Polybags are challenging, being non-rigid containers with unique physics and special requirements in sorting. To understand why the Z1 design is superior it is important to understand the physics for successful sortation. Diverting a bag from a direction of flow to a sort point requires force acting on the bag to move the bag to the chute or container. Sortation systems deliver this force to the bag by gravity, normal force and friction.
Tilt tray and bomb bay style sorters use gravity very effectively to transfer bags from the main line. These sorters are very costly, large, heavily engineered, time consuming to install, space inefficient and inflexible.
Normal force sorters consist of shoe sorters, pushers and paddle sorters. Shoe sorters can be fast but are also costly, large, heavily engineered, time consuming to install, space inefficient and inflexible. Paddle sorters and pushers are low cost and have some flexibility, but are very limited in rate. Not all polybags can be handled. All normal force sorters require some level of rigidity and minimum height of the contents in the polybags to receive and handle the mechanical transfer force.
The Z1 and Intralox ARB use friction to transfer bags from the main moving conveyor. The ARB can be scaled from small to large systems with medium throughput rates making it one of the closest competitors to the Z1. There has been much fanfare about the technology of the ARB or Activated Roller Belt but a closer look exposes limitations and makes the Z1 a clear winner when it comes to handling polybags.
The force to move the bag is determined by the bags inertia and the drag on the bag resisting the motion. Friction is a combination of the surface “grip” (COF) and the normal force acting on the surfaces (typically weight). The COF or coefficient of friction is dependent on the material and surface area in contact with the bag. Optimizing combinations of normal force and COF increases the friction and the force transferred to the bag.
Control issues aside, the primary reason for unsuccessful bags sorts is insufficient friction to drive the bag off the conveyor before the bag is carried past the sort point. Transfer force must usually overcome the drag from the bag moving over a conveying surface, inertia of the bag, and mechanical interference. Many sortation system manufacturers will limit the minimum weight of the polybag package in order to have enough normal force.
The challenge with handling polybags is generating enough friction and therefore force to overcome forces holding the bag on the conveying surface. A cross belt sorter overcomes this challenge by conveying the transfer mechanism holding the bag (e.g.). ARB is a long continuous chain conveyor carrying small rollers that are actuated by stationary motors at sort positions in the conveyor bed. ARB sorters activate rollers in the chain to carry off the bag as the chain is moving.
ARB transfer rollers only spin and do not lift the polybag. This design relies on product in the bag to be rigid enough to bridge across the rollers minimizing bag contact with the chain conveyor surface. If the bag “flows” or sags between the rollers the force imparted by the rollers must overcome the force from contact with the chain. Any drag from the chain may be just enough to slow down the transfer of the bag as it travels past the sort point to miss the sort point.
The Z1 lifts bags from the conveying surface minimizing drag on the bag during transfer.
The Z1 transfer lifts are stationary not moving forward as do the rollers of the ARB chain. The lifting motion disengages the bag from the forward motion of the conveyor rollers to insure proper sortation. Furthermore the Z1 can also briefly interrupt forward motion of the conveyor roller to stop forward motion of the bag at the sort point. This happens without slowing progress of the bags upstream. Therefore very liquid bags without flat and or rigid items inside can be sorted effectively.
GET A GRIP
The transfer rollers in the ARB design that impart the transfer force on the bags are made in an injection molding process and the materials used limit the grip of COF. The roller material must also be hard enough to contact the actuating rollers beneath the conveyor chain while moving at speed. There is a specific relationship with the slip required and wear characteristics further limiting the grip or COF of the roller material.
In contrast the Z1 material that contacts the bag surface for transfer contacts only the bag and therefore can be optimized for the maximum grip on polyethylene surfaces.
ARB systems overcome transfer slowdown from by competing forces by reducing the chain speed and or widening the chute and distance between the sort points. The affect is less sort rate, fewer sort points within a given area and or larger more expensive sorter.
ARB system overcome the transfer slowdown by reducing the width of the belt to let gravity complete the transfer process. The very narrow conveying surface limits the range of product that can be handled and puts pressure on the induction process (bag loading) to be perfect. Every bag loaded must be perfectly centered which may slow down the induction process requiring more loading stations and labor to man them.
ARB transfer roller are small in diameter to effectively fit into the conveyor chain. The small diameter means a small contact area with the poly bags. Less contact area means less transfer force. By using large wide flat transfer belts, the Z1 contact area is huge by comparison.
It can be argued the Z1 is overkill with the superior grip, contact surface and controlled forward movement, but not all bags act the same. Small light weight bags with small rounded items inside offer very little friction for consistent and positive sorting action. Large heavy bags that tend to flow can drag severely. The Z1 transfer generates superior transfer forces while eliminating competing forces to handle packages other sortation systems can’t, but the Z1 does not stop here.
FOR SURE FOR SURE
The Z1 is designed to provide the most positive and accurate sort in the market. Polybags are tracked into and out of every sort point. In the case where a bag does not transfer within the typical time of several hundred milliseconds, the Z1 will hold the bag at that sort point and make additional attempts to move the bag into the correct container. Z1 will not advance bags scheduled for sortation past the sort point to make sure packages do not end up in the wrong place or lost. If the bag will not clear into the container manual intervention will be called to that location to clear the bag. Restart will continue the sort process without need for clearing or moving package other than the trouble zone.