Casting mould temperature control cartridge heater

01

Fit the heater to the hole and the part temperature

Watlow's FIREROD cartridge heater data sheet describes nickel-chromium resistance wire, conductor pins and magnesium oxide insulation in an Alloy 800 sheath, rated for part temperatures up to 760 °C (1,400 °F), watt densities up to 62 W/cm² (400 W/in²) and up to 480 V, with mould, die and platen heating among its applications. Diameter tolerance is ±0.05 mm (±0.08 mm for 25 mm units); sheath length tolerance ±2.4 mm up to 114 mm, then ±2% (±3% for 1/8 in units); actual diameters run 3.1 mm (1/8 in) to 25.3 mm (1 in) with maximum lengths of 305–1,830 mm. [1]

Watlow’s metal-heating chart relates hole fit, watt density and application temperature for steel parts. Its inch example is 0.500 − (0.496 − 0.002) = 0.006 in clearance, printed as 0.15 mm. The same text prints the hole as 13 mm and the heater as 12.6 ±0.05 mm; these rounded metric inputs do not reproduce that clearance. Editorial calculation: 0.500 in ×25.4 = 12.700 mm, and 0.006 in ×25.4 = 0.1524 mm. Use a dimensioned drawing in one unit system and confirm the source’s rounded pairs with Watlow. Its stainless-steel chart correction adds 0.0015 in (0.04 mm) to actual fit. For aluminium and brass it prints a temperature increment of “100°F (38°C)”: as a temperature difference those are not equivalent. Editorial calculation: 100 ×5/9 ≈55.6 °C, whereas 38 °C ×9/5 =68.4 °F. Neither editorial conversion settles the intended chart correction; obtain Watlow confirmation before applying it. For square holes or grooves the maker requests consultation. Watlow’s camber limit is 0.020 in × (length in feet)² or 0.005 in, whichever is greater. [1]

Tempco’s fit example uses the maximum hole diameter minus the minimum heater diameter on the heated length: a nominal 3/8 in heater at 0.371 ±0.002 in in a 0.376 ±0.002 in hole gives 0.378 −0.369 =0.009 in. Tempco recommends drilled and reamed holes for higher watt density or temperature and says a looser fit assists installation and removal. Its stated centreless-ground option is ±0.0005 in. [3]

Tempco catalogue page 2-4 lists a maximum-temperature performance heading of 1400 °F (760 °C), while its note names a maximum recommended sheath temperature of 1200 °F for a standard heater and also limits operation by the selected termination’s ambient temperature. Both statements belong to the same page; confirm the ordered construction and termination rather than applying the higher heading to every unit. Page 2-6 recommends avoiding exposed heated sections beyond the mould, which lack a heat sink, and keeping the lead end out of immersion liquid. [2] [3]

Editorial calculation: assume a cylindrical heater of 12.6 mm diameter with exactly 100 mm heated length, uniform 400 W input, and only its lateral surface counted, excluding ends and cold sections. Heated area =π ×1.26 cm ×10 cm ≈39.6 cm²; average geometric watt density ≈400/39.6 =10.1 W/cm², approximately 65 W/in². These assumed dimensions and power are not manufacturer-published or measured operating data, and the result is not an allowed design rating. Obtain the allowable density for the actual fit, temperature and heater construction. [1]

Give the supplier the mould material, hole diameter and tolerance (and whether through or blind), heater diameter and overall and heated length, required wattage and voltage, part operating temperature, thermocouple type and position, lead length and protection (braid, conduit), seal type against oil, moisture or plastic, termination, mounting (flange, fitting) and quantities.

02

Set electrical data, leads and seals

Watlow lists wattage tolerance +5%/−10% and resistance tolerance +10%/−5% for units other than its 1/8 in size; that size instead has +10%/−15% wattage and +15%/−10% resistance tolerance. Its nominal room-temperature resistance is 90% of the Ohm’s-law calculation before use and 95% after use, with values described as approximate and application-dependent. Its electrical table ranges from 240 V /3.1 A for 1/8 in to 480 V /23 A for the larger diameters; the 1/4 in thermocouple option has a separate 3.1 A maximum. Component recognitions are listed at 240 V AC, with the warning that not all option combinations are covered. [1]

Watlow’s ambient-air example concerns a pre-oxidised FIREROD in 70 °F (21 °C) air. At 44 W/in² (6.8 W/cm²) it lists a sheath temperature of 1450 °F (784 °C), with one-year life expected if cycling is not too frequent. This is the source’s conditional air-service expectation, not a mould-heater lifetime guarantee. Select lead insulation and seals by their own ratings, separately from the part-temperature capability. [1]

Tempco page 2-4, Rev 3 (02-2022), lists actual diameters from 6.25 to 25.30 mm, maximum 240 V through nominal 1/2 in and 480 V from 5/8 in when applicable, subject to maker confirmation. Its general maximum-current row is 4.4–23 A by diameter, but the named termination types have a separate, lower row of 3.0–9.7 A; that termination-current note assumes ambient up to 250 °C with mica-insulated leads. The cold resistance is set 3.3% below calculated hot resistance. Its camber entry for lengths above 6 in prints both “0.020” per foot of length” and the squared expression 0.020 × (length in feet)². These differ for lengths other than one foot: confirm the intended acceptance rule with Tempco. [2]

Editorial calculation: assuming a purely resistive single-phase 240 V supply, a Tempco nominal 1/2 in heater at the table’s 2520 W entry gives I =2520/240 =10.5 A, matching its general current row. The named termination exceptions for that diameter instead list 7.6 A. The calculation is neither manufacturer-published operating data nor a measurement, and does not establish that the maximum-wattage entry is compatible with every termination. [2]

Editorial note: on casting moulds, protect leads and seals from molten metal splash, release agent and cooling water, and ask how the heater is removed if it seizes in the hole.

03

Compare a split-sheath heater for oversized or worn bores

Dalton Electric's Watt-Flex cartridge heaters use a split sheath whose two halves expand against the bore at operating temperature and contract on cool-down for removal, so the maker says they do not rely on a tight installation fit. Its stated approximate guideline is a bore about 0.007 in larger than the nominal heater diameter, with diameter tolerances of ±0.002 in for 1/4–3/4 in, ±0.0025 in for 1 in, ±0.05 mm for 8–20 mm and ±0.06 mm for 25 mm, and length tolerance of ±3% (3/32 in minimum), ±2% above 20 in (500 mm). [4]

Dalton lists TGGT and extruded PFA leads for applications up to 480 °F (250 °C), and high-temperature fibreglass leads up to 840 °F (450 °C). Those source pairs describe the offered lead choices; confirm the selected lead rating and environmental sealing for the mould face. Its wattage method adds heat-up watts, (weight in lb × specific heat × temperature rise in °F) / (3.412 × heat-up hours), to heat-loss watts, then multiplies the sum by 1.2 as its stated 20% safety factor. [4]

Editorial calculation: convert Dalton’s diameter-specific catalogue ceilings using 1 in² =6.4516 cm²: 225 W/in² for nominal 1/4 in gives approximately 34.9 W/cm², and 75 W/in² for nominal 1 in gives approximately 11.6 W/cm². These are conversions of separate catalogue entries, not measured operating data or a universal allowed range. The heater supplier must confirm the permitted value at the actual temperature and bore condition. [4]

04

Insertion heaters offered for mould heating

Jaye lists a plastic-injection mould cartridge heater; Sinton and Hongtai list cartridge heater product ranges. Match the ordered element and termination to the mould drawing.

05

Resistance winding, MgO insulation and lead-end protection

The FIREROD section drawing identifies the winding, conductor pins, MgO insulation and sheath. Specify the lead-end construction alongside the heated section. [1]

06

Bore drawing, cold sections and electrical acceptance

Provide the bore and heater tolerances in the same unit system. Record the selected heater construction and termination before accepting wattage, temperature or camber limits.

07

Send the hole, temperature and electrical data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Mould: [material; temperature]
Hole: [diameter; tolerance; depth]
Heater: [diameter; length; heated length]
Electrical: [watts; volts; phases]
Leads: [length; protection; seal; termination]
Sensor: [thermocouple type; position]

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