Calculation of Tie Rod Length for Three-Plate Injection Moulds
Calculating the tie rod length for a three-plate injection mold is a critical step in its design, directly impacting the coordination and safety of the mold’s opening and closing movements. Because a three-plate mold has two parting surfaces (the sprue parting surface and the cavity parting surface), tie rods are required to achieve orderly separation between the fixed platen, runner pusher, and movable platen. Insufficient tie rod length can prevent the runner from ejecting smoothly or the cavity from fully closing. Excessive tie rod length can cause problems such as tie rod bending and mold collision. Calculating tie rod length requires comprehensive consideration of factors such as runner solids length, cavity depth, and mold plate thickness. Accurate dimensional chain analysis ensures that the opening distance of each parting surface meets production requirements.
The key to calculating tie rod length is determining the distance at which the sprue parting surface opens. This distance must be sufficient to remove runner aggregate, typically 1.2-1.5 times the sum of the sprue length and the maximum runner aggregate diameter. The sprue length refers to the distance from the injection molding machine nozzle to the runner. It is equal to the sum of the fixed platen thickness and the runner pusher plate thickness, minus the overlap between the two. For example, if the fixed platen thickness is 50mm, the runner pusher plate thickness is 20mm, and the overlap is 5mm, the sprue length is 50 + 20 – 5 = 65mm. The maximum runner aggregate diameter depends on the runner diameter and the number of cavities. For single-cavity molds, the runner aggregate diameter is equal to the runner diameter. For multi-cavity molds, the runner aggregate diameter is the combined dimensions of the runner and gate. For example, in a four-cavity mold with a cross-shaped runner and an 8mm runner diameter, the maximum runner aggregate diameter is approximately 16mm (at the intersection). Therefore, the opening distance of the main runner parting surface must be at least 65+16=81mm, and 90-100mm is taken after considering the safety margin to ensure that the runner condensate can be smoothly removed without interfering with the mold.
The opening distance of the cavity parting surface is another important parameter in calculating the tie rod length. It must meet the space requirements for product removal and ejection mechanism operation. It is generally 1.1-1.3 times the sum of the maximum part height and the ejection distance. The maximum part height refers to the part’s dimension in the mold opening and closing direction. For example, for a shell-type part with a height of 50mm, the maximum height is 50mm. If the part has a protrusion or undercut, the protrusion height must be taken into account. For example, for a part with a 10mm boss, the maximum height is 60mm. The ejection distance is the distance the ejector pin pushes the part out of the mold core. It is generally 5-10mm greater than the maximum part height to ensure that the part is completely free of the core. For example, for the part with a boss mentioned above, the ejection distance is 70mm. Therefore, the opening distance of the cavity parting surface must be at least 60 + 70 = 130mm, and a safety margin of 150-170mm is recommended to prevent collision between the part and the mold during ejection.
The formula for calculating the tie rod length for a three-plate mold is: Tie rod length = sprue parting surface opening distance + cavity parting surface opening distance + fixed platen thickness + runner push plate thickness + stop distance + safety margin. The stop distance refers to the thickness of the tie rod’s upper limit stop, which controls the maximum opening distance of the sprue parting surface and is typically 10-20mm. The safety margin, which takes into account factors such as platen deformation and part clearance, is generally 10-15mm. For example, if a three-plate mold has a sprue parting surface opening distance of 100mm, a cavity parting surface opening distance of 160mm, a fixed platen thickness of 60mm, a runner push plate thickness of 25mm, a stop distance of 15mm, and a safety margin of 10mm, the tie rod length is 100 + 160 + 60 + 25 + 15 + 10 = 370mm. When calculating, it is important to note that the tie rod length must be greater than the total thickness of the mold in the closed state to ensure unimpeded mold opening and closing.
The calculation of tie rod length must take into account the specific structure of the mold. Different types of three-plate molds (such as point-gated three-plate molds and latent-gated three-plate molds) differ in details. The tie rod of a point-gated three-plate mold must pass through the fixed mold plate, runner push plate, and movable mold plate, and its length must cover the entire mold opening and closing stroke. The tie rod of a latent-gated three-plate mold may only pass through a portion of the mold plate, and the calculation must be adjusted based on the actual thickness of the mold plate passed through. The position of the step on the tie rod must be accurately calculated to ensure that the main runner parting surface is opened first when the mold is opened, and after the runner solid is removed, the cavity parting surface is opened to avoid damage to the mold due to incorrect action sequence. For example, the step distance on the tie rod that controls the main runner parting surface must be equal to the opening distance of the main runner parting surface, with an error controlled within ±0.5mm to ensure accurate limit.
Verifying and adjusting tie-rod length is the final step in ensuring proper mold operation. A trial mold is required to confirm smooth mold opening and closing, and that the opening distances of each parting surface meet design requirements. During the trial mold, observe the mold opening process: First, the sprue parting surface opens smoothly, allowing runner slurry to eject smoothly without any lag. Then, the cavity parting surface opens, the ejector mechanism operates normally, and the part is removed smoothly. If the sprue parting surface opening distance is insufficient, preventing runner slurry removal, increase the tie-rod length. If the cavity parting surface opening distance is insufficient, hindering part ejection, adjust the tie-rod stop structure. Measure the actual mold opening and closing distance, and ensure the deviation from the designed value is within ±2mm. Otherwise, recalculate and replace the tie-rod. Additionally, check the force applied to the tie-rod during the mold opening and closing process to prevent bending or breakage caused by improper length, ensuring long-term, safe mold operation. Through precise calculation and trial mold adjustments, the tie-rods of the three-plate mold achieve precise mold opening and closing control, ensuring smooth production.
