Injection molding glue is not strong and solution
Injection molding combines two different plastic materials through the injection molding process to create products with specific functions and appearances. These products are widely used in electronics, automotive, and daily necessities. However, a weak overmolding is a common quality issue in this process. This can manifest as delamination and shedding between the two materials, seriously impacting product performance and service life. The causes of weak overmolding involve multiple factors, including material compatibility, process parameters, and mold design, requiring systematic analysis and targeted solutions.
First, ensuring the compatibility of the encapsulating materials is fundamental to resolving the problem of weak encapsulation. Different plastic materials vary in molecular structure, polarity, melting temperature, and other properties. If two materials are poorly compatible, the molecules cannot form an effective bond, inevitably leading to weak encapsulation. Therefore, when selecting encapsulating materials, prioritize combinations with good compatibility, such as PP and PE, or ABS and PC. For material combinations with poor compatibility, adding a compatibilizer can improve interfacial adhesion. A compatibilizer is typically a polymer with an amphiphilic structure that acts as a bridge between the two materials, promoting molecular diffusion and entanglement. Additionally, pre-treating the substrate surface can improve compatibility. For example, chemical etching or plasma treatment can be used to increase the substrate’s surface roughness and polarity, thereby enhancing adhesion to the encapsulating material.
Secondly, optimizing the injection molding process parameters is crucial for improving the strength of the overmolding. Injection temperature is a key parameter affecting overmolding quality. If the melt temperature of the overmolding material is too low, it will not fully wet the substrate surface, making it difficult to form effective intermolecular bonds. Excessive temperatures can cause thermal deformation of the substrate or degradation of the overmolding material, affecting the bonding effect. Therefore, the barrel and mold temperatures must be appropriately set based on the characteristics of both materials. Generally, the melt temperature of the overmolding material should be slightly above its melting point, while ensuring that the substrate does not deform during the overmolding process. Precise control is also required for injection pressure and hold time. Sufficient injection pressure ensures that the overmolding material adheres tightly to the substrate surface, expelling air from the interface and improving bond strength. Prolonging the hold time appropriately promotes molecular diffusion at the interface between the two materials, strengthening the bond. Typically, injection pressure is controlled between 80-150 MPa, and the hold time is set between 5-15 seconds, depending on the thickness of the product.
The rationality of mold design directly affects the firmness of the lagging. The mold cavity design should ensure that the lagging material can flow evenly and smoothly to the substrate surface, avoiding defects such as turbulence and bubbles. In terms of gate setting, multi-point injection or fan-shaped gates should be used to evenly distribute the lagging material on the substrate surface to prevent local pressure from being too high or too low. In addition, the mold’s exhaust system must be unobstructed, and exhaust grooves should be set at the interface where the lagging material contacts the substrate to promptly exhaust air and volatiles to avoid poor interface bonding due to gas stagnation. The mold’s positioning mechanism must also be precise to ensure that the substrate is fixed in place within the mold to prevent the substrate from shifting during the lagging process, resulting in uneven lagging thickness or loose bonding. For substrates with complex shapes, structures such as bosses and grooves can be designed on their surfaces to enhance the firmness of the lagging through mechanical bite.
The quality of the substrate’s surface treatment is a key factor influencing the effectiveness of encapsulation. The presence of impurities such as oil, dust, and release agents on the substrate surface will hinder direct contact between the encapsulating material and the substrate, resulting in a decrease in bonding strength. Therefore, the substrate must be rigorously cleaned before encapsulation. Ultrasonic cleaning and alcohol wiping can be used to remove surface impurities. For substrates with smooth surfaces, sandblasting or electrospark machining can be used to increase surface roughness, increase contact area, and enhance mechanical engagement. Furthermore, the substrate’s crystallinity can also affect the encapsulation effect. Highly crystalline substrates have low surface polarity, making them less conducive to bonding with the encapsulating material. This can be improved by controlling the substrate’s molding process to reduce crystallinity or by annealing the substrate.
Quality control and testing during the production process are also crucial for ensuring the integrity of the overmolding. Before production, the mold must be carefully inspected to ensure cavity cleanliness, accurate positioning, and unobstructed venting. During production, regularly spot-check overmolded products, using methods such as peel and tensile tests to verify the bond strength of the overmolding. Problems are promptly identified and process parameters adjusted. For products with loose overmolding, the cause is analyzed and appropriate measures are implemented, such as adjusting temperature and pressure, replacing compatibilizers, or improving substrate surface treatment methods. Furthermore, equipment maintenance is strengthened to ensure that the injection molding machine’s temperature control accuracy, pressure stability, and other indicators meet requirements, thereby ensuring the quality of the overmolding. Comprehensive control and optimization can effectively address the issue of loose overmolding and improve product quality and reliability.
