Presentation by Tim Cowell, Technical Director Die Casting & Metal Working Fluids, The Hill and Griffith Company.
In the picture above Tim Cowell is presenting this Formal Presentation, "Die Lubricant Biofilms - How They Impact the Die Casting Process," at the XVI Simposium 2017 in Queretara, Mexico
Within the die casting process, where molten metal is injected into a steel die, a release/lubricant agent is applied to the die to allow for proper release. These release agents are often referred to as die lubricants.
Die Lubricants provide three major functions that are vital to the die casting process.
- Release of the Casting
- Lubrication of the moving parts of the Die
- Surface cooling of the Die
Die Lubricant Central Systems
These products are typically water dilutable and are diluted between the range of 50:1 to 100:1 in a central mixing system.
The die lubricant is delivered through a network of piping connected to the die cast machines and ultimately to the die surface by spray nozzles. Below is a simple diagram of this type of set up.
Die Lubricant Spray Nozzles
An important key to assuring that the die is evenly coated with the die lubricant is to make sure that all spray nozzles are working and there is nothing that would obstruct the flow to and from these nozzles.
To insure this, a series of filters are placed at various locations in the die lubricant supply system.
An example of a steel reinforced wire mesh filter used in Die Lubrication supply systems. Often steel reinforced filters are used to keep the filters from collapsing.
The purpose of these filters is to keep the fluid clean so that foreign materials will not plug up the spray nozzles.
Inline filters are used to keep the lubricant flow clean and protect the Spray nozzles from clogging up.
Biofilms often clog these filters reducing the amount of release agent being supplied to the spray nozzle and reducing the effectiveness of the atomization.
An example of an inline screen filter just pulled out of the line. This screen filter was given to the die lubricant manufacturer for further evaluation.
However, these filters often become contaminated with a slime (biofilms) resulting in a reduction of die lubricant flow to the spray nozzles and sometimes completely shut off.
All water dilutable die release formulations share the common problem of susceptibility to microbial attack.
This is not all bad news since we need the used dilute fluid to be biodegradable for waste treatment purposes.
The challenge for both formulators and die casters using water-based release agents is to minimize the adverse economic impact of uncontrolled microbial contamination in the die casting operations.
Biofilms are a real problem in maintaining a clean die lubricant system.
In order to address this problem, the following questions need to be answered:
What are Biofilms?
- How do Biofilms Form and Reproduce?
- What can be done to remove Biofilms?
- What can be done to prevent Biofilms?
What are Biofilms?
Biofilms are collections of microorganisms and the extracellular polymers they secrete, attached to either inert or living substrata.
Two types of microorganisms contaminate die release fluids: bacteria and fungi. Four factors predominate in controlling microbial life:
- Energy Source – Obtained by breakdown of organic compounds
- Nutrients – Water Source (minerals)
- Warm Thermal Conditions
- pH conditions – between 7-9.0
Microorganisms create "micro-environments“ called Biofilms in which conditions may be very different than they are in the bulk fluid.
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS). Biofilm EPS, which is also referred to as slime
These Biofilms are protected from changes in diluted release agent temperature, chemistry and pH by the slime that the microbial population produces.
In terms of pH, this means that even though a diluted release agent may be maintained at pH 8.5-9.0, the pH within the microenvironment may be below 7.0. PH control often affects symptoms rather than the underlying problem of microbial growth.
Biofilms: Central Systems and Piping
Die Casting Systems and Biofilms
What can be done to remove Biofilms?
Biofilms are difficult to remove, especially the mature colonies. Typically, removal involves a two-step process.
- Chemical Treatment
- Physical Removal
A residual level of biocide is essential to help control biofilms, but biocide alone cannot penetrate or dissolve a biofilm mass. Incorporating the use of specially formulated cleaners/disinfectants assist in dissolving the EPS allowing the biocide to effective kill the microbes. It also removes byproducts, such as Nitrogen (Ammonia) that promote the growth of the bacteria in the biofilm.
Normally, the system being cleaned is filled with the cleaner/disinfectant and allowed to sit for a period of 8 to 24 hours before being flushed with water (preferably hot). This allows for plenty of contact time for the surfactants of the cleaner to break down the EPS and allow the disinfectant to do its work.
Physical methods can be effective, especially in addressing “mature,” well-developed biofilms. Flushing hot water through pipes at high velocities, for example, can help remove biofilms from smooth pipe interiors, such as PVC.
Biofilms may be more difficult to address in pipes with rough interior surfaces. In those cases more aggressive physical methods are used to scour pipe interiors. Pushing a flexible swab with an abrasive outer coating through a pipe by means of hydraulic pressure is one of the most common methods employed to physically remove scale and biofilm.
If the buildup is too much, then the pipe should be replaced with PVC if possible.
What can be done to prevent Biofilms?
Biofilms are heterogeneous, inherently patchy, and colonized with diverse microbial communities — qualities that make biofilm control challenging for any die lubricant supply system. The keys are a monitoring system, a treatment system, and a preventative maintenance program.
A. Monitor the Microbes within the System
With the understanding that the microbes (bacteria and fungi) are present where water and food sources are present, it is unrealistic to think that the microbes can be eradicated. However, monitoring the number of microbes within the bulk fluid allows for a die caster / fluid manager to determine when hit the system with a kill dosage of biocide / fungicide.
The simplest way of monitoring them is by bio dip slides.
These slides allow for the both the die caster / fluid manager to determine the concentration of microbes present in the bulk fluid.
If the bacteria are greater than 104 in the bulk fluid, then the system should be treated with a dose of biocide.
If fungus shows up in greater amounts of two colonies, then a fungicide should be added. These are typically added to the diluted die lubricant reservoir.
B. Reduce the free number of bacteria by using small dosages of biocide / fungicide
There are products that keep the number of both bacteria and fungi minimal by introducing low maintenance dosages into the system throughout the week. The focus is keeping the number of microbes minimal before they begin to create biofilms.
C. Perform Preventative Maintenance every six months.
Every six months, the system should be shut down and cleaned using the specially formulated cleaner / disinfectant.
This will keep the bacteria and biofilms to a minimum before they create problems for production.
Die lubricant spray is vital to the successful production of castings within the die casting industry.
Biofilms, without the proper treatment program, can create a real problem in this area.
Die casters must have a monitoring system and a scheduled preventative program in place to assure a clean and consistent source of die lubricant supply to the die.
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