I. Project Background
Two-color overmolding products, with their distinctive appearance and versatile functionality, are widely used in various fields such as consumer electronics, automotive interiors, and medical devices. The TPE (thermoplastic elastomer) material TA75A-15, owing to its outstanding performance, has become an ideal choice for developing two-color overmolding products. With a Shore A hardness of 75, it combines excellent elasticity, wear resistance, and processability, and can effectively bond with a wide range of substrates, meeting the diverse requirements of different products.
II. TA75A - 15 Material Properties
Outstanding elasticity and soft touch: With a Shore A hardness of 75, TA75A-15 offers moderate elasticity, providing a comfortable and soft grip that is ideal for products requiring frequent human contact—such as handles for electronic devices and handheld medical instruments—thereby reducing hand fatigue caused by prolonged use.
Outstanding wear resistance: The specially formulated design gives the material exceptional wear resistance, enabling it to significantly extend product lifespan and reduce the risk of performance degradation caused by wear when used in friction-prone areas such as automotive interior components (e.g., shift ball joints, armrest box pads).
Excellent processability: The melt flow rate is appropriately balanced, enabling rapid and uniform filling of the mold cavity during injection molding. By adjusting processing parameters, product dimensional accuracy can be precisely controlled, thereby enhancing production efficiency and ensuring consistent product quality—factors that are conducive to large-scale industrial production.
Wide substrate compatibility: Exhibits excellent adhesion to common plastic substrates such as PP, ABS, and PC. The bonding is achieved through chemical bonds and intermolecular forces, ensuring that the two-color overmolding products maintain a firm bond with the substrate over extended periods of use, preventing the overmolding layer from peeling off or cracking, and thereby guaranteeing the structural integrity and functional reliability of the product.
III. Key Design Considerations for Two-Color Rubber-Coated Products
Product Structure Design
Thickness of the rubber-coating layer: The thickness of the rubber-coating layer should be determined appropriately based on the product’s application scenario and performance requirements. For products with high abrasion resistance requirements, the rubber-coating layer thickness should be slightly increased; for products that prioritize lightweight design, the thickness should be minimized as much as possible while still ensuring adequate performance. Typically, the thickness of the rubber-coating layer is controlled within the range of 0.5 to 3 mm.
Transition Zone Design: In the transition zone between the rubber-coating layer and the substrate, designs such as rounded corners and chamfers are employed to reduce stress concentrations and prevent cracking caused by uneven stress distribution during product use. The radius of curvature in the transition zone is typically no less than 0.5 mm.
Functional Structure Integration: In the overmolding design, special structures such as anti-slip textures and shock-absorbing grooves are integrated to meet product functional requirements. The anti-slip texture can be directly molded into place, enhancing the friction on the product’s surface; the shock-absorbing grooves effectively absorb vibrations, thereby improving the user experience.
Mold design
Selection of Two-Color Injection Molding Mold Types: Based on the product’s shape, size, and production volume, select an appropriate two-color injection molding mold type. For example, rotary table molds are suitable for products with complex shapes and high precision requirements, while slide-type molds are better suited for the rapid production of products with simpler structures.
Mold Material Selection: The main body of the mold is made from high-strength, wear-resistant steel, such as Cr12MoV, to ensure dimensional stability and durability of the mold during long-term high-pressure injection molding. The mold surfaces that come into contact with TA75A-15 are subjected to nitriding treatment to enhance surface hardness and improve demolding performance.
Runner System Design: Optimize the runner system to ensure balanced and uniform melt flow during injection molding of TA75A-15. Employ hot runner technology to reduce material waste and enhance molding efficiency. The runner diameter should be reasonably designed based on product size and injection volume, typically ranging from 3 to 8 mm.
Ejection System Design: Reasonably arrange venting grooves and vents to promptly discharge gases generated during the injection molding process, thereby preventing defects such as bubbles and insufficient material in the finished products. The depth of the venting grooves is typically controlled within the range of 0.02 to 0.05 mm, while the width is determined based on the mold dimensions and product structure.
4. Production Process
Raw material preparation
TA75A-15 Preprocessing: Dry the TA75A-15 raw material in an oven at 80℃ for 2–3 hours to remove moisture, thereby preventing defects such as bubbles and silver streaks in the molded products caused by moisture evaporation during injection molding.
Substrate Preparation: Clean substrates such as PP and ABS to remove surface oils, dust, and other impurities. Methods such as ultrasonic cleaning or alcohol wiping can be used to ensure the substrate surface is thoroughly clean, thereby enhancing the adhesion between the substrate and TA75A-15.
Injection Molding Process Parameters
Injection molding temperature: For TA75A-15, the injection molding temperature is typically maintained between 180 and 200℃ to ensure thorough melting of the material and good fluidity. For different substrate materials, the injection molding temperature should be adjusted accordingly. For example, when overmolding with a PP substrate, the temperature can be slightly lower, ranging from 170 to 190℃; whereas when overmolding with a PC substrate, the temperature can be appropriately raised to between 190 and 210℃.
Injection pressure and speed: The injection pressure is determined based on the product’s shape, size, and mold structure, typically ranging from 80 to 120 MPa, to ensure that TA75A-15 can fill the mold cavity quickly and uniformly. The injection speed should not be too fast, as this could lead to jetting marks that would compromise the product’s appearance quality; it is usually controlled within the range of 30 to 60 mm/s.
Holding pressure and holding time: The holding pressure is typically 60% to 80% of the injection pressure. The holding time is determined based on the product thickness, usually ranging from 5 to 15 seconds. This ensures that the product receives sufficient material replenishment during the cooling and shrinkage process, thereby reducing defects such as sink marks and deformation.
Cooling Time: The cooling time is determined based on the product thickness and the efficiency of the mold cooling system, typically ranging from 15 to 30 seconds. This ensures that the product cools sufficiently to set its shape and prevents deformation during demolding.
Secondary Injection Molding Operation
Substrate Positioning and Fixation: Before secondary injection molding, the substrate that has already been formed through primary injection molding must be precisely positioned and securely fixed in the mold. Methods such as positioning pins or locating grooves can be used to ensure accurate substrate placement during TA75A-15 injection molding, thereby preventing issues like uneven encapsulation or inconsistent wall thickness.
Injection molding sequence control: First, inject the base material. After the base material has cooled and solidified, rotate or move the mold to the second injection station and then inject the TA75A-15 encapsulating layer. During the injection process, strictly control the time interval between the two injections—generally no more than 30 seconds—to ensure optimal adhesion between the TA75A-15 and the base material.
V. Quality Control
Appearance Inspection: A combination of visual inspection and optical instrument inspection is used to perform a 100% examination of the product’s appearance. The inspection checks for defects such as bubbles, flow marks, insufficient material, and flash on the product surface. Additionally, the product dimensions are measured to ensure compliance with design specifications, with dimensional tolerances controlled within ±0.1 mm.
Bond Strength Testing: A certain proportion of products are randomly selected and subjected to peel tests, tensile tests, and other methods to determine the bond strength between TA75A-15 and the substrate. The peel strength requirement is no less than 3.5 N/cm, and the tensile strength must be no less than 4 MPa, ensuring that the rubber-coating layer remains firmly bonded to the substrate over the long term.
Performance Testing: Conduct appropriate performance tests based on the product’s application scenarios and functional requirements, such as wear resistance testing, aging resistance testing, and chemical resistance testing. For wear resistance testing, a friction testing machine is used to simulate real-world friction conditions; after testing, the wear depth of the rubber-coated layer should not exceed 0.1 mm. Aging resistance testing is carried out in an artificial climate aging chamber; after a specified aging period, the product’s performance changes should not exceed 10%. In chemical resistance testing, the product is immersed in common chemical reagents, such as alcohol and acetic acid, to observe whether any deformation, discoloration, or delamination occurs.
VI. Cost Analysis
Material Cost: TA75A—15 has a relatively moderate material price and offers a certain cost advantage compared to some high-performance TPE materials. By optimizing product design and manufacturing processes and appropriately controlling the thickness of the overmolded layer, material usage can be effectively reduced, further lowering material costs.
Mold Costs: Two-color injection molds are relatively complex to design and manufacture, resulting in higher costs. However, by selecting the appropriate mold type and materials, optimizing the mold structure, and extending the mold’s service life, the per-unit mold cost can be amortized. Meanwhile, as production volumes increase, the proportion of mold costs in the total cost will gradually decrease.
Production efficiency cost: TA75A - 15. With its excellent machinability and reasonably set injection molding process parameters, this material can enhance production efficiency, shorten the production cycle, and reduce labor costs and equipment energy consumption. By optimizing the production process and lowering the defect rate, it can also effectively cut down on overall production costs.
VII. Market Prospects
TA75A-15, with its outstanding performance and reasonable cost, is well-suited for use in two-color overmolding products, meeting the demands of various industries for product aesthetics, functionality, and quality. In the consumer electronics industry, as consumers increasingly seek personalized and comfortable products, two-color overmolded phone cases and tablet protective covers are poised for significant market growth. In the automotive interior sector, two-color overmolded steering wheel covers and seat adjustment buttons can enhance the interior’s texture and user experience, driving steady growth in market demand. In the medical device industry, two-color overmolded products can improve the operability and hygiene of medical instruments, presenting enormous application potential. As the use of TA75A-15 in two-color overmolding continues to expand, it holds great promise for creating new market opportunities across an even wider range of fields.