Mould Temperature Controller and Thermal Oil System Cleaning in Tongling

Abstract: A mould temperature controller rarely fails in its temperature loop; it fails in the heat transfer oil and the heater tubes. Oil held at 250-300 C oxidises and cracks, forming sludge, gum and carbon that deposit on heater tubes, plate heat exchangers and pipe walls, so heat transfer falls, heat-up times stretch and a coked heater tube eventually burns out. Tongling's first industry is copper-base new materials, and its copper-clad laminate presses, copper foil coating lines, enamel wire lines and injection moulding shops run a large number of oil-type controllers and small thermal oil systems. Following GB/T 34352-2017, this article sets out four on-site coking indicators, an eight-step shutdown cleaning route, acceptance figures for deposit removal and corrosion rate, and an anonymised case from a copper-clad laminate press shop.

1. Where Mould Temperature Controllers Sit in Tongling's Plants

Tongling is a national base for non-ferrous metals and electronic materials, and copper-base new materials are its first industry: the city runs complete chains from electrolytic copper through rod, foil, copper-clad laminate and printed circuit board, from copper strip to lead frames, and from rod to enamel wire and cable. Sulphur-phosphorus chemicals and fine chemicals, building materials and power, and new energy vehicle parts run in parallel in the same city, and the demand for high-temperature equipment cleaning sits on those process steps.

Oil-type mould temperature controllers and small thermal oil systems are used in four places: copper-clad laminate pressing and coating dryer rollers, where the press platen and roller have to hold a small surface temperature spread; injection and die-casting moulds in auto parts, appliance and electronic plastics shops, where mould temperature control limits sink marks and warpage; enamel wire baking furnaces and film coating rollers, heated by a circulating oil loop; and fine chemical reactor jackets and dryers, where volumes are small, temperatures high and control tight.

These units share their numbers: 6-120 kW per unit, 180 L or less of oil in many systems, pipework mostly DN15-DN50, several units in parallel, and short shutdown windows. Oil held at 250-300 C degrades faster than in a large thermal oil heater system, and the coking concentrates: heater tube surfaces and plate heat exchanger passages cake first. That is why a controller can show a normal temperature reading while heat-up times keep stretching.

2. Judging Coking: Four On-Site Indicators Before the Oil Test

Coking can be judged without opening the system. Four indicators are read in order: heat-up time, heater tube to oil temperature difference, circulation condition, and control quality.

Heat-up time first: if the time from cold to set point is 20% longer than when the unit was commissioned, or the unit can no longer reach set point on no load, that is the first signal. Second, temperature difference: an infrared thermometer on the heater tube wall against the oil outlet shows the gap widening from about 10 C to more than 30 C, which means an insulating carbon layer has built up on the tube. Third, circulation: pump outlet pressure becomes unsteady, flow drops, filter differential pressure climbs and the screen carries black carbon or gum. Fourth, control quality: mould temperature swing widens from about 1 C to more than 3 C, and parts show batch-to-batch colour or warpage differences.

Two of the four are enough to justify an oil sample: carbon residue, acid value, viscosity change, flash point and low-boiling fraction. Any indicator clearly away from new-oil values means the oil has degraded, and topping up with fresh oil only accelerates the decline; the correct route is to drain, clean and refill.

3. The Eight-Step Shutdown Cleaning Route

Cleaning is done with the system shut down; on-line cleaning suits only lightly degraded systems with little coking. The eight steps run in this order, each with its own control figures:

1. Cool down and drain: drain hot at 70-80 C, where viscosity is low and residual oil runs out; below that temperature sludge clings to the walls and stays behind.
2. Compressed air blow-down: 0.2-0.6 MPa in sections, carrying residual oil and loose carbon out of the pipework.
3. Manual sludge removal: expansion tank, filter housing and low points are cleaned by hand; a small system typically yields a few kilograms to a dozen kilograms of sludge.
4. Inspection and repair: pump, valves, flanges, electric heater tubes and the plate heat exchanger are checked; badly coked heater tubes are inspected separately or replaced, and seals are renewed for the new medium.
5. Charge the organic cleaning agent: volume set at 1-1.2 times the system volume; where stainless steel is present the medium must stay below 25 mg/L chloride, so a chloride-free formulation is used, and the full boiling range of the agent should not exceed the system's inlet oil temperature.
6. Circulate: the electric heater must not be energised during cleaning, since local overheating or dry firing can re-sinter dissolved gum into hard carbon; circulate cold for 4-8 h as the product data sheet allows, check heat distribution with an infrared thermometer every 4 h, keep cleaning pressure below system working pressure, and clean the filter often.
7. End point and recovery: stable pressure, even heat distribution, and one further hour of circulation after a filter clean with no fresh sludge means the end point is reached; the spent cleaning liquid is recovered in full and consigned as hazardous waste rather than reused.
8. Blow-down and refill: blow down with compressed air again and confirm no residual liquid (no saponification, emulsification or separation); fill with new oil and boil out in stages at 100-150-180-210-250 C to remove water and air, then run a pressure leak test.

4. Acceptance: Deposit Removal, Corrosion Rate and Oil Quality

The job is not finished when the system looks clean. Equipment-side data covers deposit removal and corrosion rate; oil-side data covers carbon residue, acid value and low-boiling fraction after the refill. Both sets are needed to close the work out.

5. Case Study: Three Controllers in a Copper-Clad Laminate Press Shop

In August 2026 we cleaned and refilled three oil-type mould temperature controllers in the press shop of a copper-clad laminate manufacturer in Tongling. Each unit is rated 36 kW with a maximum working temperature of 280 C and holds about 180 L of oil, serving four presses; the units had been in service for four years without an oil change.

Before the work: heat-up time had stretched from 35 min when commissioned to 55 min, the press platen surface spread had widened from 2 C to 6 C, the filter clogged every three days and its screen carried black carbon. The oil sample returned 1.4% carbon residue, an acid value of 0.62 mgKOH/g and an 18% viscosity change, all three past the limits at once.

The work followed the eight steps: shut down and cool to 75 C, drain and recover 520 L of used oil, blow down with compressed air at 0.4 MPa in sections, remove about 12 kg of sludge by hand from the expansion tank and filter housing, charge 540 L of organic cleaning agent and circulate cold for 6 h with the electric heaters left de-energised, recover the spent liquid, blow down and inspect again, then refill and boil out in stages from 100 C to 250 C.

Results: 92% of visible surface free of deposit and a coupon corrosion rate of 0.06 mm/a; ten days after refilling the oil returned 0.12% carbon residue, 0.05 mgKOH/g acid value and 2.8% low-boiling fraction, all inside the post-refill range; heat-up time came back to 38 min and the platen spread to 2 C. The whole job took five days, scheduled entirely inside the press line's planned shutdown window, and the plant added half-yearly oil testing and heater tube inspection to its equipment check sheet.

6. Service Radius, Shutdown Windows and What We Need From You

Tongling is about 300 km from our Danyang base via the Nanjing-Wuhu expressway, so a routine project is mobilised on a 4-5 hour drive with cleaning agent, circulation pump, compressed air equipment and corrosion coupons on board. Within Tongling we cover Tongguan, Yi'an and the suburban districts as well as Zongyang county, and Chizhou, Wuhu, Anqing, Xuancheng and Huangshan are handled on the same response.

Four items from the plant make scheduling straightforward: the equipment nameplate and heater rating, the system oil volume (or expansion tank capacity), the current heat transfer oil test report, and the available shutdown window. With those in hand the gap between fixing a shutdown date and mobilising is normally 2-3 days. For several units in parallel we recommend cleaning in batches, keeping the rest in production rather than stopping the whole line; cleaning and boil-out are scheduled in two separate blocks so that boil-out does not affect other workstations.

7. Frequently Asked Questions

Does a slow heat-up always mean the oil has coked?

No. Check three things first: heater current against the rating, pump flow and bypass valve position, and the accuracy of the controller and temperature sensor. Only when all three are normal and heat-up time is 20% longer than at commissioning should you take an oil sample for carbon residue and viscosity and pull a heater tube to measure the carbon layer, rather than assuming coking instead of reduced heater power or pump flow.

Can we just change the oil without cleaning the system?

No. Once the used oil is drained, sludge and carbon remain on the heater tubes and pipe walls, and fresh oil carries that residue back into circulation so the carbon residue climbs again; in comparable systems we reviewed, oil-only changes were back above 0.8% carbon residue within 3 months. Cleaning and the oil change have to be done in one campaign, without a gap between the two steps.

Can the electric heater be energised during cleaning?

No. Applying heat through the heater tube during cleaning risks local overheating or dry firing and can re-sinter dissolved gum into hard carbon. Under GB/T 34352-2017 some organic cleaning agents are intended for ambient-temperature use, with figures taken from the product data sheet. We circulate cold for 4-8 h, watch heat distribution with an infrared thermometer and hold pressure below system working pressure.

What has to be watched when the system contains stainless steel?

Two hard limits: chloride below 25 mg/L, which means a chloride-free formulation, and temperature and concentration set by the product data sheet and the measured deposit, with coupons in the circuit to quantify the corrosion rate. The case system above measured 0.06 mm/a against a 0.1 mm/a limit, so the formulation was safe for its carbon steel and stainless parts, and no pitting appeared after cleaning.

How often should the oil be changed and the system cleaned?

Oil testing sets the change, on-site indicators set the cleaning. Sample the oil every half year for carbon residue, acid value, viscosity change and flash point; when two consecutive samples are clearly away from new-oil values, or heat-up time is 20% longer and the filter keeps clogging, book a shutdown clean and refill. Systems running at 280-300 C generally go 2-3 years per cycle, and a press line in continuous production takes the shorter end of that range.

All cleaning processes and acceptance criteria in this article follow GB/T 34352-2017 Guide for cleaning of organic heat transfer material heaters and systems; inspection and replacement of the controller itself (heater tubes, plate heat exchanger, circulation pump) follow the equipment manual.

Service coverage: Anhui (Tongling, Chizhou, Wuhu, Anqing, Xuancheng, Hefei, Ma'anshan, Huangshan) · Jiangsu (Nanjing, Suzhou, Wuxi, Changzhou, Zhenjiang, Yangzhou, Taizhou, Nantong, Yancheng, Suqian, Huai'an, Lianyungang, Xuzhou) · Zhejiang (Hangzhou, Ningbo, Jiaxing) · Shanghai

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