As we all know, tie rods (column columns) of general toggle-type (machine-hinge type) injection molding machines are easier to break than fully hydraulic tie rods. What is the reason?
In design of toggle lever type, rigidity of system must be considered. Because toggle lever type generates clamping force by rigidity, diameter of tie rod is generally larger. But what is strange is that tie rods are easy to break, especially in certain industries, such as production of video cassettes, plastic fast food cutlery and other industries. Due to thin wall of product, good material flowability, large forming area, and fast production speed, breakage of tie rod connecting rod (hinged edge) is very common, and it may even cause mold plate to break.
Why is there such an anomaly? This article attempts to find out cause of its fracture and explore its solutions through various aspects of analysis.

Working process of injection molding machine is a process of alternating stress, so any factors that cause stress concentration may cause fatigue damage.
Such as:
1) Steps where shaft diameter changes greatly.
2) Transition part lacks rounded corners, undercuts, etc.
3) Surface scratches or machined threads accidentally damage surface of optical axis.
4) Surface roughness of thread is too large.
5) Thread is compressed for a long time, surface is damaged due to insufficient surface strength (at die adjusting nut).
6) Improper heat treatment of thread surface causes stress concentration.
7) Material defects.

Since toggle lever type relies on deformation of four tie rods to obtain clamping force, if length of four tie rods is different, deformation will be different. For example, if one of four tie rods is shorter, shorter tie rod may withstand much more than 1/4 clamping force that it should bear, resulting in breaking.

If four tie rods are different in length, for example, one is long and three are short, then thermal stress caused by sharply high temperature or drop of longer tie rod will be restrained by other three rods and be in a statically indeterminate state, which will cause rod to break.

If mold is not flat, combined error of length of connecting rod (hinge), height of bearing seat and length of tie rod is too large. When connecting rod is straightened, force on both sides will be uneven. Then
1) Rear mold plate is caused to swing and tie rod is bent, tie rod is easily broken under action of bending stress and tensile stress.
2) If product flashes, operators who are generally inexperienced will further increase clamping force, which will cause pull rod to break and mold to deform due to overload.

Since toggle lever type can only release clamping force before mold is opened,
1) Instantaneous impact when opening mold causes machine to vibrate, damage tie rods and other parts.
2) Since entire clamping part (including the mold) is kept under stress for a long time, it will lead to premature fatigue failure of part.

 

1. In design, avoid sharp changes in shaft diameter. At steps or undercuts, try to use large rounded transitions and choose materials with better comprehensive performance. Special treatment must not only solve thread surface hardness (wear resistance), but also try to reduce surface stress concentration, improve surface finish, and minimize stress damage.
2. To ensure machining accuracy, especially length of connecting rod, height of bearing seat, accuracy of crosshead, positioning accuracy of bearing seat on mold plate, etc.
3. Pay special attention to length of four tie rods when assembling or after removing mold adjusting nut, distance between mold adjusting nut and rear mold plate must also be adjusted.
4. Analyze reason when product has flash, do not increase clamping force one-sidedly. If mold is found to be uneven, it should be flattened, if length of connecting rod is uneven, it should be corrected, including copper pads, paper sheets, etc., or mold adjusting nut should be corrected.
Above solutions can only play a certain role, may not be able to solve it fundamentally, and some measures will increase manufacturing cost, such as increasing diameter of tie rod and using good materials. Therefore, to some extent depends on manufacturer's choice. For use of a fully hydraulic clamping structure, it is a better choice to solve problem of broken tie rods. However, due to problems of slow speed and high energy consumption in traditional hydraulic type, after manufacturers weighed, toggle lever type was still main choice for small and medium-sized machines. It was not fundamentally solved until emergence of two-plate type.

Two-plate type is mainly divided into four-cylinder holding two-plate type and four-cylinder direct-locking two-plate type. Two-plate type is much better than liquid-filled type in terms of clamping force, because two-plate closed force line is only in the middle of tie rod, between movable mold plate, fixed mold plate, and mold. Since length of tension rod is reduced by nearly 50%, rigidity of entire clamping system is improved, stressed parts are also reduced (such as rear mold plate, oil cylinder, etc.), which reduces fatigue damage of whole machine, so life of the whole machine is increased.
Four-cylinder direct-locking two-plate type, due to need for mechanical action conversion (such as nut enclosing), which affects speed of mold opening and closing, so it can only be used for mainframes. Four-cylinder direct-locking two-platen type does not require mechanical action conversion, is more convenient to use than direct pressure type. Speed is up to or faster than toggle lever type, clamping force can be reduced with end of injection process, so that parts (including mold) are subjected to high pressure time is reduced by nearly 80%, life of machine will be more than doubled. Therefore, four-cylinder direct-lock injection molding machine will be the best choice to solve broken tie rod. .