Critical Examination of GISS Applications
It works with every alloy, and it solves all your quality and tool wear problems. This is precisely the claim that the GISS technology approaches foundries. You don’t even need to say “abracadabra”.
The last newsletter caused a commotion when I compared GISS to the Rheometal process. Let’s give some additional information about my critique of the GISS process. I compared the semi-solid process, as I had previously posted, to the Rheometal process. With the low solid fraction of GISS, the advancement is minimal. The filling behaviour of the cavity is the same as that of liquid HPDC. Thixotropic properties don’t have an effect here.
Where is the flaw in the GISS process?
The marketing material of the GISS technology shows the following as the main selling points:
Quality improvements
Cycle time reductions
Spray cycle optimisations
Longer tool lifetimes.
All of these improvements should be achieved without changing the mould, alloy, or anything else.
When going through their handouts from conferences, they don’t even classify themselves as a Semi-Solid process. A more accurate comparison is actually with conventional HPDC, and let’s examine the marketing claims.
All the claimed advancements of casting through the GISS technology can be achieved by foundries with a 100% liquid alloy in conventional HPDC. The key is to have a proper thermal management system in the tool. Cooling the hotspots to avoid shrinkage porosity, and utilising cooling systems that can effectively remove the heat. When you do that, implement microspray at the same time.
With that combination, you will achieve a uniform die temperature, longer tool lifetimes, and a higher-quality casting. That has nothing to do with Rheocasting; that is just HPDC done right.
If shrinkage porosity is troubling you, consider redesigning your gating system to address this issue. For symmetrical parts, a three-platen tool addresses many issues.
Why is the main marketing message “no changes needed”? The world is constantly changing and evolving. Why is your tool design the only constant in our universe? The improvement is massive for foundries that have implemented these steps in their casting process. It is an even bigger step for their success story showcases.
In the presentations, they showed spray time reductions from 55 to 42 seconds and water volumes of 21 to 16 litres per shot. This is not normal! That is a washing cycle, not a spraying process! With microspray, you’re looking at a duration of around 10 to 15 seconds and a release agent volume of 1 to 5 millilitres per shot. The sprayed release agent doesn’t remove all the energy from the cool, and the cold spots in the tool steel cause thermal fatigue.
With the mentioned improvements to the peripherals in the original liquid HPDC process, you can eliminate 80 to 90% of the GISS applications where they currently sell the machines. The improvements in the peripherals have one-time costs, but you save massively afterwards. With the GISS technology, you will need to order new diffusers constantly. The one in the picture had to be replaced after 2000 to 5000 shots. That is an expensive subscription for your foundry.
And for Rheocasting?
When you want actually to utilise the thixotropic properties of Rheocasting, the solid fraction is too low to enable overall thixotropic properties. The mould still fills turbulently with GISS, and in leak-tight applications, every metal meeting is a channel for gas molecules to leak out. Additionally, you need long feeding times to achieve a shrinkage porosity of zero.
None of this is possible with the low solid fraction of GISS. You need a higher solid fraction of 35 to 45% for that, along with the changes to the peripherals I mentioned above, and some additional venting and vacuum channels. With these modifications, you have the perfect castings!
GISS is far from being physically capable of achieving that. So, that traps the process in the small intersection of the last bit of shrinking porosity, which causes me issues, and I don’t mind the diffuser bill. That is not a big subset for a commercial product.
Conclusion
That is why, to me, GISS is a compromise that doesn’t work well in either direction, despite its claim to be the best solution for everything. Upgrades in thermal management and microspray implementation significantly enhance the outcome in any HPDC process. Start there before touching any semi-solid process!
And for Rheocasting, the solid fraction is too low to achieve a meaningful effect. However, the technology is marketed as the ideal solution, which gives Rheocasting a bad reputation because of its lack of results.
This is why I built the Rheocasting Masterclass to teach the ins and outs of the Rheocasting process. Learn how Rheocasting works and how to utilise it profitably.
Share:
Casting Insights⚒️
Learn about new topics around the foundry industry each Tuesday.
Subscribe to the newsletter and be part of our community.
