Manufacturing E3 Chapter 2 - Shaping of Polymers PDF

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Notes on shaping polymers...

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Shaping of Polymers Plastics can be shaped into a wide variety of products, the plastics can also be in a wide variety of forms. In addition to this plastics are often the main ingredient in other materials such as paints, varnishes, adhesives and polymer matrix composites. Each of the different forms of plastic requires it own shaping processes.

The commercial importance of polymer shaping processes has grown as the use of polymers has increased. Applications of plastics have increased much faster than either metals or ceramics during the last 50 years.

There are a number of good reasons why polymer use has become so popular. The range of part geometries than can be produced is almost unlimited, when plastics are moulded the part is net shape, this means the part is finished and does not require further shaping or processing. Processing polymers requires less energy than for metals, this is mainly because the processing temperatures are much lower. Also lower temperature make handling easier , cooling takes less time so to production rates increase.

Plastic shaping processes are classified according to the resulting product geometry and there are eight classifications: •

Extruded products with constant cross‑section

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Continuous sheets and films

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Continuous filaments (fibers)

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Moulded solid parts

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Moulded hollow parts with thin walls

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Parts made of formed sheets

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Castings

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Foam products

The two most common shaping processes used with plastics are extrusion and moulding.

To shape a thermoplastic polymer it must be melted, this involves heating it so that it softens to the consistency of a liquid, the liquid polymer is known as a ‘polymer melt’. A polymer melt has two important properties which must be considered during processing: •

Viscosity – a resistance to flow

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Viscoelasticity – a shape “memory”

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Viscosity is a fluid property that relates to shear stress, essentially this is the force needed to rip the fluid apart and change it into a different shape.

Viscoelasticity is a combination of viscosity and elasticity, it can be seen in both solid polymers but particularly in polymer melts. It is basically the materials desire to spring back to a previous shape, an example of viscoelasticity is die swell in extrusion.

Extrusion Extrusion is one of the fundamental shaping processes for metals and is very common in polymers. It is a compression process in which material is forced to flow through a die. The result is long continuous product with constant cross sectional shape, this shape is determined by the shape of the orifice.

The machine used to produce extruded plastics is called an extruder, it consists of two main components, a barrel and a screw. The extruder barrel contains the plastic material as it moves towards the die. This ratio of diameter to length is selected depending on the polymer being used.

The feedstock for and extruder is pellets or powdered polymer, this is fed by gravity onto the screw whose rotation moves material through barrel. So long as feedstock is added production is continuous. Electric heaters are located along the barrel, the mechanical working of the material by the screw also adds heat which maintains the melt.

From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

The extruder is divided into three sections each with its own functions.

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The feed section takes feedstock from the hopper and moves it along the barrel to the first set of heaters 2

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The compression section follows - the polymer is by now a fluid, air which was mixed with the pellets is removed and the material is compressed

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The final metering section mixes the melt to ensure it is homogenised. This section also ensures there is sufficient pressure behind the polymer to pump it through die opening

Once it has travelled along the barrel the polymer melt eventually reaches the die, before reaching the die the material passes through a breaker plate, this contains a screen pack which is a series of wire meshes supported or a plate containing small holes, the material has no option but to pass through this before reaching the die. The screen pack has three main functions, to filter contaminants and hard lumps from melt, to build pressure in metering section and to straighten the flow of the polymer melt and remove its "memory" of the shape of the screw.

The die is very simple, it consists of a single hole the shape of which is the required cross section of the part. The orifice must be smaller than the required part as the part will get larger due to die swell. Due to die swell the shape of the die is often not exactly the same as the part produced, producing a square profile part does not require a square orifice.

From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons Designing a die requires skill, judgment and experience.

Hollow profiles are more difficult to extrude, they require a much more complex die. The simplest hollow extrusions are tubes, pipes and hoses. However more complex cross sections for the inside hollow can be produced. Hollow profiles require a mandrel to form the inner shape, the mandrel is held in the centre of the die orifice using a spider and prevents solid

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being produced at the centre of the tube. Mandrels often include an air channel through which air is blown to maintain the hollow form whilst it hardens

From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

Wire and cable coating is achieved in a similar way to extruding tubes. The polymer melt is applied to bare wire as it is pulled at high speed through a die. The wire is usually cooled by passing the coated wire through a water bath.

Thermoplastic sheets and film are usually produced using an extrusion method. The factor which differentiates sheets and films is thickness. A sheet has a thickness from 0.5 mm to about 12.5 mm and a film has a thickness below 0.5 mm. There are three main extrusion processes used to create sheets and films, Slit Die Extrusion of Sheet and Film, Blown Film Extrusion Process and Calendering.

Slit Die extrusion of sheet and film is the same as conventional extrusion, it uses a narrow slit as the die opening. The slit may be up to 3 m wide and as thin as about 0.4 mm. The main problem with this method is uniformity of thickness throughout the width of the sheet. There are two reasons for this, the die orifice is a very different shape from the barrel so the viscoelasticity of the polymer tries to return it to the shape of the barrel. The other problem is the polymer melt may not reach the edges of the slit die under sufficient pressure to ensure uniform thickness. The edges of film are usually trimmed because die swell often makes the film wider than intended. The process has high production rates of about 5 m/s.

The Blown Film extrusion process combines extrusion and blowing to produce a tube of thin film. The extruder includes a mandrel which ensure the part extruded is a hollow tube, air is 4

then pumped into the hollow tube which inflates it. As it inflates the still hot polymer tube increases in diameter and reduces in thickness. As the inflated tube gets further from the extruder it cools and solidifies.

From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

Calendering does not use a barrel or screw, instead the molten metal is placed between two counter rotating rollers. These draw the plastic inwards and squeeze it, the gap between the rollers is a wide and thin slit and this is the shape of the extruded film. The film passes through further rollers to further reduce its thickness, the equipment needed for this method is expensive but production rates are high. It is also known for its good surface finish and high thickness accuracy.

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From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

Fibres and filaments are very similar, a fibre is a long thin strand of material, the length of a fibre is usually in the region of at least 100 times its diameter, a filament is a fibre of continuous length.

Fibres can be both natural or synthetic and as a result textiles are either natural or synthetic, synthetic fibres account for about 75% of the total fibre market. Fibre and filament production is referred to as spinning, this is the term used historically when production was performed by hand. The spinning method draws and then group natural fibres together to form a thread

For synthetic fibres spinning is performed at the exit of an extruder, the polymer is extruded through a spinneret, this is a die with multiple small holes, the result is a number of parallel filaments of material. These are then drawn together, this means they are pulled so as to increase their length and reduce their diameter.

There are three variations of spinning depending on the polymer type being used, Melt spinning, Dry spinning and Wet spinning.

In melt spinning the starting polymer is heated to molten state, it is then pumped through spinneret in a manner similar to conventional extrusion.

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Dry Spinning is very similar to melt spinning, the main difference is the polymer is a liquid not because of heat but due to a solvent, the polymer and solvent need to be premixed. The polymer is solidified by evaporating the solvent after extrusion.

Wet spinning also uses polymer that are only a liquid when a solvent is applied. The polymer is extruded as before but it must then pass through a liquid chemical that coagulates the polymer into a solid.

Filaments produced by any of the three processes are usually subjected to further cold drawing, this is done to align the crystal structure of the polymer along direction of the filament. The result is that tensile strength is increased significantly.

Injection Moulding Injection moulding is another very popular polymer processing method, the polymer is heated to a highly plastic state. It is then forced to flow under high pressure into a mould cavity. Once the material solidifies the moulded part is then removed. The method produces components that are nearly always to net shape. The process is relatively quick taking 10 to 30 secs per part, however, production times of one minute or more are not uncommon.

Injection moulding is the most widely used moulding process for thermoplastics it is popular because complex and intricate shapes are possible. There are two main limitations to the shapes that can be produced, it must be possible to machine or otherwise make the mould and the shape must allow the mould to be removed.

An injection moulding machine has two main components, the injection unit melts and delivers polymer melt - it operates in a similar way to an extruder and the clamping unit which opens and closes the mould after each injection cycle. Unlike in an extruder the injection unit also acts as a ram to inject molten plastic into the mould. The clamping unit has three functions, it holds the two halves of mould in proper alignment with each other, it keeps mould closed during injection by applying a clamping force sufficient to resist injection force and it opens and closes the mould to allow the finished part to be removed.

A typical moulding cycle consists of the following four stages

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From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

A two plate mould consists of two halves. Each of the two halves is mounted to the clamping unit, this allows the machine to open and close the mould. The main feature of the mould are •

The cavity - this is in the shape of the geometry of part.

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Distribution channel - The channel includes a sprue which leads from the injector nozzle into the mould. It also has runners which lead from the sprue to actual mould cavity.

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Ejection system - The most common method is ejector pins which extend as the mould is opened – this pushes the solid part out

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Cooling system – The cooling system consists of an external pump connected to passageways in the mould. Water is circulated through these passageways to remove heat from hot plastic

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Air vents - these are needed to permit air in the cavity to escape as the polymer melt is injected in

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From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons A three plate mould uses three plates to form the mould. The three parts are stacked against each other, the sprue and runner are created in a cavity between the first and second moulds plates, the actual part is made in a cavity between the second and third plates. When the mould opens the three plate are separated, this method allows fully automatic operation of moulding machine. As the mould opens the runner and part are pulled in opposite directions, this breaks off the runner, the part then falls into a parts bin and the runner falls into a waste bin.

From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

Hot runner moulds eliminates solidification of sprue and runner. This is achieved by locating heaters around the corresponding runner channels. The plastic in the mould cavity solidifies but the sprue and runner remain molten. The main benefit of this method is it saves material that otherwise would be scrap.

As with any solidification process shrinkage is a potential problem, this is a reduction in linear size which occurs during cooling. The polymer has a larger volume at moulding temperature than it does at room temperature. Polymers have high thermal expansion coefficients, this means that significant shrinkage occurs during cooling in mould.

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Some typical shrinkage values for polymers are Plastic

Shrinkage (mm/mm)

ABS

0.006

Nylon 6,6

0.020

Polycarbonate

0.007

Polyethylene

0.025

Polystyrene

0.004

PVC

0.005

To account for shrinkage the mould cavity must be larger than the part .The amount a particular dimensions must be enlarged is given by the following equation Dc = Dp + DpS + DpS2 where Dc is the dimension of the cavity Dp is the required part dimension and S is the shrinkage value.

There are a number of approaches which can be used to reduce shrinkage:

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Injection pressure - as the pressure is increased more material is forced into the mould cavity (the polymer compresses slightly). When shrinkage occurs the overall size reduction is less as there was more material in the mould than needed

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Compaction time - this has a similar effect as more molten material is injected as the part starts to cool and shrink

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Moulding temperature - although counter intuitive higher temperatures lead to less shrinkage. The higher temperature make the polymer melt less viscous and so more material can be packed into the mould

A variation on basic injection moulding is thermoplastic foam injection moulding. This is used to produce thermoplastic parts that have a dense outer skin around a lightweight foam centre. The advantage of these parts is that they are have very high stiffness to weight ratios. They are produced in either of two ways, a gas is introduced into the molten plastic in the injection unit or a gas producing ingredient is added with starting pellets. Injection moulding is usually used with thermoplastics, it can be used with thermosets but the problem is if a thermoset is heated in the injector it will create cross links and start to cure before being injected. To overcome this the equipment and operating procedure must be 10

modified. To avoid premature cross linking temperatures in the injector are relatively low, enough for the polymer to flow but not enough to cause cross linking. The melt is then injected into a heated mould, contact with the hot mould causes the cross linking to occur and the plastic to harden. Curing is the most time consuming step in the cycle.

Another injection moulding method is reaction injection moulding, here two highly reactive liquid ingredients are mixed and immediately injected into a mould. A chemical reactions leading to solidification then takes place.

Compression Moulding

Injection moulding is nearly always used with thermoplastics, thermosets and elastomer use other moulding methods. Compression moulding is an old and very widely used moulding process for thermosets. The polymer material is known as the moulding compound, this can be in various different forms such as powders, pellets or a liquid. The amount of moulding compound must be precisely controlled. If too little compound is loaded the part will be incomplete, usually too much material is added to ensure the part is completed – any extra material is forced out as the mould is closed.

Compression moulding is performed in four stages. Charge is loaded into the open mould, the mould closes and the charge is compressed and spread throughout the mould. The mould and therefore the charge is heated which cures it. The part is ejected and removed.

From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

The moulds used in compression moulding are cheaper than those of injection moulding, this is because they are simpler than injection moulds. They do not have sprues and runners as the charge is placed in the mould rather than injected into it. The process is really only suited 11

to simple part geometries. This is mainly due to the fact that termosets do not flow as well as thermoplastics, this means they do not distribute throughout the mould as easily.

The mould must be heated to achieve curing of the polymer, there are three ways of achieving this: 1. Electric heating elements built into the mould 2. Hot steam in applied to the mould 3. Hot oil is circulated through channels in the mould

The advantages of compression moulding are the moulds are simpler and less expensive, there is less scrap as sprues and runners are not needed, the residual stresses in the finished part, which may lead to failure, are less.

Transfer Moulding

Transfer moulding has similarities to both compression moulding and injection moulding, the thermoset charge is loaded into a special chamber immediately ahead of the mould cavity. It is heated and then a plunger is forced into the chamber forcing the polymer into the mould cavity. The mould cavity is also heated and this causes the polymer to cure.

In Pot transfer moulding the charge is pushed by a ram from a “pot” (material holding chamber) through a vertical sprue channel into the mould cavity. The part is removed using an ejector pin. A small amount of waste material is created by the sprue.

From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

In Plunger transfer moulding a plunger injects charge from a heated well through horizontal channels into the mould cavity. The mould makes multiple parts which are all joined by waste material which must be cut off. 12

From: Principles of Modern Manufacturing 4th Edition. John Wiley & Sons

In both compression and transfer moulding scrap is produced, each cycle results in a quantity of leftover material whic...