In the context of plastic products, we can consider screen printing of injection molded parts as a form of secondary processing or reprocessing. Secondary processing refers to additional steps or operations performed on a plastic product after the primary processing methods like injection molding, extrusion, or die-casting.
Secondary processing of plastic products includes various activities such as surface finishing, additional machining, and assembly. Surface finish methods mentioned, including screen printing, pad printing, hot stamping, Thermal-transfer printing, water transfer printing, painting, spraying, electroplating, vacuum aluminizing, flocking, and cloth wrapping, are used to enhance the appearance, add decorative elements, or provide branding on the plastic products.
We can employ machining methods like drilling, turning, milling, bending, and rolling to achieve specific shapes, dimensions, or functional features that were not attained during the primary processing stage.
Assembly techniques such as hot melt welding, ultrasonic welding, and laser welding join different plastic parts or combine plastic parts with other components.
The purpose of secondary processing is to add value to the plastic product, improve its appearance, and achieve functionalities not possible through the primary processing method alone.
Today, we focus on the most prevalent surface finish process for injection molded products: screen printing.
What is screen printing?
Screen printing is the abbreviation of “silk screen printing.” The screen printing technique utilizes a mesh screen to transfer ink onto a substrate(fabric, paper, plastic, or metal), in specific areas to create the desired design or pattern.
Modern screen printing technology uses photographic materials and the photographic plate-making method to create screen printing plates. This process allows for building screen holes that correspond to the graphic part of the design while blocking the screen holes in the non-graphic areas.
During printing, ink is transferred to the substrate through the screen holes using a squeegee. The ink is squeezed through the open areas of the screen corresponding to the graphic part, resulting in the reproduction of the original design on the substrate.
Screen printing equipment is generally considered simple, easy to operate, and relatively inexpensive compared to other printing methods. Creating screens and making plates is also relatively easy and cost-effective, making it accessible for various applications.
Screen printing has various applications and is commonly used for producing colorful oil paintings, posters, business cards, book covers, signs, and printed textiles, among other printed products.
The process of screen printing of injection molded parts
Pattern Design: Create the desired design or pattern that will be printed on the injection molded parts.
Film Output: Generate a film positive of the design, which will serve as a stencil for screen printing.
Manufacture of Screen Frame: Construct a screen frame to hold the mesh screen.
Stretching: Stretch the mesh screen tightly onto the screen frame to ensure proper tension for printing.
Photopolymer Coating: Apply a photosensitive emulsion or photopolymer to the screen mesh and use a squeegee or similar tool to distribute and coat the mesh evenly.
Film Placement: Position the film positively onto the coated screen in the correct location, aligning it with the desired printing area.
Exposure: Expose the screen to ultraviolet (UV) light, which hardens the photopolymer and creates a stencil by blocking the areas not covered by the design.
Water Rinsing: Rinse the screen with water to remove the uncured photopolymer and reveal the open areas of the stencil.
Ink Selection and Adjustment: Choose the appropriate ink for the desired colors and adjust the viscosity of the ink if necessary.
Printing: Secure the injection molded part in place and align it with the screen stencil. The ink is effectively applied onto the part’s surface by utilizing a squeegee to press it through the stencil’s open areas.
Drying: Depending on the
ink type, allow the printed parts to dry, either through air drying or specialized drying equipment.
Inspection: Inspect the printed parts for quality control, checking for proper ink coverage, clarity of the design, and any defects or imperfections.
Packing: Package the finished printed parts appropriately for storage, shipment, or further processing.
Design Requirements for Injection Molded Parts:
Design Complexity: Evaluate the intricacy and level of detail in the design you plan to screen print on the injection molded parts. Complex designs may require more advanced printing techniques or equipment with higher precision.
Size and Placement: Determine the size of the design and its placement on the molded parts. Consider the dimensions of the printing area and ensure that the screen printing equipment can accommodate the required print size.
Color Considerations: Determine the number of colors involved in the design. Multi-color designs may require equipment with multiple stations or heads for accurate color registration.
Artwork Compatibility: Prepare the artwork or design files in a format suitable for screen printing.
Ink Selection: Choose the appropriate ink type based on the substrate material of the injection molded parts. Different inks are available for printing on plastics, metals, or other materials. Ensure compatibility between the ink and the substrate.
Durability and Resistance: Consider the durability requirements of the printed design, especially if the injection molded parts will be subjected to harsh environmental conditions or wear and tear. Select inks and printing techniques that provide the necessary durability and resistance properties.
The quality inspection of screen printing patterns
Adhesion: The screen-printed pattern adhesion usually uses the method specified in the GB9286-88 standard. This method provides guidelines for evaluating the adhesion of the ink to the substrate by employing specific scratching techniques.
Color: The printed colors should conform to the specified sample or color card requirements. This ensures that the pattern’s colors match the desired appearance and are consistent with the intended design.
Line: The thickness of the pattern lines should be even throughout the printed design. Consistency in line thickness contributes to the pattern’s overall quality and visual appeal.
Color Set: The position of the colors within the pattern should be accurate. This means that each color is correctly aligned and placed as intended, ensuring the desired color arrangement and visual effect.
Pattern Continuity: The screen-printed pattern should have no broken lines. The lines forming the pattern should be continuous and without interruptions or gaps, maintaining the integrity and visual cohesiveness of the design.
Position: The screen-printed pattern should align with the requirements specified in the drawings. This ensures the pattern is positioned correctly on the product, adhering to the intended placement and orientation.
Integrity: The strokes forming the pattern should be complete without missing or incomplete parts. This ensures the pattern is visually intact and accurately represents the intended design.
Other: Additional quality considerations may include evaluating friction resistance, aging resistance, solvent resistance, and other specific requirements. These properties can be assessed to ensure the screen-printed pattern meets the necessary durability and performance standards.
Automation of screen printing in injection molding In China
Companies in China commonly implement the automation of the screen printing process for injection molding. China has a well-developed manufacturing industry and is a major hub for injection molding production. The country has made significant advancements in automation technologies, including integrating automated screen printing systems into the injection molding process.
Automation in screen printing for injection molded parts offers several benefits, such as increased production efficiency, improved accuracy, consistent quality, and reduced labor costs. Automated screen printing equipment can handle high production volumes, ensuring faster and more reliable printing processes.
In China, numerous manufacturers and suppliers specialize in providing automated screen printing solutions tailored explicitly for injection molding applications. These companies offer a range of equipment options, including fully automatic printing systems that integrate seamlessly with injection molding machines.
These automated systems typically feature advanced functionalities such as automatic loading and unloading of parts, precise registration systems, programmable printing parameters, and quality inspection capabilities. The automation technology employed in these systems helps streamline the production process, minimize human error, and achieve higher productivity levels.