Cleaning metal and plastic parts using an ultrasonic cleaner.

In my search for finding methods of cleaning parts as good and easy possible when cleaning a pinball machine, I finally bought an ultrasonic cleaner. This is a great machine. It's an excellent way of cleaning parts, making them look new, without wasting many time doing manual labour. I let it run when I shop a pinball machine, run small batches whilst I disassemble new parts. That way I save a lot of time as cleaning happens whilst I'm removing parts. It's not a manual action to do after removing all parts which adds to the time. If you often shop a pinball machine and want the best results, this machine is a must have !

I had heard the past years already about ultrasonic cleaners. Heard good things about it, but mostly just that they were very good to remove crud of plastic parts like star posts. But they were specialised machines, only used by ie juwelers, and not easy (or cheap) to find. Usually I put these parts in the dishwasher, which also gives reasonable good results and is easy to do. Sometimes I let plastic parts soak in a strong cleaning solution and cleaned them manually using a brush. So I wasn't in an immedate need for a new solution.

When I had dropped off my Gottlieb Majorettes pinball machine to have it repaired by Henk De Jager, Ramon and Henk showed me another Gottlieb woodrail they had just shopped and cleaned. Many parts looked like new, including the starposts and pop bumper parts (plastic skirt and metal ring). I had tumbled the metal starposts on my machine and the result was similar. But their plastic parts looked better. Seeing the result convinced me to finally also invest in such a machine. Nowadays a lot of shops offer these machines for sale for home use, and prices have decreased.

How does an ultrasonic cleaner work ?

An ultrasonic cleaning machine uses, as its name implies, ultrasound to clean parts.
Very simplified: ultrasound waves (you cannot hear them) makes a fluid vibrate very fast. This generates microscopic small bubbles which implode. The energy of the collapsing bubbles loosens dirt. Dirt doesn't stick to the objects surface as strong as before.

Guidelines in using an ultrasonic cleaner

The objects to be cleaned have to be submerged completely in the cleaning fluid.
More heavy objects (with more mass) take more time to clean, as they absorb more energy. Therefor it's better to run small batches than to fill the container completely.
I also try to put similar parts together and not mix light plastic startposts with heavy metal parts.

When the object is too large to fit your machine, you can still clean it. Put the object to be cleaned in a large container (like a pyrex glass) and submerge it with cleaning liquid. Put this in the tank and fill the rest of the tank (the machine may not run with too little cleaner solution). The ultrasound vibrations will transfer through the liquids inside the container and clean the large part. Because of the extra mass (larger part and more liquid) it may take longer than usual.

Ultrasonic cleaners have a timer. Mine goes to 480 seconds (8 minutes). I usually let it run this long. Sometimes it is not enough and I give it another 8 minute run. The machine isn't completely silent, it makes a high-pitched sound. Not a pleasant sound you want to hear all day. But you'll like its results !

All dirt will losen its bond with the surface of the object. It will partially dissolve, but sometimes also stay on the surface. Therefor it is important after cleaning to immediately remove the parts from the machine and rinse them with clear water.
If you leave the dirt in the cleaning liquid, floating dirt will attach again to the part. Removing the parts from the cleaner machine without rinsing will leave a coat of dirt which will dry.

So always immediately rinse all dirt off with clear water. Some parts may already look clean when they come out of the tank. Small dirt will dissolve in your cleaning liquid. Parts that had thick oil or grease on it may still look dirty. It doesn't get dissolved but you'll notice if you rinse them (or wipe off with a brush) the containment doesn't stick anymore to the part but wipes off.

Take into account that ultrasounds can break up some types of glue. Also be careful with pinball parts that have paint on them, especially with plastics of older types of pinball machines. It could very well be that you're left with only a clear plastic and all the printed artwork has come off...

What cleaning solution to use in an ultrasonic cleaner

The manual of my unit said to use distilled water. While this works for very clean parts (or to clean fragile parts like jewelry or glasses), water alone is just not good enough to remove the usual dirt found on pinball machine parts.

I've tested a few types of solutions. My goal was to search for one versatile cleaner that can be used on all types of parts (plastic and metal). If you only will use an ultrasonic cleaner one specific type of material (plastic, brass, metal, chrome, ..) there may be better solutions and procedures. Some people ie use a run of vinegar, then baking soda, then regular and then distilled water on brass parts.

My first try was dishwasher soap. This does helps a bit, as everything that breaks the surface tension of the liquid helps dissolve the dirt into the water, so it doesn't stick to the part anymore. However dishwasher soap isn't very effective in an ultrasonic cleaner.
The manufacturer also doesn't advice to use it. The vibrations will generate bubbles in soap. A little bit of soap isn't strong enough to break the surface tension of all dirt, and you can't fill the whole tank with undiluted soap because the foam may become too big.

You can buy specific liquids for use in an ultrasonic cleaner. Some are even specific designed for pcbs. Unfortunately some of them are quite expensive (especially with shipping, and if you need a lot of it).

My goal was to search for an industrial, non-foaming cleaner. I found a great one at Makro (Metro in the UK). It's cheap (less than 5 euro for 5 litre). It's very strong, you can dilute it, and it's versatile, as it's useful for both metal and plastic parts.

It seems Metro / Makro has changed the packaging to this new layout.

When you look for a local industrial cleaner, it's important to note that the label specifies it contains less than 5 percent of soap. Note if the instructions warn you not to use on specific types of metal..
You may also not use any flammable liquid in these machines.

Anyway, I'm happy with the industrial cleaner I found. I dilute it half with water and this works really well. I've just shopped my Cyclone pinball machine and all parts I put in come out like new. All star posts and other plastic parts, small metal parts, .. I'm impressed how clean and shiny they all look ! I really should have bought this machine many years ago !

Health risks of ultrasonic cleaners

Ultrasonic cleaners are sold for home use and industrial use. They are relatively safe, but some precautions must be taken. The energy and associated dangers increase with the power of the unit (Hz and dB). Be more careful with a large powerful industrial type than with a tabletop home model.

Ultrasounds in the liquid don't transfer very well through the air or heavy mass. Keeping the lid closed, and positioning the unit on a sturdy table should be enough to protect you from any danger.

Do try to avoid touching the unit (and especially the cleaning fluid) when the unit is running. If you want to reposition an object in the liquid, first turn the machine off.

Just use common sense - the soundwaves can transfer through mass into nearby liquids. It's not a good idea to have it running for many hours next to your fish tank ie..

Ultrasonic cleaning compared to a tumbler

In the past I cleaned metal parts using a tumbler. So how do both machines compare to each other ? An ultrasonic cleaner is not a replacement for a tumbler, but it's a good addition. Both have their advantages and disadvantages.

A tumbler doesn't only clean but also polishes metal. Ultrasound only cleans.
The ultrasonic cleaner is more versatile - it doesn't only work on metal parts but also on plastic parts.

Metal parts that have been tumbled usually need to be cleaned afterwards as they're covered in the dust of the corn media. This dust will get in every small hook and is difficult to remove manually. Therefor an ultrasonic cleaner is very useful.

A good workflow is to first clean metal parts in the ultrasonic cleaner, to remove all dirt and thick grease using a strong cleaning solution. Then use the tumbler to polish them. This will go faster as the part is already clean. Finally again ultrasound cleaning (using water will do) to remove the fine dust left behind after tumbling.

If I would have to select between both machines, I'd pick the ultrasonic cleaner. You can also use it on plastic parts and it takes less time. I'm extremely happy with its results, everyone should get one !