Working Principle of In-Mould Labeling (IML) is a key topic in modern plastic packaging and product decoration. In-mould labeling (IML) integrates the label with the plastic part during the molding process, creating a durable, high-quality decorative surface while simplifying downstream labeling steps.
1. What Is In-Mould Labeling (IML)?
In-mould labeling (IML) is a process in which a pre-printed label or decorative film is placed inside the mold cavity before plastic is injected or blown. When the molten plastic is injected, it fuses with the label. After cooling, the label becomes an integral part of the final product surface.
This technology is widely used in food packaging containers, household product packaging, cosmetics packaging, and durable plastic parts that require high-quality decoration and information display.
2. Core Working Principle of In-Mould Labeling
The working principle of in-mould labeling (IML) can be summarized in four main steps:
- Label material selection
- Label positioning in the open mold
- Plastic melting and injection molding
- Cooling, demolding, and label–product integration
Each step directly affects the final appearance, adhesion, and durability of the in-mould labeled product.
3. Step 1: Label Material Selection
The IML process begins with selecting a suitable label material according to the type of plastic product and application scenario.
Common label materials include:
- Paper labels
Suitable for applications that prioritize cost-effectiveness and basic print quality, and where moisture or abrasion resistance requirements are not extreme. - Polypropylene (PP) labels
Very common in IML, especially for PP containers such as yogurt cups and food boxes. The label and container share similar materials, improving recyclability and adhesion. - Polystyrene (PS) labels
Used for PS products, providing good printability and decorative effects. - Labels with the same or similar material as the molded part
When the label is made from a material identical or similar to the molded product, the bond between the label and product is enhanced, improving impact resistance and recyclability.
Cavitation Labels and Laminated Films
In some applications, cavitation labels or laminated film labels are used:
- Cavitation labels
These consist of a sponge-like intermediate layer bonded between two thin layers. The micro-void structure can:- Reduce material usage and weight
- Provide a softer hand feel
- Improve visual effects such as opacity and print contrast
- Laminated film labels
When abrasion resistance and surface durability are critical (e.g., high-frequency handling, friction or cleaning), laminated film is used as the label material. Multiple layers of film provide:- Higher scratch and wear resistance
- Better chemical resistance
- Longer service life of the printed design
4. Step 2: Label Placement in the Open Mold
After material selection and label printing are completed, the label is transferred into the mold cavity.
In the working principle of in-mould labeling (IML), precise and stable positioning is crucial.
Common label fixation methods include:
- Clamps / mechanical fixtures
Physically hold the label in the desired position inside the mold. - Vacuum suction
Vacuum holes in the mold cavity suck the label tightly against the mold wall, preventing movement during injection. - Static electricity
Static charges are used to attract and hold the label onto the mold surface. - Compressed air
Airflow may be used in some systems to help position or temporarily hold the label.
These methods ensure the label remains flush with the mold cavity wall, maintaining accurate registration and preventing wrinkles, distortion, or misalignment.
5. Step 3: Plastic Melting and Injection
Once the label is securely in place, the IML process moves to the injection stage:
- The selected plastic material (e.g., PP, PS, etc.) is heated to a molten state.
- The molten plastic is injected into the mold cavity under high pressure.
- The plastic flows over the back side of the label, pressing it firmly against the mold surface.
At this moment, the label and molten plastic are in close contact, and under heat and pressure, they begin to bond. If the label and the molded part share similar materials, the interface fusion is stronger, resulting in an integrated decorative layer.
6. Step 4: Cooling, Demolding, and Final Appearance
After injection:
- The plastic gradually cools and solidifies inside the mold.
- As the part cools, the label is permanently attached to the product surface, becoming a part of the structure rather than a separate sticker.
- Once the set cooling time elapses, the mold opens and the finished product is ejected.
At this stage, the product already carries:
- A high-resolution printed design
- Required text information (brand, ingredients, instructions, barcodes)
- Possible visual effects such as matte, gloss, metallic, or textured finishes
No additional outer labeling processes are needed; the entire decoration is completed in-mould.
7. Advantages Highlighted by the Working Principle of IML
By understanding the working principle of in-mould labeling (IML), its advantages become clear:
- High integration: Decoration and molding are done simultaneously, reducing separate labeling steps.
- Enhanced durability: The label is fused with the plastic product, offering better scratch, moisture, and chemical resistance than conventional stickers.
- Excellent print quality: Pre-printed labels can use advanced printing technologies (such as offset, flexo, or gravure) to achieve high-definition images.
- Improved efficiency: Eliminates or reduces post-molding labeling, improving average production speed.
- Better recyclability: When the label and product share the same or similar material, overall recycling is simpler and more efficient.
8. Summary: Understanding the Working Principle of In-Mould Labeling (IML)
In summary, the working principle of in-mould labeling (IML) is based on:
- Selecting appropriate label materials (paper, PP, PS, cavitation labels, laminated films)
- Accurately positioning the label in the open mold with clamps, vacuum, static electricity, or compressed air
- Injecting molten plastic into the mold, allowing it to bond with the label under heat and pressure
- Cooling and demolding to obtain a finished product with an integrated decorative surface
This process transforms traditional “label-on-product” into a label–product integration, providing higher-value, longer-lasting surface decoration and information display.