How are disposable plastic cups printed?

The printing of disposable plastic cups is typically done after the cup body is formed through a specialized process, 

with the most mainstream and efficient method being flexographic printing.

Below is a detailed explanation of the printing process for disposable plastic cups:

Core Printing Technology: Flexographic Printing

Flexographic printing is a rotary printing technique that uses flexible relief plates (usually made of soft photopolymer or rubber). It is particularly suitable for high-speed, single-sided printing on non-absorbent materials (such as plastics and films), making it ideal for substrates like plastic cups.

Detailed Printing Process

Step 1: Pre-Treatment - Surface Corona Treatment

This is a critical step. The surface energy of plastic cups (especially those made of PP, PS, and PET) is very low, making them "oleophobic," which would prevent ink from adhering effectively and cause it to scrape off easily.

• Method: The plastic cups pass through a corona treater.

• Working Principle: High-frequency, high-voltage current generates a corona discharge between the electrode and the ground, ionizing the air around the cups and producing plasma. This plasma bombards the cup surface, modifying its molecular structure to increase surface energy and roughness.

• Effect: After treatment, the cup surface becomes "ink-philic," significantly enhancing ink adhesion and durability.

Step 2: Printing - On High-Speed Rotary Printing Machines

The treated cups are fed into a dedicated flexographic printing machine. This machine typically consists of a large rotary turntable with multiple printing stations around it.

Working Principle:

1. Inking: Each printing station corresponds to one color. Ink flows from the ink pan, and a metal roller known as the "anilox roller" is responsible for precisely metering and transferring the ink. The surface of the anilox roller is covered with countless tiny, uniform honeycomb-like cells (engravings) that determine the amount of ink transferred to the plate.

2. Ink Metering: The anilox roller transfers the ink to a "doctor blade," which scrapes off excess ink, leaving only the precise amount within the cells.

3. Ink Transfer: The anilox roller, carrying the precise amount of ink, contacts the flexible "plate cylinder," transferring the ink to the raised image areas of the plate.

4. Printing: The plate cylinder directly contacts the plastic cup mounted on a mold, transferring the image onto the cup body through slight impression pressure.

5. Color Registration: A cup needs to pass through multiple such stations (e.g., cyan, magenta, yellow, and black) to complete a multi-colored pattern. Precise mechanical registration ensures each color aligns accurately with the previous one, forming the complete design.

Step 3: Drying - UV Curing

Special UV (ultraviolet) curable inks are used for printing plastic cups. These inks instantaneously change from liquid to solid when exposed to ultraviolet light.

• Process: The freshly printed cups immediately pass through a UV drying unit (equipped with high-intensity UV lamps).

• Effect: Within a fraction of a second, the photoinitiators in the ink are activated, triggering a cross-linking reaction between monomers and prepolymers, causing rapid curing and drying.

• Advantages:

  • Instant Drying: Allows immediate progression to the next step (e.g., stacking, packaging) at very high speeds.

  • Durable Ink: The cured ink film is very hard, wear-resistant, scratch-resistant, and complies with food packaging safety standards (no solvent residues).

  • Vibrant Graphics: Because curing is instantaneous, the ink does not penetrate the material, maintaining high color saturation and gloss.

Step 4: Post-Processing and Inspection

The dried cups are automatically stacked and packaged. Before packaging, vision inspection systems or manual spot checks are typically used to detect defects such as misregistration, missing print, or scratches in the printed pattern.

In summary, the printing of disposable plastic cups is a precision automated process that combines surface physical modification, high-speed relief printing, and photochemical curing technology. This ensures efficient, safe, and visually adequate pattern printing while maintaining low cost.