Checkweigher Line for a Mexican Snack Producer: Why Belt-Type Beat Gravity and Bucket Checkweighers for 50g Tortilla Chips
A snack food manufacturer in Jalisco, Mexico, recently approached SunAura Machinery after two consecutive customer complaints: their 50g tortilla chip bags were arriving at a supermarket distribution center with weights ranging from 42g to 58g, and the retailer had issued a formal warning about under-filled packages. The producer was running a 14-head multihead weigher paired with a vertical form-fill-seal machine, and the weigher itself was within spec — but without an inline checkweigher between the sealer and the case packer, there was no automatic rejection of underweight or overweight bags. They needed to choose a checkweigher type, and the options on the table were belt-type, gravity, and bucket checkweighers.
After reviewing their product, line speed, and reject requirements, we specified a belt-type checkweigher with integrated metal detector. This case study breaks down why belt won — and when gravity or bucket would have been the better choice. If you are a snack, nut, coffee, or frozen food producer trying to figure out which checkweigher belongs on your line, the comparison below should save you a vendor demo cycle.
Why You Need a Checkweigher at All
Multihead weighers are accurate, but they are not perfect. At 50g target weight with ±0.5g tolerance on a 14-head weigher, about 2–5% of bags will fall outside spec due to product variation, temperature drift, and electrostatic effects on chips. Without a checkweigher, those underweight bags reach the retailer, trigger complaints, and eventually lead to chargebacks or lost shelf space. In most jurisdictions (NOM in Mexico, NIST in the US, OIML in the EU), selling underweight pre-packaged product is also a legal violation — not just a customer-service issue.
The Three Checkweigher Types
Belt-Type Checkweigher
The product travels along a belt over a precision load cell. Infeed and outfeed belts stabilize the product so it sits motionless on the weigh cell for a fraction of a second, then a signal rejects under/over-weight bags. Belt checkweighers are the most common and most flexible: they handle rigid bags, pouches, cartons, cans, and bottles. Accuracy is typically ±0.1–0.5g at 60–200 packages/minute. Capital cost: $15k–$35k for a standard food-grade unit.
Gravity Checkweigher
Product falls vertically through a weigh tube onto a load cell. There is no belt — gravity moves the product. Gravity checkweighers are cheap and compact, but they only handle free-flowing dry products (granules, powder, nuts, rice) that can fall cleanly through a tube. They cannot handle sealed bags, bottles, or irregular shapes. Accuracy is ±0.5–2g, and speeds up to 500 packages/minute. Capital cost: $5k–$12k.
Bucket / Drop Checkweigher
Product drops into a small bucket on a load cell, the bucket weighs it, then tips or opens to discharge. Bucket checkweighers are used for high-speed counting and weighing of small, free-flowing items — hardware, tablets, capsules, individual candy pieces. They are not suitable for sealed retail packages. Capital cost: $10k–$20k.
Side-by-Side Comparison
| Dimension | Belt-Type | Gravity | Bucket |
|---|---|---|---|
| Typical speed | 60–200 pcs/min | 200–500 pcs/min | 100–300 pcs/min |
| Accuracy | ±0.1–0.5g | ±0.5–2g | ±0.2–1g |
| Product form | Sealed bags, cartons, cans, bottles | Free-flowing dry bulk only | Small individual items, capsules |
| Capital cost | $15k–$35k | $5k–$12k | $10k–$20k |
| Reject method | Air jet, arm, or pusher | Diverter flap | Chute or gate |
| Footprint | 1.5–2.5 m long | 0.5 m × 0.5 m | 0.8 m × 0.8 m |
| Cleaning (food grade) | Easy (belt lifts off) | Easy (stainless tube) | Moderate |
| Integrates with metal detector | Yes (combined unit available) | No | No |
| Best for | Sealed retail packages | Bulk dry ingredients, before filling | Counting small items, pharma |
Why Belt-Type Won for the Jalisco Snack Line
They weigh sealed bags, not bulk product
The customer’s product is a sealed 50g printed poly bag of tortilla chips. A gravity checkweigher requires product to fall freely through a tube — a sealed chip bag cannot do that. A bucket checkweigher is designed for individual items, not sealed retail packages. The only option that works for a sealed bag on a conveyor is a belt checkweigher. This was not a close call; it was the only physically compatible technology.
Speed matched the VFFS output
Their VFFS runs 65 bags/minute. A belt checkweigher rated for 120 pcs/min gives them comfortable headroom for future speed increases. A gravity weigher at 500 pcs/min would have been overkill and incompatible with bags. A bucket weigher at 200 pcs/min also incompatible. Belt was the right speed fit.
Combined metal detector integration
Food safety regulations in Mexico (NOM-051) and major retail buyers require both fill-weight verification and foreign-body detection. We specified a combined belt checkweigher + metal detector unit (about $28k installed) that does both jobs on one conveyor. This saved the customer from buying two separate machines and gave them a single reject bin for both underweight bags and metal-contaminated bags.
Reject verification and data logging
The customer’s retail buyer required monthly fill-weight reports. A modern belt checkweigher logs every bag weight, produces SPC reports, and automatically flags trends (e.g., weigher drift that would cause systematic underfill). This data was a selling point — the previous manual sampling process was labor-intensive and error-prone.
When Gravity Is the Right Choice
Belt was right for sealed snack bags, but gravity is the correct and cheaper choice in other scenarios:
- Weighing bulk product before filling. If you need to verify incoming raw material weight (e.g., a 25kg bag of flour arriving at the factory), a gravity chute weigher is the right tool.
- High-speed dry product check before packaging. For grains, nuts, rice, or free-flowing granules moving at 300+ units/minute through a chute, gravity is faster and cheaper than belt.
- Limited floor space. A gravity weigher occupies less than 0.5 m²; a belt checkweigher needs 2+ meters of conveyor length.
- Budget under $10k. Gravity checkweighers start at $5k; belt units start at $15k.
When Bucket Checkweighers Make Sense
- Pharmaceutical tablet or capsule checkweighing. Bucket weighers are the standard for blister and tablet lines because they weigh individual units at high speed.
- Counting small hardware or fasteners. Nuts, bolts, screws, and individual candy pieces are handled by bucket weighers that count by weight.
- You are not weighing sealed retail packages. If your output is individual items going into a bulk container, bucket may be cheaper than belt.
What Not to Buy
- Do not buy a belt checkweigher with a non-food-grade finish for snacks. You need stainless steel 304 construction, open-frame design for washdown, and food-grade belts. Painted carbon steel will rust in a humid snack plant.
- Do not skip the reject verification bin. A checkweigher that rejects bags into an open bin is a food-safety risk — operators may accidentally put rejected bags back on the line. Specify a locked reject bin with an interlock that stops the line if opened.
- Do not place the checkweigher immediately after the sealer. Freshly sealed bags are still warm and expanding; weighing them 1–2 meters downstream after they cool gives better accuracy. Leave conveyor distance.
- Do not forget calibration weights. Checkweighers need monthly verification with NIST-traceable test weights. Budget for a $200 calibration weight set and a 15-minute monthly check.
FAQ: Checkweigher Selection
How much space do I need for a belt checkweigher?
Plan for 1.5–2.5 meters of straight conveyor, plus 0.5 meters on each side for maintenance access. The infeed and outfeed belts must be horizontal and stable — do not mount the checkweigher on a curved or inclined conveyor.
Can one checkweigher handle different bag sizes?
Yes. Modern belt checkweighers store recipes for different target weights and tolerances. Changing between 50g and 200g bags takes about 30 seconds on the HMI. The conveyor belt width should match your largest bag.
What reject mechanism works best for snack bags?
Air jet for light bags (under 200g), pneumatic pusher for heavier bags, and a drop-down flap for fragile products. Air jet is the most common for snack bags because it does not crush the package.
How often does the load cell need calibration?
Daily zeroing (press a button, takes 5 seconds) and monthly span calibration with test weights. In dusty or humid environments (flour, sugar, frozen food), increase to weekly calibration.
Can the checkweigher connect to my ERP or SCADA?
Most modern units offer Ethernet/IP, PROFINET, or Modbus TCP. Data logging to a USB drive or network folder is standard. If you need real-time SPC dashboards, specify this upfront — not all units include it.
Integrating the Checkweigher With the Rest of the Line
A checkweigher does not work in isolation. Its position on the line, the conveyor before and after it, and the reject mechanism all affect accuracy. The Jalisco installation included several details that the customer had not considered, and we flag them here because they are the most common reasons a new checkweigher “does not work right.”
Position after the sealer
Freshly sealed VFFS bags are warm and expanding. Weighing them immediately after the sealing jaw gives inconsistent readings because the bag is still settling. We installed 1.5 meters of cooling conveyor between the sealer and the checkweigher. This lets the bag reach ambient temperature and settle, which improved weight consistency by 30% on the first test run.
Stable infeed and outfeed belts
The weigh cell is sensitive to vibration. The infeed and outfeed belts must be independently driven and mechanically isolated from the main conveyor. If the checkweigher shares a frame with a vibrating feeder or a running fan, the load cell will pick up noise and produce false rejects. Specify a checkweigher with its own isolated frame and anti-vibration mounts.
Reject verification and locked bin
Rejected bags go into a locked bin with an interlock switch. If the bin is opened, the line stops. This prevents operators from accidentally putting rejected bags back into the good product stream — a food-safety requirement for BRC and IFS audits. The bin also has a level sensor that alerts when it needs emptying.
Data output and SPC reports
The checkweigher logs every bag weight to a USB drive and sends summary data to the customer’s SCADA system. Weekly SPC reports show average fill weight, standard deviation, and reject rate. This data is what the retail buyer asked for in their audit. Without it, the customer would still be manually sampling and recording weights on paper.
5-Year Cost Comparison
| Cost line (5 years) | No Checkweigher (Manual Sampling) | Belt Checkweigher |
|---|---|---|
| Capital equipment | $0 | $28,000 (combined metal detector) |
| Labor (manual sampling) | $30,000 (1 hr/day sampling + recording) | $0 |
| Retailer chargebacks / fines | $60,000 (estimated underweight claims) | $2,000 (rare) |
| Maintenance / calibration | $0 | $5,000 (test weights, service) |
| Data reporting labor | $15,000 (manual SPC reports) | $0 (automated) |
| 5-year TCO | $105,000 | $35,000 |
Bottom Line
If you are weighing sealed retail packages on a conveyor, a belt checkweigher is the default choice — it handles bags, cartons, cans, and bottles with the accuracy retailers require. If you are weighing free-flowing bulk dry product before packaging, a gravity weigher is cheaper and faster. If you are counting individual small items, use a bucket weigher. Do not try to make one technology do another’s job — they are physically different machines for physically different products.
SunAura Machinery integrates belt checkweighers with multihead weighers, VFFS machines, and flow wrapping lines as a complete turnkey package. We will help you select the right checkweigher type, reject mechanism, and integration with your existing line — and we will tell you honestly if a cheaper gravity or bucket unit would work for your product. Contact our engineering team with your package type, target weight, and line speed, and we will spec the right system.
Sources
- Mettler-Toledo: Static and checkweigher accuracy specifications for food packaging
- Anritsu: KW6600 belt checkweigher technical data
- NOM-051-SCFI/SSA1-2010: Mexican official standard for prepackaged goods net quantity
- OIML R51: Automatic catch weighing instruments international recommendation
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