A revolutionary approach to rotomoulding – Evolve
If there are no pinholes then there is a danger that the part has been overheated or heated for too long and this will result in brittleness. If there are too many pinholes then the part is ‘undercooked’.
The ideal condition is thought to be a small number of pinholes towards the inner, free surface although the optimum moulding conditions should be established by performing tests on the moulded part to verify whether the material (and the part) achieves the performance levels targeted.
Evolve - its mode of action
Evolve would speed up the coalescence of the powder in its molten phase by reducing the surface tension of the bubbles of air which are randomly formed during the sintering phase. This means that less heat (energy) is required for them to disappear. Evolve is not a flow promoter and would not have any effect on the rheological (viscosity) properties of the resin. Other factors may influence the sintering stage of the process such as particle size and its distribution, particle shape and how the heat is transferred from the oven (burner) to the polymer.
Test results
whenever the chemistry of a material is changed or even slightly modified, it is recommended to assess the mechanical and physical properties of the resin. In this study Revolve N-307 with and without Evolve were compared. To ascertain the impact properties of the Evolve N-307, rotationally-moulded hexagonal bins were produced at various moulding conditions on a Ferry RS1.90 carousel machine. The oven temperature was fixed at 250 °C and the oven time was varied.
The rotomoulded hexagonal shaped items were cut in square plaques (125 x 125 x 3 mm) and then conditioned at -40 °C for 24 hour before being impacted. The plaques were finally impacted in accordance to the falling dart method developed by ARM-I. The ARM Impact strength values at each of the moulding conditions are plotted in figure 1
The mathematical model applied to the falling dart test methodology also enables us to calculate the number of brittle failures. This number is plotted against cook time in figure2.
Fig. 1 ARM Impact comparison Fig. 2 Brittleness comparison
By evaluating the number of brittle failures it is possible to determine a range of moulding conditions within which a good quality resin has zero brittle failure. This determines the operating or processing window of a polymer.
By applying the Evolve technology of N-307 it is possible to observe that N-307 reaches the same impact energy but sooner and maintains the same operating window of N-307 without Evolve.
Gas consumption
Evolve technology can reduce the surface tension of the powder in the molten state. Therefore the powder, when molten, might require less energy to coalesce. To prove this, it was necessary to create a correlation between a material’s property and the temperature of the oven. In this study we measured the density of a moulded part and monitored the gas consumption required to cook the part at the optimum moulding conditions. N-307 with and without Evolve was used and hexagonal shape bins were moulded on the Ferry RS1.90 carousel machine. The time in the oven was kept fixed and the temperature of the oven was varied as illustrated in figure 3. For each moulding condition, the peak internal air temperature was also measured as shown in figure 4 and the gas consumed was recorded by an Actaris gas meter located on the back of the Maxon burner within the Ferry.
Fig. 3 Part-density vs. Oven Temp Fig. 4 Part-density vs. PIAT
All measurements taken during the analysis are listed in table 1.
Table 1 Summary of the measurement
Conclusions
As described in this article the Evolve technology influences the optimum moulding condition of a resin by affecting the sintering process. This leads to a number of potential benefits such as a reduction in the quantity of pinholes on the surface a moulded product, a reduction of cycle time with a potential increase of productivity per shift and reduction of gas consumption with a significant reduction of CO2 emissions. For further information about the Evolve technology please contact the author at [email protected]
