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What is a Lot Number and Why Do I Care?

We hear a lot about product recalls these days. Everything from cars to children’s toys to dog food gets recalled for various reasons. Sometimes you get a notice in the mail telling you that to take your car to the nearest dealer to have them apply a fix for some manufacturing deficiency. Other times you may hear in the news that toothpaste or some other product with a certain code stamped on it has been recalled, etc.

So how does a manufacturer know that a certain bag of chips or a particular vehicle has a manufacturing or safety issue? Of all the tubes of toothpaste made or cars produced, how did they know to recall a certain item? The answer can be found in LOT TRACKING.

Similar to a serial number which identifies a specific item, a lot number is a code (or codes) that identifies a defined quantity of material throughout the manufacturing and distribution process. Product with the same lot number is manufactured by the same equipment using the same raw material components during a specific time period.

Why does this matter?

Lot numbers play a key role in quality assurance. Manufacturers test material for quality. If a material fails to meet acceptable levels, lot numbers are the key to finding and identifying what was potentially affected by the defective product. Product with the same lot number is considered bad, whether the defect is due to a component defect or a manufacturing process equipment malfunction.

For example, consider a tube of toothpaste that fails to meet quality standards because of a contaminated ingredient. Because tubes with the same lot number were made with the same ingredients, all tubes of toothpaste with that lot number are also considered bad. However, the contaminated ingredient could have been used to make toothpaste with different lot numbers as well. The toothpaste manufacturer will have to identify all toothpaste lot numbers that were made with the contaminated ingredient. Using lot number tracking, the manufacturer can alert their customers to the bad products and know for certain which tubes of toothpaste should be recalled even after they have been distributed around the country.

Challenges of lot tracking

As you can see, the traceability of lot numbers can quickly become quite complex. A manufacturer may be tempted to take shortcuts to simplify the process. For example, a company may consider an entire month’s production as a single lot number, but this can result in undesirable consequences. An entire month’s production may have differences in composition and been manufactured on different equipment. In this case, if they need to notify customers about a defective product, the smallest distinguishable group is a whole month’s worth of production. This can be very costly.

Different manufacturing processes require different methods of lot tracking. An item made up of unique, traceable components can be tracked as a single lot number or serial number. A car is a good example of this. Other processes involve mixing specific quantities of raw materials to make batches of product. These batches can be assigned convenient lot numbers for traceability. A third type of process requires mixing specific rates of raw materials together with extensive processing to continuously produce a stream of product. Here, lot numbers typically are assigned to product that is produced over a specified period of time.

Handling bulk materials presents unique tracking challenges. Bulk material can be delivered in small individual packages or placed in large storage silos. A large storage unit can contain material with multiple lot numbers or materials from different vendors. During production, portions of material in a silo or tank can be transferred out to other containers. As a result, bulk material with different lot numbers may become mixed, both purposefully and unintentionally. Even when empty, a storage container may need additional cleaning steps to completely remove trace elements from previous lots, especially for material that could be allergenic, toxic or reactive.

Here’s an example: liquid oil can be purchased in small totes, each with a different lot number. These totes can be pumped into a large bulk storage tank, resulting in a mixture of oil with different lot numbers. In this case it is important to understand how lot numbers are tracked and reported. As liquid is pulled out of the tank, it will be a mixture of all lot numbers added to the tank since it was last completely emptied out. In practice, it may not be practical to completely empty a tank. This causes a continuous buildup of lot numbers in the tank and reduces the effectiveness of the lot traceability program.

To deal with these kinds of situations, manufacturers use different models of how material mixes to determine when the amount of material with a particular lot number becomes a negligible factor. For liquids, assuming a perfectly blended mixture of all the materials is usually the most accurate. When the fraction of material with a particular lot number becomes lower than a certain threshold, it is assumed to be a negligible component. Dry bulk material in a storage silo often tends to rat hole from the top to the bottom. In these situations, a last-in-first-out model of tracking lot numbers is effective. Specially designed mass-flow silos are sometimes used so that material moves through the silo in a first-in-first-out manner.

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