. Depending on how many materials you’re weighing out, consecutive measuring can take significantly longer. 


How a GIW System Works

These systems also use a metering device (valve, feeder, etc.), but the scale is part of the downstream system. This can be a simple platform scale that a bin or hopper is placed on directly under the unloader, or the scale may be built into a blender, mixer, or scale hopper and material is conveyed up and into it. 


Aside from the location of the scale, the process works similarly:

  • Step 1: The operator enters in the desired batch to the scale controller
  • Step 2: The downstream scale tares to zero.
  • Step 3: The metering device actuates bulk mode (Fast mode in the case of feeders or conveyors, Fully open in the case of a valve).
  • Step 4: As it nears the desired amount, the device shifts to slow mode (feeder) or dribble mode (in the case of a valve).
  • Step 5: Then the flow is stopped at the target weight. 

Figure 3. FormPak gain-in-weight platform scale & flexible screw AND gain-in-weight platform scale & slide gate valve. Equipment shown here can be modified to fit in height-restricted environments.


Hybrid Weight-Based Systems for Multi-Ingredient Batching

For many industries, like food processing, creating a recipe with multiple ingredients requires the speed and traceability of the loss-in-weight systems and the accuracy of the gain-in-weight batching. We often end up creating a hybrid systems for many operations. 


Hybrid System for a Sausage Manufacturer | Case Study

  • The Problem: One great example of this is a system we created for a large sausage manufacturer that needed a better way to batch their flavor blends. Before working with the Formpak team, they relied solely on manual labor — team members would manually pour in 50lb bags of material into the meat blender. The problem is this left a lot of room for human error. When repeatedly dumping bag after bag, it’s easy to lose count. The company was having to throw away batch after batch due to inaccurate measurements and zero traceability. 


  • The Solution: To maximize speed and accuracy, we devised a system where the sugar, salt, onion powder, and proprietary spice would be measured out concurrently using a loss-in-weight system and then fed into a mini hopper where the gross material weight is captured as a verification of batch (using a gain-in-weight scale) before dumping it into the blender. 


Figure 4. Hybrid system created for sausage manufacturing.


Designing the Right System for Your Operation 

To maximize speed and accuracy in your operation, it’s best to work with a team that has a deep understanding of the material you work with and the ability to engineer a system that fits in your environment. At FormPak, we have over 50 years of combined experience in the dry materials field, we can assist your company with bulk bag filling, unloading, lifting, as well as fully integrated batching systems, bag break stations, and flexible screws. We’ll even provide free no-obligation systems drawings and run flow tests on your material. To discuss the requirements of your bulk bag or dry material handling project, request a quote or reach out to our team.