How is a lunch box made? From raw material to finished product – step by step

Raw material incoming – Not all PP is the same

Lunch boxes are mainly made of PP (polypropylene). But the price of PP can vary by double. Why?

Good food-grade PP uses virgin material – no recycled content. The lunch box looks clear, has no smell, and won't turn yellow in the microwave. Recycled material gives a dull gray color, a weird smell, and deforms after a few uses.

When we receive raw materials, every batch comes with a test report. We don't just trust the supplier's certificate – we do our own checks: melt flow index, ash content, flexural strength. Basic items.

One more thing: color masterbatch. Bright colors like pink or blue come from masterbatch. Cheap masterbatch leads to uneven color or fading. We use imported masterbatch for stable color and light fastness.

You can also request specific certifications like BPA Free, FDA, or LFGB. We lock down the material batch to match those standards from day one.

Step 2: Injection molding – Machine parameters make or break the product

Raw material goes in, next is injection molding.

Sounds simple: melt plastic pellets, inject into a mold, cool and shape. But there is a lot to it.

Temperature: PP melts between 160-220°C. Too low, the plastic doesn't fully melt – you get flow marks and weak strength. Too high, the material degrades – the lunch box becomes brittle and cracks easily. Each mold has its own optimal temperature curve. That's factory experience.

Pressure: Injection pressure, holding pressure, back pressure – these affect dimensional accuracy and internal stress. Too little pressure, the part sinks; too much, it sticks. We have a parameter manual for every mold. 20+ years of data.

Mold: The mold is the heart of injection molding. A good mold runs millions of cycles without issue. A bad mold shows flash and leaks after thousands. Our in-house tooling shop maintains molds regularly – checking vents, cooling channels, ejector pins.

Common defects: sink marks (dimples on surface), flash (excess plastic on edges), bubbles, black spots. We avoid these by controlling machine parameters and maintaining molds. Our QC checks every 2 hours – stop the line if something's off.

Step 3: Gasket making – The key to leak-proof

What's the worst thing for a lunch box? Leaking soup.

The gasket is what keeps it sealed. We use food-grade silicone that meets FDA and LFGB.

The process for silicone is different – it's compression molding. Put silicone material into the mold, apply heat and pressure to vulcanize. Time, temperature, pressure – all need precise control.

The most important property for a gasket is compression set. How well does it bounce back after being compressed? We test it: compress the gasket for a set time, measure how much it recovers. Our internal standard is tougher than the national standard.

Also hardness. Too hard, hard to close. Too soft, not tight enough. We adjust the silicone hardness (usually 40-60 Shore A) to match the latch strength.

What you may not know: We make the gasket molds ourselves. Different shapes need different cross-sections – O-ring, square, custom. We design it based on the gap between lid and base.

Step 4: Assembly and inspection – The final defense against bulk issues

Molded parts and gaskets come together in assembly.

Simple, but watch these points:

Latch test: Latches need to open and close smoothly but not loose. We do cycle tests – at least 5,000 cycles. Cheap lunch boxes snap after two months because the latch root is too thin or the plastic is weak.

Leak test: The most direct test. Fill with water, close the lid, invert and tilt. See if any drips. We have a dedicated leak test station – every single lunch box goes through it.

Visual inspection: Check for scratches, black spots, sink marks, flash. This relies on experienced QC staff. They are trained to know what's acceptable and what's not.

Microwave test: Take samples, microwave on high for 2 minutes. Check for deformation or cracking. Simulates real use.

Step 5: Packaging and shipping – Will lunch boxes get damaged in transit?

Good products need proper packaging to arrive safely.

Scratch protection: Clear PP scratches easily. We put foam or bubble wrap between layers.

Stacking stability: In the carton, lunch boxes must be stacked neatly so they don't shift during transport. We design the packing pattern based on carton dimensions.

Labels and cartons: Barcodes, certification marks, "MADE IN CHINA" – all must be printed correctly. Outer cartons need warnings like "Don't stack too high" and "Keep dry".

Pre-shipment confirmation: We take photos and videos during container loading and share them with the client. Proof in case something goes wrong in transit.

Final thoughts

A lunch box looks simple, but there are over ten steps from raw material to finished product. Any weak link hurts bulk quality.

We've been in this business for over 20 years. Nothing high-tech – but we control every step tightly. Clients trust us not because we talk big, but because we stay consistent.

If you're looking for a lunch box supplier, ask about their raw material sources, mold maintenance records, and QC procedures. The more details you dig into, the lower the risk.