The full screen printing process for injection molded parts consists of three main stages: pretreatment, screen printing operation, and post-treatment with quality inspection. It covers the entire workflow from preparing the molded parts to delivering finished products.
I. Pretreatment Stage
1. Substrate cleaning and degreasing
Remove mold release agents, oil, dust, and other contaminants from the surface of injection molded parts. Commonly used cleaning agents include isopropyl alcohol, white spirit, or dedicated plastic cleaners. Heavily soiled workpieces may be cleaned with ultrasonic equipment to ensure no residue affects ink adhesion.
For low surface energy substrates such as PP and PE, corona or flame treatment is required to raise the surface tension above 38 dyn. Printing should be completed within 8 hours after activation to prevent the surface tension from declining.
2. Positioning fixture preparation
Design dedicated screen printing fixtures based on the shape of the injection molded parts. Vacuum forming, snap-fitting, or positioning pins may be used to achieve fast and accurate positioning and avoid print misalignment.
Calibrate the relative position between the screen stencil and the fixture to ensure the printed pattern exactly matches the intended location on the workpiece.
For non-standard positioning fixtures, vacuum plates, or custom alignment components used in this stage, Hansheng Automation can provide precision machining based on customer drawings or sample parts.
3. Ink preparation
Ink mixing should be carried out in a well-ventilated dedicated area to prevent safety risks from solvent vapor accumulation. Select the appropriate ink for the substrate material, such as PP-specific ink for polypropylene or general plastic ink for ABS. Some UV inks require the addition of a dedicated curing agent.
Mix the main agent and curing agent according to the manufacturer's specified ratio. Stir thoroughly and allow the mixture to stand for 10–15 minutes to release air bubbles, avoiding defects such as bubbles or sagging during printing.
II. Screen Printing Operation Stage
1. Equipment setup and trial printing
Install the screen stencil onto the screen printing machine. Adjust screen tension to 20–25 N/cm for nylon mesh and 25–30 N/cm for metal mesh. Set the squeegee angle between 45° and 60°. Adjust squeegee pressure according to ink viscosity to avoid excessive force that may deform the screen or cause excessive ink deposition.
Perform trial printing to check pattern position accuracy, ink layer thickness, and defects such as missing prints or pinholes. Only begin batch printing after all parameters meet requirements.
2. Batch printing
Place the workpiece onto the positioning fixture and start the machine. The squeegee moves evenly across the screen and transfers ink onto the workpiece surface. Printing speed is generally controlled at 10–30 cycles per minute to ensure a uniform ink layer.
Conduct sampling inspection every 50–100 pieces to monitor print quality and adjust equipment parameters or replace ink as needed.
III. Post-Treatment and Quality Inspection
1. Curing treatment
Solvent-based inks can be cured naturally at room temperature for 24 hours or force-dried. Infrared drying is typically performed at 60–100°C for 15–30 minutes. UV inks require curing under a UV lamp. For example, with a 1000 W lamp unit, exposure of 5–10 seconds is usually sufficient. Protective shielding must be installed during UV curing to prevent direct exposure of skin and eyes to ultraviolet light.
After curing, allow the workpieces to cool to room temperature to prevent cracking or sticking of the ink layer caused by high temperature.
2. Quality inspection
Visual inspection: Check pattern completeness and look for misalignment, scratches, bubbles, or exposed substrate using the naked eye or a 10x magnifier.
Adhesion test: Use a cross-cut tester to create a 1 mm × 1 mm grid on the ink layer, apply 3M 600 tape, and remove it quickly. Evaluate the adhesion level according to GB/T 9286-1998, with level 0 being the best.
Performance testing: Carry out additional tests such as abrasion resistance, solvent resistance, and weathering resistance according to customer requirements to ensure the product meets usage conditions.
3. Packaging and warehousing
Wrap qualified screen-printed parts with anti-static film or foam padding to protect the printed surface from scratches, and package them by batch.
Mark the batch number and production date before transferring to storage.
