Checkweigher Rejection Methods | Air Blast, Paddle, Swing Arm, Drop Belt
The reject mechanism is often the last thing considered when specifying a checkweigher, and it is the part most likely to cause product damage. Choosing the wrong mechanism does not show up in the accuracy specification; it shows up as crushed packs on the reject conveyor three months after installation.
The four standard reject methods
Air blast rejector
A short burst of compressed air pushes the product off the belt. It is the simplest, fastest and cheapest mechanism, with almost no moving parts to wear. Its limitation is force: the air must be strong enough to move the product, which means it is only appropriate for very light, largely empty pouches. Against a filled 100 g sachet, an air blast either fails to move it or, at higher pressure, tears the film.
Best for: empty or near-empty flexible pouches, lightweight film packs, sachets under about 50 g.
Paddle rejector
A flat blade pushes the product sideways along the belt plane. It is slower than an air blast but far gentler, because the product is moved rather than thrown. For a filled or fragile product this is usually the correct default. The blade is adjustable for stroke length and timing, and adding a side guide baffle prevents soft pouches rolling over instead of being pushed.
Best for: filled pouches, blister packs, chocolate bars, biscuits, small cartons, anything that must not be dropped.
Swing arm rejector
An arm sweeps across the conveyor in a wide arc and pushes the product into a reject bin. The motion covers the full belt width, so it handles wider and heavier products than a paddle. It is a positive mechanical movement, so it does not depend on product lightness — but the arc means the product is accelerated, and very delicate items can still be damaged. Arms are normally fitted as a second mechanism on machines that handle mixed product types.
Best for: cartons, cans, tubs, trays, heavier rigid packs, and lines running several formats.
Drop belt rejector
A short belt segment below the main conveyor drops the product downward. The product is not pushed laterally at all, which makes this the gentlest option. It suits glass, porcelain or any product where even a paddle contact would be unacceptable. It needs more space below the conveyor and a lower collection point, so it is not always physically available.
Best for: glass bottles, glass jars, ceramic containers, extremely fragile packs.
How to choose
Ask three questions. How heavy is the lightest product on the line? If the answer is under roughly 50 g and the packs are empty, air blast will work and it is the least expensive option. How fragile is the product? If it is filled, soft or easily bruised, use a paddle. Is the product rigid and heavy? Use a swing arm. Is the product made of glass or ceramic? Use a drop belt.
For lines handling more than one format, the practical answer is to specify a machine with at least two mechanisms and confirm from the specification that they can be switched per recipe, rather than changed mechanically.
Common problems and their causes
| Product not rejected reliably | air blast pressure too low for the product weight; or side guides too wide |
|---|---|
| Product topples instead of being pushed | no side guide baffle; bag too soft; paddle stroke too fast |
| Film torn on the reject | air blast pressure too high for a filled pouch; use a paddle |
| Reject bin fills during a shift | reject rate higher than expected; fit a takeaway conveyor |
| Product damaged at the reject exit | drop height too large; lower the reject chute or add padding |
Frequently asked questions
Can I change the reject method after installation?
Mechanically, a supplier can retrofit a different mechanism, but the cost is significant compared with specifying correctly. It is always cheaper to specify the right mechanism on the original order.
Does the reject method affect weighing accuracy?
The mechanism itself does not, provided it is triggered after the weighing zone and the product has left the load cell. A poorly timed mechanism that strikes the product while it is still on the weighing section will cause an accuracy loss.
Which reject method is most common in food plants?
The paddle, because food products are usually filled and relatively fragile. Air blast remains common where the line handles empty sachets or very light items.