Diederlik Hellingh
Business Unit Manager Prosthetics
GC Europe
As a high quality and future oriented company, GC is continuously making efforts to improve the quality of existing products and the development of new products. Based upon input from our customers, GC Europe started looking for a method which provides consistent quality and reproducibility when using GC’s casting investments.
The prerequisites for a stable investment material are:
- Stable input of raw materials
- Stable production process
- Reproducible analytical methods
- Standardized way of working
The first 3 points are controlled by GC’s production facilities in Leuven: talking and making agreements with raw material suppliers, checking and controlling each and every parameter of production, testing and validating analytical equipment to know the boundaries of the analytical methods used in the QC lab. These are hundreds of parameters which have to stay in balance to come to the same quality of product!
Unfortunately point 4 ‘Standardized way of working’ cannot be controlled throughout our production processes. There are a number of parameters which influence results and behaviour of all investments, many of these are human factors. As a company striving for non-stop improvements, Renfert developed the Twister evolution, a new high-tech mixing device which is able to exclude part of the human factors which can influence some of the final properties of our investment material, with special attention to expansion.
The Twister evolution is designed to perform premixing as well as vacuum mixing, in order to avoid dust and to standardize the mixing method. When using this device in a daily practice in the QC-lab at Leuven, we found it very easy to use. The most important factor is to determine a standard program and then always use this program. We tested the device extensively and found that even a slight difference in parameter can change the final properties, especially the expansion!
The first sequence of tests with the Renfert Twister evolution: 200g of powder was mixed with 44ml of liquid in a 500ml bowl at 23°C. Fujivest II, Fujivest Premium, Stellavest, Fujivest Super, Multipressvest as well as Fujivest Platinum were tested with different programs. The aim was to find one standard program for customers to use for all our GC investment materials, whereby the premixing is carried out by the device instead of by hand in order to avoid dust and the influence of human factors.
The following standardized program was found to be effective for all of our current investment materials when using 200g of powder:
The first challenge was to produce a homogeneous mixture without premixing by hand. It is of paramount importance that no powder is left on the mixing blades. We came to the conclusion that an interval of 15” gave optimal results. We also learned that an increase in premixing time meant shortened setting time and less expansion.
The effect of pre-vacuum is negligible for the setting time and flow – apart from with Fujivest II pre-vacuum where there is an influence on the expansion if 100% vacuum is not selected. 15 seconds pre-vacuum is recommended for optimal degassing.
The speed of the Twister affects setting and thermal expansion; they expand with the increase of the speed. The graphs below show the effect on Fujivest II:
Once again, please be aware that alternative combinations will influence the final product, especially expansion, because the pre-mixing time must be re-adapted to achieve a homogeneous mixture.
In the second sequence of tests we wanted to find the maximum powder load of the current Renfert Twister evolution mixing bowls. The same parameters were used as described above on the mixing device, only the premixing time was adjusted. We know from the previous tests that the premixing time will influence the setting time and setting expansion. It is very important to get a homogeneous powder-liquid mixture in order to create reproducible results. We recommend using 200g (+ 44ml liquid) of powder in a 500 ml bowl. When other amounts of our GC investment powders are used, please observe the following combinations in order to avoid inhomogeneous mixtures:
