History of Casting Impregnation

Peter Young's historical account

History of Casting Impregnation

Weather, Time and Salamonic

Casting porosity has always been a problem and no doubt will always be a problem. In days gone by, cast iron and steel would be left out to weather. If that did not work then the casting was filled with salamonic and forcibly rusted – it was as simple as that! The problem became acute when aluminum was introduced at around the turn of the last century. A somewhat inert material not succumbing to oxidisation in the same away as iron.

Linseed Oil

Certain hot dips such as linseed oil were regularly employed with varying degrees of success. Often the fault was only found after fully machining. The result of this was a highly contaminated component almost needing re machining before it could be used. During the war years the Rolls Royce Merlin engine cylinder block was treated this way.

Bathtub of Bakelite Solution

Brentford Works
Brentford Middx UK Circa 1946
click for larger view

My first introduction to impregnation was in the late 1940's when my father opened a job shop in Brentford Middlesex. There he had a bath tub bought from a neighbour who had it sunk in her back garden for her ducks. It was filled with Bakelite solution (normally used for winding insulation), an oven made from fire bricks, a metal bench and dish holding methylated spirits.

The castings were first placed into the oven and brought up to a temperature that was sometimes almost blue through lack of oven controls. The castings were removed with tongs and plunged into the varnish (that had been diluted with solvent) and allowed to cool. The object being that as the component cooled, the air in the porosity would shrink, drawing in the impregnant. How we never had a fire I do not know! – PDY

Recovering the components from the bath, they were then placed onto the metal table and washed with the mentholated spirits to remove surplus varnish. They then revisited the oven, this time to cure the residue drawn into the porosity. This in turn caused exudation by the out gassing of the solvent, which had to be removed using a knife whilst hot and replunged again into the varnish. This whole process generally occurred three times before a seal could be assured.

Autoclave and Vacuum Impregnation

Industrial Impregnations Ltd leaflet, 1956
Industrial Impregnations Ltd leaflet, 1956
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By 1948 components were being treated using vacuum. Components were packed into an autoclave and subjected to a vacuum, as we do today. During this time the sealant would be drawn into the vessel to flood the components. This would again be followed by washing and baking. Unfortunately, such varnishes were diluted with solvent that boiled out during stoving, leaving the casting contaminated.

Oven baking continued after impregnation with a gang of female workers scratching away at exudation removal before re-impregnation. Solvent washing by hand continued using Isopropanol alcohol. Some concern was given to health and safety with the provision of rubber gloves and aprons, but more often than not the gloves had holes in them. And ventilation was leaving the door open!

Sodium Silicate Sealant Gives Way to Polyester

In the USA sodium silicate was generally the chosen sealant and remained in limited use until recent times. It was resisted here in the UK, initially due to its poor chemical resistance. However, it must be said that it was attractive from the point of view that it was simple to apply, it was water soluble and it didn't smell!

Industrial Impregnations Ltd, Blackthorne Rd, Colnbrook
Polyester Impregnation Plant
Industrial Impregnations Ltd.
Poyle Trading Estate, Colnbrook, Bucks. UK
Circa 1962
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The 1950's Bring Polyester Resin

It wasn't until the mid 1950's with the arrival of polyester resin that we could contemplate fewer than three impregnations to achieve a regular level of sealability. The main advantage with polyester was that it was 100% polymerisable and therefore a higher level of sealant deposit in the porosity could be assured. Other advantages included the ability to wash off the sealant in hot detergent water rather than using solvents. The only problem being the smell of the obnoxious styrene being the diluent in the sealant and the regular blocking of drains.

Twenty Years Later, Was Polyester Resin History?

For Ultraseal, polyester continued in use right up to the time that PC 504 was introduced in 1976. Competing companies continued its use through to the late 1980's and there are indications that although somewhat rare now, it can still be found in use today. High cost, long processing times (in the order of one hour) of polyester, and environmental regulations eventually put an end to this era


Methacrylates continue to grow in popularity as the best all-round sealant. Developments of late have shown that there is much still to be learned and understood with the present sealant technology – with the emphasis increasingly being placed fair and square on application and sealant management.

Coming Of Age

For a variety of reasons the impregnation industry would appear to have taken a long time to have broken with tradition of the past. Perhaps it's because it has always been looked upon as a reclamation process that no-one wanted but everyone needed!

In PDY's time, Ultraseal invested a high proportion of its income in improving process performance with many landmarks to its credit, having been copied world wide – a form of flattery for which you receive no pay!

It is a unique industry to be in because there are no rules to speak of, there are guidelines, indeed approvals, most of which are written in the absence of first hand application knowledge and advised by suppliers led by commercial interests! The problem is simply that if you cannot see or feel what you're getting for your money how can you be sure that the process has been carried out correctly and that the components being treated are treatable?