The equipment used to manufacture the nickel shells tested has been as follows: 

• Polypropylene tank: 600 mm × 400 mm × 500 mm in size。

• Three teflon resistors, each one with 800 W。

• Mechanical stirring system of the cathode。

• System for recirculation and filtration of the bath formed by a pump and a polypropylene filter。

• Charging rectifier。 Maximum intensity in continuous 50 A and continuous current voltage between 0 and 16 V。

• Titanium basket with nickel anodes (Inco S-Rounds Electrolytic Nickel) with a purity of 99%。

• Gases aspiration system。

Once the bath has been defined, the operative parameters that have been altered for testing different conditions of the process have been the current density (between 1 and 22 A/dm2), the temperature (between 35 and 55 °C) and the pH, partially modifying the bath composition。论文网 

4。 Obtained hardness

   One of the most interesting conclusions obtained during the tests has been that the level of hardness of the different electroformed shells has remained at rather high and stable values。 In Fig。 2, it can be observed the way in which for current density values between 2。5 and 22 A/dm2, the hardness values range from 540 and 580 HV, at pH 4 ± 0。2 and with a temperature of 45 °C。 If the pH of the bath is reduced at 3。5 and the temperature is 55 °C those values are above 520 HV and below 560 HV。 This feature makes the tested bath different from other conventional ones composed by nickel sulfamate, allowing to operate with a wider range of values; nevertheless, such operativity will be limited depending on other factors, such as internal stress because its variability may condition the work at certain values of pH, current density or temperature。 On the other hand, the hardness of a conventional sulfamate bath is between 200–250 HV, much lower than the one obtained in the tests。 It is necessary to take into account that, for an injection mold, the hardness is acceptable starting from 300 HV。 Among the most usual materials for injection molds it is possible to find steel for improvement (290 HV), steel for integral hardening (520–595 HV), casehardened steel (760–800 HV), etc。, in such a way that it can be observed that the hardness levels of the nickel shells would be within the medium–high range of the materials for injection molds。 The objection to the low ductility of the shell is compensated in such a way with the epoxy resin filling that would follow it because this is the one responsible for holding inwardly the pressure charges of the processes of plastics injection; this is the reason why it is necessary for the shell to have a thickness as homogeneous as possible (above a minimum value) and with absence of important failures such as pitting。 

Fig。 2。 Hardness variation with current density。 pH 4 ± 0。2, T = 45 °C。

5。 Metallographic structure

   In order to analyze the metallographic structure, the values of current density and temperature were mainly modified。 The samples were analyzed in frontal section and in transversal section (perpendicular to the deposition)。 For achieving a convenient preparation, they were conveniently encapsulated in resin, polished and etched in different stages with a mixture of acetic acid and nitric acid。 The etches are carried out at intervals of 15, 25, 40 and 50 s, after being polished again, in order to be observed afterwards in a metallographic microscope Olympus PME3-ADL 3。3×/10×。 

   Before going on to comment the photographs shown in this article, it is necessary to say that the models used to manufacture the shells were made in a FDM rapid prototyping machine where the molten plastic material (ABS), that later solidifies, is settled layer by layer。 In each layer, the extruder die leaves a thread approximately 0。15 mm in diameter which is compacted horizontal and vertically with the thread settled inmediately after。 Thus, in the surface it can be observed thin lines that indicate the roads followed by the head of the machine。 These lines are going to act as a reference to indicate the reproducibility level of the nickel settled。 The reproducibility of the model is going to be a fundamental element to evaluate a basic aspect of injection molds: the surface texture。