Checkweigher Machines

A checkweigher works by weighing each product while it moves across a load-cell-mounted conveyor, comparing the result to preset limits, and rejecting out-of-tolerance packs. The working principle has four stages: an infeed belt spaces products apart; the weighing conveyor samples the load-cell signal during the product's transit and filters out vibration; the controller classifies the pack as under, accept, or over against the active recipe; and a reject device (air blast, push rod, swing arm, or drop-flap) removes failed packs without stopping the line. Because the weight is captured dynamically, in motion, throughput is limited only by belt speed and product length, with MIQI machines running at 5-90 m/min.

Dynamic checkweigher accuracy typically ranges from ±0.02 g for light products on precision machines to ±30 g for heavy cartons. Accuracy depends on the weighing range, belt speed, product stability, and environment: MIQI's multi-lane MQ-CWP6 achieves ±0.02-0.05 g over 0.1-100 g, the MQ-CW4523L3 achieves ±0.3-2 g over 1-3000 g, and the 80 kg MQ-CW10070L80 holds ±10-30 g. As a rule, accuracy is quoted as a band because it varies with product weight and speed within the machine's range. Vibration isolation, wind shielding, and correct product spacing are the practical factors that keep real-line accuracy close to the rated figure.

Checkweigher calibration has two parts: static calibration with certified test weights, and dynamic verification with real product. First, the weighing platform is zeroed and spanned using standard weights placed on the stationary belt, confirming the load cell reads correctly across the range. Then reference packs of known weight are run across the belt at production speed, and the dynamic reading is offset-corrected against the static value, because belt motion introduces a small systematic shift. Calibration should be verified at shift start, after product changeover, and after any mechanical adjustment. MIQI machines store per-recipe dynamic corrections in the 100-recipe memory, so a verified product setup is recalled rather than rebuilt at each changeover.

The most common checkweigher reject systems are air blast, push rod, swing arm, drop-flap, and flip plate, chosen by product weight, shape, and fragility. Air blast suits light sachets and sticks below a few hundred grams; push rods handle cartons, trays, and bottles; swing arms sweep mid-weight packs continuously at higher speeds; drop-flap and flip-plate designs let delicate products fall gently onto a lower conveyor. Heavy-carton machines also offer powered-roller diverts and lift-and-transfer units. MIQI fits any of these as standard or customized options, with 1- to 3-stage sorting sections so under and over rejects can be separated, and each reject is confirmed against the count log so no failed pack passes unrecorded.

A checkweigher enforces net content rules by weighing 100% of packs and rejecting any that would fail the applicable underfill limits. In the United States, NIST Handbook 133 governs net-content testing: inspectors sample a lot, the average net weight must meet or exceed the labeled quantity, and no sampled package may be short by more than its Maximum Allowable Variation (MAV). In the EU average-weight system (the e-mark), three rules apply: batch average must be at least the nominal quantity, no more than 2.5% of packs may exceed the Tolerable Negative Error (TNE), and none may exceed twice the TNE. Setting checkweigher limits at the MAV or TNE thresholds, and using its logged records as evidence, lets a packer prove compliance while cutting overfill giveaway to the minimum the rules allow.

A checkweigher scale weighs products in motion and checks 100% of production automatically, while a static scale weighs one stationary item at a time and depends on manual sampling. The dynamic checkweigher trades a small amount of absolute precision for complete coverage: every pack is classified and out-of-tolerance items are physically rejected, so defects cannot slip through between samples. Static scales remain useful for calibration reference and spot checks, but they cannot keep up with a line running at 90 m/min, and sampled inspection can only estimate, never guarantee, the compliance of a full batch. For net-content enforcement, completeness checks, and giveaway reduction, an inline conveyor checkweigher is the standard tool.