The Silent Cycle Killer: Varnish Induced Failure in Injection Molding Machines

Introduction

In a high precision injection molding process, consistency is paramount. When cycle times drift even by a few milliseconds, the result is increased scrap rates. Maintenance personnel may look to issues with the resin, heaters, or mechanical settings. You may experience burnt-out solenoids that may be viewed as a simple electrical fluke. However, the solenoid failure may be due to varnish. Varnish forms a thin sticky coating that results in valve stiction. The increased friction from the varnish forces the solenoid to work harder resulting in increased coil temperatures.

What is varnish?

Varnish is byproduct of lubricant degradation that is polar and sticky. The primary causes of varnish are oxidation, micro-dieseling and electrostatic discharge. The varnish will tend to coat machine surfaces especially in tight clearance zones and cooler regions of the lubricating system. The tendency for deposits to form in tight clearance zones is the reason that the solenoids draw more current and higher burn-out rate. Varnish may go undetected with traditional oil analysis tests such as particle counting. Varnish precursors are submicron in size

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How is varnished formed?

We previously mentioned the three main causes of oxidation, micro-dieseling and electrostatic discharge.

Oxidation: Is a chemical reaction of the lubricant or the additives react with oxygen. This reaction results in acidic byproducts and insoluble molecules that eventually agglomerate into varnish deposits.

Figure 1. Oxidation of Oil Molecule

Micro-dieseling: Is a pressure induced degradation that occurs when air bubbles trapped in a hydraulic oil. As the oil moves from the low-pressure suction side of the oil pump to the high-pressure side, the bubbles are rapidly compressed, which results in an increased temperature of about 1,800 ⁰F. The intense heat cracks the surrounding oil molecules resulting in carbon soot and scorched oil molecules. Micro-dieseling has become more common as lube system capacity is smaller than it has been historically, which results in lower dwell times so there is less time for entrained air to be released from the lubricant.

Figure 2. Adiabatic Compression

Electrostatic Discharge (ESD): ESD has become a major concern in the last 10 or 15 years. There are a couple of factors that have caused this is that the more highly refined oils have a lower conductivity as the oil flows at high velocity through tight clearance zones and the tighter filtration that are being used.

Figure 3. Electrostatic Discharge

Varnish Detection

As we mentioned previously, traditional oil analysis is blind to varnish. We need to use specialized testing to detect and measure varnish.

Millipore Patch Colorimetry (MPC): This test is the primary method for detecting varnish. The fluid is mixed with a nonpolar solvent and passed through a 0.45-micron filter. The varnish that is insoluble in the nonpolar solvent is captured on the filter patch. The deposit captured on the filter membrane’s color is measured using a spectrophotometer. The result is reported as delta-E.

Table 1. delta-E Alarm Limits

RULER™: Measures the antioxidants in the sample relative to the new oil. This method uses Linear Sweep Voltammetry to determine the remaining levels of antioxidants in the fluid. Low levels of antioxidants leave the oil susceptible to degradation.

Ultracentrifuge:  A sample of the oil is placed into a centrifuge tube and spun at an extremely high rate. The centrifugal force that is generated causes heavy, semi-soluble molecules to precipitate to the bottom of the tube. The amount of deposit at the bottom of the tube is rated on a scale of 1 to 8.

Table 2. Ultracentrifuge Alarm Limits

Air Release:  This test measures how quickly the oil can separate itself from entrained air bubbles. In an injection molding machine, poor air release is a primary driver of micro-dieseling (pressure-induced combustion of air bubbles), which rapidly accelerates varnish formation.

FTIR: A molecular "fingerprint" of the oil. By shooting infrared light through a sample, we can detect specific chemical changes. For varnish monitoring, we are specifically looking at oxidation -- the chemical precursors to varnish.

Varnish Impact and Symptoms

Valve Stiction: Varnish is polar and coats metal surfaces. Valve clearance may be as small as 3 microns. Even a very thin layer of varnish can result in valve drag or stiction.

Coil Burnout: When the spool is stuck, the solenoid draws more current to break stiction, the increased current results in excessive heat that breaks down the coil insulation.

Energy Penalty: The varnish forms a coating on oil coolers resulting in thermal insulation from the varnish this results in reduced cooling in addition to the increased current from the solenoid.

Table 3. IMM Symptoms and Causes

Filter Analysis

The filters capture wear particles, seal fragments, contaminants, and degradation products that have been generated or contaminated the system. Some of these may have not found their way into a sample bottle. We recommend submitting both the supply side filter as well as the return filter. Filter analysis can be used on newly commissioned equipment, annually or when a potential problem is suspected. Because the filters function as a concentrator, it often traps "soft contaminants" and sludge that have not yet reached a high enough concentration in the bulk oil to trigger an "abnormal" rating on a standard lab report. Analyzing the color and texture of the residue trapped in the depth media can confirm the presence of varnish precursors, such as nitration or oxidation byproducts. When paired with your UC and MPC testing, this data allows you to move from reactive maintenance to a proactive strategy, identifying the exact point where the oil’s chemistry has begun to break down before it results in a sticking valve or a burned-out solenoid.

Conclusion

The evolution of modern injection molding machines has prioritized speed and efficiency, but these advancements come at a cost to the lubricant. With smaller reservoirs, higher pressures, and more sensitive proportional valves, the window for error has closed. As we have seen, varnish is not just a nuisance; it is a "silent killer" that bypasses standard filtration and traditional oil analysis, manifesting only when cycle times drift or solenoids begin to smoke. By the time a valve seizes, the damage to your production schedule and your bottom line is already done.

Do not wait for increased scrap rates or burned-out coils to tell you that your oil is failing. Precision molding requires precision fluid management. Contact MRT Laboratories today to schedule a comprehensive Varnish Potential Analysis. Our expert diagnostic team will help you evaluate your MPC levels, antioxidant health, and filter debris to keep your cycle times consistent and your production lines moving.