Even if parts come off the press with extremely close dimensional and weight tolerances, the trip through the furnace can have a great effect on dimensional stability. The sintering operation is what develops the physical properties and size of the part, and many things happen in the furnace.

The final size and properties of the as-sintered part are influenced by the following:

• The material, its method of manufacture, and all basic attributes

• The alloys or additives and their uniformity

• The pressed density and the density distribution

• The pressed sizes

• The furnace conditions

• The time in delube or preheat, the time in high heat, and the cooling time and rate

• The temperatures reached at each stage

• The atmosphere, its composition, amount and flow rate, and flow pattern over the work

• The work load and its distribution in the furnace

For consistent work, the time-temperature profile that each part "sees" must be very uniform within the furnace and on an hour-to-hour and day-to-day basis. The part that comes off the press can be measured, weighed, evaluated, and examined as it is made. Parts being sintered can generally be measured and examined only after they leave the furnace, which means several hours of production before adjustments can be made based on the part itself.

Normal events that can occur to alter the sintering and therefore the tolerances could be:


• Variation in time due to change in belt speed. Most belt furnaces have a driving drum that has som e form of rubber facing to grip the belt and usually a pinch roll to hold the belt tight to the drum. However, if the facing is worn or the gripping roller loses spring tension or the load varies too much, the effective speed can vary regardless of the speed of the drum.

• Speed control of the drive may have variables, and the setting of the speed may not be accurate. If the speed is measured by a ruler and a watch, errors of 5% or more are possible unless great care is taken.

• Inconsistent spacing of parts on furnace belts can alter the net effect of the heat-up and cooling rate.

• Variations in the temperature profile. In furnaces with a small number of control zones, variation in gas burners or change or deterioration of heating elements can cause a change in the temperature profile that may not be picked up by the temperature controller. More zones of control will minimize this, but not eliminate it. The newer electronic controls can be programmed to or actually learn how to obtain the most stable temperatures.

Parts that are relatively thin and flat, such as valve plates used in compressors, are often sintered placed on graphite or ceramic plates that are maintained very flat. Although this may reduce the yield of the furnace, it produces flatter parts than can be achieved by placing the part directly on the belt or on metal screens or trays that tend to go out of flat.

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