Polystyrene
- The clearest option and the lowest cost
- Best for viewing and imaging through the base
- Brittle, and softens at modest heat
Molded part guide: petri dishes
Injection Molded Petri Dishes
A petri dish looks like the simplest part in a lab: a shallow round base and a loose lid. In use the base has to be flat and clear enough to image through, the lid has to let air in without sealing on condensation, and a tall stack of dishes has to stay put on a bench or in an automated handler.
Base and lid
A shallow part with a demanding base
Round and square culture dishes, divided dishes, contact plates, and their lids.
The base is a thin, flat disc with a short wall. It is usually gated at the center of the underside so melt spreads out evenly in all directions and the wall fills at the same moment all the way round. An off-center gate leaves the far side filling last, which shows up as flow lines in the clear base and as a wall that is taller or thinner on one side.
The gate leaves a small vestige in the middle of the base, exactly where the dish rests. If it stands proud, the dish rocks and the base is not level under a microscope. The gate area is recessed slightly into the base so the vestige sits below the resting plane, and the dish stands on a ring near its edge.
Flatness and clarity go together. A base that cools unevenly dishes or twists, which distorts images and lets media pool on one side. An even section across the base, balanced cooling on both faces, and a polished cavity give a clear, flat floor. Sink and flow marks that are invisible in a stack become obvious when light passes through the base.
The lid is meant to sit loosely. Small vent lugs on its underside hold it just above the base rim so air can exchange. Without them, condensation from warm media forms a liquid seal between lid and base. Stacking rings on the lid top and the base underside key each dish to the one above, so tall stacks do not slide.
Features
Dish features and what each one is for
Each feature is small, and each one changes how the dish behaves in use.
| Feature | Purpose | Design note |
|---|---|---|
| Flat clear base | Viewing and imaging through the floor | Even section, balanced cooling, polished cavity |
| Recessed gate area | Lets the dish sit flat | Vestige kept below the resting ring |
| Vent lugs on the lid | Air exchange and no condensation seal | Lugs sized so the lid sits level |
| Stacking rings | Stacks that do not slide | Fit tolerant of small differences between lid and base |
| Grid lines | Counting colonies | Molded on the underside, fine and even |
Resin
Polystyrene is clear and brittle
Most dishes are general purpose polystyrene, and its brittleness shapes the tool.
- Polystyrene gives glass-like clarity at low cost, which is why most dishes use it
- It cracks rather than bends, so ejection must push evenly on the rim and the base
- A sharp corner where base meets wall starts cracks in handling and drops
- Static charge attracts dust, so dishes are handled and sleeved quickly after molding
Resin choice
Polystyrene dish or polypropylene dish
Clarity usually wins. Toughness and heat resistance sometimes matter more.
Polypropylene
- Tough, and handles higher heat and many solvents
- Hazy, so viewing through the base is poorer
- Suits dishes that must survive handling or heat more than be seen through
Drawing
What to send with a petri dish
How the dish is used decides which features matter most.
Dish and lid CAD, or the diameter and height you need
Viewing or imaging requirements on the base
Vent style, and stacking needs for benches or automated handlers
Grid, text, or marking on the base
Resin, and how dishes are sterilized and packed
Expected annual volume
Keep reading
Related guides
Other lab parts, flat parts, and the defects clear parts show.
Pipette tips
Thin cones with a clean orifice.
Pipette tipsTrays
Flat parts that hold position.
TraysLids
Wide lids and panel flatness.
LidsContainers
Tubs and jars with a center gate.
ContainersFlow lines
Streaks in a clear base.
Flow linesWarpage
Bases that dish or twist.
WarpageGate location
Why dishes are gated at the center.
Gate locationPolypropylene
The tougher, hazier alternative.
PolypropyleneMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded petri dishes
Why do our dishes rock on the bench?
Either the gate vestige stands proud of the resting ring, or the base has dished from uneven cooling. Recessing the gate area and balancing the cooling usually fix both.
Why does the lid fog and stick to the base?
Condensation from warm media forms a liquid seal where lid and base touch. Vent lugs on the lid hold a small gap so air can move and the lid lifts freely.
Why do some dishes arrive cracked?
Polystyrene is brittle. Cracks usually start at a sharp base corner or from uneven ejection. A rounded corner and ejection spread around the rim and base reduce them.
Can dishes have compartments?
Yes. Divided dishes are molded with internal walls, kept thin and even so the base still cools flat. The walls also act as extra flow paths, so the gate stays at the center and each section fills at the same pace. Otherwise one compartment finishes filling late and shows flow lines or a short wall.
Next step
Quote the dish and lid together
Upload the dish and lid, or send the size you need. DFM feedback covers gating, base flatness, vent lugs, stacking, and ejection.