Injection clamping force
Injection clamping force is a critical parameter of an injection molding machine. It refers to the maximum clamping force applied by the machine’s clamping mechanism to the mold. Its magnitude is directly related to the stability of injection molding production and the quality of the molded part. During the injection molding process, the plastic melt fills the mold cavity under high pressure, generating an outward expansion force. The clamping force must be greater than this expansion force to prevent the mold from expanding and defects such as flash and overflow. If the clamping force is insufficient, gaps will appear on the mold parting surface, allowing the melt to overflow through the gap, forming flash. This not only affects the appearance quality of the molded part, but also increases the workload of mold cleaning and reduces production efficiency. Furthermore, the formation of flash can lead to insufficient melt filling, resulting in problems such as missing material and undersized parts. Therefore, accurately calculating and appropriately setting the clamping force is crucial to ensuring smooth injection molding production.
The magnitude of the clamping force is affected by a variety of factors, the first of which is the structural parameters of the plastic part and the mold. The projected area of the plastic part is one of the main factors affecting the clamping force. The larger the projected area, the greater the expansion force generated after the melt fills the mold cavity, and the greater the required clamping force. The complexity of the plastic part’s shape also affects the clamping force. For plastic parts with complex shapes, the melt has greater flow resistance during the filling process, and the required injection pressure is higher. The corresponding expansion force will also increase, thus requiring a greater clamping force. The number of parting surfaces in the mold also affects the clamping force. Molds with multiple parting surfaces may generate expansion forces in multiple directions during the injection molding process, requiring a higher clamping force to ensure a tight fit between the parting surfaces. In addition, the mold’s guiding accuracy and mold plate rigidity will also affect the effective use of the clamping force. Poor guiding accuracy or insufficient mold plate rigidity will lead to uneven distribution of clamping force and localized insufficient clamping force.
The injection molding process parameters also significantly influence the required clamping force. Injection pressure is a key factor influencing expansion force. Higher injection pressure increases the melt pressure in the mold cavity, generating greater expansion force and requiring more clamping force. Therefore, while ensuring part quality, injection pressure should be minimized to reduce the required clamping force. Holding pressure and holding time also affect clamping force. Excessively high holding pressure or prolonged holding time will cause the melt pressure in the mold cavity to remain consistently high, increasing expansion force and requiring greater clamping force. Excessively high melt temperature reduces the plastic’s viscosity and increases its fluidity. At the same injection pressure, melt pressure distribution in the mold cavity is more even, but this may also increase expansion force due to volume expansion. Therefore, properly adjusting injection molding process parameters can effectively reduce the required clamping force.
The properties of the plastic raw material also affect the clamping force. Different types of plastics have different fluidity and shrinkage rates. Plastics with good fluidity can fill the mold cavity at lower injection pressures, resulting in less expansion force and requiring less clamping force. Plastics with poor fluidity, on the other hand, require higher injection pressures to fill the cavity, generating greater expansion force and requiring more clamping force. The shrinkage rate of the plastic also affects the clamping force. Plastics with high shrinkage rates experience significant volume changes during the cooling process, which can cause pressure fluctuations in the mold cavity and indirectly affect the clamping force required. In addition, additives in plastics, such as fillers and reinforcements, can also change the fluidity and mechanical properties of the plastic, thereby affecting the clamping force.
Proper calculation and selection of clamping force requires comprehensive consideration of the above factors, and empirical formulas can generally be used for estimation. A commonly used formula is: Clamping Force = Part Projected Area × Number of Cavities × Average Melt Pressure × Safety Factor. Part Projected Area refers to the projected area of the part on the parting surface, measured in square centimeters; Number of Cavities refers to the number of parts molded in the mold; Average Melt Pressure is generally selected based on the type of plastic and the complexity of the part, typically between 20 and 50 MPa; and the safety factor is generally set between 1.2 and 1.5 to ensure sufficient margin for clamping force. In actual production, clamping force should be adjusted based on mold trials. If flash or overflow occurs, the clamping force is insufficient and should be increased appropriately. Excessive clamping force increases equipment energy consumption, accelerates mold and equipment wear, and may also cause internal stress in the part, impacting part quality. Therefore, while ensuring the mold does not expand, a low clamping force should be used to improve production efficiency, reduce costs, and extend the life of the equipment and mold.
