In the manufacturing of wheels using overmolding techniques—where a soft TPU (thermoplastic polyurethane) outer layer is bonded to a harder substrate like PP (polypropylene) or nylon—engineers often face a critical challenge: the TPU may fail to properly adhere or fully encapsulate the harder material, leading to defects and reduced product durability. One effective method to overcome this is the two-shot injection molding process, where the TPU outer layer is molded first, followed by injecting the hard plastic (PP or nylon) into it. This approach relies on high-temperature bonding to create a secure, integrated part. Let’s explore why this issue occurs and how to implement this solution successfully. Common Causes of Overmolding Failure When TPU is overmolded onto PP or nylon, several factors can lead to poor encapsulation or adhesion: Material Incompatibility: TPU, PP, and nylon have different chemical polarities and surface energies, making natural adhesion difficult. Insufficient Melt Temperature: If the hard plastic substrate is not hot enough when the TPU is injected, the materials won't fuse properly. Shrinkage Mismatch: Different cooling and shrinkage rates can cause the TPU to pull away from the rigid core. Mold Design Flaws: Inadequate venting, gate placement, or insufficient injection pressure can prevent complete filling and wrapping. The Two-Shot Molding Solution: TPU First, Then Hard Plastic The traditional method injects the hard material first, then overmolds TPU onto it. Reversing this sequence—molding the TPU outer shell first, then injecting PP or nylon inside it—can solve many encapsulation problems. Here's how and why it works: Process Breakdown: Step 1: The TPU is injected to form the flexible outer wheel (tread, sidewalls). Step 2: While the TPU is still very hot and within its processing temperature window, the rigid plastic (PP/nylon) is injected into its core or designated cavity. Step 3: At this high interface temperature, the incoming melt of the hard plastic slightly remelts the inner surface of the TPU, allowing the materials to interdiffuse and mechanically lock together at the molecular level. Key Advantages of This Method: Superior Mechanical Lock: The high-temperature injection of the hard plastic creates a strong fusion bond as it melts into the TPU's inner layer. Better Encapsulation: The TPU outer shell acts as a defined mold cavity for the hard core, ensuring complete containment and eliminating flow fronts that cause incomplete wraps. Reduced Stress: Since the TPU cools and shrinks around the already-solidified hard core (which cools last), residual stresses are minimized, improving dimensional stability. Critical Parameters for Success To implement this method effectively, precise control of process parameters is non-negotiable: Temperature Control: The interface temperature is the most critical factor. The inner surface of the TPU must be above its softening point when the hard plastic is injected. This o...

Jan 30, 2026

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We are thrilled to announce that KUNSUN POLYMER MATERIAL (FUJIAN) CO., LTD. successfully participated in the 28th Wenzhou International Leather, Shoe Materials, and Shoe Machinery Exhibition, held from August 22nd to 24th, 2025, at the Wenzhou International Convention and Exhibition Center. It was an incredible platform to showcase our innovative TPU shoe material solutions and connect with industry professionals from around the globe. A Major Attraction: Our TPU Shoe Materials Our booth became a focal point of interest, drawing numerous domestic and international clients and professionals specializing in shoe materials. Visitors were particularly captivated by our high-performance TPU samples, which highlight the versatility, durability, and aesthetic appeal of our products. From enhanced elasticity and abrasion resistance to eco-friendly recyclability and stunning transparency, our TPU materials are engineered to meet the evolving demands of the modern footwear industry. Whether for athletic sneakers, fashion-forward casual wear, or rugged outdoor boots, KUNSUN TPU offers cutting-edge solutions that combine performance with sustainability. Learning and Growing Together Beyond promoting our brand and products, this exhibition was also a profound learning experience. We had the privilege of engaging with potential clients of different ages, backgrounds, and professional experiences. Each conversation provided us with new insights into market needs, design trends, and technical challenges—helping us grow not only as a company but as innovators committed to serving the global footwear community. We are inspired by the feedback we received and energized by the opportunities to collaborate with partners who value quality, creativity, and sustainability.

Aug 29, 2025

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(July 10-13, 2025) – From July 10th to 13th, the 22nd Asia-Pacific Plastics & Rubber Exhibition was grandly held at the Qingdao World Expo City International Convention and Exhibition Center. KUNSUN POLYMER MATERIAL（FUJIAN）CO., LTD., an industry pioneer with over 30 years of dedicated focus on TPU R&D and manufacturing, made a grand appearance at Booth S3C195. Showcasing its renowned brand Koslen® and innovative achievements, the company became a focal point in the TPU sector at this year's exhibition, leveraging its profound technical expertise and cutting-edge new product portfolio. The KUNSUN booth experienced a continuous flow of visitors and sustained surge in popularity. Professionals and partners from diverse fields including footwear materials, pipes, medical products, wearable electronics, wheels, automotive parts, injection molding, and modification stopped to explore the rich array of application samples and core material exhibits. They engaged in in-depth discussions and enthusiastic negotiations with the KUNSUN technical team. Visitors gained an immersive experience of the exceptional performance and broad applicability of Koslen® TPU materials, seeking new opportunities for win-win collaboration. With thirty years of deep cultivation in the TPU field, KUNSUN Polymer has consistently focused on technological innovation and quality enhancement. The Koslen® brand has earned widespread trust from global customers through its superior physical and mechanical properties, processing stability, and eco-friendly characteristics, successfully penetrating numerous critical application scenarios. At this exhibition, Kunsheng Polymer powerfully launched two innovative product lines: Aliphatic TPU Series: Delivers an ideal solution for fields demanding stringent color stability and appearance. SCF (Supercritical Foaming) Dedicated TPU Series for Micro/Macro Foaming: Tailor-made for advanced supercritical physical foaming processes, this series overcomes technical barriers. It empowers customers to achieve large-scale, stable production of high-performance, lightweight foamed products (like premium sports shoe midsoles, lightweight cushioning materials), pushing the boundaries of performance.

Jul 23, 2025

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