Virtually all cast metals that need to retain liquids or gases or prevent the ingress of moisture through porosity can be impregnated, including:
What size porosity can be sealed?
The totally variable nature of porosity through the wall of a casting makes it impossible to state what size void can and cannot be sealed by impregnation. The low viscosity and low surface tension of the sealant will prevent it from staying in large defects during the draining, washing and curing stages, and as such modern day impregnation sealants can only seal micro-porosity. This can be described as a ‘fine stream of bubbles’ when castings are tested air under water, or ‘weepers’ when castings are pressurised internally with water. Gross defects in castings are described as ‘gushers’ and cannot and should not be sealed by impregnation.
When is the best time to impregnate?
This depends whether machining operations will open up porosity that is not accessible for impregnation in the as-cast state, and which could result in a leak path through the casting wall. This is typically the case with die-castings and therefore the best time to impregnate is after machining. On the other hand impregnation before machining can be effective for sand castings and powder metal parts, which have more open porous structures.
Can castings be pressure tested immediately after impregnation?
Yes. The sealant polymerisation (curing) process takes only 2-4 minutes at 90°C and the cycle time in the hot water cure tank of an impregnation system is typically 10 minutes, which includes time for the castings to heat up from ambient temperature to 90°C. As soon as the castings have cooled down after removal from the hot water cure tank they can be pressure tested. If castings fail on pressure test straight after impregnation but pass if left for a couple of days before testing, this should be a concern to the customer as it will be due to inefficient impregnation. The main cause of this is high shrinkage of the sealant during curing and the retention of water in the porosity that is slowly absorbed into the sealant over a few days causing the sealant to swell and fill the porosity. The result will be a sub-standard seal that could break down in service with expensive consequences. MIL follow strict and thorough procedures to ensure their sealants are maintained in accordance with the original specifications. Castings impregnated by MIL can therefore be tested straight after impregnation and the sealing performance will not deviate over time.
Why is sealant often left in blind holes and machined threads after impregnation?
Sealant contamination on impregnated castings is due to ineffective cleaning methods in both the cold water wash tank and also the hot water cure tank. This problem is typical of using older impregnation equipment which does not have a good mechanical washing process and processes large capacity baskets where multiple layers of parts mask the washing action of the parts in the middle of the basket. Castings with hardened sealant in machined holes and threads will need to be re-machined resulting in additional costs and production delays. If the sealant contamination is not noticed during inspection and not removed before assembly, the sealant could break away in service and block water, oil and fuel ways causing the seizure of an engine.
Midland Impregnations Ltd have invested in the latest equipment that features very effective washing technology and processes smaller loads on faster cycle times. The result is that cleanliness is guaranteed.