Alternative materials in combination with metals are becoming more ubiquitous in our day to day life for their enhanced properties. But manufacturing such composite material objects have been difficult and costly. This is because the bonding between metal and other materials is a challenge due to various parameters. And further machining such components is cumbersome. Back injection molding is a manufacturing technique wherein this bond can be achieved with considerable strength and ease but optimum parameters like nature of adhesives, nature of material bonded, coating technologies, coating conditions are under researched. This thesis addresses the lacuna with the aim of optimizing the above said parameters for back injection molding technique for various materials. Preparation for back injection molding involves Lamination which is one of the methods for applying the heat activated film to the metal. Since adhesive properties are sensitive to external factors like temperature, cooling period and pressure, and nature of adhesive itself is a factor for effective bondage, a series of pre-testing followed by narrowing down to optimal parameters which ensures an effective bondage can be calibrated and standardized. The adhesive which ensures a maximum bonding strength for a specified metal-plastic combination can also be determined. Screen printing which is an alternative process of applying adhesive to the metal has further more variable parameters like adhesive composition, thickness of adhesives, curing temperature and time, mesh number. As applying and curing is time consuming, it is highly essential to find the optimum composition which meets the bond strength requirements with easy workability. Extensive evidence based testing has resulted in selecting the best adhesive composition for different metal-plastic combinations. These methods has resulted in finding the optimal parameters for obtaining the best possible output by using back injection molding for different metal-plastic combinations. With effective implementation of the results of the thesis, manufacturing metal-plastic hybrid materials can be made more efficient and effective in terms of product quality and considerable reduction in cost and time savings.
Keywords: Adhesives, Heat activated film, Lamination, Screen Printing, Back injection, Metal plastic combination, Bond strength.
Wilhelm Gronbach GmbH
Wilhelm Gronbach GmbH has been one of the specialised companies in metal working, anodising and powder coating. The company was founded in 1962 by Mr. Dipl.-Ing. Wilhelm Gronbach which started manufacturing first parts under patents for freezer hinges and started producing components for domestic appliances. Now, Gronbach GmbH is a leading innovator in the field of surface finishing and stepped in wide variety of sectors, for example in domestic appliance, consumer electronics and automobile.
The company has four branches of specialisations in Germany, Austria, Italy and Slovakia. The site in Wasserburg am Inn, Germany controls supply chain, like product development, metal processing and on to the surface finishing using anodising and powder coating. It helps the company to create Innovation through systems and be the competence centre for surface technology.
With outstanding performance, professional collaboration and reliability, the company does not want to merely satisfy the clients, but want to delight them. Some of the many clients that trust on Gronbach are brand names like AEG, BORA, B/S/H, Miele etc.
1. Introduction 1
2. Scope 3
3. Aim 4
4. State of Art and background 5
4.1. Material 5
4.1.1. Stainless steel 5
4.1.2. Aluminium 5
4.1.3. Acrylonitrile Butadiene Styrene (ABS) 6
4.1.4. Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS) 7
4.1.5. Polybutylene terephthalate (PBT) 8
4.1.6. Polypropylene (PP) 8
4.1.7. Polyamide 6 (PA6) 9
4.2. Bonding methods 11
4.3. Adhesives 12
4.3.1. Film adhesives 14
4.3.2. Liquid adhesives 17
4.4. Lamination 20
4.5. Screen printing 25
4.6. Back injection 29
4.7. Testing method 33
4.7.1. Cross Hatch adhesion test 33
4.7.2. Adhesive bond tension test 34
4.7.3. Moisture exposure test 36
4.7.4. Surface energy test 37
5. Result and Discussion 38
5.1. Lamination result 38
5.2 Screen printing result 44
6. Summary and outlook 47
7. References 48
Table 1 specifications of various plastics 11
Table 2 Specification of the tested heat activated adhesive films 17
Table 3 Specification of the tested liquid adhesives 20
Table 4 Process parameters of lamination 24
Table 5 Process parameters of screen printing 28
Table 6 Process parameters of back injection 33
Figure 1 metal-plastic component using 1mm double-sided adhesive tape 1
Figure 2 metal-plastic component using back injection technique 2
Figure 3 Adhesive and sealant selection considerations 11
Figure 4 Bonding methods employed in the company Gronbach 12
Figure 5 Schematic representation of adhesive bonding two adherends 13
Figure 6 Joints with cohesive failure, adhesive failure and combination of both 14
Figure 7 Picture of Tesa HAF 8410 HS film with liner 15
Figure 8 Possible molecular structure of thermoplastic adhesive film 16
Figure 9 Effect of catalyst on process frame 19
Figure 10 Meyer KFK – L 400 lamination machine 21
Figure 11 Schematic representation of the conveyor 21
Figure 12 Schematic representation of lamination system used in Wilhelm Gronbach Gmbh 22
Figure 13 Sample before and after lamination 23
Figure 14 Process flow chart of lamination 24
Figure 15 Final sample after lamination and stamping 25
Figure 16 Picture of the mesh used 25
Figure 17 Screen printing machine 26
Figure 18 Working principle of screen printing 26
Figure 19 Ultrasonic thickness gauge 27
Figure 20 Punched and deep drawn sample after screen printing 28
Figure 21 Process flowchart of screen printing 29
Figure 22 Demag Ergotech 50 – 200 injection molding machine 30
Figure 23 Schematic representation of film insert molding (FIM) 30
Figure 24 Finished metal-plastic combined component after back injection 31
Figure 25 Testo infrared thermometer 32
Figure 26 example of cross hatch test values 34
Figure 27 samples tested using cross hatch method and the cross hatch cutting apparatus 34
Figure 28 Universal tensile testing machine 35
Figure 29 Sample prepared for tensile testing 36
Figure 30 Accelerated weathering apparatus 37
Figure 31 Accu Dyne test marker pens 37
Figure 32 Bond strength of Tesa HAF 8410 HS 39
Figure 33 Bond strength of Tesa HAF 58470 40
Figure 34 Bond strength of Nolax S22.2202 41
Figure 35 Bond strength of Nolax A22.5010 SP 41
Figure 36 Bond strength of Nolax TEX 2790 – 3 42
Figure 37 Bond strength of Nolax A21.2294 43
Figure 38 Bond strength of NoriPET 093 44
Figure 39 Bond strength of Noripress SMK 45
Figure 40 Bonding strength of Huehoco 351099 – 80L 46
Equation 1 Laplace-Kelvin equation of capillary 13
Equation 2 Equation to calculate bond strength 35
Back injection moulding is a hybrid technology of combining different materials into one component, thus achieving the properties of all combined material in one part. The process involves injection of a plastic material to a metal sheet and bonded with the help of a bonding agent, which is activated by the heat of the plastic melt. The bonding agent can be applied by two different methods depending on the type of the bonding agent. This project involves developing methods to bond different plastics to metal, testing the bond strength between the metal and the plastic and optimising the bonding effect by varying the parameters.
Figure 1 metal-plastic component using 1mm double-sided adhesive tape
The interest in this project started, when a new decorative design with metal-plastic combination was needed for a coffee machine cover panel. Previously the process of combining the plastic component with the sheet metal was done with a 1 mm double-sided adhesive tape. However, this method was not preferred as the resultant component is thick and it needs more time as the two components must be produced and processed separately and then combined together. The disadvantage of the adhesive tape is that bonding is not strong and 1 mm is too thick, that it makes an air gap between the metal and the plastic, where there is possibility of flow of water. Therefore, when looking for a new method with thinner bonding agent and integrated processing led to back injection technique to overcome this problem.
Figure 2 metal-plastic component using back injection technique
Back injection technique involves coating the metal sheet with heat activated bonding agent and then plastic melt is injected on the coated surface, which activates the adhesive to bond the plastic and the metal. The commonly used coating methods are powder coating, coil coating and so on. These methods are not cost effective because only finished coated products can be procured. Consequently, a new method of lamination and screen-print were developed to coat the adhesive on the metal within the company. This thesis engages in testing the effectiveness of this new methods by checking the resultant bond strength of the adhesives that are coated by these methods.
Since the industrial age, metal has been dominantly used because of its stable properties. Though metal has advantages like toughness, strength and so on, it comes with its own set of disadvantages say weight, corrosion etc. To tackle this, lightweight and composite materials were developed, but it lacked the physical strength and look of the metal. This lead to the hybrid combination of metal and plastics to bring out the physical attributes of metal combined with plastics in a light weight construction. Injection molding technique is a significant process method existing for plastic production and here in comes the need to further develop the back injection technique to join the plastics with the metal to produce one component with both the properties of metal and plastics.
The aim of this project is to select a method to bond different plastics to metal, test the bond strength between metal and plastic and to optimize bonding effect by varying the parameters.
The basic tasks performed are,
1. Choosing suitable methods for specific bonding
2. Selection of different adhesives (Bonding agents)
3. Determining the optimal composition for screen-print adhesives
4. Determining the process parameters
5. Selection of plastic that is most suitable for the respective method and the bonding agent
6. Possible way to achieve strong bond strength
4. State of Art and background
The most common metals used in metal stamping are steel and aluminium. Each material has distinct set of properties that is right for the end application. The cost, strength and malleability makes the two metals as a choice for production of various products and therefore, we choose these two metals for testing the back injection technique with various plastics. The plastics are chosen depending on the requirements, properties and its area of application.
All the materials used are explained in detail in further sections. 
4.1.1. Stainless steel
Stainless steel 1.4016 is commonly used stainless steel grade for everyday applications. Grade 1.4016 is a ferritic stainless steel which is magnetic in nature and has high strength, toughness, good formability and ductility that can be used in deep drawing process. It is a carbon alloy steel with 15-17% of chromium, which gives a good heat and corrosion resistance and hence, it is chosen for the testing purpose. Grade 1.4016 is typically used with a polished or brushed finish.  
Aluminium is an almost ideal substrate for adhesives and sealants. It has high surface energy and is very resistant to most environments. Deep drawn aluminium parts which are being used have high strength to weight ratio (1/3rd density than steel) with desirable properties such as light weight, corrosion resistance (thin film of naturally occurring aluminium oxide), high strength and durability. Cold working process like blanking has the effect of work hardening material. As the material is stretched into its final shape, structure of metal is altered which gives it the desirable greater strength than the origin metal. Because of its ductility (70 kN/mm2) it is well suited for stamping process. Aluminium is used with purity of 99.7% alloying with 0.2% iron and 0.1% silicon. The reason for using Aluminium 99.7 in testing is that it does not require painting or coating, but can be anodized to create decorative finish and to improve its already excellent resistance to corrosion. As a result, adhesive bonded aluminium joints are commonly used in the aircraft and automotive industries. Aluminium joints are also commonly used in adhesive studies and for comparison of different adhesive materials and processes.   
4.1.3. Acrylonitrile Butadiene Styrene (ABS)
Acrylonitrile Butadiene Styrene (ABS) is an opaque thermoplastic, amorphous polymer. Thermoplastics become liquid (i.e. have a “glass transition”) at a certain temperature (221 degrees Fahrenheit in the case of ABS plastic). They can be heated to their melting point, cooled, re-heated without significant degradation of material properties. Instead of burning, thermoplastics like ABS liquefy which allows them to be easily injection molded and subsequently recycled. Whereas, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy), resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials not suitable for recycling. ABS is also an amorphous material as it does not exhibit the ordered characteristics of crystalline solids. 
ABS contains discrete, cross-linked butadiene rubber particles (PB) which are grafted with poly (styrene-co-acrylonitrile) SAN and embedded into SAN matrix. The method of production include emulsion, mass suspension and mass polymerisation. There are several variables such as rubber particle size and distribution, amount of SAN grafted to rubber, the styrene to acrylonitrile ratio, the molecular weight, the cross-linked density of rubber and amount of rubber which are manipulated in various methods of production accounts to several ABS grades. 
ABS has a strong resistance to corrosive chemicals and/or physical impacts. It is very easy to machine and has a low melting temperature making it particularly simple to use in injection molding manufacturing processes or 3D printing on an FDM machine. ABS is also relatively inexpensive, typically fall somewhere between those of Polypropylene (“PP”) and Polycarbonate (“PC”). ABS plastic is not typically used in high heat situations due to its low melting point. All of these characteristics lead to ABS being used in a large number of applications across a wide range of industries. 
4.1.4. Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS)
PC-ABS (polycarbonate-ABS) is one of the most widely used industrial thermoplastics. PC-ABS parts are ideal for conceptual modelling, functional prototyping, manufacturing tools and end-use-parts. The PC/ABS property balance is controlled by the ratio of PC and ABS in the blend, the polycarbonate molecular weight and the additive package. The ratio of polycarbonate and acrylonitrile-butadiene-styrene affects mainly the heat resistance of the final product. PC/ABS blends exhibit a synergic effect resulting in excellent impact resistance at low temperatures that is better than impact resistance of ABS or PC. Main PC/ABS properties include High impact strength even at low temperatures, Heat resistance, high stiffness, easy processing, Low overall shrinkage and high dimensional accuracy, colourable & printable. PC/ABS resins fit into applications that require a high heat distortion temperature (95-125°C) and good toughness. They have excellent low temperature toughness, making them ideally suited for products that will see a broad range of temperature. 
The high heat distortion of the alloy is an improvement over ABS, while the low temperature impact resistance gives it an advantage over polycarbonate. By reinforcing the alloy with glass fiber, nickel-coated carbon fiber and/or stainless steel, required structural, static dissipative and EMI/RFI shielding application can be done.
The reinforcing material most commonly added to thermoplastics is glass fiber, since they have excellence stiffness and higher temperature application than unreinforced thermoplastics. 
The fibers are made by melt drawing of various grades of glass and are comprised of strands of filaments that can be further processed by size reduction, twisting or weaving into fabrics. They are often surface modified with coupling agents to improve bonding with plastic matrix and improve properties. 
4.1.5. Polybutylene terephthalate (PBT)
Polybutylene terephthalate (PBT) is a thermoplastic (semi-) crystalline polymer, and a type of polyester. PBT is resistant to solvents, shrinks very little during forming, is mechanically strong, heat-resistant up to 150 °C (or 200 °C with glass-fibre reinforcement) and can be treated with flame retardants to make it non-combustible.
Polybutylene Terephthalate is the synthetic thermoplastic compound of 1-4-Butylene glycol with DMT or PTA. There are two kind of PBT product – PBT Resin and PBT compound. PBT Resin is the base resin while PBT compound are combination of PBT resin, fiber glass filling and other additives such as UV protection agent, and flame retardant. They have properties such as fast crystallization, easy to mold, high deflection temperature, fiberglass reinforced, good mechanical strength, low water absorption rate, low shrinkage rate, resistant to chemical, low warpage and good electrical resistance. Both uncompounded PBT and compounded PBT with glass fiber 30 are used for testing purpose. 
4.1.6. Polypropylene (PP)
Polypropylene (PP) is a thermoplastic “addition polymer” made from the combination of propylene monomers. It can be used both as plastic and as a fiber. Polypropylene can be made from the monomer propylene by Ziegler-Natta polymerization and by metallocene catalysis polymerization. Polypropylene has remarkable properties, making it suitable to replace glass, metals, cartons and other polymers. These properties include low density (weight saving), high stiffness, heat resistance, chemical inertness, steam barrier, food protection, good transparency, impact/rigidity balance, stretchability (film and fibre applications), good hinge property, high gloss, easy to weld, recyclability. The majority polypropylene is produced as a homopolymer.  
For the purpose of experiment, both PP with Talc filling and Glass fiber filling are used. Mineral filled polypropylene provides high mechanical stiffness, thermal stability, good low temperature properties and good dimensional stability over a wide temperature range. Available as a natural colored, colorable, UV stabilized compound. 
4.1.7. Polyamide 6 (PA6)
PA 6 (unreinforced) is a tough and strong material affording parts with good damping characteristics and high shock resistance even in the dry state and at low temperatures. PA6 is distinguished by particularly high impact resistance and ease of processing. They have good chemical resistance properties and are perfectly suited for injection molding applications. It is very tough, having good impact properties which can be increased even at low temperatures by adding impact strength modifiers. Can be filled with glass fiber for high stiffness and rigidity. 
The material possesses excellent processing properties in combination with good general and thermal performance. It has greater impact resistance than polyamide 6.6 (PA6.6), at the expense of a lower modulus and higher moisture absorption. PA6 is excellently resistant to oils, greases and aliphatic and aromatic hydrocarbons, as well as to most halogenated hydrocarbons. It is not resistant to acids and alkalis. The UV stability of PA6 is better than that of most other engineering polymers. They have advantages such as excellent surface finish even when reinforced, Strength, Stiffness and Chemical resistance to hydrocarbons. They have limitations such as high water absorption and poor chemical resistance to strong acids and bases. 
The orientation and the surface properties of the Polyamide injection moulded part is determined by the filling phase. The aliphatic chain segments in the polymer chain of amides provides an amount of flexibility in the amorphous region. Flexibility in the amorphous zones and high inter-chain attraction in the crystalline zones provides the polymers to be tough above their glass transition temperatures. High melting point is because of the high intermolecular chain attraction. The melt viscosity is low above the melting point because of the polymer flexibility. At temperatures high typically, more than 200°C above the glass transition temperature. The relatively low molecular weight is also found to be relatively low. 
Since PA absorbs more moisture, they are employed with Glass fibers to reduce moisture absorption and to operate at high temperatures.
Table 1 specifications of various plastics
4.2. Bonding methods
In choosing the correct bonding agent and developing an optimized bonding method, there are several critical decisions that need to be made as shown in Fig. 3. Steps such as substrate selection need to be analysed and optimized with respect to their influence on the final, desired result.
Figure 3 Adhesive and sealant selection considerations
A significant problem is that each of these decisions cannot be made independently. The bonding method, for example, may influence the substrate selection and processing conditions that is necessary. Therefore, the adhesive selection must be made considering all of the parameters involved in the bonding method. 
The coating method primarily depends on the type of the adhesive used. The bonding methods that are used within in the company is shown in the fig. 4. The focussed methods are the lamination method and the screen printing method, as the warm press method comes with a pre-coated coil.
Figure 4 Bonding methods employed in the company Gronbach
Adhesive is a substance that is capable of holding at least two substrates together strong and permanently. Adhesive must work like a liquid during bond formation in order to wet (make contact with) the adherends. The adherends must have high surface energy to make good contact with the adhesive. They form surface attachment through the development of intermolecular forces between the surface and the adhesive.
When two surfaces are wet by a thin liquid (adhesive), as a result strong adhesion between the surfaces is achieved. This can be explained with the help of Laplace-Kelvin equation of capillary.
Equation 1 Laplace-Kelvin equation of capillary
Where p1 and p2 are respectively the pressure within the adhesive and the pressure in the vapour outside the adhesive; R1 and R2 are respectively the radius and half the thickness of a thin, circular layer of adhesive in contact with the solid; and γLV is the surface tension of the adhesive. If R1 is greater than R2, then p2 will be greater than p1. Hence, the two plates will be forced together because of the pressure difference p1 and p2. From this equation, it explains that adhesives which spontaneously wet two solid surfaces will hold them together.
Figure 5 Schematic representation of adhesive bonding two adherends
Adhesive function primarily by the property of adhesion. Adhesive materials generally have high shear and tensile strength. Adhesion is the intermolecular forces between two different substances which cause the attraction of the substances that hold it together. It is totally different from cohesion, which happens only by the intermolecular force of attraction within a single substance. For better understanding, joints that are failed either adhesively or cohesively or combination of both is illustrated in fig. 6.
Figure 6 Joints with cohesive failure, adhesive failure and combination of both
Adhesive failure is caused by the bond failure between the adhesive surface and the adherend surface. In adhesive failure, the adhesive either does not stick to both adherend surfaces or it just sticks to the one surface completely without sticking to other. Cohesive failure is caused by the failure or breakage within either the adhesive or the adherend. In cohesive failure, the adhesive is subjected to stress fracture and breaks internally while it’s still in contact with the both surfaces (bond strength to the adherend is stronger than the internal strength of the adhesive itself). 
4.3.1. Film adhesives
Film adhesives are thin heat activated adhesives that are coated by lamination process. The adherend must be resistant to heat to withstand the process. The film based adhesive is a dry film which is activated by the application of heat with or without pressure. It is dry and rubbery at room temperature, but becomes tacky or liquid by the application of heat and pressure. This kind of heat activated adhesive is also referred to delay tack adhesives, as it requires external energy to activate tackiness.  
The film used are relatively thin and have a paper support called liner on one side to avoid tackiness of the film to the lamination system. During lamination the adhesive tape is laminated on the metal surface and then the liner is removed to proceed next step of bonding the plastic to the metal. Also after lamination, the shelf life is not affected and it remains the same period of time as the shelf life of adhesive tape which is normally 12 – 18 months. This ease up the storage of the samples after lamination. Only few companies produce these kind of heat activated films. One of them is well known company “Tesa” which is a leading manufacturer of adhesive tapes and the other one is a Swiss based company called “Nolax”.
Two types of Tesa heat activated films are used for the experiment namely Tesa HAF 8410 HS and Tesa HAF 58470. Tesa HAF 8410 HS is a double-sided heat activated film (HAF) with a thickness of 60 μm. Tesa HAF 8410 HS is a heat activated double-sided amber adhesive film based on reactive phenolic resin and nitrile rubber, covered with a paper liner. The material is non-sticking at room temperature and can therefore be cut and punched well. For initial fixing, the activating temperature of the film is about 90 °C. For the next processing step, the product is reactivated under high pressure and temperature. After curing, the adhesive remains elastic and achieves an extremely high bond strength as well as excellent temperature and chemical resistance. Tesa HAF 8410 HS is mainly designed for implanting chip modules in smart cards. It is suitable for materials such as PVC, ABS, PET and PC and it fulfils the requirement of durability and good bond strength.
Figure 7 Picture of Tesa HAF 8410 HS film with liner
Tesa HAF 58470 is a black double sided tape which is a reactive heat activated adhesive with a thickness of 50 μm. It is also based on phenolic resin and nitrile rubber covered with a strong paper liner, but made especially for bonding of metal components to various plastic or metal surfaces. For example, bonding of Stainless steel or Aluminium to PC, ABS or PMMA. It is used in constructive bonding in electronic devices, decorative metal components and so on.  
Nolax produces various single layer and multilayer adhesive films depending on the requirement and purpose. Nolax has different adhesive film for different plastics and the film are produced by blown film extrusion or multilayer extrusion. The single layer film are thermoplastic adhesive based on modified polyolefin. The multilayer film has thermoplastic polyurethanes (TPU) on one side and modified polyolefin on the other side. 
Figure 8 Possible molecular structure of thermoplastic adhesive film
These film are soft, slightly rubber-elastic film for the injection molding of metal with polar plastics for bonding of incompatible substrates in sandwich panels (lightweight construction). The polyolefin side adheres to the metals, as well as PVC, PU, ABS, PA and the thermoplastic polyurethanes (TPU) side adheres to the plastic.
Both Nolax S22.2202 and Nolax A22.5010 SP are multilayer adhesive films with a thickness of 45 μm which are made especially for ABS and PC/ABS, where the former film is without a protective liner and the later A22.5010 SP is developed with a liner. Nolax TEX 2790 – 3 is a single layer adhesive film with a thickness of 65 μm which is made to bond Polypropylene PP with metal. Nolax A21.2294 is also a multilayer film with a thickness of 60 μm and it helps to bond Polyamide PA6 with metal. 
Table 2 Specification of the tested heat activated adhesive films
4.3.2. Liquid adhesives
Liquid adhesives are solvent based adhesives that require curing in room temperature or elevated temperature after coating it on metal and then reactivated by the application of heat and pressure. Liquid adhesives are normally classified into single component and multiple component systems. Single component adhesives have a pre-mixed components such as hardener or catalyst and therefore it eliminates the time required to metering and mixing the components. Also the pot life and process window is prolonged in single component adhesives, as it has the same time period of the adhesive itself. Pot life is the time taken for the mixed components to lose its chemical stability and its effectiveness of bonding. Process window is the time frame between drying and injection molding after the application of liquid adhesive. Multiple component adhesives comprise of two or more components that must be kept separate until just before the coating process. These components must be mixed in proper ratio before coating and once mixed, it has a limited pot life depending on the storage environment. The few of the various components that are used in multiple component system are base or binder, catalyst, retarder, hardener and so on.
The base or binder is the principal and largest component of an adhesive. The base provides the main characteristics such as wettability, strength, environmental resistance and curing properties. Catalyst is a component that causes the primary base to crosslink and solidify and it remains unchanged in the curing reaction. Unlike hardener, catalysts are used in small quantities as it is critical that it can result in poor bond strength if the base is over or under catalysed. Retarder is a critical component that can control curing rate, storage and working life of the mixed component. It is incorporated into the system to accelerate or decelerate the curing rate. Hardener is mixed to the base to promote the curing reaction by chemically combining with the adhesive and taking part in it. Hardener are chosen carefully to react with a certain base and helps to improve its curing characteristics. Heat activated liquid adhesives obtained from Proell and Huehoco were tested. 
NoriPET 093 and Noripress SMK are two solvent based adhesive inks developed by well-known screen printing ink manufacturer Proell. NoriPET 093 is a solvent based three-component screen printing ink for IMD/FIM technology. NoriPET has characteristics like formability, elasticity, temperature resistance in injection molding and so on.
The three components of NoriPET are base – NoriPET 093, catalyst – NoriPET 005 and retarder – NoriPET 097. Catalyst NoriPET 005 has to be mixed prior to printing in the amount of 1 – 3% and the mixture has a pot life of 8 – 12 hours. Retarder NoriPET 097 can be mixed in any ratio to obtain an optimized drying. To achieve optimum bond strength, drying must be done immediately after printing.
The important factors that influence the adhesion and peel strength of the injection molded parts are the amount of catalyst, process frame and drying conditions. Process frame (time gap between drying and injection molding) depends on amount of catalyst and melt temperature of the plastic. Generally the best adhesion is achieved when the melt temperature is high during injection. The effect of catalyst is shown in the fig. 9 and it can be seen that increasing the amount of catalyst does not improve process window and so an optimum range of catalyst must be used say 1 -2 %. 
Figure 9 Effect of catalyst on process frame
Noripress SMK is a new two component solvent-based bonding agent which is compatible with screen printing. It is tailor made for IMD/FIM technology and can be back molded for different resins like ABS, PA, PC and PMMA. The two components are the base – Noripress SMK and the hardener – 8125. Hardener 8125 must be mixed homogeneously in right proportion with the Noripress SMK without any air bubbles. Hardener stabilizes the inner structure of the bonding agent and therefore improving the bond strength and long term resistance. The pot life of the mixture is 12 hours and it must be dried immediately after printing. For best results, back injection must be done as soon as possible after printing and drying with Noripress SMK. The bonding also depends on the layer thickness of the printed adhesive. The adhesive activates when the melt temperature is more than 240 oC and the process window can be extended by using higher melt temperature. 
Huehoco 351099 – 80L is a single component thermoplastic adhesive layer based on a vinyl resin combination. 351099 – 80L is developed especially for co-extrusion process, but can also work for back injection technique. This adhesive is suitable for plastics such as ABS, PC and PMMA. After the application of the adhesive on the metal, it forms a thin yellow layer on the metal once it is dried. Since it is a single component adhesive, it has a pot life of 6 months and the process window is the same as the pot life. 
Table 3 Specification of the tested liquid adhesives
Lamination is a coil coating process used to coat the adhesive onto the metal by application of heat and pressure. Heat activated films are employed as adhesive coating. The lamination process is performed with Meyer KFK – L 400 lamination machine. The photo of the Meyer lamination system can be seen in fig. 10. Meyer lamination system can handle metal sheets or coil of maximum 1 mm thickness and 400 mm width.
Figure 10 Meyer KFK – L 400 lamination machine
The lamination process involves coating the adhesive on the metal by sending metal and adhesive film in a conveyor through the heating zone for a certain time under the application of pressure and then cooling it to get the laminated metal sheet as an end result. The paper liner that covers the adhesive helps to protect the conveyor belt from getting tacky and after lamination, the liner can be removed and the adhesive can be reactivated by application of high temperature and pressure of plastic melt during injection molding. The laminated sheet should not have any air bubble between the adhesive layer and metal surface.
Figure 11 Schematic representation of the conveyor
Figure 12 Schematic representation of lamination system used in Wilhelm Gronbach Gmbh
Fig. 11 illustrates the method taking place within the machine and Fig. 12 shows the entire coil coating unit with Meyer lamination system that is employed in the company Gronbach. From Fig. 11, the working of elements can be understood. Firstly, the height of the top conveyor belt can be adjusted manually when a thicker sheet needed to be coated. But normally for sheet within 1 mm, the height is set default as 0 mm.
Next, to adjust the pressure applied by the roller, two elements are required namely Pressure and Niveau (level). The pressure is normally set to maximum (50 N/cm2) and Niveau is adjusted to increase or decrease the pressure on the metal sheet. Niveau is the gap between the top and bottom rollers and increasing the Niveau, increases pressure on the metal sheet. The machine has two symmetrical heating elements on the top and bottom of the conveyor belt, where the temperature of the heating zone can be changed from room temperature to a maximum of 200 oC. The time through the heating zone is dependent on the velocity of the conveyor belt. Both heat and pressure is applied throughout the heating zone. Following comes the cooling zone, where the metal and adhesive still experience the pressure but the heat is removed by passing cold water through the cooling elements. The cooling elements are also placed symmetrically on both top and bottom of the conveyor belt. The velocity of the conveyor belt can be adjusted from 0.2 m/min to 9 m/min. This helps to increase or decrease the time taken for the metal sheet to pass through the heating zone. Ultimately, the finished laminated metal sheet with adhesive film comes out of the cooling zone.
The sample metal strip before lamination and after lamination with Tesa HAF adhesive is shown in the fig. 13. After multiple trials of lamination, the process parameters for different adhesives on both stainless steel and aluminium were figured out and the respective process parameters of lamination is presented in the table 4.
Figure 13 Sample before and after lamination
Table 4 Process parameters of lamination
After lamination of the adhesive film on the metal sheet, the liner is removed and the metal is punched and deep drawn to form the insert sample which is later back injected with plastic. The process flow chart is shown in fig. 14 and the final sample after lamination and stamping is shown in fig. 15.
Figure 14 Process flow chart of lamination
Figure 15 Final sample after lamination and stamping
4.5. Screen printing
Screen printing is a process of coating liquid adhesive ink on metal with the help of a screen (mesh). Mesh is a material made of a network of thread which act as a screen. The mesh and the screen printing machine are shown in fig. 16 and fig. 17 respectively.
Figure 16 Picture of the mesh used
Figure 17 Screen printing machine
The process involves pouring and applying the liquid adhesive over the meshed screen which stays right over the metal sample which in turn gets coated with the adhesive. The flow of liquid through the mesh depends on the viscosity of the liquid and the distance between the mesh and sample should be very less in order to avoid air bubbles. The schematic working of screen printing can be observed in fig. 18.
Figure 18 Working principle of screen printing
The important process parameters of screen printing are adhesive composition, drying, process window, mesh number and adhesive layer thickness. In a multiple component adhesive, the right proportion of components must be found to achieve a strong bonding. Then the adhesive composition should be mixed homogeneously with less or no air bubbles and coated immediately onto metal before the pot life expires. Right composition helps to increase the process window, which is the time gap between drying and injection molding. Next important parameter is the drying process which helps to vaporise the solvents present in the adhesive and enables curing the adhesive to form a thin layer on the metal. Drying is carried out in a rack oven. Precise drying temperature and drying time should be carried out, as over drying may cause reduction in process window and bond strength and under drying may cause residue of solvent in the sample which also reduce bond strength.
Mesh number defines the fineness or coarseness of the screen. For example, Mesh number 18 – 180 represents the mesh has 18 opening per cm and the thread diameter is 180 µm. The thickness of the adhesive layer depends on the mesh fineness. Adhesive layer thickness acts as an important factor affecting the bond strength. The final layer thickness is measured after drying as the solvents evaporate during drying. The thickness is measured with the help of an ultrasonic thickness gauge and it is shown in fig. 19.
Figure 19 Ultrasonic thickness gauge
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