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(d) Removal of the shell from the model.
(e) Production of the core by filling the back of the shell with epoxy resin resistant to high temperatures and with the refrigerating ducts made with copper tubes.
The injection mold had two cavities, one of them was the electroformed core and the other was directly machined in the moving platen. Thus, it was obtained, with the same tool and in the same process conditions, to inject simultaneously two specimens in cavities manufactured with different technologies.
3. Obtaining an electroformed shell: the equipment
Electrodeposition [5] and [6] is an electrochemical process in which a chemical change has its origin within an electrolyte when passing an electric current through it. The electrolytic bath is formed by metal salts with two submerged electrodes, an anode (nickel) and a cathode (model), through which it is made to pass an intensity coming from a DC current. When the current flows through the circuit, the metal ions present in the solution are transformed into atoms that are settled on the cathode creating a more or less uniform deposit layer.
The plating bath used in this work is formed by nickel sulfamate [7] and [8] at a concentration of 400 ml/l, nickel chloride (10 g/l), boric acid (50 g/l), Allbrite SLA (30 cc/l) and Allbrite 703 (2 cc/l). The selection of this composition is mainly due to the type of application we intend, that is to say, injection molds, even when the injection is made with fibreglass. Nickel sulfamate allows us to obtain an acceptable level of internal stresses in the shell (the tests gave results, for different process conditions, not superior to 50 MPa and for optimum conditions around 2 MPa). Nevertheless, such level of internal pressure is also a consequence of using as an additive Allbrite SLA, which is a stress reducer constituted by derivatives of toluenesulfonamide and by formaldehyde in aqueous solution. Such additive also favours the increase of the resistance of the shell when permitting a smaller grain. Allbrite 703 is an aqueous solution of biodegradable surface-acting agents that has been utilized to reduce the risk of pitting. Nickel chloride, in spite of being harmful for the internal stresses, is added to enhance the conductivity of the solution and to favour the uniformity in the metallic distribution in the cathode. The boric acid acts as a pH buffer.
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.
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.