When using PETG shrink film in cold foil printing, the following key considerations should be addressed to ensure printing quality, production efficiency, and final product performance:

1. PRETREATMENT OF MATERIALS AND CONTROL OF SURFACE CHARACTERISTICS

1. Surface tension meets the required standards.

The surface tension of PETG shrink film must reach 38-42 dyn/cm to ensure uniform application and adhesion of cold foil adhesive. For optimal performance, surface activity should be enhanced through corona or plasma treatment prior to printing. Printing must be completed within 24 hours after treatment to prevent surface tension degradation.

2. thickness uniformity

The thickness deviation of the control film should be within ±3μm. Thickness non-uniformity will cause difference in shrinkage rate, which may lead to pattern deformation or uneven foil transfer. The substrate with longitudinal/latitudinal thickness tolerance ≤2% should be preferred.

3. Heat shrinkage rate matching

Perform pre-test of the film’s shrinkage during post-printing processes (e.g., heat sealing, forming), with longitudinal shrinkage ≤3% and transverse shrinkage 15-25%. This ensures dimensional synchronization with the cold foil pattern to prevent stretching or wrinkling.

II. Cold foil adhesive and coating process compatibility

1. Glue type selection

Select solvent-based or UV-curable cold foil adhesives to ensure compatibility with PETG substrates (to prevent adhesive corrosion of film surfaces). We recommend using ester-based solvent systems to minimize swelling risks in PETG.

2. The amount of coating applied is precisely controlled.

The coating amount should be adjusted according to the foil layer thickness (typically 8-12g/m²). Insufficient coating may result in poor adhesion, while excessive coating may cause glue overflow or incomplete drying. The amount should be precisely controlled by the mesh size of the screen roller (150-200 lines).

3. Optimization of drying parameters

The drying temperature of solvent-based adhesive should be controlled at 60-80℃ with 3-5m/s wind speed to ensure the residual solvent content is ≤5mg/m². The curing energy of UV adhesive should reach 800-1000mJ/cm² to avoid foil transfer failure caused by uncured adhesive.

III. Pressing and Foil Transfer Process

1. Compaction temperature and pressure

The temperature of the pressing roller is controlled at 50-60℃ (to avoid premature shrinkage of PETG) and the pressure is set at 0.3-0.5MPa. Uneven pressure will lead to local loss of foil layer. It is recommended to use a silicone pressing roller (hardness 70-80 Shore A) to ensure uniform contact.

2. pressing speed matching

The speed is controlled at 80-150 m/min, which is adjusted according to the drying speed of the adhesive. If the speed is too fast, the foil transfer may be incomplete, and if the speed is too slow, the film may be stretched and deformed.

3. Release film peeling angle

The peeling angle should be maintained at 30-45°. If the angle is too large, the foil layer will tear. If the angle is too small, the peeling resistance will increase. The film material should be stabilized by the tension control system (tension fluctuation ≤±2N).

IV. Post-processing and Environmental Control

1. stability of tension control

Throughout the printing process, the film tension should be maintained at 10-15N (adjusted according to film width). Sudden tension changes may cause pattern registration errors (registration accuracy must be ≤0.1mm). A closed-loop tension control system is recommended for real-time monitoring and adjustment.

2. Environmental temperature and humidity management

The workshop maintains a temperature of 23±2℃ with 50±5% relative humidity. Excessive humidity may cause condensation on the film surface or moisture absorption by the adhesive, while insufficient humidity could generate static electricity (requiring an electrostatic eliminator with a voltage ≤500V).

3. Post-cooling process coordination

For subsequent hot shrink molding, ensure the cold foil layer remains intact and color-stable at the shrinkage temperature (typically 80-100℃). Conduct a pre-test for temperature resistance (conditions: 100℃ for 30 minutes, with adhesion loss ≤10%).

V. Quality Inspection and Problem Identification

1. Key indicators detection
   • Adhesion: The foil layer must not detach by more than 5% when tested with 3M 610 adhesive tape.
   • Friction resistance: The 450g weight paired with suede fabric maintains its color without noticeable fading after 50 friction cycles.
   • Pattern integrity after shrinkage: When the shrinkage rate meets the standard, the pattern deformation should be ≤2%, with no cracks or peeling.
2. Common Issues Resolution
   • For foil layer bubbles: Check if the coating amount is excessive or the pressing pressure is insufficient, and adjust to the standard range.
   • Imprint error: calibrate the parallelism of printing plate roller and pressing roller, control the tension fluctuation;
   • Pattern distortion after shrinkage: Optimize the ratio of longitudinal to transverse shrinkage of the film to ensure uniform shrinkage.

VI. Compatibility of Equipment and Auxiliary Materials

1. Selection of printing materials

The laser-engraved ceramic mesh roller (chromed layer thickness 5-10μm) is used to avoid scratching the PETG surface. The diameter deviation of the plate roller is ≤0.01mm to ensure the registration accuracy.

2. compatibility of release film

Use PET release film (release force 3-5g/in). Excessive force may cause foil residue, while insufficient force could lead to premature peeling. The release force should be tested and matched with the type of cold foil adhesive.

3. roll up and roll down device

The winding tension should be 5-10% higher than the unwinding tension, and the winding hardness should be controlled at 40-50 Shore D to prevent wrinkling or creasing of the film roll.

Through precise control of the process parameters and optimization of material properties, this method effectively addresses common issues in PETG shrink film cold foil printing, including insufficient adhesion, pattern distortion, and uneven foil transfer. The final product thus meets stringent requirements for high definition, strong adhesion, and stable shrinkage performance.

 

• When applying PETG shrink film with cold foil printing technology, in addition to thickness and shrinkage rate, the following aspects require special attention:
• 1.Membrane tension control

The PETG shrink film must maintain a surface tension of 38-42 dynes/cm to ensure optimal adhesion between the cold foil adhesive and the film substrate. Insufficient tension may cause pattern detachment or edge warping during hot stamping. For optimal results, apply surface-active treatment (e.g., corona or plasma treatment) before printing, and use a tension meter to monitor the film’s stability in real time, preventing uneven foil transfer due to tension fluctuations.

2. Temperature matching for contraction

The heat resistance of cold foil adhesive and hot stamping foil must align with the shrinkage temperature profile of PETG shrink film (typically operating within 60-90°C). Insufficient heat resistance in the adhesive may cause pattern distortion or discoloration during subsequent heat shrinkage processes. If the dye or coating in the hot stamping foil is incompatible with the shrinkage temperature, it could result in loss of metallic luster or unpleasant odors. Optimal hot stamping parameters and subsequent shrinkage process parameters should be determined through stepwise temperature testing.

3. Die-cutting Accuracy and Compensation of Shrinkage Deformation

After cold foil printing, PETG shrink film requires die-cutting processing. The pre-deformation pattern must be designed in advance based on the film’s transverse/longitudinal shrinkage rates (typically 30-50% transverse and 5-10% longitudinal). The die-cutting blade dimensions require reverse compensation calculations. For instance, when the transverse shrinkage rate is 30%, the pattern’s transverse size should be enlarged by 30%, while ensuring uniform die-cutting pressure (recommended range: 8-12 kgf/cm²) to prevent foil layer cracking caused by uneven pressure distribution.

4. Environmental humidity sensitivity

PETG film exhibits inherent moisture absorption properties. When ambient humidity exceeds 60%, issues such as surface fogging and adhesive curing defects may occur. The cold foil printing workshop requires maintaining humidity levels between 40-55%, while storage should employ sealed packaging with built-in desiccants. During printing, sudden humidity fluctuations necessitate adjusting the drying temperature (recommended increase of 5-8°C) to accelerate adhesive cross-linking, thereby preventing post-printing bubbles or mist patterns.

5. The uniformity of the adhesive coating

Cold foil adhesive must be applied using a high-precision screen roller (recommended line count: 150-200 LPI, depth: 25-35μm) to maintain a wet adhesive thickness of 3-5μm. Uneven coating may cause uneven adhesion of the foil, resulting in localized burn-through or smearing. It is recommended to monitor coating volume every 30 minutes (using weighing or a coating meter) and clean the screen roller regularly to prevent clogging and ensure application accuracy.

6. Offset printing accuracy after shrinkage

For multi-color overprinting after cold foil processing, the impact of PETG shrinkage-induced dimensional changes on overprinting must be considered. The “pre-shrinkage compensation method” is recommended: during plate design, scale adjustments for each color plate should be made based on measured shrinkage rates, with horizontal scaling ratio calculated as 1/(1+horizontal shrinkage rate) and vertical scaling ratio as 1/(1+vertical shrinkage rate). Additionally, installing a tension closed-loop control system on the printing press helps maintain overprint errors within ±0.1mm, preventing pattern misalignment beyond visual tolerance (typically requiring overprint deviation ≤0.3mm).

7. Thermal stability of thin films

Some PETG grades may undergo thermal degradation after prolonged heating (such as compounding or secondary processing), resulting in yellowing of the cold foil layer. It is necessary to select PETG raw materials with stable melt flow rate (MFR) (2.5-3.5g/10min,230℃/2.16kg is recommended) and control the maximum temperature of the cold foil drying section to no more than 80°C to avoid premature thermal shrinkage or degradation of the film, which may affect the transparency and mechanical properties of the final product.

8. Compatibility of recycled materials

When using PETG film containing recycled materials, the recycled content must not exceed 30%, and the recycled materials must be thoroughly dried (moisture content ≤0.02%). Impurities or small volatile molecules in recycled materials may cause abnormal curing of cold foil adhesive, resulting in reduced peel strength between the foil layer and substrate after hot stamping (minimum requirement: ≥1.5N/15mm). It is recommended to filter the recycled materials using a screen with a minimum mesh size of 40μm to remove impurity particles.

9. Control of peeling force of hot stamping foil

The compatibility between the release layer of the hot stamping foil and the adhesive directly affects the peeling effect. It is recommended to control the peeling force at 0.8-1.2N. Too little peeling force can lead to incomplete transfer of the hot stamping foil, while too much can cause tearing at the edges of the pattern. The peeling effect can be optimized by adjusting the hot stamping pressure (recommended 0.3-0.5MPa) and the hot stamping speed (suggested 80-120 m/min). If necessary, test with hot stamping foils using different release agent formulations.

10. Weather resistance testing verification

The cold foil-printed PETG shrink film must undergo weather resistance testing to evaluate its long-term performance, including:

• Thermal cycling test (-20℃~60℃,10 cycles)
• UV resistance test (UVB-313 lamp, irradiation intensity 0.71W/m², duration 168h)
• Friction resistance test (load 500g,500 cycles, no noticeable discoloration)

Post-testing, the cold foil layer must demonstrate no cracking, discoloration, or reduced adhesion (with peel strength retention ≥85%) to meet durability requirements for applications such as food packaging and cosmetic labeling.

 

In cold foil printing of PETG shrink film, special attention should be paid to corona treatment, as detailed below:

• Corona treatment intensity control: The surface tension of PETG material must reach 38-42 dynes/cm to meet adhesive adhesion requirements in cold foil printing. Insufficient corona intensity (surface tension below 38 dynes/cm) reduces adhesion between aluminum layer and film during cold foil transfer, causing foil leakage at graphic edges and localized delamination. Conversely, excessive corona (surface tension exceeding 45 dynes/cm) causes excessive molecular chain breakage on the film surface, leading to quality defects like crystalline spots and patterns during subsequent heat shrinkage. This may also result in unstable heat shrinkage rates and affect registration accuracy.
• Corona Treatment Uniformity Control: Cold foil printing requires extremely high uniformity of film surface energy, with transverse corona deviation controlled within ±1 dynes/cm. Inconsistent contact pressure between corona rollers and film, fluctuating electrode spacing, or insufficient gas purity (dust content>0.1mg/m³) may cause localized low-tension zones on the film surface. During printing, these areas result in inconsistent adhesive layer strength after UV curing, leading to “snowflake” or “fog-like” defects in cold foil hot stamping, particularly noticeable in large solid-color patterns.
• Corona Aging Management: After corona treatment, the surface active groups in PETG film gradually degrade over time. The optimal usage window at room temperature (25°C) typically lasts 48-72 hours. If printing is not performed within 72 hours, the surface tension may drop below 36 dynes/cm, requiring re-corona treatment. Direct use will reduce the cold foil transfer rate to below 85% (the standard should be ≥98%), and may cause delamination during subsequent processes like lamination or bagging.
• Corona Surface Identification and Treatment: Cold foil printing requires direct contact between the corona surface and adhesive. The surface tension difference between the corona surface and non-corona surface of PETG film typically exceeds 10 dynes/cm. Using the non-corona surface as the printing surface may result in uneven adhesive coating, causing the cold foil adhesion strength to fall below 0.5N/15mm (the standard requires ≥5N/15mm). Additionally, during friction resistance testing (500g pressure, 30 reciprocations), noticeable aluminum layer detachment will occur.
• Surface cleanliness after corona treatment: If dust accumulates on electrodes or oil stains remain on the film surface during corona treatment, “corona shadow zones” may form. The surface tension in these areas typically drops below 35 dynes/cm. Prior to cold foil printing, ion wind gun cleaning must be performed to ensure dust particles ≤10 particles/㎡ (≥01mm²) and oil residue ≤5mg/m². Failure to meet these standards may cause localized cold foil transfer failures, resulting in pinhole defects that compromise product appearance qualification rates.
• Compatibility with UV-curing adhesives: The number of polar groups (hydroxyl and carboxyl) on the PETG surface after corona treatment must match the active components in the UV-curing adhesive. Excessive polar groups (>1.2×10¹⁵/cm²) from over-corona treatment may accelerate adhesive curing by 5-10%, potentially causing screen blockage. Conversely, insufficient polar groups (<0.8×10¹⁵/cm²) result in incomplete curing, reducing cold foil adhesion strength by over 30%.
• Impact of Thermal Shrinkage on Corona Treatment: After cold foil printing, PETG shrink film undergoes a 120-150℃ thermal shrinkage process with a typical shrinkage rate of 15-25%. Inconsistent corona treatment causes uneven stress distribution in the film, resulting in “arching” or “twisting” deformations that exceed the standard tolerance of 0.3mm (allowed ≤1mm). This may also induce stretching deformation in the cold foil’s printed patterns, potentially leading to cracks in severe cases.
• Corona parameter optimization for film thickness: For PETG shrink films with 15-30μm thickness, the corona power is typically set at 3-5kW (at 100-150m/min). When thickness increases to 30-50μm, the power must be raised to 5-7kW to prevent insufficient deep corona, which would cause uneven adhesion strength distribution of the cold foil along the film thickness and result in localized foil detachment during deep stretch testing.
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