The more plastics seen in the market today ranging from plastic bags, the plastic pipes and most sheets are products of this technique. The research groups have done evaluation on this technique and recommended some of the latest improvements being witness today. A mong the key steps made include the enhancement in the dissolution of the slowly soluble drugs. This has modified the drug release and the transdermal passage of the drug. The extrusion refers to a process of converting a raw material into a product of uniform size, density and shape. This achieved by forcing it through a die but done under controlled conditions. It is very possible to make this process a continuous one by allowing the temperatures to be constant. The extruding machine has two parts. The first part consists of the conveying system whose major task is to transport the material and put a degree of distributive and dispensation of the mixing. The second part consists of the die itself that forms the material to the required form of the shape.
The melt extrusion has been broadly classified into a molten system which uses melt under high temperature and a semi solid viscous system. The application of heat in the molten system aims at controlling the viscosity and allowing flow into the die. The semi solid systems are the multiples of the phases which concentrate dispersions containing high proportions of liquid solid mixture
The hot melt extrusion equipment is made up of an extruder, the secondary equipment for the extruder, tools for doing the monitor, lower stream processing equipment and the product evaluation parts or components. These parts include hopper for the introduction of the content, barrels, twin screws, the screw and the die Petrick, I.J. and Simpson, T.W., 2012.
The auxiliary equipment for the monitoring processes include a heating or cooling device installed for barrels, a conveyor belt for cooling down the product and the solvent delivery pump that pushes the content outside. The parameters to be monitored include the temperature of the gauges, the speed of the of the screw, the torque of extrusion and the pressure of the operating system
The extrusion process can be broken down into the following stages in order to enhance the understanding. These stages may include;
It consists of one or two rotating screw inside a stationary cylindrical barrel. The barrel is made of a very robust material of sections which are bolted or clamped together. The die is connected to the end of the barrel. The end of the barrel will always determine the shape of the product. The required heat to melt the product is provided by the friction between the melt and the shearing surface. Most of the commercially available extruders have a modular design. Such designs allow for the choice of the screws or the interchangeable sections that normally alter the configurations of the feed system. The modification of the process to meet a specific requirement is thus very possible. These screws allow the extruder to perform a mixing and reduction of the induced particles.
Conventionally, the extrusion channel is divided int three main sections or channels. These sections include feed section, transitions zone and finally the metering zone. The raw material is fed from the hopper directly into the so-called feed section the section has a deeper flight with greater pitch. This kind of configuration enables the feed material to be fed easily into the screw. The pitch and the helix angle provide a constant rotation speed of the screws. The material is usually introduced as a solid plug to the transition zone. In this section the materials are mixed, melted and compressed before being plasticised
The compression process is achieved through decrease of the thread pitch of the screw while maintaining the flight depth. The result is an increased pressure as the material move along the barrel. The transverse flow of the melt is made possible by the helical screw. The drag flow, pressure flow and leakage are all applied. The material is then introduced at the metering zone but already in the form of homogenous plastic melt which is ideally suitable for the extrusion. In order to obtain a uniform thickness, the flow must be kept constant without any stagnation at any point in the of tube or channel. The metering zone thus serve to reduce the functioning of pulse activity that may interfere with the delivery rate in the die cavity.
In the twin screw extruder exist two agitator assemblies mounted on the shafts that are parallel to each other. These shafts rotate in the same direction or in the opposite direction corresponding to the position of the material adjacent to the shaft. This arrangement allows the agitator to wipe on the both sides of the narrow part of the barrel. The co-rotating allows for the better mixing of the material being mixed as the surfaces usually move against. As the screw rotate, the flight of one screw element will always wipe the flank of the next screw thus causing the material transfer from one screw to another. These movements collectively make the content to move in the barrel.
The features of the twin screw are;
The hot extrude must be allowed to cool after leaving the die in order to harden it into the desired dimension. The commonly used way of cooling is by the introduction of the extrudate int a water bath that is usually circulated or just being sprayed on the product. The temperature of the cooling water should vary depending in the need for cooling. The treatment of the extrusion will be an issue of the polymer under investigation.
The melt flow index is used to analyse and to determine the flow properties of the melted plastic. This method calculates the flow of the melt out of the die after every 10 minutes. The grams of the sample polymer are heated and the melt forced to flow through the capillary using a weighted piston. The weight of the material that has been extruded in the time interval indicated is the flow rate. It is therefore very obvious that if the MFR is a large value then the flow is high, the polymer under investigation has short chains and a relatively low molecular weight. The height melt index therefore is used to indicate a low molecular weight while a low index indicates the large molecular weight. The viscosity is also shown in the same set up. The high MFI indicates the polymer flows very quickly through the apparatus and this implies a lower material viscosity. It is important to note that though the melt flow index gives solution to this particular task, it is just but an approximation of the melting conditions whose values are subject to considerable values of error. It is also not reliable when it comes to the determination of the actual velocity.
The purpose of this project is to investigate the practicality of manufacturing 3D printer filament the normal recycled PP. The project focuses on finding out why the commonly used plastics are never used in the manufacture of the 3D printer. The major objective is finding out the feasibility of producing a good quality PP filament suitable for the 3D printing.
This is an upcoming technology that aims to produce products from the raw materials under the instruction of the computer. The very idea of this technology is where the 3D printers directly fabricate a three -dimensional tangible object by putting one layer over the other layer with time. The prototype or the desired object is first of all drawn or created using Computer Aided Design software and later used in the Additive Manufacturing.
Although this has been applied even with the metals, the plastic usage has been very common. The high demand for the plastics in the 3D printing can be as result of the desired qualities of the product which ranges from the durability, flexibility and the durability. The plastic is fed to the print as a solid that is normally heated into a molten state.
The growth of the 3D is still angered on the plastics a s the major raw material. This has however raised a lot of environmental issues. There is a need to look into this and make the 3D printing an environmentally friendly activity. In order to a achieve this, a 3D Printer filament that is made from the recycled plastic can be used instead of the virgin raw plastic material. The extensive research that has been done by many companies has led to the successful conversion of the plastic waste into a 3D printing filament with some perfect quality.
The most commonly used technologies are of the ABS and PLA thermoplastics in the 3D printing industry. The PP is not commonly used in the industrial consumer industries. Despite its admirable properties like high chemical resistance e.g. resistive to the corrosion by the acids and other solvents that are organic in nature, it has not taken the proper rooting in the industries of the 3D printing. The turning of the plastics into a PP filament for the later application in in the 3D printing is a very good idea.
The primary objective of this project is to find the suitability of the recycled PP 3D printing filament. To enhance faster understandings, the work has been divided into the following sub objectives
The Additive Manufacturing has existed in the manufacturing industry for more than three decades as Avery established and strong technology. Upon its introduction, the layered technology was referred to as the Rapid Prototyping. This was so because the technology the only used in the production of the sticky and brittle parts. These brittle substances were majorly used as the prototypes for the new product design before they could be subjected to the real applications and this helped in saving of money and time. In a round the year 2000, the attention of the manufacturers together with the users has been finally shifted from just prototyping to making end product using the 3D printers. This is because the 3D had developed the much-needed accuracy in its operation. There was an improved accuracy in the quality of the output and the software performance. Today the 3D printers have a broad spectrum of uses in nearly every corner of manufacturing firm. Other than the prototyping use, other areas have also benefitted from the latest development including tooling and casting, the automotive industry and medical and dental sector.
The idea is just but an improvement of the 2D printing technologies that has been seen in the inkjet that have their operations in the office. In the 3D technique, it includes any manufacturing process in which the virtual solid model data is converted into a 3D output layer by layer. The expected geometry of the figure is copied well into the AM machine without any alteration such as drafts, undercuts etc for the manufacturing process to be carried out.
To 3D print an object, the first thing is to create the virtual model in the computer. This is usually accomplished by using the computer Aided and design applications. After this stage is complete which the digital part is, it is taken through slicing software that will break it down into distinctive layers. The interpreted data is then sent to the 3D printer that does the fabrication and completes the 3D printed product.
In the three most common 3D printing technologies, SLS.SLA and FDM, the first two always use lasers to produce the prints in the industries. The project however focusses on the Fused Deposition Modelling. This choice is guided by the consumer 3D printers’ desktop present. The FDM is an extrusion technique that exclusively handles the plastics as the raw material. This is the kind of print works by melting the plastic filament and this done through a heated extruder head. The layers are made one after the other to ensure proper bonding. This process will be executed according to the supplied data to the printer.
The materials for the 3D printing process are of the wider range including the plastics, composites, resins etc. These raw materials can be used in various states such as filament, powder and pellets. The main raw material however is the plastics or the polymers. These products have had the wide range of application and this has them part of the modern life. The FDM process exploits the two most common polymers and these include PLA and ABS which are consumed in the filament form. The other known types of the materials used for the commercial 3D filament include the PVA, HIPS, PET and PA.
Plastics are all divided into two groups. These groups are the thermoplastics and the thermosets. The thermoplastics are usually solid at room temperature. When they are heated, they become soft and flexible and can be placed inside the mould or other shaping device to obtain the desired shape after cooling has been allowed. The distinctive characteristic which allows it to be reheated several times makes them unique on their own and preferred for the various work. The thermosets on other hand cannot be remoulded once they have been heated.
When introduced inside the mould, thermosetting plastics will undergo a crosslinking reaction that take longer to cool. This means that such plastics cannot be remelted after they have been once shaped. The ABS and PLA are actually the leading polymers consumed in the 3D printing, and the PP material though under research is classified here as a thermoplastic too. Based on the morphology, thermoplastics can be grouped into two. Amorphous carbon polymer a crystalline polymer
The amorphous polymer has the molecules coiled randomly with distinctive entanglement. The polymers with the crystalline structures have their molecules arranged in a regular and structural pattern. The degree of crystallinity has a lot of influence on the characteristic of the polymer.
This is a thermoplastic polymer that is produced when a propylene molecule is polymerised. The polymerised propene molecule usually forms into three basic chain structures. These structures are determine using the methyl groups present in it. In the second PP chain, the methyl group is attached antenatally in the backbone chain. The third structure has that random distribution of the methyl group in the chain. It is important to note that of all the three structures, only isotactic arrangement as its molecule packed together. It is therefore commonly preferred for the commercial uses. It is crystalline a lot and has the properties required for the plastic work Petrick, I.J. and Simpson, T.W., 2013. 3D. The properties of the polypropylene will depend on the tactility, molecular weight, the proportion of the comonomer and finally the molecular weight distribution. The PP products offer a very good chemical resistance like the acids, perfect environmental stress cracking resistance, good detergent resistance and a lot of ease to operation. The unique ability to be manufactured by different methods such’s extrusion and injections very important.
The PP has numerous applications which include in the automotive industries, building construction, pipes and fitting fibres and furniture. It has also replaced the PET, PC and ABS in the kitchen applications or making the appliances. As for the PP application in the 3D printing, it may be strange and difficult to print due to the serious warping and poor surface adhesion in the individual layers. The reason why PP is of low Flexural and Youngs Moduli is de to its ability to elongate explains why it is difficult to print the living hinges for the PLA lids which are brittle and always tend to break rather than to bend. When the impact resistance is looked into, the PP has a significant more or high amount of impact strength than PLA.
A polymer of the PP
The material provided to facilitate this research is called CIRCO recycled PP that has been produced by Fortum company. The main physical and chemical properties of this compound are as shown below.
Density at the temperature of 23 degrees at a value of 905kg/m, The Tensile modulus of 1.3 GPA, Impact strength of 60 among others. The first step in the recycling starts by separation from other sources of plastics. The recognition of the best type is done using the resin identification coding system. This code is always printed on the bottom of any product that has been dealing with that particular plastic. The codes are used for different operation so that different polymers can be recycled separately to avoid any possible contamination. The items made from the PP have the universal identification number “5” that is enclosed in a triangle.
In the most cases, the PP is blended with PET to produce the plastics that are used for the making of beverage bottles. In the above requirement, the identification is done by using float/sink method. In the separation of the PP and the PET, the PP floats while the PET sinks. It is recommended that the sorting out is done on the basis of the colour. A number of companies separate the plastics before they are processed. The separation process gives a better-quality Ladd, C., So, J.H., Muth, J. and Dickey, M.D., 2013. If the plastic was uniform, it is taken through shredding process that involves turning into flakes that can be resold as recycled goods. The recycled PP can be further processed by extrusion to produce a very denser plastic pellet. The major challenges in the recycling of the homogeneous PP come from the easy degradability of this polymer during the recycling process. The PP is affected by the thermal degradation and the bonds between the carbon and hydrogen are broken. The mechanical stress and ultraviolet radiation also contributes to this structural alteration. The elongation makes the impact strength reduced.
The level of success while using the 3D printing is basically determined by the quality of the filament which is dependent on several aspects. The diameter of the filament is very crucial. It describes the variation in the quality. The filament therefore should be maintained at the constant value. In most of the spools the filament diameter is 0.75 mm to 3 mm. The best tolerance should be at 0.05. The other important characteristic is the filament roundness as distributed a long the length. The filament irregularity in the diameter will make the extruder to fail and therefore no plastic goes to the hot end. When the extruder motor is not strong enough to push the filament through will definitely give an irregular product. The important characteristic for any successful 3D print is the constant filament thickness and roundness as distributed across the full length.
Sometimes the present of the impurities may lead to the poor melted plastic viscosity with the solid particles blocking the extruder nozzle and this is translated to the high cost of time and money.
Their sizes are measured by the size of the 3D they are printed on. The Dutch Manufacturers invented the printing welder so to get a machine that look like the 3D printer. Federal suggested that the innovation should be an armed robot with a print head that has a strong metal that can support the structure. Though they were able to print a model of abridge that span canal in Amsterdam through the work of Autodesk and Heijman Construction Company and this invention encourage field of Construction Automation
This the use of the3D printing technologies layer by layer leading to addiction. This method though as some drawbacks e.g. the time taken for completion of the work and tactics of building –up limits the potential material, lead to introduction of flaws and decreases an object ultimate strength.
Joseph Desimone the Carbon 3D CEO was inspired by a morphing bad guy from a molten puddle in Terminator 2 and their continuous product which he changed to monolithic forms of out a pool of polymer. The Desimone printer was to be 1000 times faster whereas the printer that was in the market was 25 to100 times.
The success while using the 3D printing is basically determined by the quality of the filament which is dependent on several aspects. The diameter of the filament is very key
Power Personal Application is where the 3D printing technology is enhanced to better people’s lives through biomedicine. Many people are helped through 3D printed implant and prosthetic where paralyzed people have walked. Federal introduced the Ekso bionics which is fixed to a person’s body perfectly to enable a person which a spinal disorder walks without the use of their leg ability to walk and to Amanda Boxtel a wheelchair victim of many year was given a 3D system which scan his whole body where the parts were fitted e.g. the spine, shin and the thigh which were integrated seamlessly with an exoskeleton moving parts and he walks again.
The engine in rocket is a no game-piece to be sold on Etsy. . Therefore 3D has got an option of text and retest at a quick and cheaper cost than any other techniques in manufacturing. Elon Musk and his engineer at space X been using the text and retest with their 3D printed Super Draco rocket engine. According to Federal , primarily 3D printed and reusable rocket has brought new opportunities in mining colonization and in space travel.
Being that 3D brought about space travel, to the injured , and to manufacturers but to the household phase of 3D printer allow the anybody willing to produce as many goofy plastic stuff as they can. The XYZ stands out all the industries which aimed to produced everyman market.
Federal also said that innovation is the price point and massive sales distribution. Simon Shen commented in china, the government planned to deliver 3D to every school.
This is the introduction of technology that I based on cell phone. A printer that uses light from cell phone for polymerization of liquid resin was invented by Jeng-YWAN Jeng, a professor of mechanical engineering and the dean of student of the engineering department at the National Taiwan University of Science and Technology. Federal claims that the innovation of phone printer brought a ma
A tensile test of the materials is a mechanical test that is normally performed to get the idea or information about the behaviour of the material. The test always provides the measurements on the properties such as tensile strength, ductility etc
The dumbbell sample is used to carry out the tensile test. It possesses an enlarged shoulder for the gripping purposes Sun, K….et.al, 2013. The gauge length, an area where the measurement is taken is centrally located. It should always be large enough to accommodate the diameter of the material under investigation.
The practical part was performed using the following methods;
Apart from the CIRCO recycled PP, which was used in this experiment. This was necessitated to analyse the mechanical and flow properties of the rPP and relation to the virgin PP. Sabic PP served as a cleaning material during the extraction.
The processor screw has six temperature zones 1 and 2 as the feed section, zone 3 and zone 4 as indicated below. The filament die was utilised throughout the work. During the cooling several cooling methods were used including water bath and cold air.
The production was done using KFM Eco Ex extruder
During the experiment, a number of tests were done so as to optimise extrusion. The aim was to produce a filament of very good quality. According to the expectations, the filament should be round shaped. With the diameter of 1.75mm in which the deviations are missing and the surfaces very smooth without any impurities. The pulling speed is always adjusted to meet or match the extrusion speed as well.
The first test that was done in the first experiment in which the filament pressurised air cooling system was used. The values of the temperatures for the extrusion and speed were noted. It was the exact temperatures as those recorded in the earlier while the virgin SabicPP was used.
During the second testing, to cool down the extruded filament cooling system was fitted with the heating element and the cold- air gun was replaced with the thermostat. The other adjustments were made on the different parameters as compared to the first experiment Petrick, I.J. and Simpson, T.W., 2013. The temperatures were increased at least to obtain a very smooth surface finish and to remove swelled parts that could be present in the first experiment be removed. The results were as those obtained in the experiment 4 previously with the exact parameters.
The operating parameters can be changed easily and continuously and this makes the component to be very versatile. The versatility property allows fort the processing of very many different formulations using the same device or machine
Further adjustments were made to the parameters Zhu, D., Mobasher, B. and Rajan, S.D., 2012. The water cooling bath temperature was increased to try reaching the more roundness of the filament. The extruder was also cleaned up using an upgrade purging compound that is normally used in the plastic laboratory of Arcada. This was done so as to confirm the contamination effects of the compound that usually remains inside the extruder.
The obtained results included the diameter that was measured twice at one point as dimensions of the interval at 1m with the digital calliper. Some diameter parameter such as average diameter and the deviations of max and min reading were obtained.
The MFI test is conducted to determine the flow properties of a melted plastic by the characteristic follow up on the how many grams of polymer flow through the die in the 10minutes.This can therefore be used to deduce the units of this parameter as g/10min.
The extrusion plastometer was used in performing the test for the virgin SabicPP Filament and the CIRCO recycled PP to compare the flow properties besides taking the same comparison for the filament and the ABS and PLA that are available for commercial consumption and to see if the there is any correlation. This test was scientifically done and conventionally accepted. Then 5 grams of the sample were placed inside the barrel with the piston being inserted later on. The material substance was pre heated for 5 mins in the pre-set initial temperatures before applying a load of 2.16kg with the cut off time set to 5 seconds. The cut off time was increased to 10 seconds. The trial and error results analysis
The shrinkage rate is a term that is used to describe the value for predicting how much difference there will be between the plastic product when it is first moulded and the plastic product after it has cooled to the room temperatures or when the heat source is removed. In the science practice, everything except water will always expand when it is heated and upon cooling shrinks. In this line not even, the plastics are exempted. It is however important to note that the value of the shrinkage rate will always vary depending on the type of the plastic used. The time that is allowed for the cooling and heating is also a factor of the consideration. This value is referred to as the shrinkage rate and it will always be given in the form of inches per inch. This expression implies that the material under the investigation will change its dimensions in the inch while the change is in terms of the inches.
It is of common knowledge to the engineers that shrinkage is unfortunate and unavoidable fact of life especially plastics which shrink as they cool from viscous liquids to solids though their shrinkage depends on the material of the plastics. Shrinkage cannot be eliminated but minimizing it is an essential for moulding parts.
Controlling shrinkage can be done when a tool is complete and parts are in motion therefore by use of combination of personal experience, education guesswork, and trial and error, mould engineers intuit ways to change the process setting to eliminate issues.
Shrinking is said to be time consuming since it needs a more detailed understanding how shrinkage behaved and from simulation of software, mold engineers dressed shrinkage as a design process that save time and improve quality of parts
The plastics have been classified as having low, median and high shrinkage. The low shrinkage is from the value of 0.000 to 0.005 inch/inch. The medium shrinkage is from 0.006 to 0.010. The plastics that have the shrinkage value above this is referred to as the high shrinkage plastics. Shrinkage always happens during melting and cooling of plastic depending on the type of material intern of whether filler or fibre reinforcement present while considering the processing and parts design factors.
This done by considering the state of equilibrium, polystyrene, ABS polycarbonates and many amorphous polymers have many and entangled orientation of molecules like a bowl of spaghetti during process of material melting.
The intermolecular force weakens and moves away then shear force is experienced during injection stage making the individual molecules uncoil and aligned flowing direction. The molecules relax and return to a state of random orientation immediately as the flow ceased while the intermolecular forces compact them together until the drop is enough to freeze them in place. The forces result to a uniform shrinkage, but the relaxation effect the factor significant to further contraction in the direction of flow. The amorphous material and crystalline material linkages will defiantly vary from their values. In the amorphous material, the range of the shrinkage control is just but very high. The materials that are crystalline have the tendency towards a higher shrinkage rate. Their response to the processing parameter changes however is very slow.
They differ from amorphous in such that they have region of high order and their molecule structure are tightly bundled. Instead of using a bowl of spaghetti as an example, these materials resemble spring connected by bungee cords Petrick, I.J. and Simpson, T.W., 2013. They lose their crystalline structure when melted and align to the direction of flow just like amorphous polymer and they do not relax on cooling. They only maintain their orientation in the direction of flow while their molecules recrystallize resulting in high rate of shrinking. However, in this case the direction perpendicular to the flow is the most affected. Most of crystalline materials such as high density polyethene, polytetrafluoroethylene (PTFE) and isotactic polypropylene shrink more than semi-crystalline materials. The molecules form crystalline region as the polymer cools by allowing the structure to fit together very tightly, making them denser thus more shrinking.
Fibres counteract shrinkage when introduced into plastic due to molecular orientation. Fibre neither contract nor expand with the change in temperature but they tend to reduce shrinkage in their orientation direction and increase shrinkage into their transverse orientation. The best example is a polymer with a glass fibre which will mostly shrink in the cross direction than in longitudinal direction thus unsuitable for close tolerance projects Ramesh, M., Palanikumar, K. and Reddy, K.H., 2013. The unfilled resin usually has a higher shrinking rate than the filled resins. This is because the filled resins are made of different material such as glass fibre, wood and mica that take part in their property changes.
Wall thickness is factor shrinkage due to the quantity of crystallinity in the material which may contribute in affecting the total shrinkage potential. Walls that are not uniformly thick always have different rates of cooling throughout their parts Mueller, B., 2012. While thin walls cool faster though have a lower crystallinity and shrinkage rates. Thick walls have slower cooling rate but have a high crystallinity and shrinkage rates. However, in amorphous materials the orientation effect is reduced by the increasing thickness of a wall. Therefore, maintained uniform thickness of a wall help in avoiding variation in shrinkage which can result to warpage.
The mechanical properties of lignin-PP and lignin-RPP composites are as shown in the table below. From the results it can be observed that there is a close resemblance in the values of the tensile strengths of composites based on RPP and those based on PP. a kind of crosslink that exits in RPP and is likely to be generated during the process of production of RPP is responsible for the slightly higher value of the tensile strength of the composite. The findings also established that there is a proportionate increase in the tensile strength of both RPP and PP as the amount of lignin in the two compounds was increased. Such a reduction may be owed to the fact that there is low compatibility between lignin and matrices of PP.
The tensile properties of recycle PP is also influenced by the compatibility of the MAPP and the lignin hydroxyl griups. The findings ffrom this study however illustrated that there is a relatively very low and insuffiecnt amount of MAPP that can be usable in making improvements on the tensile strengths of the recyled PP composites. Agglomeration of lignin is responsible for the lower properries of tensile strength with the various composites. There is an insignifcant different between the strength of recyled PP and RPP all of which having been lignified. Such findings ilustrate a possiblility of the use of RPP as a substitute in the recycled PP applications.
In most cases adjustment in condition of processing is the most familiar way for mould engineer when addressing shrinkage. Mitigation of shrinkage is possible when the change in pressure, temperature, and packaging and cooling time Ramesh, M., Palanikumar, K. and Reddy, K.H., 2013. Liquid plastics are compressing the molecules into a small volume and then inject more materials into mould to compensate for shrinkage when pressure is applied on them (plastic liquid). Gating should be done from a thick area to a small area to provide efficient package for the thicker section since gating thin to thick may lead to freezing of the thin section therefore limiting the packaging of the thick section. Speeding up the rate of cooling is also an approach to mitigating shrinkage in that it allows less time in development of crystals and in increase shrinkage. It noted that increase in cooling rate sometimes lead to reduce crystallinity so much that compromises its performance parts. In processed condition, most of materials lose the properties of crystalline that made it an appropriate choice as a part of first place. The cooling must be evenly distributed since non-uniform cooling also lead to variation in shrinkage and subsequent warpage.
Comprised into two types: volumetric and linear.
Volumetric shrinkage
This is a shrinkage result from thermal contraction which affects all polymers and /or crystallization of semi- crystal polymers. It is defined by the extent on which materials change the volumes during solidification process. Plastics generally shrink up to 25% during the injection moulding process on other hand volumetric contacts the part in all dimensions
This shrinkage result in production of a type of warpage called bowl. The bowl occurs in perimeters with more volumes thus stay hot and shrinks more resulting to pop up in area of the centre. Whereas a saddle is caused by freezing of the perimeter but the centre continues shrinking thus pulling perimeter leading to buckle and/or twist to maintain its length.
Plastics are subjected to a new set of features which affect their way of shrinking when they are injected to mold. This is due to a specific shear and extension force production action of polymer during the packing and the filling phase. At this stage the plastic molecules align themselves to the direction of flowing polymer. Their alignment defines a linear shrinkage but the orientation may vary in their direction and magnitude which mean that most polymers shrink more in a parallel flow than in a perpendicular flow
The alignment of the molecules can be sometimes unpredictable or just becomes too difficult to predict. In such cases the extensional forces do play a major role in aligning the molecules that are usually perpendicular to the flow. The extensional flow is usually generated from within the centre and the flow spreads outwards and this causes the chains which orient themselves in the multiple directions. There is no distinct pattern in the skin laminates. A high shear rate characterises the outer laminates with the frozen layer. The transition laminates have relatively low shear medium rates without specific orientation. The inner laminates have equally lower rates and usually tend to be oriented in the perpendicular to the transverse section flow Wen H. 2013.The influence of the flow is dependent on the thickness of the part under the effect. The effect can be distributed using the gate type and the location. When the part is characterised by both the extensional orientation which is usually at the middle and the radial orientation that only affects the outer laminates, the shrinkage will therefore determine the direction of the warpage. This will also be dependent on the material being filled.
The transient flow refers to the flow front that usually changes the direction during the filling that results from a filling imbalance. This change in the direction usually creates the directions in the shrinkage and the result is creation of the residual stress in the internal part.IT is important to note that the intensity of the stress and them being parallel and perpendicular will directly influence the flow rate and increase the warpage. This situation or the scenario can be explained as anisotropic shrinkage since it only describes the shrinkage that varies with the direction. This is different from the isotropic that describes the flow or the shrinkage that take place in the same direction. In order to control the shrinkage in the plastics whether for the increased value or for the decreased value depending on the required or desired value, the following parameters must be regulated or their combinations in the right quantity achieved.
The shrinkage of the plastic can be controlled by playing with the temperature values when plastics are still within the hot barrel. The high temperatures are obviously responsible for the higher shrinkage values that are in some of the plastics. This is because in the plastics, as the temperatures increase the individual particles expand and take more and more spaces Kempen, K. et al 2012. It therefore follows that the higher the expansion the greater the expansion. When the temperatures are lowered in the barrel, there is lower degree of the amount of the shrinkage in the plastics too. The general conclusions in the project are that the shrinkage value can be up to 10% that is proportional to the temperatures.
The shrinkages can therefore be controlled by regulating the temperatures of the mold when it is inside the barrel. Under the normal conditions the hot mold will always create less shrinkage as compared to the cold mold. This phenomenon is as a result of the cold mold that solidifies the plastic surface sooner than the hot mold resulting in shrinkage before the full injection pressure is applied to the barrel. A hot mold however, allows the individual molecules to continue being mobile and be compressed by the injection pressure before solidifying.
The pressure for the injection has a direct influence on the rates of the shrinkage. The increase injection pressure implies lower shrinkage rate. The effects of the high temperatures are that it pacts the individual particles of the plastic together. The tightly packed molecules are not able to move and this translates into lower shrinkage as the cooling process continues Hassan, A.M.T., Jones, S.W. and Mahmud, G.H., 2012. As the pressures are controlled so are the shrinkage rates. This means that controlling the shrinkage will only be possible before the solidification is achieved. If the controller decides to relax the pressure, the shrinkage will increase since the molecules have been allowed to be mobile once again.
Conclusion
The results that were obtained were actually in much agreement with the existing values. It is very evident that the properties of the plastics can be improved by addition of the components that are usually regarded as the impurities. These materials however, have no significant effect on the plastic shrinkage. The knowledge of the properties of the 3D printing was found to be very useful in the industries.
References
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