CHAPTER 3 ELECTROPLATING OF FDM-ABS

Transcription

1 31 CHAPTER 3 ELECTROPLATING OF FDM-ABS 3.1 INTRODUCTION Electroplating can be referred to as, an electrodeposition process for producing a thick and consistent coating, using of metal or alloys, upon a surface by the act of electric current (ASTM International, 2003). The coating is produced for decorative or protective purposes, also enhancing specific properties of the surface. The surface can be either conductors, such as metal, or nonconductors, such as plastics (Helen H Lou & Yinlun Huang 2006). In many industrial applications, metallization of non-conductive surfaces play a vital role, as it lowers costs, allows more flexibility in designing of parts in comparison with parts fabricated by using metals (Kuzmik 1990). Therefore, Plating on plastics (POPs) has been developed and widely used in the manufacture of printed circuit boards (PCBs), automobile parts, electromagnetic interference (EMI) shielding application (Kuzmik 1990, Weng Landau 1995 & Viswanathan 1993), weight reduction, electrical conductivity, formability enhancements, high impact resistance and weatherproofing (Wimonrat et al 2011). Chemical vapor deposition (Long et al 2000), physical vapor deposition ( Bruyn et al 2003), metal-powder coating

2 32 (Warshawsky, et al 1989) and electroless plating are some of the commonly available methods for preparing plastics for electroplating. Electroless plating has been used for years for POPs due to the several advantages it offers like, low cost, easy coating of surfaces with complex shapes, building a platform for electroplating plastics etc., (Bruyn et al 2003, Oh et al 2002, Khoperia 2003, XU Zhi-mou et al 2000). Copper (Cu) and nickel (Ni) plating have been one of the most commonly used electroplating techniques for many of the applications as discussed in the previous chapters. As years so far, electroplating has also been used in the area of rapid prototyping. It is also to be noted that electroplating not only adds strength and rigidity to parts, but also decrease the effects of ageing caused by absorption of water or ultraviolet light; however, it may adversely affect feature resolution and tolerance (Zhou et al 2007). Saleh et al (2004) in their work on, Effects of electroplating on the mechanical properties of stereolithography and laser sintered parts, have concluded that the UTS and modulus of elasticity improved for SLA and SLS parts plated with Cu and Ni, with less of an effect on % elongation. Metalcoated RP models are being developed by several multinational OEM s for development of new products. Aerospace companies are using it for building large RP models to generate mock-ups of modules or vehicles in development. In the present research, the task of electroplating (Cu + Ni) FDM- ABS samples have been undertaken with the purpose of adding strength to FDM-ABS models for using in real time applications.

3 METALLIZATION OF FDM-ABS SAMPLES Plating on FDM-ABS samples are carried in two stages (1) Electroless (2) Electroplating. Metallization involves transformation of nonconductive surface into conductive or more commonly it is a technique of coating metal on the surface of a non-metallic objects. Figure 3.1 indicates the various steps carried in the metallization of plastics. Figure 3.1 Steps involved in the metallization of ABS Plastics Electroless plating on FDM ABS sample The present work incorporates electroless copper deposition, this process can be treated as an electrochemical reduction process or in other words, it is the reduction of metal ions by chemical reactions (Mishra & Paramguru, 2010). In the electroless deposition of copper on FDM-ABS substrates, the surface preparation and electroless bath composition plays an important role in electroplating of FDM-ABS substrates (Sam et al 2004). In the surface preparation stage the FDM-ABS samples were made to undergo pre-treatments processes like etching, sensitization and activation.

4 Etching The FDM-ABS samples were dipped in a vat containing concentrated chromic and sulfuric acid (H 2 SO 4 ) for an hour. The etching solution was prepared by mixing 220 gms of H 2 SO 4 and 440 gms of chromic acid (H 2 CrO 4 ) for a litre of demineralized (DM) water (H 2 O). Demineralized H 2 O is used deliberately to control the impurities. Necessary care should be taken while preparing the etching solution, H 2 SO 4 was initially mixed with demineralized H 2 O. Since, this mixing liberates lot of heat and hence the solution should be allowed to cool down. Later H 2 CrO 4 is mixed with the solution (DM H 2 O+ H 2 SO 4 ). For etching, the temperature was maintained in between C using temperature controlling devices. The etching solution creates small micro holes by selective removing of butadiene content from the surface of the FDM- ABS samples. The holes thus created acts as site for holding the metallic particles in activation and electroless process. Once the etching was over the specimens were rinsed with 30% hydrochloric acid (HCl) solution and later rinsed with DM water. Dipping in HCl and DM water helps in removing and neutralizing of etchants. If etchants are not removed, it may cause problems like skip plate due to the bleed out in subsequent stages Activation Activation process also known as catalyzing process. It is a process which is chemically activated layer / film is made to deposit on the etched FDM-ABS surface. In this stage FDM-ABS samples is ready for electroless plating. The activator solution comprises of two parts (Solution A and Solution B). Solution A is a mixture of 20 gms of tin chloride (SnCl 2 ) and

5 35 15 ml of HCl in a liter of DM water and Solution B is a mixture of 2 ml of silver nitrate (AgNO 3 ) and 5 ml of ammonia solution in a liter of DM water. The FDM-ABS etched samples are initially dipped in solution A for about 2-3 minutes and then rinsed in DM water. Then the rinsed samples are dipped in solution B about 2-3 minutes again and rinsed in DM water. SnCl 2 and AgNO 3 gets absorbed into the pores created due to etching. Further the copper electroless process copper is bonded with SnCl 2 and AgNO 3. Figure 3.2 represents the activation treatment for FDM-ABS samples. Figure 3.2 Samples dipped in a vat consisting of activator solution B Electroless plating The activated FDM-ABS samples are now subjected to electroless copper treatment. Figure 3.3 presents the electroless treatment for FDM-ABS

6 36 samples. During electroless treatment, a thin layer of copper by saying 1µm is developed. This gives the required conductivity for the further electroplating process. The electroless solution was prepared by mixing 8 gms of copper sulphate (CuSO 4 ), 35 gms of sodium potassium tartrate (KNaC 4 H 4 O 6 4H 2 O), 40 ml of formaldehyde (CH 2 O) and 10 gms of sodium hydroxide (NaOH) in a litre of DM water. Normal room temperature was maintained for electroless treatment. The FDM-ABS samples were dipped in the prepared solution for about 20 minutes. Finally electroless plated FDM-ABS samples were rinsed in DM water. Copper sulfate added to the solution acted as a source of copper in electroless copper solution, sodium, potassium tartrate added into the solution acts as a complexing agent for the formation of coordinate bonds with a metal atom or ion (Cu). Formaldehyde added acts as a reducing agent in the solution and Sodium hydroxide acts as a source of alkalinity in the solution. Figure 3.3 Samples dipped in electroless solution

7 Electroplating on FDM-ABS samples The electroless plating procedure was adopted for successful electroplating of FDM ABS plastics with apt conductivity. The electroless plated FDM-ABS samples are immersed in a vat containing a solution of metallic salts, connected to a positive and a negative electrode. The FDM-ABS samples acts as an electrode and the anodic conductor is chosen based on the type of metallic salt present in the solution (For example: copper anode if copper salts are present, nickel anode if nickel salts are present). For the deposition to take place on the FDM-ABS samples a potential difference is applied and due to this the anodic conductor dissolves into the solution and replaces the depleted metallic salts due to the deposition. Acid copper plating and nickel copper plating has been carried on the FDM-ABS samples. The details of electroplating have been elaborated in the following paragraphs Acid copper plating Acid copper plating or simply copper electroplating is a process in which copper from the anode is made to deposit on the cathode (electroless FDM-ABS samples) via the use of the electricity. Figure 3.4 shows the copper plated samples used in the study. Copper plating has two main functions in plating on plastics viz., levelling and elasticity / ductility. Levelling is important for the reason to have a clear bonding between electroless copper and electroplated copper, which helps in having a uniform deposit. The most important property of copper is its ability to withstand contraction and expansion during varying temperatures. Copper s ductile property makes it possible to move with ABS plastics when exposed to varying temperatures. Also the coefficient of linear thermal

8 38 expansion of ABS makes it possible to sustain the temperature variation (Joseph R Arnold, 2013). An acid copper plating solution was prepared by mixing 30 ml of H 2 SO 4 and 200 gms of CuSO 4 in a litre of DM water. 1 ml of coumarin (C 9 H 6 O 2 ) and 1 ml of sodium allylsulphonate (C 3 H 5 NaO 3 S) were also added as additives. The C 9 H 6 O 2 and C 3 H 5 NaO 3 S acts as brightener and leveller solutions. Which are basically organic compounds. Oniciu & Muresan (1991) have noted the use of a leveller and brighteners solutions. They describe levelling as the capacity of an electroplating solution to produce coatings of fairly thick deposit in small depressions and comparatively thinner deposits in undersized protrusions with an objective of having uniform deposits throughout the surface. Brightening is defined as the solution s (electroplating) ability to produce fine deposits of crystallites smaller than the wavelengths of visible light. The prepared solution with all the chemicals added is stirred with a stirrer manually to obtain homogeneity. Then the electroless plated FDM- ABS samples are immersed in the vat of acid copper. The operating temperature for the acid copper bath was maintained in between C. A current of mamp / cm 2 was applied. The current efficiency was between 95 99%. The electroless plated samples, depending on the requirements of electroplating thickness was kept dipped in the vat. The deposition / coating is a time dependent process and it was observed that for building 1µm layer, the time required was a minute. Therefore, as an example for building 5 µm layers it took 5 minutes. All the samples irrespective of nickel layer thickness had a copper coating of 5µm thickness. The acid copper plated samples were then rinsed in 30% HCl solution to remove the excess

9 39 solution adhering to the surface. Finally the samples were dipped in a DM water. Figure 3.4 Copper plated samples

10 Acid Nickel Plating The FDM-ABS copper coated samples were dipped in the vat containing acid nickel solution. Figure 3.5 shows the nickel coating treatment on copper plated samples. Figure 3.6 and 3.7 show various nickel plated samples used in this research. The acid nickel electroplating solution was prepared by adding gms of nickel salt in a litre of DM water. The temperature of 45 0 C, has to be maintained for nickel plating. The anode (Nickel) was then placed in the vat containing acid nickel solution. A current density of mamp / cm 2 was applied with a current efficiency of 95%. Nickel plating is relatively a fast coating process and takes only one third of the time taken for copper plating. Therefore, for building 10µm thickness it approximately takes 5 minutes and similarly for building 30 and 50 µm it takes 10 and 15 minutes respectively. The nickel plated FDM-ABS samples were then rinsed with 30% HCl solution and then with DM water. Figure 3.5 Samples undergoing Nickel plating treatment

11 41 Figure 3.6 Rinsing of nickel plated flexure FDM-ABS sample. (a) Izod test sample. (b) Hardness test sample. (c) Flexure test sample. (d) Tensile test sample. Figure 3.7 (a) (d) Nickel Plated samples

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