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 LESSONS TABLE OF CONTENTS 1. AIM OF THE PRACTICE THEORETICAL BACKGROUND THE INJECTION MOULDING MACHINE THE WAY OF THE PLASTIC DURING INJECTION MOULDING THE INJECTION MOULDING CYCLE THE PARTS OF THE INJECTION UNIT AND THEIR ITS FUNCTIONS THE STATE VARIABLES OF MELT DURING THE INJECTION PROCESS THE MACHINES AND EQUIPMENT USED DURING THE LABORATORY PRACTICE REPORT Injection moulding of thermoplastics 14/2

3 1. The goal of lab practice The aim of the lab practice is to learn and analyse the technology of injection moulding and the injection moulding cycle. Additionally, the injection moulding machine and its parts are studied, as well as the features and characteristics of thermoplastic polymers and injection moulded products. 2. Theoretical background Injection moulding is the most versatile technology used for producing polymer (mainly thermoplastic polymer) products. Thermoplastic polymer pellets are heated above their melting temperature; the resulting viscous polymer melt is then injected at high speed and pressure into a closed mould at a controlled temperature. In this mould the high pressure polymer melt cools down and forms a simple or complex, dimensionally accurate (3D) part, practically without waste. A feature of the technology is that only one kind of product can be injection moulded with a mould, thus these moulds are not universal. Although an injection mould is very costly, mainly due to the machining cost of the metal, a large number of products (at least 100,000) can be produced with the same mould, thus the specific price of a product is low and the whole technology is economical The injection moulding machine Injection moulding is the process of choice for a large number of products varying in size, shape and material. A great number of injection moulding machine types were developed for optimal manufacturing. In general, the main parts of an injection moulding machine are the following (Fig 1.): Injection moulding of thermoplastics 14/3

4 Fig.1 The main parts of the injection moulding machine The function of the machine stand (or bearer) and hydraulic system (Fig. 1 1) is to unite the other main systems of the machine and ensure a vibration free connection between them. The function of the clamping system (Fig. 1 2) is to move the movable mould half, while the stationary mould half is fixed to the mould platen. During the injection moulding phase (when the cavity is filled with the thermoplastic melt) the mould must be closed against the high pressure of 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 opened so that the product can be removed. The clamping system can be mechanical (hydro-mechanical or electro-mechanical) (Fig. 2.) or fully hydraulic. Injection moulding of thermoplastics 14/4

5 Fig.2 The schematics of the hydro-mechanical clamping system The injection system (Fig 1 3) is the most important unit in the injection moulding machine. Its function is to melt the thermoplastic polymer pellets (create a homogenous melt), and to inject this melt into the mould. The function of the control unit (Fig 1 4) is to control the main parameters of the injection moulding machine, the whole production process and to provide an interface between man and machine. A mould is also required for injection moulding (Fig 1 +), but it is not listed as a main system of the injection moulding machine, since a mould can be used in several machines, moreover, a machine can operate with various moulds. The mould can typically be opened into two halves (a stationary and a movable mould half), which is necessary to remove the product from the mould cavity The route of the plastic during injection moulding Mostly thermoplastic polymers are processed by injection moulding. In this case, the process consists of the following steps: Filling the hopper with the raw material (thermoplastic polymer pellets) (Fig 1. 3), Conveying, melting and homogenisation of the material (bringing it into a formable state), Injecting the melt into the closed mould with high speed and pressure (shaping), The cooling of the melt in the controlled temperature mould (fixing the shape), Removing the completed product from the mould. Injection moulding of thermoplastics 14/5

6 2.3. The injection moulding cycle The cycle of the injection moulding process consist of the steps listed above, but their order and interaction can be understood through the theoretical process diagram (Fig. 3.). Fig. 3 The injection moulding cycle Between two cycles, the mould is always open, there are no products in it and the injection unit is in its backward position; it is pulled away from the mould so there is an air gap and thus no thermal conduction from the injection unit (it is hot) to the mould (it is cooler). Besides, the injection unit already contains the plasticized (molten and homogenized) thermoplastic melt. When an injection moulding cycle starts, the mould and after that, the injection unit close. After the mould and the injection unit are closed, the melt (plasticized one cycle before) is injected with high speed and pressure into the mould by the axial (piston-like) movement of the screw (injection phase) the pressure of the injected material is kept (holding pressure phase). After the melt is injected into the mould, it starts cooling and solidifying immediately, while parallel to this, Injection moulding of thermoplastics 14/6

7 the injection unit prepares the melt for the next cycle. The screw rotating in the barrel of the injection unit conveys the material forward, which is melted by both the heat and the shearing action (plastification). During the rotational movement of the screw, the pressure of the plasticized material pushes the screw backwards. When sufficient material is melted, the rotation of the screw stops, and the injection unit is drawn away from the mould so that there is no thermal conduction between the heated nozzle of the injection system and the cooler mould. After the part has cooled and solidified in the mould, the mould opens and the product can be removed (de-moulding). After this, the injection moulding cycle stars all over again (Fig. 3.) The parts of the injection unit and their functions The injection system (Fig. 4.) is the main system of the whole injection process. Its functions are as follows: melting the raw material, conveying and homogenization of the melt, storing the melt, injecting of the melt into the mould, providing the necessary holding pressure. Fig. 4 Injection system The main purpose of the plasticizing process is that the solid polymer pellets fed into the hopper with the necessary additives (colorants, fire retardants, antistatic agents etc.) get into a homogenous melt state, have the proper temperature and viscosity, and the additives are uniformly dispersed in the melt. The plastification process consists of the following steps. The solid pellets drop (due to gravity) from the feed hopper into the feed zone of the screw within the heated barrel. The screw is a Injection moulding of thermoplastics 14/7

8 threaded part fitted loosely to the barrel. As the screw rotates, the solid pellets are transported in the direction of the screw tip and the pellets are heated by the barrel, (external heating) and due to viscous friction (internal heating), and finally they become molten. The homogeneity of the melt (both material-wise and viscosity-wise) can be controlled with the heating of the barrel, the geometry of the screw and the rotational speed of the screw. The goal is to have optimum melt homogeneity in a short time (since short cycle time is preferred), thus the screw is separated into three sections (zones) (Fig. 5.). The polymer pellets first travel through the feed zone (Ød=const.), where the solid pellets are compressed and conveyed in the direction of the screw tip and nozzle. Fig. 5 Three-zone screw (core-progressive) The middle part of the screw is the compression (Ød ØD) or transition zone, where the inner diameter (the core) of the screw increases. In this section, the pellets melt completely and compressed sufficiently to force trapped air back to the hopper. In the final zone, the metering zone (ØD=const.), the melt is further homogenized. The melt reaching the nozzle does not enter the mould immediately. The melt must enter the mould at high speed in order to fill it completely while it is still in the melt state. A plasticizing unit cannot melt the solid pellets and produce adequate melt homogeneity, while at the same time fill the mould cavity at high speed. Therefore, the melt should be stored before injection. For this reason, the screw of reciprocating screw injection moulding machines can move axially. The screw rotates so the melt flows along the screw in the direction of the nozzle, and accumulates in front of the screw tip. As there is more and more melt in front of the screw tip, a pressure builds up called backpressure, which forces the screw backwards, while it is still rotating. The screw rotates and moves backwards until it produces enough melt from the solid pellets. After this the screw stops rotating and moving axially. At the end of the barrel, an adapter called nozzle is fastened to it. The nozzle provides mechanical and thermal connection between the hot barrel and the much cooler mould. The melt enters the mould through the nozzle, but apart from the injection and holding phases (discussed later), melt must not exit through it. Accordingly, the nozzle must be closed during plastification (otherwise Injection moulding of thermoplastics 14/8

9 melt would flow out through it); therefore, plasticizing is done when the injection unit (with the nozzle) is in its forward position. thus the cooling and solidifying part in the mould itself blocks the flow of the melt. Injection, the second most important task of the injection system, is injecting the plasticized and homogenized melt in front of the screw tip into the closed mould. This should be carried out rather fast so that the hot melt does not freeze before it fills the much cooler mould. Since the polymer melt has high viscosity, high pressure is needed to inject it into the mould. The axial movement and the high pressure are ensured by the hydraulic piston of the injection system (Fig. 4). Naturally, the whole injection process should be controlled to avoid damage to the mould or to the melt. During the injection phase, it must also be ensured that the melt does not flow backwards to the hopper instead of flowing forward, filling the mould cavity. The non-return valve prevents this unwanted backflow of the melt, while the pressure is higher in front of the screw tip than behind it. (Fig. 6) Fig. 6 The part of a ring type non-return valve (a) and its principle of operation during the plasticizing, as well as the injection and holding phases (b) After the melt is injected into the mould at high speed, it starts cooling down in the controlled temperature mould and its volume decreases (it shrinks). In order to produce a void-free product, this loss in volume must be compensated with additional melt flow. Accordingly, in the holding phase, the screw still moves forward axially, pushing some melt into the mould, and thus keeping the melt in the mould under pressure. This phase lasts until the melt solidifies (most probably at the gate, where wall thickness is the lowest). In order to transmit the pressure from the screw to the mould, a melt cushion should exist in front of the screw tip during the whole injection cycle. After the holding phase, the screw starts rotating and it moves backwards due to backpressure until the melt for the next cycle is produced from the pellets. Meanwhile, the cooling of the product still continues within the Injection moulding of thermoplastics 14/9

10 mould cavity. When the product has cooled enough and is stiff enough, the mould opens and the product can be removed from the cavity. After this, the mould closes and a new cycle can start The state variables of the melt during the injection process The process in the mould can be understood as the function of the p, v, T state variables. The specific volume (v=1/ρ) of polymers is largely affected by outer (hydrostatic) pressure (p) and by temperature (T). At the same pressure, the specific volume change of polymers as a function of temperature is higher even in a solid state than that of all of the other structural materials, which manifests in the higher heat expansion coefficient of polymers. Spencer and Gilmore were the first to publish that the state variables of the polymer melt can be described with an equation, analogous to the universal law of gases, known from thermodynamics: ( p )(v ) RT M, (1) where p is hydrostatic pressure, v is specific volume, R is the universal gas constant, T is absolute temperature, M is the weight of the monomer unit of the polymer chain, π, and ω are correction constants. The injection process as a function of the state variables p, v, T is as follows (Fig. 7): Fig. 7 p-v-t diagram of the injection cycle Before analysing the state variables of the polymer melt during the injection moulding cycle, first let us investigate the p, v, T relation of amorphous polymers. As can be seen, when temperature increases, the specific volume linearly increases just like in the case of any other structural material, Injection moulding of thermoplastics 14/10

11 but the rate of increase is significantly higher (higher heat expansion coefficient). When a certain temperature called Glass Transition Temperature (Tg) is reached, specific volume continues to increase linearly but its steepness increases, since above Tg the mobility of the polymer molecular chains increases. As the temperature is increased further, no other break points can be seen on the curve because an amorphous thermoplastic polymer does not go through a first-order phase transformation during melting (unlike semi-crystalline thermoplastic polymers, where the melting of the crystalline phase represents the first-order phase transformation). Finally, it can also be seen that polymers are compressible and thus their specific volume will decrease, although only when high pressure is applied. Based on all this, let us investigate the pressure, temperature and specific volume of the polymer during an injection moulding cycle (Fig. 7 ): 1-2: The polymer melt fills the mould cavity (injection phase), and its pressure increases, 2-3: Switchover to holding pressure, which results in some pressure drop, 3-4: Holding pressure, which is maintained until the thinnest part (usually the gate) of the product is frozen (point number 4). The beginning of the holding pressure also means the beginning of the cooling phase, 4.: The gate is frozen, and so pressure can no longer be transmitted to the mould. This is the end of the holding phase, 4-5: The product cools at nearly constant specific volume (izochor state), while its pressure drops. The specific volume decrease of the product due to cooling is compensated by its inner pressure, 5.: The product reaches atmospheric pressure, thus its dimensions are exactly the same as the dimensions of the mould cavity, 5-6: The product further cools, this time under constant pressure (isobar), while its volume decreases (shrinkage), 6: The mould opens de-moulding (removing) of the product, 6-7: The product further cools in the open air and continues to shrink. Injection moulding of thermoplastics 14/11

12 3. The machines and equipment used during laboratory practice ARBURG ALLROUNDER 320C INJECTION MOULDING MACHINE (FIG. 8.) Maximum shot weight: 88 g (PS) Maximum clamping force: 40 t Clamping system: Hydraulic Bar distance: 320X320 mm Screw diameter: 30 mm Maximum injection pressure: 2290 bar Fig. 8. ARBURG Allrounder 320C type injection moulding machine Injection moulding of thermoplastics 14/12

13 REPORT Name: Grade: Neptun code: Date: Checked by: Lab practice supervisor: 1. The task Set up the ARBURG Allrounder 320C injection moulding machine to produce quality products, Investigate the cycle, measure the parameters required to draw the cycle diagram (cycle time, mould closing and opening time, temperature profile etc.). Draw the cycle diagram using the measured parameters. 2. Basic and measured data, and calculated results: Temperature profile Polymer: At the nozzle: [ C] Density: [g/cm 3 ] In the middle: [ C] Mass: [g] At the throat: [ C] Injection volume: [cm 3 ] Injection pressure: [MPa] Holding pressure: [MPa] Injection time: [s] Holding time: [s] Screw displacement at injection: [mm] Screw displacement during holding: [mm] Mould closing time: Mould opening time: [s] [s] Injection unit closing time: Injection unit opening time: [s] [s] Screw rotation time: Cooling time: Cycle time: [s] [s] [s] Injection moulding of thermoplastics 14/13

14 3. Cycle diagram of injection moulding Injection moulding of thermoplastics 14/14