How we discovered the successful art of automatic tray feeding

Is denesting really that difficult?

Geometry, material, and rigidity are just a few properties a tray or pot have that can shut the door to one denesting solution that may have worked for another product. Until we see the invention of robotic arms with near human-like dexterity and texture the process will remain elusive to automate with broad solutions. In a way it can almost feel like a black art.

Denesting isn’t complicated.

Imagine an operator doing the job of denesting a stack of trays into a production line. They have two hands, one hand is holding the stack of trays, and the other hand is removing the trays from the stack to place on the conveyor. The process is easy to automate when we look at the work of each of the operator’s hands.

Hand 1 is holding the stack. The hand isn’t holding the stack in a way that prevents hand 2 from doing its job, but it’s holding the stack secure so that the stack of trays doesn’t fall and collapse all over the production line.

Hand 2 is both picking and placing the tray. The operator isn’t throwing the trays, he’s probably putting them down on the conveyor gently. Quickly, but controlled.

An operator can move the stack as it gets smaller so that hand 2 can always reach a tray, and for automating we can use gravity to achieve the same effect by putting our stack of trays into a magazine that holds the stack of trays not parallel with the ground.

With our gravity fed magazine, we only need two technologies to automate the denesting process:

·       A “holding” technology, that is strong enough to hold a stack of trays, but compliant enough to allow the second technology to do its work.

·       A “pick and place” technology, that picks a tray from the stack and places it onto a production line.

Denesting is somewhat complicated.

For our A-Series we use “clips” or “retainers/escapements” as our “holding technology”. Clips are little blocks that the tray can ride up on that hold the stack of trays inside the magazine. These clips usually come with some sort of attachment that acts as the mechanism that resists the trays from falling into the machine. Some trays may require a softer touch than other trays and so more compliant clips are needed. Other trays might be quite dense and become heavy when stacked so a stronger clip is needed.

Our “pick and place technology” involves using suction cups to pick and hold the trays. Depending on the rigidity of a tray, the location you place the suction cup might be critical. The surface or geometry of the inside cavity may also mean a certain size of suction cup is required.

The human hand is an amazing piece of hardware that can do an immense number of different tasks but is something we haven’t managed to fully replicate yet. While a human hand will hold any stack of trays correctly, when we come to our “holding technology” we find that what was a suitable solution for one tray or pot isn’t suitable for another. It just isn’t as robust as a human hand. We have the same problem with our “pick and place technologies” too.

A solution to this problem is simply having the technologies easily replaceable. Have one set of “holding” parts for one tray that can be easily swapped for a set of “holding parts” for another tray. Have a set of “pick and place” parts for one tray, and another for another tray.

Our “holding technology”, clips, are fixed to a “mask” that doubles as an interlocking safety guard that guards the area of the machine where the stacks of trays enter. These have a fixed set up and are easily swapped to other masks so that each tray can have its own dedicated mask, broadening the number of different trays a denester can handle.

Our “pick and place technologies”, suction cups, are apart of assemblies we call “pickframes” that, like the masks, have a fixed set up and can be easily swapped so that each tray can have its own dedicated pickframe. 

This way we can have one machine be a broad solution to many denesting problems. Different trays just require different change parts.

Denesting gets easier with experience.

Having handled hundreds of trays and pots over the years, Partners in Packaging have a portfolio of clip and suction cup options to use for our denesting solutions. To streamline the process of selecting the most suitable solution, and to improve our design of these parts, we have constructed a Test version of the A-Series.

A production A-Series will have masks and tooling pickframes pre-set up and dedicated to the trays it is handling so that operators don’t need to adjust the machine when swapping trays out, speeding up the change over process. Unlike a production A-Series, the test rig A-Series is completely adjustable.

The test rig hosts just one magazine but has the flexibility to run a product with maximum dimension of 300 mm high by 500 mm wide, down to a minimum of 50 mm high by 80 mm wide.

The holding technology, clips, can be quickly replaced to different types and several clips can be tried top and bottom to get the ideal support for the stack.

The pick and place technology, our pickframes, come in three, easily replaceable assemblies that are themselves easily adjustable to find that perfect spot to apply a suction cup to the cavity of a tray, and host several suction cup heads if more are needed. There are even quick release fittings added to these parts to tidy away loose pneumatic piping from unused suction cup heads.

All adjustable parts are fitted with scales so that their locations can be recorded to later be used in the design of masks and tooling pickframes on the production A-Series trays the trays will later be denested from.

Here is a list of what the test-rig finds:

·       Maximum stack sizes: Dense trays might create heavy stacks that require stronger clipping to hold up. Stronger clipping makes the trays more difficult to pull out of the stack and therefore a maximum stack size must exist when the strength required to hold up the stack in the magazine is stronger than the strength required to pull a tray out of the stack through the clips. If this occurs, a scale on the side of the magazines tells us how full a magazine is with a particular tray before reaching maximum stack size. With this knowledge we can provide magazines on the production A-Series that operators cannot overfill, which maintains the reliability of the denester.

·       Minimum stack sizes: Conversely, light trays can have the opposite problem where when a stack gets too low the stack becomes too light and trays don’t readily fall down the magazine under their own weight. This produces a minimum stack size requirement. By knowing at which point a stack becomes too light we can set a low-stack detection sensor on the production A-Series to alert operators to fill the magazines again just before the machine reaches the minimum stack size. A second low-stack sensor can stop the machine is the minimum stack size has been reached. This will maintain the reliability of the denester.

·       Clip type, location, and quantity: As previously mentioned, clip location and quantity required to correctly hold the stack of trays in a magazine can be recorded by the test rig. As the clips are easily replaceable, we can test different types of clips too. Presently the test-rig has five different types of clips which we consider standard for our machines. These clips have been innovated and iterated on over the hundreds of trays we’ve denested over the years and are the five that are most used. We will innovate more of these holding technologies with a planned sixth standard clip coming to the test rig soon.

·       Pick type: Our adjustable pick frames come in three standards.

o   Straight pick, where a suction cup goes straight into the tray to pick it out of the stack. This is the only option without pneumatic cylinders so the cheapest option to run.

o   Actuated pick, where the suction cup gets some extra reach with the help of a pneumatic cylinder. This is often required to pick deeper, or more rigid trays.

o   Sidewall pick, where the suction cup picks up on the bottom wall of the tray cavity, instead of the base. Ideal for card trays and can vastly reduce the number of change parts if running several trays of similar top out but different depth.

The test rig gives us the freedom to try all three pick types on a tray before a quote has been put together. This has the effect of reducing the cost of a denester by finding commonality in pick frames between different trays. A deep tray, if flexible enough, may not require actuated pick and therefore testing the tray early can avoid the additional costs of buying the pneumatic cylinders.

·       Suction cup type, location, and quantity: Like with clips, we can test a tray to see the best location and how many suction cups should be used when picking a tray. Also, like clips, we can test different types of suction cups. We’ve tried well over 50 different types of suction cup over the years, and we’ve picked the most successful of these to run on our test rig.

·       Tray dangle: Depending on how best to pick a tray, the tray itself could dangle off the suction cups (not sit square to the pickframe) when the pick arm is in the down position. We can measure if and how much a tray dangles off a pick frame to ensure trays held by the denester are clear of anything on the conveyor they’re deposited onto before the tray is finally placed.

Like the production version of the A-Series, the test rig is HMI operated. This ensures trays can be tested at intended production speed. Faster machines will pull a tray out of a stack with greater force, meaning stronger suction cups must be used but also stronger clips may be necessary.

Measure twice, cut once.

As discussed, the benefits to testing trays early can reduce the cost of the denester. By testing early parts can be manufactured correctly the first time. Design time of these parts is reduced as the locations of clips and pick frames are already known before opening CAD software.

The only other available time to test trays on an A-Series is when the production machine is built. Testing early reduces the time spent at this stage of the construction, improving lead times on getting machines out of the door and to the customer. It also reduces chance of parts like pickframes being changed late into a project.

Denesting isn’t difficult with experience and knowledge.

Robotic arms with dexterity and deftness of a humans may appear sometime in the future, and even further forward they may become affordable, but until then there will be a need for dedicated, single task machinery like denesters.

The test rig is a product of years of development on our A-Series denesters. It is a culmination of our experience so far and will become a new knowledge base going forward. Results from tests run on the machine can be easily codified and trends in clip types and suction cups will be more easily spotted. This new database will be used to inspire new designs, improving our denesters going forward.

Denesting isn’t a black art, but it is an art.

If you want to know more about our A-Series denester, click here. We also have a wide range of denesters that this test rig will help improve the design of. If you’re interested in having your products tested on the A-Series test rig, please get in contact with sales@partnersinpackaging.com.