Regarding water transfer printing on plastic bottles, pay attention to these details:

I. Water Transfer Printing Process Flow

Production of water transfer paper → Soaking of the paper → Pattern transfer → Drying → Finished product

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1. Production of water transfer paper: The pattern is processed by computer and output as a film to make a printing plate. Then, various colored patterns are printed on the water transfer base paper (which has been specially treated and requires no further processing). A cover varnish is printed over the patterned base paper. Another method is to use a color laser printer to directly print the pattern onto the water transfer base paper before proceeding with water transfer.

2. Soaking the paper: Place the printed paper into water, soak for 30 seconds, then remove it from the water. Wait until it can slide smoothly. Note: Soaking time should not be too long, otherwise the glue on the paper surface will dissolve.

3. Pattern transfer: First, treat the surface of the object to be transferred. Then, transfer the soaked paper onto it, scrape off any excess water, and dry.

4. Drying: For items such as metal, ceramics, and glass, place them in an oven and bake at 140°C for 20 minutes. For plastic items, bake at 70°C for 60 minutes. Candles, helmets, or tempered glass do not require baking.

5. Finished product: Spray a layer of transparent topcoat over the baked pattern on the item. After drying for 12 hours, the pattern will permanently bond with the object. This step can be skipped, but the adhesion will be poor.

II. Water Transfer Printing Operation

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1. Film application: Pre-print the desired pattern on a water-soluble film.

2. Activation: Use a special solvent to activate the pattern on the film into an ink state.

3. Transfer: Use water pressure to transfer the pattern onto the object.

4. Rinsing: Rinse off any residual impurities on the object with water.

5. Drying: Dry the object.

6. Topcoat spraying: Spray a PU transparent varnish to protect the surface of the object.

7. Redrying: Dry the surface of the object after spraying the topcoat.

III. Two Types of Water Transfer Technology

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One is water decal transfer technology, and the other is water film transfer technology. The former is mainly used for transferring text and photographic patterns, while the latter is more suitable for complete transfer over the entire surface of a product. Film transfer technology (Cubic Transfer) uses a water-soluble film to carry the patterns and text. Due to the excellent tension of the water transfer film, it easily wraps around the product surface to form a pattern layer, giving the product a completely different appearance, as if it were painted. Film transfer technology can apply colorful patterns to workpieces of any shape, solving the problem of printing on three-dimensional products for manufacturers. Curved surface transfer can also add different textures to the product surface, such as leather, wood, jade, or marble patterns, while avoiding misalignment often seen in flat surface printing. Moreover, during the printing process, the product surface does not need to come into contact with the printing film, avoiding damage to the product surface and ensuring its integrity.

Water transfer printing uses a specially chemically treated film printed with the desired color patterns. The film is floated on the water surface, and water pressure is used to evenly transfer the color patterns onto the product surface. The transfer film automatically dissolves in water, and after rinsing and drying, a transparent protective coating is applied. The product then presents a completely different visual effect.

IV. Inspection Items for Water Transfer Printing

1. Particles

   • A-side:

     • Particles with an area ≥ 0.5 mm² (white/black spots) are not allowed.

     • Particles below 0.1 mm² are acceptable if they do not significantly affect appearance.

     • Particles between 0.1 mm² and 0.5 mm² are allowed up to 3 on the entire A-side, with a distance of more than 6 cm between particles.

   • B-side:

     • Particles with an area ≥ 0.7 mm² are not allowed.

     • Particles below 0.2 mm² are acceptable.

     • Particles between 0.2 mm² and 0.7 mm² are allowed up to 3.

2. Oil spots

   • A-side:

     • Oil spots between 0.1 mm² and 0.5 mm² are allowed up to 2, with a distance of ≥ 5 cm between them.

     • Oil spots below 0.1 mm² are acceptable if they do not significantly affect appearance.

   • B-side:

     • Oil spots between 0.2 mm² and 0.7 mm² are allowed up to 2.

     • Oil spots below 0.2 mm² are acceptable if they do not significantly affect appearance.

3. Common appearance issues

   • Pattern: Must be complete and clear.

   • Color spots: Significantly affecting appearance is not allowed.

   • Foreign matter: Significantly affecting appearance is not allowed.

   • Oil flow: Significant oil flow is not allowed.

   • Scratches: Significant scratches are not allowed.

   • Color difference: Significant color difference (compared to the sample) is not allowed.

   • Cracking: Paint cracking is not allowed.

   • Pitting: Significant pitting is not allowed.

   • Deformation: Deformation is not allowed.

4. Appearance standards:
   (1) Grade surface division standards:

   • Grade A surface: The outer surface often seen after assembly, including the top surface visible to a person standing normally and side surfaces visible without bending.

   • Grade B surface: Surfaces not often seen but visible under certain conditions, such as sides and bottom surfaces.

   (2) Inspection conditions:

   • Light source requirements: North sky daylight or two indoor high-efficiency fluorescent lamps (illuminance about 1000 lumens).

   • Visual inspection distance: 300 mm for Grade A surfaces, 500 mm for Grade B surfaces.

   • Inspection standards: Based on the light source standards, distinguish the grade surfaces of the product. All grade surfaces should have no exposed substrate, peeling, or other defects. All surfaces should be free of scratches, bubbles, wrinkles, pinholes, powder accumulation, or other defects. Inspect by scanning at a speed of 3 cm/s from the standard distance for each grade surface.

5. Performance standards:
   (1) Non-destructive inspection items:

   • Gloss: Measured with a gloss meter at a 60° angle, with a ±5% error tolerance.

   • Color: The color must match the design drawing and show no significant difference compared to the color sample.

   (2) Destructive inspection:

   • Adhesion test: Cross-cut test method. After spraying, take a sample and make 11 parallel cuts in both directions at 1 mm intervals on the coated surface (with enough force to expose the substrate), creating 100 squares. Apply 3M tape firmly and pull it off at a 45° angle at a speed of 100 mm/min. Check if any coating falls off. The acceptance standard is grade 0–1, meaning脱落面积不超过5%为合格 (peeling area not exceeding 5% is acceptable).

   (3) Hardness test:

   • Use a sharpened HB pencil fixed horizontally at a 45° angle to the coated surface. Push it forward along a straight line for 15–30 mm. After erasing the mark with a rubber, check the coated surface. The standard: no visible scratch is acceptable.

   (4) Solvent resistance test:

   • Use analytical alcohol (99.8% anhydrous ethanol) to wet a cotton swab. Apply 1 kg of force and rub the coated surface back and forth 50 times. The standard: the coating must not show any peeling, discoloration, or swelling. Slight changes in gloss are acceptable.

   (5) Impact resistance test:

   • Using test equipment, drop a 500 g weight from a height of 500 mm. The standard: after a 1/4 positive impact from the punch, the surface coating must show no cracking or peeling.