As we all know, the pull rod (Colin column) of the general elbow (machine hinged) injection molding machine is easier to break than the full hydraulic type tie rod. What is the reason?
In the design of elbow bar, the rigidity of the system should be considered, because the elbow bar is based on the rigidity to produce the locking force, so the diameter of the general tie rod is larger. But it is strange that the pull rod is easy to break, especially in some industries, such as video box production, plastic fast food knife and fork. Due to the thin wall of the product, good material fluidity, large forming area and fast production speed, the fracture of the rod link (hinge edge) is very common, even the formwork fracture will be caused.
Why is this anomaly? This paper attempts to find out the cause of fracture and explore the solution through analysis of various aspects.
1、 Analysis of the cause of the fracture of the tie rod
1. Fatigue failure
The working process of injection molding machine is the process of alternating stress, so any factor which causes stress concentration may cause fatigue failure.
For example:
1) The step where the shaft diameter changes greatly.
2) The transition is short of fillet, undercut, etc.
3) Surface scar or machining thread carelessly damage the surface of optical axis.
4) The surface roughness of thread is too large.
5) The thread is pressed for a long time, and the surface extrusion damage (at the die adjusting nut) is caused by insufficient surface strength.
6) Stress concentration is caused by improper heat treatment on the thread surface.
7) Material defects.
2. Overload pull off
Because the elbow rod type obtains the mode locking force by deformation of four pull rods, if the length of four tie rods is different, the deformation is different. If one of the four is short, the shorter tie rod may bear a large amount of 1/4 mode locking force which it should bear, which leads to the breaking.
3. Temperature strain leads to breaking
If four tie rods are different in length, for example: one long and three short, the long tie rod will break due to the high temperature or the decrease of thermal stress caused by sharp temperature and constrained by the other three rods.
4. Tensile failure caused by composite strain
If the die is uneven, the comprehensive error of the length of the connecting rod (hinge edge), the height of bearing pedestal and the length of the tie rod is too large. When the connecting rod is extended, the force on both sides is uneven. that
1) The back formwork swings, the pull rod is bent, and the pull rod is easy to pull off under the action of bending stress and tensile stress.
2) The operator who is lack of experience will further increase the locking force, which will cause the tension of the rod overload and the mold overload deformation.
5. Damage caused by instantaneous impact stress
Because the elbow bar can release the locking force before the die is opened, the force can be released
1) The instantaneous impact of the opening mode causes the machine vibration and causes the damage of the pull rod and other parts.
2) The premature fatigue failure of the whole die closing part (including the mold) will be caused by the long-term stress state.
2、 Solutions
1. In design, it is necessary to avoid the sharp change of shaft diameter. In the step or back groove, try to use large fillet to transition, and choose better comprehensive performance materials. Special treatment should not only solve the hardness of thread surface (wear-resistant) but also try to reduce the stress concentration on the surface, improve the surface finish and minimize stress damage.
2. The machining accuracy, especially the length of connecting rod, the height of bearing seat, the accuracy of crosshead, and the positioning accuracy of bearing seat in the template, etc.
3. Special attention should be paid to the length of the four pull rods after assembly or after the adjusting nut is removed. The spacing between the adjusting nut and the rear formwork should also be adjusted.
4. When the product is flying, the reasons shall be analyzed, and the locking force shall not be raised unilaterally. If it is found that the die is uneven, it should be ground flat, such as uneven length of connecting rod, including copper pad, paper, etc., or the die adjusting nut shall be corrected.
The above solutions can only play a certain role, not necessarily fundamentally, and some measures will increase the manufacturing cost, such as increasing the diameter of the tie rod, using good materials, etc. So to some extent, it depends on the manufacturer's choice. For the use of full hydraulic mode closing structure, it is a better choice to solve the broken rod. However, because of the traditional hydraulic type still has the problems of slow speed and high energy consumption, after weighing by the manufacturer, the small and medium-sized machines are still mainly elbow bar type, and only after the appearance of the two plate type, can they be fundamentally solved.
3、 Two plate type and four cylinder straight lock two plate
Two plate type mainly consists of four cylinder holding two plate and four cylinder straight lock two plate type. Two plate type is much better than liquid filling type in terms of force on the locking die, because the two plate type sealing force line is only in the middle of the tie rod, between the moving formwork, fixed formwork and die. Because the force length of the tie rod is reduced by nearly 50%, the rigidity of the whole die closing system is improved, and the stress parts are also reduced (such as the rear formwork and cylinder), which reduces the fatigue damage of the whole machine, so the whole machine life is improved.
Four cylinder package drinks two plate. Because of the need for mechanical action conversion (such as nut holding), which affects the opening and closing die speed, it can only be used in large-scale machines. The four cylinder straight lock two plate type does not need mechanical action conversion, such as the direct pressure type is convenient, the speed reaches or faster than the elbow rod type, and the mold locking force can be reduced with the end of the injection process, which reduces the high pressure time of the parts (including the mold) by nearly 80%, and the machine life will be more than doubled. Therefore, the four cylinder straight lock injection molding machine will be the best choice to solve the broken rod.
