Casting answers & advice.

Q In our aluminum permanent mold casting facility, we experience casting dimensional problems related to mold wear and maintenance. What might cause this misalignment?

Background: Proper care and maintenance of permanent mold tooling and equipment optimizes the useable life of the mold. A maintenance plan can help prevent tooling damage and lost production time for repair, but, more critically, can cause mold misalignment and variability that can affect the dimensional accuracy of the cast component. The mold ejection and alignment systems are most at risk during maintenance. Effective maintenance can protect these systems for extended periods, but the lack of proper maintenance can lead to problems with the alignment and ejector pins.

Answer: Several common maintenance mistakes can cause mold misalignment.

Ejection System--Lack of maintenance can lead to galling or cause pins to rust. Many facilities use graphite with an oil carrier. When the oil burns off, the graphite is left as a lubricant. A simple way to confirm the pins are receiving adequate lubrication is to look for rust or galling following the application of the lubricant. Another indicator is to look for a flame at the mold parting line when the mold opens, which shows the oil covering the end of the pin is igniting.

Another common error in maintaining the ejector system is allowing mold blasting media to become lodged in the counter bores of the headed return pins. This can cause "mushrooming" of the return pin head, which can prevent the mold from closing fully, creating flash and dimensional problems. Blast media also can become caught in the ejector pin and mounting plate holes, leading to binding and galling, which can cause bent pins and castings (Fig. 1).

[FIGURE 1 OMITTED]

Alignment System--Early alignment system component failure leading to dimensional irregularity is a common error caused by ineffective maintenance and lubrication. New leader pins and bushings often are designed with clearances from 0.001-0.005 in. (0.0025-0.0127 cm), a tolerance that leaves little room for error. If the casting requires strict dimensional accuracy, even minor wear can cause the casting to be out of tolerance. It is important to know the shift limits of each mold and change the bushings before they exhibit signs of wear.

Q We spend a lot of money replacing failed electric motors that run our production equipment. Are there any secrets that can extend the life of these motors in the harsh metalcasting environment?

Background: Electrical motors are critical components to many metalcasting operations. Yet the conditions in which these motors operate (exposure to heat, dust and sand) are less than ideal for electric equipment and can cause premature failure of electric motors. Some motors are essential components whose failure can shut down an entire operation. Maintenance of these "prioritized motors" is especially critical.

Answer: A recent study by the U.S. Dept. of Energy identified several common causes of premature motor failure that can be minimized with proper maintenance.

Environmental causes--Blocked ventilation passages by particulates, damaged or destroyed external cooling fans, and the accumulation of dust interfere with the ability of the motor to cool itself. Similarly, motors operating in heat (direct sunlight, adjacent to furnaces, etc.) allow temperatures to rise and interfere with motor cooling. Also, chemicals and acids can attack cooling fans. All these conditions contribute to motor overheating and premature failure.

Electrical causes--The three most common failure modes are overloading, unbalanced voltages and over-voltage. An overload condition that stalls a motor is rare, but what often goes unnoticed are overloads where the torque load on the motor is increased beyond the rated full-load torque. The output torque of a motor will match the load torque, but the motor runs at a higher current that generates unwanted heat. A small 3.5% voltage imbalance will cause a 25% increase in temperature. When a supply voltage is greater than 10% of the motor-rated voltage, a motor is susceptible to flux saturation responsible for core losses that generate heat.

Mechanical causes--Bearing failures account for 75% of mechanical-related failures. An insidious form of bearing failure is false brinelling, which occurs when an unused motor is exposed to vibrations. To avoid this, primary and backup systems should share the load equally, switching at least once a week. Three other mechanical failure modes include misalignment, imbalance and soft foot, which is when one of the multiple mounting feet is insecure, causing unwanted vibration.