Temperature settings for cylinder and mold
Desmopan® and Texin™ should be processed at melt temperatures of between 190 and 220 °C. With some hard grades, a melt temperature of up to 240°C may be needed. The melt temperature ranges for the individual Desmopan® grades can be found in the relevant product information sheets.
Figure 1 shows guide values for the settings for cylinder and nozzle heating in relation to the Shore hardness. As a rule, small injection molding machines need a higher temperature setting than large ones.
Fig. 1: Temperature profile according to hardness range
The mold temperature has a major influence on the quality of the surface and the demolding behavior. It also affects shrinkage and internal (frozen-in) stresses in the final component. Mold temperatures of between 20 and 40 °C are generally employed. A number of modified and glassfiber grades of Desmopan® and Texin™ require mold temperatures of up to 60 °C in order to achieve an optimum surface finish.
With thick-walled articles, cooling down to approx 5 °C can bring a reduction in cycle time.
Optimum settings of the correct temperature are not only important for an easy and unobstructed processing. The correct processing temperature can also have, depending on the TPU – grade, a high influence on the final physical properties of the parts. The following graphs show exemplarily the stress at 100% strain, the ultimate tensile strength and the ultimate elongation as a function of the processing temperature. The study was done on an injection molding machine with 30mm screw diameter.
Valuation of clamping force
Clamping force ≥ mold opening force in kN =
projected surface in cm2 • mean cavity pressure in bar
The actual clamping force required is determined first and foremost by the two variables included in the formula. Over and above this, the clamping force is influenced by other factors, such as the rigidity of the machine and the mold, the design of the molded part, the permitted breathing, the processing parameters and the molding compound itself.
As a guideline from the experience, it is assumed that the mean cavity pressure is between 300 and 700 bar.
-Projected surface = sum of all the surfaces subject to pressure projected on to the plane of the clamping platen Example: truncated cone-shaped disc
Mean cavity pressure
For material grades with very good flow behavior, it may be necessary to use the higher pressures (greater clamping force) in order to prevent flash formation.