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48 | VACUUM IMPREGNATION www.mmcpublications.co.uk

No need to turn up the pressure

According to the following contribution from Ultraseal International, there is no need to turn up the pressure on vacuum
impregnation with modern recycling sealants.

Diecasters the world over have long used vacuum
impregnation with porosity sealants to improve the
quality of die cast components and to ensure that

they are leak-free. Indeed, the process has been
around for decades.
Porosity in cast metal parts is a natural
phenomenon that arises during the casting process

for a variety of reasons and it is extremely difficult
to eliminate by purely controlling the casting
conditions alone. Early on in its adoption, vacuum
impregnation was viewed as a remedial process for

recovering defective castings. Today, after decades of
chemical and equipment developments, vacuum
impregnation is widely adopted and considered an
integral part of the component production process.

Evolution of impregnation
At its heart, impregnation is simple. Porosity Ultraseal full closed loop impregnation system.
comprises microscopic holes in a cast metal part, disposal of waste water are strict.
a few specific applications. However, modern

which can lead to serious problems, most As the name suggests, up to 95% of recycling
recycling sealants, which have low viscosity,
commonly the creation of leak paths that sealant can be recycled, in stark contrast to the perform outstandingly well with a straightforward
ultimately can cause a part to fail in the field and performance of non-recycling sealants, where much dry vacuum impregnation process, without any
result in costly warranty issues for manufacturers.
of the chemical is simply washed down the drain need for applying added pressure.

Porosity sealants fill up the holes completely; they with the wash water. Recycling sealants also Research carried out by the University of
enter the porosity in liquid form and are then produce minimal waste water, as the wash water is Plymouth (‘Modelling of casting impregnation
cured, or set, into a solid, impermeable, durable recycled.
using a single open pore model’, by Dr G Peter
plastic, without affecting the dimensions or surface ‘Recoverable’ sealants: Some non-recycling Matthews, 2003) for Ultraseal International has

finish.
sealants are described as ‘recoverable’ but a crucial shown that the effect of adding pressure during the
distinction has to be made because recoverable impregnation process has a negligible effect for
sealants are not the same as a recycling sealant. porosity down to one micron in diameter (one
They simply do not deliver the benefits or superior thousandth of a millimeter).

performance of recycling sealants.
All sealants, including anaerobic ones, can be
‘recovered’ to some extent by using a centrifuge
after the impregnation stage and before the

washing stage but the volume of sealant recovered
is extremely low. Moreover, with non-recycling
sealants, the wash water has to be replaced after a
limited number of cycles to prevent contamination

and a large amount of waste water is produced.
Recycling sealants are now adopted globally
Example of a cast metal component with porosity.
within the automotive supply chain by OEMs and The effect of pore size and overpressure on pore filling times.
Anaerobic sealants: Outdated anaerobic sealants, suppliers. With global OEM and industry Dr Mark Cross, Technical Director of Ultraseal

which set slowly at room temperature, are still used approvals, recycling sealants are the natural first International commented: “Application of
in some parts of North America and Mexico. choice for the impregnation of powertrain and overpressure serves to reduce sealant penetration
However, these can take up to a day to cure, other component types.
time but has no practical benefit to the process as
making them unsuitable for just-in-time typical penetration times are in the order of less

production lines. Anaerobic sealants also require Choice of process
than five seconds at ambient pressure.”
constant refrigeration, aeration and reactivity With modern thermocure sealants, there are two Using well-established theories of fluid flow and
checks.
choices of process; historically dry vacuum with known mathematical relationships, a model of pore
Thermocure sealants: Most manufacturers now pressure was popular but nowadays, the simpler impregnation was created to allow the variables of

prefer modern methacrylate-based, thermocure dry vacuum process is producing excellent results, the impregnation process to be investigated for the
sealants, which were introduced in the 1970s. These thanks to the physical properties of modern effect on pore penetration. The model
cure within a few minutes when exposed to heat recycling sealants.
demonstrated that at ambient pressure it would
under controlled conditions.
Back in the 1960s, many porosity sealants were take less than one thousandth of a second for a
Recycling impregnation sealants: The next truly far more viscous than their modern counterparts modern recycling sealant to fill a porosity 0.5mm

significant advance in technology was the and in order to ensure that the sealant was across and 2.5mm deep. For porosity of the same
development of recycling sealants, which use less thoroughly driven into any porosity, the practice depth but 500 times narrower (one micron across),
water, less chemicals and produce far less waste was adopted of applying pressures of up to six bar it would take only a few seconds.
water than non-recycling sealants. These sealants immediately after the application of sealant to force While theoretically adding pressure would speed

are particularly suited for use in territories where the liquid into all the cavities. This practice is the process up, the advantage gained would be
water is expensive or difficult to obtain, such as known as ‘dry vacuum with pressure’ and is still negligible.
parts of Mexico, or where regulations about the
used in some countries, including the USA and for
www.ultraseal.co.uk

Cast Metal & Diecasting Times January/February 2014