Disc-Shaped Graphite Heaters: The Slots Cut In from the Outer Edge – Why Does the Heat Crowd Toward the Inner Ring?

XRD Graphite · Disc-Shaped Graphite Heaters · Technical Column

Disc-Shaped Graphite Heaters: The Slots Cut In from the Outer Edge – Why Does the Heat Crowd Toward the Inner Ring?

A disc-shaped heating element spreads its heating surface into a large flat plane, which looks simpler than a cylinder or rod. But it carries an unavoidable geometric account: when slots are cut at equal angles, the same sector block continuously narrows as it runs from the outer edge toward the inner ring—the cross-section is changing, so the resistance per unit length is changing, and the heat consequently crowds inward. This article breaks down this account, then discusses how to arrange the slots, how to leave the bridges, how to position the holes, how to grind the faces, and which items left blank on the drawing often lead to disputes at both ends.

Written by: XRD Graphite · Technical Column

Let us put the conclusion first: A disc-shaped heater is not simply “a circular plate with a number of slots cut into it.” It is a resistor spread flat into a disc. Current must travel from the outer edge to the inner ring and back out again, and how many turns it makes is determined by the slots. Along the radial direction, the width of the sector block is not constant. Width is changing, so cross-section is changing, and the resistance and current density over the same length change accordingly. Making this component requires first accepting this non-uniformity, then arranging it through slot arrangement, bridge width, and inner-ring compensation—rather than expecting that “cutting it evenly makes it even.”

I. First, Positioning: Heating Elements Have Three Basic Postures; the Disc Is the Flattened One

Graphite heating elements come in a wide variety of shapes, but by current path they fall into only three basic postures. Distinguishing them prevents forcing experience from other types onto the disc.

The disc shape is usually chosen for three reasons: a large-area planar heat source is needed (when opposed above and below, the disc face is a uniform radiating plate); an axisymmetric temperature field is needed (equal distance from centre to outer edge in all directions—the prerequisite for “same all the way around”); and space must be freed at the centre (temperature measurement passages, support columns, crucible bottoms, and material flow often fall exactly on the centre). Once these three converge, the shape can only be circular.

Its position also determines its account: as a supporting heating element, the disc face is often very close to a charge plate or crucible bottom, and heat is transferred by radiation plus the gap. The flatness of the disc and the uniformity of the gap between disc face and charge bottom directly determine the temperature difference in the layer beneath the workpiece. Precisely because it is close, once the disc face warps or locally overheats, the problem shows up first in the charge, not first in the heater itself.

II. Its Essence Is Still a Resistor, Just Flattened

Heat output is determined by power, power by resistance and current, and resistance by material resistivity, path length, and cross-section. This relationship holds for any shape. Therefore, designing a heating element ultimately means doing two things: making the path length reach the required value, and keeping the cross-section from varying.

The convenience of the cylinder is this: with vertical slots, the strip width is constant along the axial direction. As long as slot width and cylinder wall thickness are controlled, the cross-section is constant, and the length is the number of turns multiplied by the circumference—the account is easy to calculate. The disc shape does not have this convenience. The same sector block is wide at the outer edge and narrow at the inner ring—because with equal-angle division, arc length is proportional to radius. Halve the radius, and the width is roughly halved, and the cross-sectional area is halved accordingly. Length is increasing while cross-section is decreasing, and both push in the direction of “higher resistance.” This is not a machining error; it is a fact given by geometry itself.

A note on the lead-out: for the disc shape, the lead-out usually runs from the back of the disc or the disc rim, through a thickened solid section, to the electrode. The task of this section is opposite to the heating section—generate less heat—so the cross-section is made thick, the path short, and the contact surface flat and clean. This follows the same principle as other heating element shapes and is not expanded on here.

III. Core One: At the Same Angle, the Outer Edge and Inner Ring Are Not the Same Width

Translating this geometric fact into three accounts in use makes it clear why disc-shaped problems always relate to the “inner ring.”

• Resistance varies along the radius: Where narrow, the cross-section is small and resistance per unit length is high. Over the same arc length, the closer to the inner ring, the more resistance is contributed and the more concentrated the heat.
• Current density rises toward the inside: Current passes through a sector block in series; as the cross-section shrinks, current density rises. Where current density is high, Joule heat per unit volume is higher and temperature rise is faster.
• Inner-outer temperature difference becomes circumferential stress: The inner ring is hotter and the outer edge relatively cooler; the radial temperature difference creates circumferential compression and tension within the disc. The slots are exactly the outlet for this stress, and slot ends and slot bottoms therefore become crack initiation points.

Therefore, a common phenomenon in disc-shaped heaters is that the disc face temperature does not decline smoothly from centre to edge, but shows a high band somewhere in the inner ring, while the outer edge is relatively cool. This band is often exactly the ring where the sector blocks narrow and approach the inner bore. If the drawing says only “uniform disc face temperature” without stating the allowable temperature difference and at which radii to measure, this statement is unenforceable at acceptance.

Faced with this non-uniformity, there are three common approaches in the industry, each with its own cost:

None of these three is universally applicable. Equal-angle slotting suits applications with low power density and relaxed radial temperature difference requirements; variable-angle slotting suits applications with high requirements where machining and inspection can keep up; inner-ring compensation often appears where the disc face must also bear some mechanical load. What must be decided is not “which is better,” but “who bears this non-uniformity”—if the material bears it, leave more safety margin; if the structure bears it, spend more process steps; if the use end bears it, state the temperature difference clearly on the drawing.

IV. Core Two: Where the Slots Start and Where They Stop

The slot has only one function: to lengthen the current path. But different opening directions produce completely different passage shapes, and this is easier to get wrong on a disc than on a cylinder.

• Outer-edge opening: The slot cuts in from the outer diameter and stops before the inner ring. The outer ring is broken, while the inner ring remains a continuous whole. Current entering from the outer edge can only travel inward along the sector block and turn back at the inner ring.
• Inner-edge opening: The slot cuts outward from the inner bore and stops before the outer diameter. The inner ring is broken, while the outer ring remains a continuous whole. Current travels outward along the sector block and turns back at the outer ring.
• Alternating inner and outer: The two opening types are arranged alternately, forcing current to zigzag “outer—inner—outer.” The equivalent path is multiplied, and power at the same voltage drops noticeably. This is a commonly used arrangement for achieving high resistance in disc shapes.

There is one point that cannot be ambiguous: on the same radius, if slots are cut from both the inner and outer sides, only a ring of connecting rib remains in the middle. If the sum of the two slot depths exceeds the disc thickness, the sector blocks are cut apart. The consequence is direct—that path does not conduct, the whole disc’s power drops, and it does not drop uniformly: starting from the break, the farther sector blocks generate less heat. Conversely, if the remaining rib is too narrow, it cannot hold mechanically and will be pushed apart by expansion in the hot state. Slot depth and remaining rib are a set of numbers that must be given together; giving only one leaves the machining end to guess.

Slot width is also not a case of “the narrower, the better.” Narrow slots save space for more heating area, but thermal expansion brings slot openings closer together. Once they touch, current takes a shortcut through the contact, that section is effectively short-circuited, and nearby sector blocks generate less heat instead. Wider slots avoid contact, at the cost of reduced effective heating area and lower disc rigidity. Therefore, slot width is usually considered together with expansion at operating temperature and disc diameter, not copied from an old drawing.

Slot bottom shape deserves separate mention: fillets are better than sharp corners. Sharp corners are stress concentration points; sharp corners left during slot cutting will initiate cracks first in service. The machining end should pay even more attention to slot cutting sequence—cut symmetrically and alternately, not several slots in succession in one direction. Cutting in succession accumulates stress in one direction, and the disc slowly becomes “cut crooked”; by the time it is noticed, flatness is already out of tolerance.

V. Core Three: That Ring of “Turns” Is Where the Whole Component Is Most Stressed

When current turns past the slot end, it passes through a short section of connecting rib. This short section plays a heavier role on the disc shape than on the cylinder shape, because it is where three “concentrations” overlap: the current path turns here and the cross-section narrows here; heat from the sector blocks on both sides conducts here and accumulates here; geometry changes abruptly here and stress concentrates here.

Bridge width therefore becomes a typical trade-off. A wide bridge is mechanically stable and thermally shock resistant, but current “takes a shortcut” through it—part of the current that should travel the full sector block diverts directly through the bridge. The diverted portion does no useful work, yet the bridge itself heats up first due to high current density. A narrow bridge reduces diversion and returns heat generation to the sector blocks, but mechanical strength and life margin drop together, and it is more easily squeezed by expansion from both sides in the hot state.

In practice, there are several easy observation points for judging whether the bridge has a problem:

• Watch which location glows first during heating. If the bridge turns red before the sector blocks, diversion is excessive and power is not landing where it should.
• Watch the bridge surface after a period of use. If the bridge scales first, thins first, or its edges round off first, temperature and atmosphere attack are heavier there.
• Watch the temperature difference between sector blocks. Noticeable differences between adjacent sector blocks on the same ring often indicate inconsistent bridge width or a slot cut too deep.

One more easily overlooked point: the number and position of bridges must be symmetrical. Bridges are structural connection points and mandatory current paths. Once a bridge is moved or its width changed to avoid a hole or a support, the current distribution across the entire disc shifts, and one side of the disc face runs hot while the other runs cool. This deviation cannot be compensated by adjusting power.

VI. Those Two Holes: Inlet and Return, and the Two Endpoints of the Current

A disc-shaped heater usually has two holes, one for inlet and one for return; some are made with one hole for inlet and the other also serving as a fixing point. They look like just two mounting locations, but they are actually the two endpoints of the current path. Change their positions and the entire disc’s current route changes.

• Distance from hole to slot: If the hole is too close to a slot, current is squeezed into a bundle at the hole edge, causing local overheating there; over time the hole opening chips and enlarges first. If the hole is too far from the slot, the path returning to the endpoint becomes longer, creating an extra “cold section” that does not generate heat as designed, and a low-temperature area appears near the disc edge.
• Relative position of hole and bridge: Whether the hole is on one side of the bridge or symmetrically straddles both sides determines how the final section of current divides. If this is not defined on the drawing, the machining end drills at the hole position centre, and the actual route may be completely different from the designer’s intent.
• Whether the hole wall can bear load: If the hole is for a through-bolt, the bolt threads into the metal component opposite, and the graphite only provides clearance—the hole wall does not bear thread force. If made as a threaded hole, graphite threads do not withstand repeated assembly and disassembly, and are suitable for light loads or applications with very few assembly cycles.
• Edge distance: The distance from hole edge to disc rim and to the inner bore is fixed on the drawing and cannot be remedied at the machining stage. If edge distance is too small, cracking propagates from the hole edge as soon as assembly applies load.

Therefore, for these two holes, the drawing should specify three things: hole diameter, hole position (relative to the centre, with an angular datum, such as the angle relative to a particular slot), and the net distance from hole edge to the adjacent slot. Of these three, the angular datum is often omitted—and this is precisely the basis for “whether the holes and slots line up.”

VII. Flatness and Slot-Cutting Sequence: A Process Account for Thin Discs

Disc-shaped heaters mostly require a flat disc face. The reason is not complicated: it is often very close to a charge plate or crucible bottom. Once the disc face warps, the gap becomes uneven, and a temperature difference appears first in the layer beneath the workpiece. If the disc face warps significantly, it will also press or bind during furnace loading, introducing thermal stress prematurely.

The trouble is that after slot cutting, the disc’s rigidity drops noticeably. A solid circular plate is continuous; once slotted, it becomes a set of sector blocks connected only by the inner or outer ring, behaving like a ring of cantilevers under load. More troublesome, slot cutting itself releases residual stress in the blank. The disc face begins to warp slowly after the slots are cut—if the faces are ground flat first and then slotted, no matter how flat they were ground, they will move after slotting.

Therefore, there are three sequence options, each corresponding to different requirements:

As important as sequence is the measurement method. For thin disc components, flatness is meaningless without “where it is measured and how it is supported”: measuring in the free state on a surface plate versus measuring after mounting in tooling and being pressed down yields two different numbers; where the three support points are placed also changes the reading. The agreement should specify: free state or installed state, support point locations, and measurement point distribution (along how many radii, how many points on each). With these clarified, acceptance does not have to rely on “it feels about right.”

VIII. Points to Watch in Machining and Acceptance

The acceptance items for this type of component are not many, but each must be traceable to a measuring tool; otherwise it is empty talk.

• Concentricity of inner and outer diameters: Eccentricity of the inner bore relative to the outer diameter makes the width of the sector blocks around the ring wide on one side and narrow on the other, directly causing one side to run hot and the other cool.
• Uniform disc thickness: Measure at multiple points around the circumference, not just one location. Disc thickness variation equals cross-section variation, which equals heat generation variation.
• Consistent slot width and depth: Measure each slot, not just the first and last. A slot cut too deep is a break in the circuit; cut too shallow, the path is shortened and power runs high.
• Slot bottom fillets and root rib: Whether fillets are produced and how much rib remains cannot be judged accurately by eye; measuring tools or profile templates are needed.
• Uniform sector block indexing: The angular position of each slot must be verified against the drawing. If indexing is off, current distribution is off.
• Bridge width and number: Measure each bridge width and verify the count. Inconsistent bridge width is a direct source of uneven current diversion.
• Hole positions and edge distances: Verify hole diameter, hole position (including angular), and net distance from hole edge to slot together.
• Disc face flatness and two-face parallelism: Measure according to the agreed method and support condition, and record on the inspection sheet.
• Deburring and dust cleaning: Graphite powder in the slots must be blown clean. Residual powder may be carried by hot gas flow to slot openings and accumulate after furnace loading, becoming a source of contact short-circuiting, or may fall into the furnace chamber and contaminate workpieces.

Of these nine items, the first three are routinely checked; the last four are often checked only after problems arise. The truly labour-saving approach is to write them all at once on the drawing and inspection sheet, so that supplier and buyer speak from the same checklist.

IX. How to Select the Material

Material selection for disc-shaped heaters has a shape-related special requirement: isotropy. Because slots are cut on the disc face and the slot directions go all the way around the centre, the same component contains sector blocks that run “along a certain direction” and “across a certain direction” simultaneously. If the blank itself has pronounced directionality, the strength and resistance of the cut sector blocks will differ, creating variation around the same ring. Isostatically pressed fine-grain material is more suitable in this respect, with uniform structure and small directional differences.

• Purity and ash: The heating element itself is inside the furnace chamber; impurities migrate at high temperatures and contaminate workpieces and furnace internals. For semiconductor, optical, and high-purity material applications, this item is more sensitive than power itself.
• Resistivity: Determines power. Beyond the nominal value itself, batch consistency is more important—if resistance differs between sector blocks on one disc, the temperature field cannot be uniform.
• Flexural strength: Sector blocks after slotting should be considered as cantilevers; what matters is flexural strength, not compressive strength. Handling, clamping, and thermal shock all test this item.
• Thermal expansion and thermal shock resistance: Heating and cooling rates are often set by the process; the material can only adapt. Materials with higher thermal expansion and weaker thermal shock resistance are more prone to cracking from slot ends on this type of slotted thin disc.

The above indicators vary with grade and specification; the technical conditions for the corresponding grade apply. Under the same phrase “high-purity graphite,” fine grain versus coarse grain, and isostatic versus moulded, produce slot opening condition and service life on entirely different levels. During selection, the grade should be written on the drawing, not just the material name.

X. After Installation in the Furnace: Drift, Short-Circuiting, and Oxidation

Failure of disc-shaped heaters is rarely “sudden breakage”; it is mostly several slow accounts running together.

• Resistance drift: Slot openings slowly widen in service—sometimes from oxidative loss, sometimes from sublimation and migration of material at high temperature, sometimes from repeated thermal expansion and contraction wearing away slot opening edges. As slots widen, the effective cross-section of the path decreases, resistance rises, and power drops at the same voltage. The manifestation is “heating gets slower and slower, and the same process curve can no longer reach the original temperature.”
• Contact short-circuiting: Material powder, condensate, spalled coating, or graphite debris falls into the slots; or thermal expansion brings slot openings into contact. Short-circuiting is a local phenomenon, manifesting as one section running particularly hot while other areas cannot heat up; in severe cases that section ablates first.
• Thermal stress cracking: Slot ends and slot bottoms are stress outlets. When heating and cooling are too fast or the inner-outer temperature difference is too large, cracks initiate from these locations. Once a crack penetrates to the adjacent slot, that sector block fails.
• Atmosphere attack: Graphite is stable under vacuum and inert atmospheres; but if the furnace contains trace oxidising components—air introduced by leakage, water vapour, gases released by certain processes—oxidative loss still occurs at sustained high temperatures. This type of loss is faster on thin-walled, multi-slotted components than on thick solid components, because there is more surface available for attack.

The criterion for replacement should be established before furnace loading. Measure the resistance of a new component in the cold state, or the voltage drop at a known current, and record it as a baseline; thereafter, re-measure at each maintenance and judge by an agreed ratio. This is far more reliable than “it looks fine, keep using it.” Several other items can be checked together: whether slot openings have noticeably widened, whether the disc face has warped enough to affect furnace loading, whether bridges have noticeably thinned, and whether there are through cracks. If any one of these reaches its limit, continued use is uneconomical—not because it will fail immediately, but because its power is no longer at the design point and the temperature field in the furnace has shifted accordingly.

XI. On Drawings and Orders, It Is Advisable to Specify the Following

For disc-shaped heater drawings, the common problem is not “cannot be drawn” but “several key numbers are missing.” Based on experience, the following are often left blank:

• Inner and outer diameters and disc thickness, with thickness tolerance and measurement point distribution.
• Number of slots, slot width, slot depth, including slot bottom shape (whether fillets are required) and the lower limit of remaining root rib.
• Slot opening direction: outer-edge opening, inner-edge opening, or alternating; and the starting position—the angle relative to which hole or datum line.
• Bridge width and number, and whether bridges are permitted to be relocated to avoid holes.
• Hole diameter, hole position, and net distance from hole edge to slot; hole position must include an angular datum.
• Disc face flatness and two-face parallelism, including measurement method and support condition.
• Material grade, and requirements for data to be supplied with the goods, such as resistivity and ash content.
• Marking and packaging: part number, batch, orientation marks; transport upright and separated to avoid pressure on sector blocks.

If only two items can be added, add the third and the sixth—slot opening direction and measurement method. Without these two, the supplier can only follow their own habits, and the resulting component may be “dimensionally correct but functionally wrong,” a dispute that cannot be resolved by post-hoc measurement.

About XRD Graphite’s Disc-Shaped Graphite Heaters

With 30 years of experience in disc-shaped graphite heater products, XRD Graphite has accumulated extensive process expertise in thin-disc slotting, slot width and depth control, and disc face flatness preservation. We machine precisely to customer drawings and strictly control slot width, slot depth, bridge width, and disc face flatness. Our products offer relatively long service life under normal use and maintenance and have gained recognition from many industry clients. The factory is located in Baofeng, Pingdingshan, Henan, with both graphite material production capacity and machining lines. Material preparation and machining for such components can be completed in-house.

If your component is stuck on several unclear points—where the slots should stop, how wide to leave the bridges, whether hole positions need an angular datum, and in what state flatness should be measured—you can send us the existing drawing and operating conditions, and we will review them together with the disc’s loading and current path. Graphite undergoes oxidative loss under oxidising atmospheres at sustained high temperatures; slot openings widen accordingly and power declines. This is more pronounced on thin, multi-slotted discs, and both selection and replacement cycles should account for it.