Technology Notebook: Oil cleanliness standards for injection moldingTechnology Notebook: Oil cleanliness standards for injection molding

Editor?s note: Author Ted Naman is technical coordinator of industrial lubricants at ConocoPhillips. This short technical article is a follow-up to our September/October 2002 article: Supplemental oil filtration protects IMM hydraulic systems, by Jack Berg of Serfilco Ltd. ConocoPhillips and Serfilco are not affiliated. The earlier article is available at www.pma-magazine.com by searching the archives for past issues in 2002. Scroll down to September, and click on the Technology Notebook and article title link.

The smallest particle that can be seen with the naked eye is about 70 µm (1 µm = one millionth of a meter), which is about the diameter of a human hair. Hydraulic oils normally contain many thousands of particles smaller than the naked eye can see. In injection molding machine hydraulic systems, hydraulic oils are routinely filtered to as low as 5 µm to minimize damage from contaminants.

Injection molding machine users rely heavily on oil analysis for the maintenance of their hydraulic systems. An operator takes an oil sample as part of a routine maintenance schedule, and sends it to a local laboratory for complete analysis. The lab conducts detailed analysis on the oil sample as an indirect way of determining the condition of the machine. The lab then sends the report to the processor.

The report describes the oil?s condition and how it reflects on the condition of the injection machine. This is very similar in concept to the medical practice of using human blood samples to determine body condition.

A key test run on the hydraulic fluid is the particle count test, which determines how many dirt particles are present in the hydraulic system, and the size of the particles as well. Until recently, particle size has commonly been reported by the ISO 4406:1987 cleanliness standard, which measures particles in two units, such as 16/13 or 14/11. The International Organization for Standardization (ISO) revised ISO 4406 in 1999 to provide a universal method for reporting the particle contamination in circulating oils.

This is done by using an optical particle counter (see photo) to count the particles in a 1-ml sample of oil. One milliliter is roughly equal to one drop of oil from a common dropper. Then a two-unit cleanliness designation is assigned to the sample to establish the level of cleanliness. First, all particles equal to or greater than 5 µm are counted, then all particles equal to or greater than 15 µm are counted.

For example, if the sample contains 400 particles >/= 5 µm, and 60 particles >/= 15 µm, it would have an ISO cleanliness level of 16/13. (Range numbers are determined from a chart published by ISO). In some cases, a three-digit cleanliness code is reported if requested by the processor, by counting the number of particles >/= 2 µm, i.e. ISO 18/16/13.

In 1999, a new ISO instrument calibration procedure, ISO 11171, was introduced. However, industry adopted it only recently, when reference material became widely available. What is ISO 11171?

ISO 11171 is a new calibration procedure for optical particle counters. It is designed to reduce their ambiguity and increase reproducibility from lab to lab. Previous calibration techniques by ISO 4406:1987 Standard have relied on calibration fluids and test dusts that have not been certified to contain an absolute size distribution, namely the AC Fine Test Dust (ACFTD). This allowed for variances within the particle counting industry that are no longer acceptable. ISO 11171 corrects this by requiring that calibration of particle counters be done using suspensions traceable to the National Institute of Standards (NIST). That is, calibration fluids must be derived from NIST-certified fluids.

Another issue affecting this area since the early 1980s is the accuracy of particle sizing of ACFTD in the In addition to ensuring that all particle counters are calibrated to the same standard, the ISO 11171 procedure requires a comprehensive calibration of all components of the optical particle counters. This new calibration standard ensures the accuracy and reproducibility of all particle counters, such that the accuracy of particle counts is not dependent on the laboratory from which the particle counts originated.

What Has Changed?

As a result of ISO 11171 implementation, particle size results, which were previously reported in two-digit format, are now reported in three-digit format. ISO 11171 specifies Medium Test Dust (MTD) as the standard reference material for optical particle counters, replacing the previously used material ACFTD.

The ISO MTD material is now traceable by NIST, which makes the quality of this material more consistent as a reference when conducting particle count testing. The net result of these changes is a new calibration procedure for optical particle counters that provides more consistent and accurate particle counting results in different labs around the world.

Cleanliness and Reporting Test Results

Cleanliness targets remain the same. Lab test results reporting by ISO 4406:1999 Standard have changed. Instead of the old two-digit code (16/13), test results contain the new three-digit code (ISO 20/16/13), with the first set of digits reported value at 4.6 µm, the second digits reported at 6.4 µm, and the third set at 13.6 µm levels as shown in the table. This compares with 2, 5, and 15 µm by ISO 4406:1987 Standard. It is important to note that none of the procedure and code changes resulted in any change in the actual contamination level of a given sample.

The Hydroclear AW hydraulic fluids ISO 32 and 46 are ConocoPhillips products that are designed and recommended for injection molding machines. A high level of antiwear and neutral detergent additives in these fluids, blended in premium base stocks, provide lubricants that readily separate water, control wear, keep deposits to a minimum, and keep servovalves and spool valves free of sticky deposits, minimizing downtime and contributing to smooth hydraulic operation.