Why do Tie Bars (Tie Rods) Break?
Most experts agree that tie bars break at a particular location most commonly due to corrosion or a pre-existing metallurgically "bad area" in the bar including cracks, inclusions, improper alloying and poor heat treatment. While this tells you why it broke a particular location it does not explain "why it broke". When a die cast machine or injection molding press breaks a tie bar, virtually always only one tie bar breaks; it broke because it was loaded beyond it capacity. Why was only one tie bar loaded beyond its capacity? Consider the following possibilities:
Misalignment of the machine (running the machine in an un-parallel condition), commonly caused by:
- Purposefully running the machine "out of square" to compensate for a poorly designed die or mold, i.e. the die does not exert even pressure on the platens when set because of the location of the cavity (cavities) in the die.
- Purposefully running of the machine out of square to compensate for a die in poor condition, i.e. die does not exert even pressure because the die is no longer "square" and the machine is purposefully mis-adjusted to compensate for this condition. A die that is not parallel is going to wreak havoc with your machine in every way; parallelism of your die is critical to the life of the mechanicals of your die cast machine.
- The die cast machine or injection molding press is not level, this is an often overlooked problem. This condition can cause the machine to run out of square causing linkage to exert uneven pressures on the tie bars and causing breakage of a bar.
- Your die cast machines is not well secured to the floor, this can create a condition in which the machine "seems to be level" when checked statically but once the machine opening and closing it "looses" it level condition.
- Poor maintenance; everything mechanical in a die casting machine or injection molding machine including the machine linkage, tie bars, tie bar nuts and platens are wearing; as an example the threads on the die height nuts actually wear from the continuous loading and unloading of the machine, these worn threads cause "slop" between the tie bars threads and the and tie bar die height nut threads. Because in real life machine loads are never perfect, mechanical areas of the machine wear at different rates causing the machine to be "out-of-square" creating un-even loading. Machines should be checked for squareness using a "squaring block" or "test ring" on a regular basis and adjusted as necessary.
- Purposeful mis-adjustment of the machine to allow the die to "spit" in a particular way.
Un-even loading of the machine, commonly caused by:
- Purposeful "overloading" of the machine, you have a chance to get a job and die that your engineers says should be run in an 1100 ton machine but you have time available in 1,000 ton machine so you run the machine beyond its limit.
- Coining in the platens does not allow the die to be set properly. You can have a new linkage and a properly squared machine but coining will keep the die from being set correctly.
- Another common cause for uneven loading is that the location of the die in the die cast machine is not correct because the exact or "ideal" shot hole location (shot position) does not exist on a particular machine, i.e. the tool was built to be run at 250 mm (9.84 inch) below center but the available machine only has a 9 inch (228 mm) shot position so you "make due" with what you have available.
In conclusion, only perfect machine and mold alignment can ensure the quality of the parts. This will also greatly extend the lifetime of tool and mechanical aspects of the machine.
What do you do now that you have a broken tie bar or nut?
There are several possible solutions to this question. The gut reaction is to order up a new tie bar or nut. However, the first step (or at least concurrent first steps) should be to identify the problem. It could have been a metallurgical flaw in the tie bar that caused it to break and its breaking was a "fluke". Perhaps it was the first time the machine was loaded to this point and even though it was loaded well within the limits of the machine the bar broke due to the flaw. Most of the time, this is not the case and consideration should be given to the above items. If you don't fix the situation you are going to keep breaking tie bars and nuts. Admittedly, sometimes it is a business decision to run a machine beyond its rated capacity or to run a "bad" tool to the end of the production run.
What other options are there besides "ordering up a new tie bar or nut" How about repairing the broken nut or bar? In the difficult economic times that we have and are experiencing, shops are looking at repairs in a different light. Repairing tie bars and nuts is becoming common place for many shops. These repairs provide important alternative solutions for getting your machine back on-line again. Advantages can include:
- Often faster delivery than waiting for factory replacement parts
- Potentially more economical than factory replacements parts
The repair of tie bars and nuts usually involves welding, machining and potentially heat treating depending on the metal and alloy used in the part to be repaired. While the concept of weld repair of these components make some die casters "uncomfortable" the technology utilized to repair these items has come from the repair of multi-million dollar components. You do not "throw away" a 50 million dollar turbine generator because you have some wear on a shaft, you rebuild it, you guessed it using welding, precision machining and heat treatment techniques.
A good full service repair shop can repair all types and styles of tie bars, tie rods, A-frame bars as well as front and rear (die height) nuts. It is important to note that both steel and aluminum bronze tie bar nuts can be repaired. If blue prints or drawings are not available in-house to repair or manufacture a tie bar or nut a good shop can reverse engineer your broken part. Repairing an existing tie bar or nut includes: assessment of the broken part, weld repair and machining of the tie bar or tie bar nut. Other operations might include polish the tie bar and potentially chrome plating the tie bar. Tie bars and tie bar nuts for virtually every brand of die casting machine have been repaired including B&T, Buhler, Prince, HPM Castmaster, Harvill, Kux, American, Frech, National, Reed-Prentice, Birch, Wotan, UBE, Toshiba, Quantum, L.K. Machinery, Producer, Weingarten, Italpresse, Agrati, THT, Hauser rotor casting machines. While it is generally more economical to repair tie bars and nuts from higher tonnage machines tie bars from 100 ton machines all the way to 4,000 ton machines are commonly repaired. Please fell free to call our office or use our "contact us" form to discuss shops that we know have provided excellent results for these types of repairs.
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