8 Reasons your Coolant is Foaming & How to Stop it.

Coolant foaming is a common issue faced by metalworking fluid users, and can be caused by various factors. In this blog, Pennine Lubricants will go into detail about why coolant foaming takes place and what steps to take to avoid it happening.  

So, why is my coolant foaming?  The main reason coolant foaming takes place is because the machine and the coolant in use are not being maintained correctly. Examples include blocked sump filters that restrict the flow of the coolant or running the coolant at too high of a percentage mix. 

Keep reading to find out more about coolant foaming including how to stop coolant from foaming and what the negatives of coolant foaming are. 

Why is My Coolant Foaming?

As manufacturers of lubricants and machine coolants, we receive many technical queries relating to why coolant foaming takes place. There are multiple factors that contribute to coolant foaming, therefore we have detailed them below to help you identify why your coolant could be foaming.

  • Running the Coolant at Too High % Mix:

Running the coolant percentage too high is a common cause for coolant foaming. Most products will have an operational band determined by the manufacturer which takes into account a number of factors for optimal performance, including defoaming performance. Running your coolant at a concentration significantly higher than the range specified can increase the risk of foaming due to the higher volume of oil, emulsifiers, detergency additives and other chemicals within the coolant formulation.

The operational band will be unique to each product, but as an example Ultrasol X-777 is a popular coolant as it has exceptionally low foaming foaming properties. The data sheet for Ultrasol X777 details a typical dilution range (min – max) of 4% – 10%. It is important to gain technical advice on your application and coolant, our technical team can advise on the correct dilution range for your application. Get in touch with our technical team easily here.

To find a datasheet for your coolant, click here and you will be taken to our helpful technical information page where you can search for your specific product’s data sheet. Alternatively, if you are seeking some more specific advice, be sure to get in touch with us today

  • Low Sump Levels Causing Air Entrainment:

If the volume of coolant in the machine sump is low, this will significantly increase the risk of foaming occurring due to air entrainment. If the fluid level is approaching or below the pick up point for the coolant pump then there is a high risk of air being sucked up which will cause the coolant to foam excessively. This will have a detrimental impact on tool life, surface finish and swarf washing as air has poor cooling, lubricating and flushing properties. 

  • Blocked Filters Restricting Flow to the Pump:

Most machines will have different types of filters in the sumps including simple lift out gauze plates, filter cartridges and bag type filters. These are in place to protect the pumps and remove impurities which if circulated would have a detrimental impact on tool life and surface finish. As filters start to blind or block, this can restrict coolant flow leading to air entrainment which will cause excessive coolant foaming.

  • Paper Filtration Systems “Stripping” Antifoam From the Coolant:

Certain coolants are produced with specific antifoam additives within the formulation. However, it is possible that systems employing fine micron/mesh filter paper can physically strip the antifoam from the emulsion as it passes through. This then leads to blinding of the filter paper which may cause flooding along with increased risk of coolant foaming.

  • Coolant Not Suited to High Pressure Coolant Systems:

Not all coolants are designed for use in high pressure coolant systems. This does not mean it is a “bad” coolant, it is just not suitable for high pressure applications. Older technology products, or products designed for low speed applications are less likely to contain low foam additives. If you use the same coolant today as you did 20 years ago, it’s unlikely to contain additives suited to modern CNC machines and high pressure coolant systems. 

  • Ultra-soft Make Up Water:

Although soft water is excellent for making long lasting, stable emulsions with low corrosion potential, you are much more likely to experience coolant foaming issues when using soft make up water or pure demineralised water. Some products are not manufactured for use in soft water too, and using a coolant produced for medium to hard make up waters will also increase the risk of coolant foaming. 

  • Hard Water Soaps:

Hard water soaps are formed in areas with high levels of insoluble calcium in the make up water. As the coolant ages, the calcium levels build up as the water evaporates and can create greasy, slimy calcium soaps on in the coolant tank and even inside the machine. In severe cases, the soap can form a blanket on the coolant surface (almost like tramp oil) which will support foam as it prevents air from being released from the fluid. 

Ultrasol X-NT products are a new range of boron-free metalworking fluids, and are suitable for both soft and hard make up water and are low foaming. For more information on this new range, and whether it would suit your application, get in touch with our technical team today.

  • Heavily Contaminated Sumps:

Fluid displacement caused by heavy swarf contamination in sumps is another cause of coolant foaming. If a significant proportion of the fluid volume is reduced by built up swarf, this reduces the ability of the coolant to dissipate air effectively. As the reduced volume of coolant is being pumped around the system the air bubbles build up to a point where the coolant may become a mass creamy foam.

How to Prevent Coolant from Foaming? 

There are several ways you can help prevent your coolant from foaming. Read our helpful checklist below to find out how you can avoid the problems caused by coolant foaming:

1. Check coolant dilution rates frequently:

It is quick and easy to check the dilution rate of your coolant with a refractometer. A refractometer is a handheld instrument used to determine the concentration of water soluble fluids such as machine tool coolants. By adjusting and maintaining the coolant within the manufacturers dilution range this will help to reduce the risk of the coolant foaming. 

To see an example of a form of testing involving a refractometer that can help avoid coolant foaming be sure to click and watch our informative video below.


2. Keep coolant sump levels topped up:

The job of coolant is to cool, so if your sump volume is only half full, the coolant will be pumped around the machine too quickly. This means the coolant will have little time to dissipate the heat from the cutting area before it’s circulated back around the system, trying to cool it again. By ensuring that the sump level of your coolant is more than 75% of the sump volume, your coolant will be able to do its job effectively, avoiding coolant foaming which is a common problem caused by low sump levels. 

3. Use the correct coolant for your application:

If you’re running a high pressure coolant system your coolant needs to be able to cope with this. High pressure coolants contain low foam additives which help to reduce the risk of coolant foaming during use. Our technical team can help identify the correct coolant for your application and machine.

4. Use a coolant suited to paper band filtration systems:

If your machine uses a paper filtration system, it is important to use a coolant suited to paper band filtration systems. Your coolant provider should be able to identify the correct coolant for your system. 

5. Choose a coolant tailored to your water hardness:

Using a coolant that isn’t suitable for the make-up water can have a huge impact on coolant foaming. Find out what water type area you are in and seek advice from your lubricant manufacturer on the right product for your application, our technical team can help you with this.  Our popular product Ultrasol X-777 is very low foaming in soft water, whereas Pencool Ultra S102 is suitable for hard water areas, as it prevents the formation of insoluble soaps, which cause foaming.

6. Heavily Contaminated Sumps:

If the machine sump is heavily contaminated with swarf or tramp oil, use a filtration system like a Freddy coolant vacuum or a Tramp Oil Separator to remove contaminants that cause bacterial growth. Your coolant will then be able to dissipate heat effectively and your coolant is less likely to foam.

What are the Disadvantages of Coolant Foaming?

Disadvantage  Explanation 
Poor tool life & surface finish Air has much lower cooling and lubrication properties compared to soluble coolant. Foam will therefore be detrimental to tool life and surface finish.
Increased drag out When coolant foams, it creates a thick, creamy substance which will cling to components and swarf. It will also sit on swarf conveyors. These three factors all lead to increased “drag out” of the coolant and thus increased coolant consumption and costs.

Coolant Foaming with Pennine Lubricants:

We’ve been helping metalworking fluid users get optimum performance from their coolant, which results in increased fluid life, improved product performance and manufacturing quality as well as assisting with HSE recommendations. If you are experiencing coolant foaming and would like some advice from our technical team, get in touch with us today. 


Pennine are more than just a supplier, they offer technical support and expertise. They provide full regular monitoring of metal working fluids and any aspects that support the safe use and operation of the oils and coolants we use on our site. Kevin Martin, Health & Safety Manager, Abbey Forged Products

They offer an excellent service, are polite and offer helpful advice. Mathew Kelly, Operations Manager, Barrett Steel

The communication from Pennine is great, we are always fully informed of our order status and they always deliver on time. I have been really impressed with the quality of their oil. Mark Wood, Fitter, Betney Cop Restoration Co.

We've recently changed to Pennine Lubricants, I can't thank Bob enough for the effort and service that's provided every time we order. Always happy to do business. Mylo Charlesworth, Purchasing and Warehouse Operative, CNC Rotary.com

In fifteen years of dealing with Pennine Lubricants, I have found the service to be impeccable. Jeremy Gosling, Director, Fine Grinding LTD

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