How Reusable Plastic Bakery Trays Are Made by Injection Molding

Injection molding makes a plastic tray by melting resin pellets in a heated barrel, closing and clamping a steel or aluminum mold, forcing the melt into the mold’s cavity, holding pressure while the plastic packs in and begins to shrink, letting it cool until it is solid enough to keep its shape, then opening the mold and ejecting the tray. The cycle repeats for every tray. It is one established way to make plastic trays, not the only one, and the details (resin, mold design, machine settings) decide what the finished tray looks like and how consistently it is made.

The Process and What Enters It

• Resin pellets. The polymer arrives as pellets or granules, sometimes blended with colorant or other additives.
• Feed system. A hopper feeds the pellets into the machine’s barrel, with dryers or blenders ahead of it where the material requires them.
• Heated barrel and screw. A rotating screw inside a heated barrel melts and mixes the material and moves it forward. The Wikipedia entry on injection moulding describes material that “is fed into a heated barrel, mixed (using a helical screw), and injected into a mould cavity.”
• Tool or mold. Two halves that close to form the tray’s shape, with channels for the melt and for cooling water.
• Clamp. The machine’s clamp holds the mold shut. Presses are rated by clamping force, which, per the same entry, “keeps the mould closed during the injection process.”
• Auxiliary material handling. Conveying, drying, color dosing, and regrind handling where used.

Process Diagram

PELLETS -> HOPPER -> HEATED BARREL + SCREW (melt and meter the shot)
                              |
                              v
        [1] MOLD CLOSES AND CLAMPS
                              |
                              v
        [2] INJECTION / FILL: melt pushed through sprue, runners, gate into cavity
                              |
                              v
        [3] PACK / HOLD: pressure held to add material as it shrinks; gate freezes
                              |
                              v
        [4] COOLING: no packing pressure; mold stays shut
                 |                         (screw recovery: screw turns and retracts,
                 |                          preparing the next shot, during this time)
                 v
        [5] MOLD OPENS -> [6] TRAY EJECTED -> mold closes for next cycle

Mold and Part Design Basics

• Cavity geometry: the cavity is the negative of the tray, including walls, floor, ribs, handles, and openings.
• Draft: walls parallel to the direction the mold opens are angled slightly so the part can release. The Wikipedia entry explains that this “draft” eases release and that “Insufficient draft can cause deformation or damage.”
• Cooling: channels in the mold carry coolant; their layout affects how evenly the tray cools.
• Gates: the points where melt enters the cavity; their number and location affect filling and appearance.
• Ribs: add stiffness to floors and walls without thickening the whole part.
• Handles and perforations: formed by features in the tool. An opening in the tray means a corresponding shape in the mold, which affects how the melt flows around it.

The Molding Cycle

In plain terms, one cycle runs like this: the mold closes and is clamped; molten plastic is injected until the cavity fills; pressure is held while the plastic packs in and the gate freezes; the tray cools in the closed mold while the screw prepares the next shot; then the mold opens and the tray is ejected. The sources below support each step.

MIT’s Design and Manufacturing II course notes (2.008, Spring 2025, Lecture 2: Injection Molding I) list the steps of injection molding as mold closing; mold filling; packing, holding, and cooling; and mold opening and part removal with ejector pins. The same notes state that “Cooling time generally dominates cycle time,” and their typical-cycle slide places screw return within the part-cooling period.

Before each shot: feed and plasticizing. The screw melts and meters the material for the next shot. This is preparation that runs alongside the cycle, not a separate step after ejection. The numbered steps below match the numbers in the diagram.

1. Mold closes and clamps. Mold closing is the first step in the MIT sequence, and the clamp holds the mold shut before any melt enters.
2. Fill. The machine injects melt until the cavity is full, controlling the flow rate as it goes.
3. Pack and hold. Pressure stays on to push in more material as the plastic shrinks, and the gate is meant to freeze during this stage so material cannot flow back out. Packing and holding are two names for the same stage.
4. Cooling. Packing pressure is released, the mold stays shut, and the tray keeps cooling until it can be ejected; cooling is normally the longest stage of the cycle.

Steps 2 to 4 follow the stage definitions in Autodesk’s Moldflow documentation (Autodesk Moldflow: Cycle time) and the MIT notes above.

1. Mold opening. The mold opens once the tray is solid enough to keep its shape.
2. Ejection. The tray is pushed out by ejector pins or plates, and the mold closes for the next cycle.

Overlap in a real cycle. These steps are not strictly one after another: the screw turns and prepares the next shot while the current tray is still cooling in the closed mold, and the mold opens only once the part is cool enough. The MIT typical-cycle slide shows screw return during part cooling, and the Wikipedia entry on injection moulding describes the same order.

After Ejection

• Gate and flash handling: gate remnants are trimmed if the design leaves them; thin flash at parting lines is removed or the tray is rejected, depending on the plant’s quality rules.
• Inspection: visual checks and dimensional checks against the drawing.
• Marking: a tray may receive labels or marks after molding, if they are not molded in.
• Preparation for distribution: stacking or nesting for shipment.

Production Constraints

• Wall thickness: thick sections take longer to cool and can sink; very thin sections can be hard to fill.
• Warpage: uneven cooling or shrinkage can bend the part out of shape.
• Cycle time: cooling sets much of it; shorter cycles raise output but can raise warpage or sink.
• Material shrinkage: the plastic shrinks as it cools, so the mold is cut larger than the finished dimensions by an allowance for the chosen resin.
• Mold maintenance: wear at parting lines, vents, and ejectors can change part quality over time.
• Tooling cost: a large, multi-feature tray mold is a major cost, which is one reason design changes after tooling are expensive.

Feature, Manufacturing Consideration, Possible Defect

The MIT course notes cited above include a process-window slide that places flash toward the high-pressure limit and short shots toward the low-pressure, low-temperature corner, with degradation and burn at high temperature. The defect names and listed causes in the table below follow the defect table in the Wikipedia entry on injection moulding. Read them as possible contributing factors, not diagnoses; a real defect investigation uses the molder’s process records and the tool and material documentation.

Quality Checks on Finished Trays

• Visual integrity: inspect for flash, sink, short shots, cracking, and burn marks against the product’s written acceptance criteria, and reject defects that compromise function, cleanability, food-contact requirements, or the agreed specification.
• Dimensions: key dimensions checked against the drawing and its tolerances.
• Fit: nesting and stacking with production trays; seating on dollies or racks the tray is meant for.
• Assembly interfaces: features that meet machines or other equipment are within their tolerances. Automation-related tolerances are covered in what makes a tray compatible with conveyors and destackers.
• Physical tests: drop, compression, or impact tests as the plant’s quality plan specifies.

Which resin the tray is molded from shapes stiffness and impact behavior; that comparison is covered in HDPE vs. polypropylene bakery trays. Being molded does not make a tray suitable for food contact; that depends on the material evidence for the finished tray.

Alternative Processes

Plastic trays and crates can also be made by other methods, such as thermoforming plastic sheet or fabricating from sheet parts. The choice depends on part design, volume, and required features. This article describes injection molding only and does not compare the methods.

Reading the Cycle in a Finished Tray

The tray features discussed here trace back to steps in the cycle: wall thickness and ribs to filling and cooling, crisp edges to packing and clamp, flatness to even cooling, and clean release to draft. When a tray shows sink, flash, warpage, or short fill, the defect table offers initial investigation leads, not proof of the root cause: the same defect can come from the machine, the mold, the material, or the measurement, so check the lot, the settings, mold temperature, and material records.