Preheating of injection molded inserts is a key process step to improve the bonding strength between the insert and the plastic and reduce the internal stress of the product. It is especially suitable for scenarios where metal inserts and plastics are composite molded. The thermal expansion coefficients of metal inserts and plastics are quite different. If a cold insert is directly placed in the mold for injection molding, uneven cooling and shrinkage will generate large internal stresses at the bonding interface, causing cracks and warping in the product, or even loosening and falling of the insert. Preheating the insert can reduce the temperature difference between it and the melt, slowing down the cooling rate of the plastic around the insert, allowing the melt to better cover the insert and improving the interfacial bonding strength. In addition, preheating can also remove oil, moisture and impurities on the surface of the insert, avoiding bubbles or silver streaks during injection molding, and ensuring stable product quality.
The temperature control for insert preheating needs to be determined based on the insert material, size, and plastic properties. The key is to balance preheating effectiveness with operational feasibility. For metal inserts such as steel and copper, the preheating temperature is typically 80-150°C. The specific value should be referenced to the plastic molding temperature. When the plastic melt temperature is high (such as approximately 250°C for PA66), the insert preheating temperature can be increased to 120-150°C to reduce stress caused by temperature differences. When the plastic melt temperature is low (such as approximately 200°C for PE), the preheating temperature can be controlled at 80-100°C to prevent overheating of the insert and degradation of the surrounding plastic. Small inserts (such as screws with a diameter of less than 5mm) have a smaller heat capacity, so the preheating temperature can be lowered to prevent oxidation and discoloration. Large inserts (such as bushings over 100mm in length) require a higher preheating temperature of 150-200°C to ensure even heat distribution. If the preheating temperature is too high, the surface of the insert will oxidize, forming an oxide layer that affects the bonding strength; if the temperature is too low, the stress relief effect cannot be achieved. Therefore, the insert temperature must be monitored in real time using a thermocouple to ensure that it is within the set range.
The preheating method for inserts should be selected based on the production scale and insert characteristics. Common methods include oven preheating, induction heating, and infrared heating. Oven preheating is the most common batch processing method. The inserts are placed in a hot air circulating oven and held at a set temperature for 10-30 minutes (adjusted according to insert thickness) to ensure uniform internal and external temperatures. This method is suitable for inserts of various shapes, but the heating rate is slow, requiring advance planning of production schedules. Induction heating is suitable for rapid preheating of metal inserts. It uses electromagnetic induction to generate heat within the insert itself, reaching the target temperature in 30 seconds to 2 minutes, making it suitable for automated production lines. For example, in the production of automotive connector inserts, brass pins are heated using induction coils, achieving both rapid temperature increase and precise temperature control, improving production efficiency. Infrared heating uses infrared radiation to directly heat the insert surface. It is suitable for flat or thin-walled inserts, offering fast heating speeds and low energy consumption, but careful distance adjustment is required to avoid local overheating. Regardless of the method used, the preheated insert should be placed in the mold as soon as possible (usually within 1 minute) to avoid excessive temperature drops that could affect the product quality.
The process coordination of insert preheating is crucial to the final effect and needs to be coordinated with the injection molding parameters and mold design. During the injection molding stage, the melt temperature and injection pressure in the area around the insert should be appropriately increased to ensure that the melt fully flows and wets the insert surface. For example, when embedding a steel insert in a nylon product, the melt temperature can be increased by 10-20°C and the injection pressure can be increased by 5-10 bar to promote a close bond between the plastic and the insert surface. In mold design, locating pins and heat conduction grooves need to be set at the insert placement. The locating pins ensure the accurate position of the insert, and the heat conduction grooves help the heat of the insert to be evenly transferred to the mold to avoid local overheating. For inserts with threads, an overflow groove needs to be set at the root of the thread to allow a small amount of melt to enter the thread gap. After cooling, a mechanical locking structure is formed to enhance the connection strength. In addition, the holding time needs to be appropriately extended, especially for large inserts, to compensate for melt shrinkage and reduce interfacial stress through holding pressure.
Quality control and troubleshooting of common issues during insert preheating are key to ensuring process stability. Before production, the insert surface should be inspected to remove oil and rust. If necessary, surface treatments such as phosphating and sandblasting should be performed to increase roughness and enhance bonding strength. After preheating, inserts should be spot-checked to confirm temperature uniformity. For example, using an infrared thermometer to measure temperature differences at different locations should ensure the temperature is within ±5°C. If cracks are observed at the interface between the insert and the plastic, this could indicate insufficient preheating temperature or excessive cooling; in this case, the preheating temperature should be increased and the hold time extended. If bubbles appear, this could indicate moisture on the insert surface; preheating should be strengthened. For automated production, temperature sensors can be installed on the insert conveyor to automatically reject under-temperature inserts to prevent compromised product quality. Ovens or induction equipment used for extended periods should be regularly calibrated to ensure that the displayed temperature is consistent with the actual temperature. For example, this can be done monthly with a standard thermometer, with adjustments made promptly if the error exceeds ±3°C. Through rigorous quality control, insert preheating can significantly improve the reliability of composite products and reduce the risk of later failure.
