Metal insert defects and solutions
In injection molding, combining metal inserts with plastic is a common process, improving properties such as strength, wear resistance, and conductivity. However, metal inserts often exhibit defects during use, such as loose inserts, cracking at the plastic-insert junction, and insert corrosion. These issues not only affect product performance but can also pose safety risks. Addressing metal insert defects requires comprehensive optimization across multiple aspects, including insert design, surface treatment, and the injection molding process.
Improper design of metal inserts is one of the main reasons for the adverse phenomenon. The shape and size of the insert directly affect its bonding strength with the plastic. If the surface of the insert is too smooth, the plastic will not be able to form an effective mechanical bite force with the insert when cooling and shrinking, and it is easy to loosen. Therefore, the surface of the insert should be designed with anti-slip structures, such as knurling, grooves, bosses, etc., to increase the contact area and friction between the insert and the plastic. For example, cylindrical inserts can be provided with annular grooves or diamond knurling on the surface, and rectangular inserts can be provided with bosses on the side. These structures can make the plastic tightly bite with the insert after molding to prevent loosening. In addition, the length and diameter ratio of the insert must also be reasonable. Inserts that are too long or too thin are prone to bending and deformation during the injection molding process, affecting the bonding effect. It is generally recommended that the aspect ratio of the insert should not exceed 5:1.
Improper surface treatment of metal inserts can lead to cracking and rust at the joint. Oil, oxide layers, rust, and other stains on the metal insert surface can hinder the bond between the plastic and the metal, resulting in gaps at the joint and prone to cracking when subjected to stress. Therefore, inserts must undergo surface treatment before injection molding. Rust removal can be achieved by pickling or sandblasting to remove the oxide layer and rust on the surface. Degreasing can be done with organic solvents or alkaline solutions to remove oil stains. For applications requiring increased bond strength, inserts can be electroplated with treatments such as zinc or chrome. This coating not only improves the insert’s corrosion resistance but also enhances its compatibility with the plastic. Furthermore, easily oxidized metal inserts, such as aluminum and magnesium, can be anodized to form a dense oxide film that prevents oxidation while increasing surface roughness and improving adhesion to the plastic.
Improper setting of injection molding process parameters will affect the bonding quality between metal inserts and plastics. When the melt temperature is too low, the fluidity of the plastic is poor and it cannot fully fill the gaps around the inserts, resulting in loose bonding; when the melt temperature is too high, the plastic will decompose, produce gas, and form bubbles at the joints. Therefore, it is necessary to set a suitable melt temperature based on the characteristics of the plastic and the size of the insert, generally 5-10°C higher than when there is no insert, in order to improve fluidity. The injection speed also needs to be controlled. Too fast will cause the melt to impact the insert, causing the insert to shift or a gap to form between the plastic and the insert; too slow will cause the melt to cool too quickly during the filling process, resulting in incomplete filling. Segmented injection molding is usually used. Initially, the area around the insert is filled at a low speed to ensure full contact between the melt and the insert. Later, the speed is appropriately increased to complete the cavity filling.
An unreasonable mold structure design can also lead to poor metal inserts. Inaccurate positioning of the insert in the mold will cause the insert to shift during the injection molding process, resulting in uneven plastic thickness at the joint and easy cracking. Therefore, a reliable insert positioning mechanism should be set in the mold, such as positioning pins, positioning sleeves, etc., to ensure that the insert is stable in position during the injection molding process. At the same time, the mold cavity around the insert should be equipped with sufficient exhaust structure to exhaust the air between the insert and the plastic to avoid the generation of bubbles. In addition, the thickness of the plastic around the insert should be uniform to avoid uneven cooling shrinkage due to excessive thickness differences, which will cause stress concentration. It is generally recommended that the plastic thickness should not be less than 1/3 of the insert diameter, and the difference between the maximum thickness and the minimum thickness should not exceed 2mm.
Poor material compatibility between the metal insert and the plastic is the root cause of poor bonding. The significant difference in thermal expansion coefficients between the metal and plastic leads to different shrinkage during the cooling process, causing stress at the joint and potentially cracking. For example, the thermal expansion coefficient of steel is approximately 12 × 10^-6/°C, while that of PA6 is approximately 80 × 10^-6/°C. This significant difference is prone to stress. Therefore, metals and plastics with similar thermal expansion coefficients should be selected. For example, aluminum (approximately 23 × 10^-6/°C) and PP (approximately 60-100 × 10^-6/°C) offer relatively good compatibility. Furthermore, reinforcing materials such as glass fiber can be added to the plastic to lower its thermal expansion coefficient and minimize the differential shrinkage with the metal insert. If a material combination with significantly different thermal expansion coefficients is unavoidable, an elastomer coating can be applied to the insert surface to mitigate shrinkage stress and prevent cracking.
