In addition to their flexibility, batch furnaces are popular because you run them only when you need them, and you can run different temperature profiles for different materials as you wish. These advantages work out when the processor has relatively small lot sizes of many different products.

Product quality, on the other hand, depends on the parts being processed seeing the “same” temperature irrespective of their position in the furnace. As furnaces get larger, it becomes more difficult to know and define the “sweet spot” within a furnace because when a thermocouple reads a certain temperature, it does not mean that the whole furnace is at that temperature. This is especially true for a large batch furnace heating up with a full load when there is a large temperature gradient between the outside of the load and the center of the load.

Temperature uniformity inside a furnace is a necessity for all thermal processing, but it is critically important for metal injection molding (MIM). The binders in the MIM component are removed by holding at particular temperatures for a certain time. If the correct temperature is not achieved in the entire load, the profile could move on to the next segment, which is usually a ramp. Binders would be evolving out of the part during this ramp. Depending on the amount of binder remaining in the part and the temperature during the ramp, the sudden evaporation of the binder can cause unacceptable cracks or blisters. In some cases, soot formation occurs, which would cause the composition of the material to change.

AMS 2750D

AMS 2750D[1] is an Aerospace Materials Specification that specifies temperature uniformity inside a furnace and how this should be measured. It specifies six furnace classes and defines the temperature uniformity for them. Table 1 provides the temperature-uniformity levels for these furnace classes.

Temperature uniformity is defined as follows:

• For conventional furnaces (non retort), it is the temperature variation expressed in ± degrees within the qualified work zone with respect to a set-point temperature.
• For retort furnaces, where a sensor in the retort is used to control the temperature, it is the temperature variation with respect to the sensor in the retort and not the furnace set temperature.

The Furnace

Figure 1 shows a schematic diagram for typical atmosphere or vacuum furnaces, which may be horizontal or vertical. The details of the furnace used for testing are as follows:

• It is a horizontal furnace with a square hot zone with molybdenum heat shields and tungsten heaters with six-zone temperature controls.
• It has a 6.75 ft³ molybdenum retort that holds 66 8-inch x 12-inch load shelves.
• The retort atmosphere is either partial pressure with laminar gas flow of argon or high vacuum.
• The intended use for the furnace is to sinter titanium or titanium-based alloys.

TestingPer AMS 2750D specification, furnaces with work zones greater than 3 ft3 but less than 225 ft3 must have eight temperature-uniformity survey (TUS) sensors located in the eight corners of the work zone and one TUS sensor located in the center. Eight thermocouples were placed at the eight corners of the furnace – four in front and four in the back. Figure 2 shows the placement of the four thermocouples in the front of the retort in the furnace. The ninth sensor is the center thermocouple built into the retort that is used to determine whether the load has reached the set temperature or not and hence control the soak time.

The AMS 2750D guidelines for the TUS data collection are as follows:

• The furnace shall be held at the test temperature until all sensors have stabilized.
• After stabilization, data is to be collected for 30 additional minutes minimum.
• All survey thermocouples must be within the temperature range.
• Data may not drift above the maximum or below the minimum temperature.

ResultsFigures 3 and 4 show the readings of the control and survey thermocouples for 400 mBar and vacuum, respectively.

It is seen that the maximum variation for the furnace tested is a total of 5 degrees with respect to the center. This is closer than the allowed temperature variation for this class of furnace at this temperature range.

Discussions

The furnace has four features that result in achieving the close temperature uniformity inside the retort:

1. Six-zone temperature control: Six thermocouples control six heating zones, which allow for uniform heating and temperature control.

2. ACCU-Temp[2]: Use of certified thermocouple data corrects the temperature reading by the “ACCU-Temp” control. ACCU-Temp control reduces the variability of the recorded temperature to within ±1oC of the actual temperature, which results in the following advantages:
a. Provides a more uniform temperature within the retort of the furnace
b. Minimizes variations in size and density anywhere inside the retort
c. Allows attaining temperatures a few degrees below the solidus
d. May obtain density of MIM parts as high as 99.8%, or better, consistently

3. Gas management and laminar gas flow[3]:
a. The process gas flows from the gas-distribution holes across the shelves and parts to the center of the retort.
b. The process gas is preheated by the heating elements and flows into the retort at a higher temperature than the internal retort temperature.
c. They are designed for short gas flow distance from the retort wall over the product to the center of the retort to ensure constant clean gas flow across the parts.
d. The “spot in the center” of the retort where the gas is evacuated through the manifold vacuum ports is a few degrees colder, therefore guiding the gas evenly across the parts.

4. The center thermocouple:
a. This takes the guesswork out when the entire furnace temperature is equalized.
b. We recommend that it be used to start the timing of temperature holds for debinding and sintering.
c. This eliminates changing the program for different amounts of furnace load.

Conclusions

The MIM 3675 furnace tested showed a total variation of 5oC for the survey thermocouples during the temperature-uniformity test. This is less than half of the variation allowed for this class of furnace for this temperature range. The furnace incorporates six-zone heating and temperature control, ACCU-Temp, gas-management features with laminar flow and a special center thermocouple, which combined make this close temperature uniformity possible. IH

For more information: Contact Satyajit Banerjee at DSH Technologies, 107 Commerce Road, Cedar Grove, NJ 07009 or Claus Joens at Elnik Systems, Div. of PVA MIMtech, LLC, 107 Commerce Rd., Cedar Grove, NJ 07009; tel: 973-239-6066 ext 12; fax: 973-239-3272; e-mail: cjoens@elnik.com; web www.elnik.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: metal injection molding, MIM, AMS 2750, temperature uniformity, work zone, laminar gas flow, sintering