Calculation of injection molding ejection force
Injection molding ejection force refers to the force required to release a plastic part from the mold cavity or core. Its accurate calculation is fundamental to ejection mechanism design and directly influences pusher/push tube sizing, layout, and power source configuration. Insufficient ejection force can lead to mold release difficulties, deformation, or strain on the part. Excessive ejection force increases mold load, causing ejector pin bending, mold plate deformation, and other faults. For example, a POM gear mold suffered from frequent breakage during production due to an inaccurate ejection force calculation, resulting in an undersized ejector pin (φ4mm). After recalculation, a φ6mm ejector pin was replaced, completely resolving the issue. Calculating ejection force requires comprehensive consideration of factors such as part material properties, molding parameters, and mold structure, establishing a scientific calculation model.
The main components of demolding force include the part’s clamping force on the core, the part’s adhesion to the cavity, and the force required to overcome part deformation. The clamping force is dominant (70%-90%). This clamping force stems from the radial clamping of the plastic against the core due to cooling shrinkage. It is calculated as: Fwrap = π × d × h × σ × f, where d is the core diameter, h is the part’s clamping length, σ is the plastic’s shrinkage stress (MPa), and f is the coefficient of friction (typically 0.15-0.3). Shrinkage stress varies significantly among different plastics. For example, PP’s shrinkage stress is 8-12 MPa, PC’s is 15-20 MPa, and POM’s is 20-25 MPa. For example, to calculate the demoulding force of a POM plastic part with a diameter of 20mm and a wrapping length of 50mm: Fwrap = 3.14×20×50×25×0.2=15700N (15.7kN). A 20% safety factor needs to be added to this, and the designed demoulding force is 18.8kN.
The calculation of adhesion force is related to the contact area between the part and the mold and the surface tension. The formula is: F_adhesion = A × τ, where A is the contact area between the part and the mold cavity (mm²) and τ is the adhesion force per unit area (typically 0.01-0.03 N/mm²). For molds with rough surfaces (Ra > 1.6 μm), the τ value should be taken at the upper limit; for polished molds (Ra ≤ 0.8 μm), the lower limit can be used. For example, if the contact area between an ABS part and the mold cavity is 5000 mm² and the mold surface roughness is Ra 0.4 μm, the calculated F_adhesion is 5000 × 0.015 = 75 N, which only accounts for approximately 0.5% of the total demolding force. This is a significant consideration for large parts but can be ignored for small parts.
The force required to overcome part deformation primarily applies to thin-walled or complex-structured parts. The calculation formula is: Fchange = E × ε × A, where E is the plastic’s elastic modulus (MPa), ε is the maximum strain during demolding (typically 0.001-0.005), and A is the ejection area (mm²). For example, for a 1mm-thick, thin-walled PP part (E = 1500 MPa) with an ejection area of 1000 mm², Fchange is calculated to be 1500 × 0.003 × 1000 = 4500 N, representing over 30% of the total ejection force. This requires careful consideration in ejection mechanism design, typically by increasing the number of ejection points or expanding the ejection area to disperse the force.
Correction of the demolding force must take into account the influence of mold temperature, part structure, and molding process. A 10°C decrease in mold temperature increases plastic shrinkage by 0.2%-0.5%, correspondingly increasing demolding force by 5%-10%. Parts with reinforcing ribs or bosses require a 10%-20% correction factor. For parts molded using gas-assisted molding, the demolding force can be reduced by 20%-30% because internal pressure partially offsets the shrinkage force. For example, the calculated demolding force for a PA66 gear at a mold temperature of 60°C is 20kN. When the mold temperature drops to 50°C, the actual demolding force increases to 22kN, requiring the ejector mechanism to be designed for 22kN. Furthermore, the demolding force must be evenly distributed, with ejector pin spacing ≤100mm. Large parts require a central ejector pin to prevent deformation caused by off-center loading. With the application of CAE simulation technology, the demolding process can be simulated using software such as Moldflow. The error between the calculated and actual results can be kept within 5%, significantly improving the accuracy of ejector mechanism design.
