Designing a plastic tray that denests. Our recommendations.
Most denesting problems are designed into the tray before a machine ever sees it. The good news is that the reverse is also true: a handful of decisions made at the tooling design stage cost nothing and make a tray reliable on high performance packaging lines for the rest of its life.
These are the recommendations our engineering team gives to packaging manufacturers and food producers designing thermoformed plastic trays. They come from more than 1,000 machine installations and a great many stacks of trays that did not want to come apart.
Start with the wall taper
The single biggest factor in whether a tray denests is the angle of its cavity walls. A tray with a 10 degree non locking taper on all cavity walls will usually denest without any special features at all. Some trays will denest with a 7 degree taper, and shallow trays, less than 16 mm deep, usually need a 15 degree taper.
Get the taper right and the stack supports itself without wedging. Get it wrong and every other feature is working uphill.
General specifications
Alongside the taper, we recommend the following for any tray intended for automated denesting:
- Cavity corners should have at least a 3 mm radius.
- Flange to cavity tolerance of plus or minus 0.9 mm.
- Outside flange to outside flange tolerance of plus or minus 0.4 mm.
- Cavity to cavity size tolerance of plus or minus 0.4 mm.
- Cut or punch one layer of trays at a time. Do not multiple stack cut.
- Adding a silicone emulsion release agent to one surface will improve denesting.
- Trays should be consistent in wall thickness, without wrinkled flanges or cracks.
- For shipping and handling, stack in columns, under pack by 1 per cent and lay the stacks on their sides.
- A flange width of 9.5 mm is generally desirable.
Denesting lugs
Where the taper alone will not keep trays apart, denesting lugs hold a small, consistent gap between nested trays so the machine always has something to work with.
We suggest semicircular lugs with a minimum diameter of 2.5 mm and a minimum depth of 3 mm, with one lug in each of the four corners. Larger trays, 400 mm square and above, need additional lugs in a matrix at approximately 50 mm centres.
One detail that is often missed: provide at least four different die cavities, staggering the lug positions by at least 6 mm. If every tray in the stack has its lugs in exactly the same place, the lugs simply nest inside each other and the benefit disappears. In some situations a common position reverse lug can be used instead.
The table top test
Before any machinery is involved, you can learn a great deal about a tray with a simple test that we use ourselves:
- Build a stack approximately 600 mm high, separating and restacking each tray first so that none of them start nested.
- Manually compress the stack by approximately 25 mm.
- Slowly remove each tray, one at a time, from the top of the stack, through the entire stack.
If the trays separate without blocking all the way down, the tray is typically acceptable for high performance packaging lines.
When the table top test fails
A tray that fails the table top test is not necessarily a lost cause, but it does need a machine test to measure the achievable efficiency. Actuated mechanical clips or pneumatic assistance may be required to run it reliably. Establishing exactly that is what our in-house test rig is for: we run your actual production samples, record the trial and show you what happens.
A note on fibre and board
These recommendations are written for thermoformed plastic trays. Card, fibre and moulded pulp formats bring a different set of variables, porosity, moisture sensitivity, surface friction and greater dimensional variation, and deserve their own conversation. You can read more on our Packaging Materials page.
Designing a new tray? The cheapest time to get denesting right is before production tooling is cut. Send us drawings or samples at sales@pipautomation.com, call +44 (0)1706 369000, or book a trial on our Tray Testing page.
Quick answers
What taper angle stops plastic trays locking together?
A 10 degree non locking taper on all cavity walls usually allows trays to denest without special features. Some trays will denest with a 7 degree taper, while shallow trays under 16 mm deep usually need a 15 degree taper.
What are denesting lugs and when does a tray need them?
Denesting lugs are small semicircular features, minimum 2.5 mm diameter and 3 mm deep, that hold a consistent gap between nested trays. They are needed when the wall taper alone will not keep trays apart, with one in each corner, extra lugs at approximately 50 mm centres on trays 400 mm square and above, and positions staggered by at least 6 mm across at least four die cavities.
How do you test whether a tray will denest reliably?
Start with a table top test: a 600 mm stack, compressed by approximately 25 mm, with each tray removed one at a time through the entire stack. Trays that separate without blocking are typically acceptable for high performance lines. Trays that fail need a machine test to measure efficiency, which is what PiP’s in-house test rig provides.