Round cells
- Even section all the way round, so each cell cools evenly
- Releases cleanly with less taper
- Wastes some space between cells
Molded part guide: seed trays
Injection Molded Seed Trays
A propagation tray is a sheet of deep, thin cells. Each cell has to let go of the tool, and later let go of a root plug without tearing it. Stacks of empty trays have to come apart one at a time at the start of the season, and the whole tray has to survive months of sun without going brittle.
Deep cells
Every cell has to release twice
Plug trays, propagation trays, carry trays, and nursery inserts.
A cell is a deep, tapered pocket with thin sides. The taper is what lets the tray come off the tool, and the deeper the cell, the more taper it needs, because a long thin pocket grips the steel along its whole length as it shrinks onto it. Too little taper and the cell scuffs or tears on the way out of the press.
The same taper has to work for the grower. A root plug is pushed or knocked out of the cell, and a cell that holds the plug tightly tears roots. So the taper is set for both jobs at once, and the inside surface is polished in the pull direction so nothing catches. Drain openings at the bottom of each cell double as the place a finger or a pin pushes the plug out.
Stacks are the next problem. Identical tapered cells nest so closely that two trays can stick together by suction and by friction, and a grower pulling trays apart one at a time all morning will not buy them twice. Small lugs or steps on the outside of each cell hold the trays a little apart, so a stack separates one tray at a time.
Sunlight is what ends most trays. Ultraviolet light breaks down polypropylene, and an unprotected tray becomes brittle after a season or two outdoors, cracking at the corners when it is picked up. UV stabilized grades slow that down, and a darker color holds up better than a natural one. Trays are also washed and sometimes steamed, so the resin has to take hot water.
Features
Tray features and their jobs
The cells do most of the work, and the rim does the rest.
| Feature | Purpose | Design note |
|---|---|---|
| Tapered cells | Release from the tool and the plug | Taper set for both, with a polished inside |
| Drain openings | Drainage and plug removal | Sized so a pin or a finger reaches the plug |
| Denesting lugs | Stacks that separate one at a time | Small steps on the outside of the cells |
| Rim | Stiffens the tray for carrying | Return flange around the whole edge |
| UV stabilized resin | Survives seasons outdoors | Darker colors hold up better |
Stacking
Nested trays that will not come apart
Identical tapered cells nest so well that a stack behaves like one solid block.
- Two nested trays can stick together by suction and by friction along every cell
- A grower separating trays by hand all morning notices this immediately
- Small lugs or steps on the outside of the cells hold each tray a little apart
- The same lugs let air in, so the stack releases instead of pulling a vacuum
Cell shape
Round cells or square cells
Square cells hold more soil per tray. Round cells behave better in the tool.
Square cells
- More soil in the same tray area
- Corners cool differently from flat sides
- Corners need more taper and a generous fillet
Drawing
What to send with a seed tray
Cell count, handling, and the season decide the design.
Tray CAD, or the cell count and cell size you need
Cell depth and soil volume per cell
How plugs are removed, by hand or by machine
Stacking and denesting needs
Years outdoors and washing method
Expected annual volume
Keep reading
Related guides
Other trays, deep thin parts, and the taper that releases them.
Trays
Handling trays that hold parts.
TraysContainers
Deep tubs and their cores.
ContainersPackaging inserts
Protective inserts in cartons.
Packaging insertsLids
Wide flat parts and flatness.
LidsDraft angle
Taper that releases deep pockets.
Draft angleSticking parts
When a part will not leave the tool.
Sticking partsPolypropylene vs polyethylene
Choosing the tray resin.
Polypropylene vs polyethyleneIndustrial and transportation
How we support industrial programs.
Industrial and transportationMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded seed trays
Why do our trays stick together in a stack?
Nested tapered cells grip each other and can pull a vacuum. Small lugs or steps on the outside of the cells hold each tray slightly apart so the stack separates one at a time. Trying a stack of first parts by hand tells you more about denesting than any measurement does.
Why do plugs tear when they come out?
The cell is gripping the plug. More taper, a polished inside surface, and a drain opening big enough to push the plug from below usually fix it without changing the cell size.
How long will a tray last outdoors?
It depends mostly on sunlight and the grade. UV stabilized polypropylene lasts far longer than an unprotected grade, and darker colors hold up better than natural ones.
Can trays be molded from recycled material?
Often yes, with some care. Recycled streams vary batch to batch, so the tray design leaves margin on the thin sections and the color is usually darker.
Next step
Quote a tray with its cell layout
Upload the tray, or tell us the cell count and depth. DFM feedback covers taper for release, denesting features, flatness, and resin choice.