Design and test of a spherical fruit and vegetable net sleeve packaging machine

Foam net sleeves have been widely used in cushion packaging of spherical fruits and vegetables, such as apples and tomatoes. Yet the current packaging has relied heavily on manual labor, leading to low efficiency and high costs. In this study, a net sleeve packaging machine was developed for spherical fruits and vegetables. The triplex scheme was also used in the machine, including embedded counter-roller conveying, dual-blade reciprocating cutting, and two-axis linkage adjustable sleeving. A programmable logic controller was selected to centrally control the coordinated multi-axis motion and sensor workflow. A touch-screen interface also enabled online adjustment of cutting length and expansion width, particularly for rapid changeover between fruit varieties. In stable net feeding, a roller gap of 3.2 mm was determined to integrate the torque distribution along the roller-net contact arc, with the stress-strain relationship from uniaxial compression tests; The constitutive equation of the foam net was fitted as σ=176.54ε-1.69ε²+0.09ε³ with an R² of 0.999 85. A polyurethane coating was applied to the roller to maximize its friction coefficient of 4.40, compared with 2.35 and 1.99 for the candidate materials. The total driving torque was required to be 8.46 N·m, leading to the selection of an 86BYG250D stepper motor. The cutting was completed within 0.1 s, fully meeting the high-throughput requirement of 50 packages per minute and a batch cycle of 6 s. Accordingly, a double-acting reciprocating cutter driven by two eccentric wheels with a 180° phase difference was designed to generate two complete cuts per revolution. Kinematic analysis of this dual slider-crank mechanism confirmed that there was an eccentricity of 3 mm, thereby yielding the maximum relative blade displacement of 6 mm. Dynamic simulation in ADAMS indicated that a stable motor torque of 39 N·mm was required at the target wheel speed of 600 r/min, corresponding to a reciprocating frequency of 10 Hz. Thus, a 25 W single-phase motor of model 4IK25A-C was selected with an optimal safety factor. The sleeving unit shared the coordinated horizontal expansion via a reverse-threaded screw and vertical descent via a linear module. Two motion paths were simulated: Path 1 was defined as horizontal expansion followed by vertical sleeving, and Path 2 was a diagonal “figure-eight” motion. Among them, path 1 was preferred to avoid the unintended horizontal displacement during the sleeve or prevent the collision with the fruit during reset. Path 2, with the continuous motion profile, was used to generate additional horizontal displacement during the vertical descent phase, thus causing net width deviation and potential interference with the packaged fruit. Furthermore, ANSYS static analysis was performed on the resin expansion plate under a 2 N transverse load. There was a maximum deformation of 0.828 mm and the maximum stress of 2.17 MPa, which was well below the material's yield strength of 40 MPa, indicating high reliability. Prototype tests were conducted on 150 samples each of apples, tomatoes, and navel oranges with diameters from 65 to 80 mm. The machine achieved an average packaging success rate of 97.3% with a low average damage rate of 0.6% at 50 pieces/min, primarily consisting of minor scratches on tomatoes and bruises on elongated oranges. Compared with existing netting machines, the speed of the prototype increased from 25% to 67%, i.e., from 30-40 pieces per minute in previous studies to 50 pieces per minute. The success rate increased from 92% to 97.3%, with a 5.3 percentage point increase in the cross-species damage rate for the first time. An optimal combination of parameters can offer a practical reference to develop versatile, high-speed horticultural packaging equipment using mechanical analysis and motion simulation. Feeding mechanisms and vision adaptive control can be expected to reduce the dependence on manual fruit orientation in future refinements.