Injection moulding BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS FACULTY OF MECHANICAL ENGINEERING DEPARTMENT OF POLYMER ENGINEERING

Transcription

1 B3 BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS FACULTY OF MECHANICAL ENGINEERING DEPARTMENT OF POLYMER ENGINEERING Injection moulding INJECTION MOULDING OF THERMOPLASTICS

2 LOCATION OF THE PRACTICE 2 TABLE OF CONTENTS 1. AIM OF THE PRACTICE THEORETICAL BACKGROUND THE HISTORY OF INJECTION MOULDING THE INJECTION MOULDING MACHINE MAIN STEPS OF INJECTION MOULDING THE SEQUENCES OF THE INJECTION AND THEIR UNITS THE INJECTION PROCESS AS A FUNCTION OF THE PARAMETERS THE MACHINES USED AT THE PRACTICE REPORT Injection moulding of thermoplastics 13/2

3 1. Aim of the practice The aim of the practice is to come to know, and to analyse the technology of injection moulding. The injection moulding machine and its parts are to be studied. In addition, the injection moulding technology, and effect of the parameters (injection pressure, cooling time, shot volume) on the part are to be studied too. 2. Theoretical background The injection moulding is the most versatile technology of producing polymer (mainly thermoplastic polymer) products. Its principle is that the raw material (polymer pellets) is heated above its melting temperature (creating a viscous polymer melt) and is injection moulded with high speed and pressure into a closed mould. In this mould the high pressure polymer melt cools, and forms an optionally complex, dimensionally accurate (3D) part, practically without waste. With the mould used, only one type of product can be injection moulded, these moulds are not universal. A mould for injection moulding is not cheap, but because of the numerous products (at least ) made by the same mould, the price of a product is low The history of injection moulding The injection moulding process has its roots in the injection moulding of metals as well as in the extrusion of polymers. In the early days the development of the process almost entirely focused on perfecting and increasing the melting capacity. The melting cavity used in the metal injection moulding was equipped with a torpedo in order to increase the heat exchange surface and decrease the melt thickness. This increased melt homogeneity. These were the piston type injection moulding machines. A theoretically new method is the pre-plasticating of the polymer to increase the melting capacity. Formerly this method was applied in a piston driven pre-plasticating barrel equipped with a torpedo. In the later designs the piston-driven barrel was replaced with a screw driven one. The main problem with both the piston driven and screw driven machines was to make the outlet of the melting space lockable, and to maintain a controllable back-pressure. Before finding solutions to these technical difficulties, the reciprocating screw injection machine was invented. Injection moulding of thermoplastics 13/3

4 This was a big advance in the melt homogeneity and melting capacity of the machine. We will analyze the injection moulding process on a reciprocating screw injection moulding machine The injection moulding machine Injection moulding is the process of choice for a tremendous variety of parts in size, shape and material. To accomplish optimally this task, a great number of injection moulding machine types were developed. In general, the main parts of an injection moulding machine are (Fig 1.): Fig.1. The main parts of the injection moulding machine The function of the machine bearer (Fig. 1 1) is to join with the other main systems of the machine. The function of the clamping system (Fig. 1 2) is basically the moving of the movable mould half. While injection moulding the mould must be closed against high pressure from the molten polymer (the clamping force is one of the main parameters of an injection moulding machine). When the part is cooled down to a certain temperature in the mould, the mould must be Injection moulding of thermoplastics 13/4

5 opened in order to remove the product. The clamping system can be mechanical (toggle mechanism) or hydraulic (Fig. 2.). Fig.2. The scheme of the toggle mechanism (mechanical clamping system) The injection system (Fig 1. 3) is the main system in the injection moulding machine. Its function the melting (homogenous melt) of the raw material, and to inject into the mould. The function of the control unit (Fig 1. 4) is to control the main parameters of the injection moulding machine, and to assure the contact between man and machine. A mould is also required for injection moulding (Fig 1. +) is, but it is not a main system of the injection moulding machine, because it can be used in more machines, and a machine operate with different moulds Main steps of injection moulding Mostly thermoplastic materials are processed by injection moulding into products. In this case the process consists of the following steps: Filling the hopper with raw material, Conveying, melting and homogenisation of the material, Injecting of the melt into closed the mould with high pressure, The cooling of the melt in the cold mould (certain temperature), Removing the complete product from the mould The cycle of the injection moulding process consist of course the steps above, but their order and interaction can be understood by the theoretical process diagram (Fig. 3.). Injection moulding of thermoplastics 13/5

6 Fig. 3. Process diagram of injection moulding After closing the mould and the injection unit, the melt (prepared one cycle before) is injected into the mould by the axial movement of the screw (injection). After injection, the melt cools down and solidifies in the mould, while the injection unit prepares the melt for the next cycle. The screw rotating in the barrel conveys the material forward, which is melted by the heating and by the shearing action (plastification). When sufficient material is melted, the rotation of the screw stops, and the injection system diverges form the mould in order to avoid the heat transfer between the (cold) mould and the (heated) nozzle (part of the injection system). After the part is cooled in the mould, the mould opens and the product can be removed. (removing). The injection moulding cycle recommences (Fig. 3.) The sequences of the injection and their units The injection system (Fig. 4.) is the main system as the view from the process. Its functions are as follows: Melting the raw material Melt conveying and homogenization Melt accumulation Injection moulding of thermoplastics 13/6

7 Injecting of the melt into the mould Provides the necessary holding pressure Fig. 4. Injection system The essence of the plastification process is: The solid pellets drop (due to gravity) from the feed hopper into the feed zone of the screw. As the screw rotates the solid pellets are conveyed in the direction of the screw tip. The material is heated due to the (external) barrel heating and due to the (internal) viscous friction, and finally melts. The melt homogeneity (both physically and thermally) can be controlled with the barrel heating, the geometry of the screw and the screw speed. In order to achieve optimum melting homogeneity in short time, the screw has three separate sections (zones) (Fig. 5.). In the feed zone (Ød=const.), solid resin granules are compressed and conveyed in the direction of the screw tip and nozzle. Fig. 5. Three-zone screw The middle part of the screw is the compression (Ød ØD) or transition zone. Here material melts completely and compressed sufficiently to force trapped air back to the hopper. In the final metering zone (ØD=const.) the melt is homogenized and stabilized further. The melt reaching the nozzle does not enters into the mould immediately. The melt should enter into the mould at high speed in order to fill completely the cold mould. It is not feasible to build such plasticating unit that can melt the solid material with the required high speed and at the same time provide good melt homogeneity. So the melt should be stored before injection. In the reciprocating screw injection moulding machines the screw can move axially. As the melt Injection moulding of thermoplastics 13/7

8 accumulates in front of the screw tip, pressure is building up which forces the screw backwards. As the screw reaches a certain distance (it already had produced sufficient amount of melt), it stops rotating. On the end of the barrel next to the mould an adapter called nozzle is fastened to the barrel. This nozzle provides mechanical and thermal connection between the hot barrel and the much cooler mould. The melt flows through this nozzle. The injection, the second most important task of the injection system is to inject the melt that is accumulated in front of the screw tip, into the closed mould. This should be carried out rather fast in order to avoid premature freezing of the melt. The melt has high viscosity, so the injection needs high pressure; this is why the screw should move forward fast and with high pressure. The whole injection process should be controlled to avoid damage to the mould or to the melt. During this phase the melt must not flow backwards. The non-return valve prevents the melt to flow back while the pressure is higher in front of the screw tip than behind it. (Fig. 6.) Fig. 6. The non-return valve After the injection the melt cools down in the tempered mould, solidifies (at first near the mould walls) and shrinks. A part of the volume decrease (shrink) can be compensated for applying a holding (packing) pressure. During the holding pressure phase the screw moves forward (slowly) and melt is forced into the mould. This phase lasts until the melt solidifies (most probably at the gate where the channel is the narrowest). In order to transmit the pressure from the screw to the mould a melt pool should exist during the whole injection cycle in front of the screw tip. After the holding pressure phase the screw starts rotating and a new cycle starts. The solidified melt our end product should be removed from the mould cavity. The main task of the clamping side is to open and close the mould, which is achieved by toggle mechanisms or by hydraulic clamping systems. Typically, a short stroke hydraulic cylinder operates the toggle mechanism. The clamping side should withstand the high injection pressure (during the injection cycle the mould should be closed). The parts are cooled down until they can be ejected without distortion. At this time the mould opens and a separate knocking mechanism removes the parts. After a while the mould closes Injection moulding of thermoplastics 13/8

9 and a new cycle can be started. In the up-to-date injection machines all phases of an injection moulding cycle can be controlled very precisely The injection process as a function of the parameters The process in the mould can be understood as the function of the p, v, T parameters (Fig. 1 7.). The specific volume v of the polymers is largely affected by the outer (hydrostatic) pressure (p) and by temperature (T). At the same pressure, the specific volume change of the polymers as the function of the temperature is higher even in solid state than all of the structural materials: which also results in higher heat expansion coefficient. Spencer and Gilmore (1950) were the first to publish, that the parameters of the polymers can be described as an equation, analogous to the law of the gases, known from thermodynamics: RT ( p )(v ), (1) M where p is the hydrostatic pressure, v is the specific volume, R is the universal gas constant, T is the absolute temperature, M is weight of the monomer unit of the polymer chain, π, and ω are correction constants. The injection process as the function of the p, v, T parameters are as follows (Fig. 7.): Fig. 7. p-v-t diagram of the injection cycle Injection moulding of thermoplastics 13/9

10 1-2: The polymer melt fills the mould cavity (injection), 2-3: Switchover to holding pressure, 3-4: Holding pressure, stands till point 4, where the thinnest part of the product is frozen, 4-5: The melt at constant specific volume cools, while pressure decreases, 5-6: Isobar cooling, 6: Removing of the product, 6-7: The product cools and shrinks further in the open air Injection moulding of thermoplastics 13/10

11 3. The machines used at the practice Maximum volume: 100 g (PS) Maximum clamping force: 25 t Clamping: Toggle mechanism (Hydromechanic) KUASY 100/25 INJECTION MOULDING MACHINE (FIG. 8.) Distance of columns: 280*280 mm Screw diameter: 35 mm Fig. 8. KuASY type injection moulding machine ARBURG ALLROUNDER 370S INJECTION MOULDING MACHINE (FIG. 9. ) Maximum volume: 172 g (PS) Maximum clamping force: 70 t Clamping: Hydraulic Distance of columns: 370*370 mm Screw diameter: 35 mm Maximum injection pressure: 2500 bar Fig. 9. ARBURG Allrounder 370S type injection moulding machine Injection moulding of thermoplastics 13/11

12 REPORT Name: Grade: Neptun code: Date: Checked by: Head of practice: 1. The task Set up the injection moulding parameters for the KuASY injection moulding machine to make a perfect product. Inspection of the cycle, measuring of the parameters required to draw the cycle diagram (cycle time, mould closing and opening time, temperature profile, etc.). Draw the cycle diagram, from the measured parameters. 2. Basic, measured, and counted data: Temperature profile Raw material: At nozzle: [ C] Density: [g/cm 3 ] Middle: [ C] Mass: [g] At throat: [ C] Injection volume: [cm 3 ] Injection pressure: [MPa] Holding pressure: [MPa] Injection time: Holding time: Screw shift at injection time: Screw shift at holding [mm] time: [mm] Mould closing time: Mould opening time: Injection unit closing time: Injection unit opening time: Screw rotation time: Cooling time: Cycle time: Injection moulding of thermoplastics 13/12

13 3. Cycle diagram of injection moulding Injection moulding of thermoplastics 13/13