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Core plugs: bonded or threaded, it’s the seal that counts

11 October 2012

When an engine block or cylinder head is cast, the process involves the use of sand to form the internal coolant passages and lubricant channels, and this sand is subsequently removed via external holes or 'cores' in the casting. Clearly, these have to be capped to seal the internal cavities and achieve a totally leak-free structure - hence the 'core plug'. The problem arises: how to secure the plug in the core.

In the 1970s the automotive industry switched to anaerobic cup plug bonding in preference to the more costly method of screwed plugs; Ford was a typical early adopter. However, regardless of whether the core is closed with a cup plug or a threaded assembly, it needs to be sealed effectively and remain leak-free despite the harsh operating conditions to which it will be subjected. Henkel's anaerobic retaining products are widely used by Ford for this purpose, not just in the UK but for its engine production throughout the world.

At its Bridgend plant, for example, Ford uses Loctite 243 threadlocker to seal core plugs during the manufacture of the Sigma SI6 short, six cylinder engine. This thixotropic product not only achieves a watertight cup plug seal, it also lends itself to process automation for which Henkel developed the ‘Rotaspray’ system. This applies the product via a spinning cup to the inside of the core - a simple but clever technique that results in a highly uniform bead of sealant being applied.

As well as sealing casting core plugs, this method is also suitable for the permanent sealing of any hole, even those at the ends of a hollow bar. Tubular material is increasingly being used in structures to reduce weight but if the ends are left open, there is danger of water ingress. Anaerobic cup plug bonding is a simple and cost effective solution to this problem.

HVAC follows the automotive lead
On seeing how almost the entire automotive industry had switched to cup plug bonding, the boiler industry started to question its own methods. Was it applying an over-engineered solution that unnecessarily added to production costs? If the automotive industry could justify the use of cup plugs on components handling water glycol at around 110?C then surely that method would be suitable for their structures containing water and additives at no more than 90?C?

Traditionally the boiler manufacturing industry had steadfastly used threaded assemblies to seal cores. “This certainly did the job but was an expensive solution,” says Henkel’s technology specialist, Bob Orme. “In high pressure applications threaded plugs are the clear choice but where pressures are less than 5bar, an efficiently sealed cup plug is entirely suitable.”

In recent years Henkel has provided the adhesive expertise that has helped many boiler manufacturers make the transition from screwed to cup plugs. And the savings that the industry has made as a result of the switch have been significant. Early estimates suggested that around £1 was being saved per plug but there have been other benefits, as Bob Orme explains:

“Although a sealed cup plug still needs engagement length it doesn’t require the same mass as a threaded plug. As a result, the walls of the structure can be thinner and carry less weight. The performance is the same despite lighter construction and reduced costs.”

For both core plug sealing methods users can call on a number of products in Henkel’s anaerobic retaining range. Among the most popular is Loctite 243 (the threadlocker that Ford is currently using at Bridgend). This works on all metals including passive substrates such as stainless steel, aluminium and plated services. Another is Loctite 648, which provides the additional benefit of increased temperature resistance.

All products in this particular Loctite range are widely used to secure threaded fasteners against vibration or shock loads; however, as we have seen, these benefits are equally applicable to cup plug sealing. They are easy-flowing or semi-solid adhesives which completely fill the gaps between the mating faces, preventing unwanted movement, loosening, leaks and corrosion.  

As Bob Orme points out, while improvements in casting techniques are minimising the number of cores needed per casting, sealing those that remain is nonetheless critical to the engine’s future performance.

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