All heat transfer fluids (also called hot oils) have a finite service life in an extruder. The primary factor that determines this lifespan is the type of fluid selected. While operating temperature and production rates play a role, they are secondary factors in how often fluid changes are required.

Using a fluid specifically designed for extrusion is always the best practice. Although many options are available, only select premium fluids are formulated with the additives needed to enhance performance and significantly extend the interval between oil changes in an extruder.

Additionally, implementing a fluid analysis program is highly recommended. This helps monitor the condition of the oil and identify the optimal change intervals for your application. Most fluid suppliers provide this type of oil analysis service at little to no cost.

Optimizing Fluid Life

All fluids degrade over time, and if they are not replaced as needed, they will leave behind deposits. The challenge lies in balancing regular maintenance with production demands.

To illustrate the stress an extruder places on oil, a fluid that may last 2500 hours in an extruder could potentially last 12–15 years in a larger, closed system without air exposure.

Several factors contribute to oil breakdown, but there are also simple strategies to help extend oil life.

Heat transfer fluids typically fail in two ways:

Oxidation
This occurs when oil at temperatures above 93°C (200°F) is exposed to air. While some level of oxidation is unavoidable, limiting exposure is critical. Fluids above 93°C in open-air reservoirs oxidize very rapidly, which can significantly shorten oil life. To prevent this, heat exchangers are built into systems to cool the oil before it returns to the reservoir. If these exchangers become fouled on either the water or oil side, their reduced efficiency can accelerate oxidation.

The simplest way to check a heat exchanger’s performance is by measuring the reservoir temperature with a probe. A properly functioning system should maintain oil temperatures below 93°C in the tank.

Thermal Degradation
This takes place when oil is heated beyond its rated maximum operating temperature. While rare, certain scenarios increase the risk.

• Extended idle time. Leaving equipment in “heat only” mode with barrel heaters on for long periods can expose slow- or non-circulating oil in barrel wraps to prolonged high heat. While short idle periods or startup heating is acceptable, extended idle heating should be avoided.
• System shutdowns. Cooling the oil before shutting off circulation pumps is critical. If pumps are stopped too soon, stagnant oil can be trapped near heaters and overheat. This is especially important for extruder barrel temperature controllers, whether built-in or portable.

As oil ages, it darkens and oxidizes, forming acids that eventually polymerize into sludge. Establishing a proper oil change cycle—similar to maintaining a car—is key to keeping the system clean and efficient.

Optimal Draining Techniques

When changing oil, ensure the maximum amount is removed before refilling. This includes not just the reservoir but also circulation lines, filter housings, and heat exchangers. Leaving excessive used oil behind will contaminate the new fluid, darkening it quickly and accelerating its breakdown.

One way to evaluate the effectiveness of a drain is through “before and after” oil analysis. Comparing the total acid number (TAN) of the new oil against the used sample can reveal how much residual oil remained after the change.

What to Do If Oil Changes Are Delayed

If the oil’s service life has been stretched but deposits have not yet formed, flushing fluids (available from most suppliers) can be used to reduce residual oil and prevent contamination when refilling with fresh fluid.

If deposits are already present—or suspected—specialized cleaning products are available. Some allow continued operation while cleaning takes place. Otherwise, care should be taken to remove sludge and residual fluid before refilling in order to maximize both the lifespan and the value of the new oil.