A Injection moulding by mikeholy

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                   A2                       Published: 11.02.2008




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




        Injection moulding




     INJECTION MOULDING OF THERMOPLASTICS




                WWW.PT.BME.HU
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                                                       LOCATION OF THE PRACTICE




                                                                                         2




                                                               TABLE OF CONTENTS

          1.       AIM OF THE PRACTICE ...................................................................................................................... 3

          2.       THEORETICAL BACKGROUND......................................................................................................... 3

               2.1.        THE HISTORY OF INJECTION MOULDING.............................................................................................. 3
               2.2.        THE INJECTION MOULDING MACHINE ................................................................................................. 4
               2.3.        MAIN STEPS OF INJECTION MOULDING................................................................................................ 5
               2.4.        THE SEQUENCES OF THE INJECTION AND THEIR UNITS ........................................................................ 6
               2.5.        THE INJECTION PROCESS AS A FUNCTION OF THE PARAMETERS .......................................................... 9

          3.       THE MACHINES USED AT THE PRACTICE.................................................................................. 11

          4.       THE MOST IMPORTANT TERMS RELATED TO THE INJECTION MOULDING IN
HUNGARIAN, IN ENGLISH AND IN GERMAN...................................................................................................... 12

          5.       LITERATURE (IN HUNGARIAN) ...................................................................................................... 12

          REPORT ........................................................................................................................................................... 13


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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 a great number of products (at least
100.000) are made by the same mould, the price for one product is low.


2.1. 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.



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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.


2.2. 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 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. During injection moulding the mould must be kept closed against the 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



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must be opened in order to remove the product. The clamping system can be either 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 is the melting (homogenous melt) of the raw material, and to inject it 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. – +), but it is not a main system of
the injection moulding machine, because it can be used in more machines, and a machine can
operate with different moulds.


2.3. Main steps of injection moulding

       Most 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 mould with high pressure,
               – The cooling of the melt in the cold mould (to a certain temperature),
               – Removing the complete product from the mould
       The injection cycle consist of the above steps; their order and interaction can be understood
by the theoretical process diagram (Fig. 3.).




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                               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 retracts 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.).


2.4. The sequences of the injection and their units

       The injection system (Fig. 4.) is the main system considering the process. Its functions are as
follows:
               – Melting the raw material
               – Melt conveying and homogenization
               – Melt accumulation

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               – 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 the
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 enter 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

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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 by
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

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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.


2.5. 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)
                         ⎜
                         ⎝    ρ⎟
                               ⎠

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 other
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 + π)(v − ω) =        ,                                       (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




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– 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




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3. The machines used at the practice


              KUASY 100/25 INJECTION MOULDING MACHINE (FIG. 8.)

Maximum volume: 100 g (PS)
Maximum clamping force: 25 t

Clamping: Toggle mechanism
(Hydromechanic)

Distance of columns: 280*280 mm

Screw diameter: 35 mm


                                                    Fig. 8. KuASY type injection moulding machine

     ARBURG ALLROUNDER 320C INJECTION MOULDING MACHINE (FIG. 9. )

Maximum volume: 132 g (PS)
Maximum clamping force: 60 t

Clamping: Hydraulic

Distance of columns: 320*320 mm

Screw diameter: 35 mm
Maximum injection pressure: 1820 bar
                                              Fig. 9. ARBURG Allrounder 320C type injection moulding
                                                                    machine




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4. The most important terms related to the injection moulding in
    Hungarian, in English and in German

Hungarian                   English                       German
fröccsöntés                 injection moulding            Spritzgiessen
fröccsegység                barrel                        Plastifizier-einheit
beömlőcsatorna              Sprue                         Anguss (Zapfen, Kegel, Stange)
elosztócsatorna             runner                        Verteilungskanal (Angussverteiler)
csigadugattyú               reciprocating screw           Schnecke
fröccsnyomás                injection pressure            Spritzdruck
utónyomás                   holding/packing pressure      Nachdruck
szerszám                    Mould                         Werkzeug
hűtés                       cooling                       Kühlung
zsugorodás                  shrinkage                     Sohrumpfen
vetemedés                   warpage                       Verziehen
összecsapási vonal          Weld line                     Bindenaht
záróerő                     clamping force                Schliesskraft
szerszámzáró egység         clamping unit                 Schliesseinheit


5. Literature (in hungarian)

1. Czvikovszky T., Nagy P., Gaál J.: A polimertechnika alapjai, Műegyetemi Kiadó,
   Budapest, 2000

2. Dunai A., Macskási L.: Műanyagok fröccsöntése, Lexica Kft., Budapest, 2003




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                                                 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/cm3]
                    Middle:                       [°C]                       Mass:                       [g]
                   At throat:                     [°C]           Injection volume:                       [cm3]

           Injection pressure:                    [MPa]          Holding pressure:                       [MPa]
               Injection time:                    [s]               Holding time:                        [s]
                                                             Screw shift at holding
 Screw shift at injection time:
                                                  [mm]                        time:                      [mm]

                                                              Injection unit closing
         Mould closing time:                      [s]                          time:                     [s]
                                                             Injection unit opening
         Mould opening time:                      [s]                          time:                     [s]

         Screw rotation time:                     [s]
               Cooling time:                      [s]
                 Cycle time:                      [s]


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3. Cycle diagram of injection moulding




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