Some Studies On Microcellular Thermoplastics Biology Essay


Microcellular injection molding is the latest technology for Indian injection molding industry. Although first microcellular injection molding was introduced to world market by Trexel Inc. USA, in 2002 but in our country this technology is still not known to even many big injection molding processors. Indian Institute of Technology, Delhi is the first among all research institutes and industry to have microcellular injection molding machine.

This Ph.D. work primarily aims to develop improve the microcellular injection molding technology for the fabrication of open cellular cell morphology. Although there are some research group working on the same task but till date no breakthrough have been reported yet. Although the task is not easy but we firmly believe that big success comes in a hard way. It was observed that microcellular injection molding is well developed in terms of process control and most of the key processing parameters close loop controlled for better accuracy and repeatability. But we observed that there are negligible efforts for the process control at mold level. One of the obvious reasons for this technological gap is because of lack of automation requirement at mold maker's level. Therefore it has been decided to fill this gap in technology. In proposed work, effort will be made to incorporate state of the art process control and data acquisition system so that foaming process could be controlled and followed inside the mold also.

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A expandable cavity mold, equipped with cavity pressure cum temperature transducers, rotary position encoder, servo electrical motor along with heating/ cooling arrangement on the both the halves of the mold, was designed. The fabrication of mold is under progress. For the process control and data acquisition NI - Labview software and hardware cards are proposed to be used. There is need to synchronize the controller of injection molding machine (i.e. PLC) with mold controller so that acquired data from mold could be analyzed on same time line of molding process.

It is expected that this work will fill the gap in control technology at mold level and proposed mold is expected to generate thermodynamically favorable foaming conditions suitable for open cellular cell morphology. It is also expected that the proposed setup will yield the processing window for open cellular morphology for various materials. It is anticipated that this work will serve many research work world wide as a reference for the development of open cellular structure with the help of microcellular injection molding process.


1.1 Background and overview

In early 1980s Eastman Kodak was facing a tough competition in United States for highly profitable business of photographic film. Eastman Kodak approached Dr. Nam Suh, Massachusetts Institute of Technology to develop a new technology that could save material without compromising on properties and manufacturability. Dr. Nam Suh developed the technology that incorporates tiny bubbles into the polymer. This was beginning of microcellular technology and continued during the 1980 and early 1990s at MIT.

The credit for commercialization of microcellular technology goes to Trexel, Inc.(earlier named as Axiomatics Corp.). Under the exclusive patent rights from MIT, Trexel Inc. developed the microcellular process for extrusion, blow molding and injection molding applications. Trexel calls the new technology the MuCell microcellular process; MuCell and Trexel are registered trademark of Trexel, Inc. Trexel, Inc. also began to license the MuCell technology to other companies for use in their commercial process. At present most of the reputed equipment manufacturers like Engel, Demag, JSW, etc., have got the license from Trexel and producing injection molding machines with MuCell technology.

Introduction to Microcellular Injection Molding

Physical blowing agents such as CO2 or N2 gas (above their critical point) are used in microcellular injection molding process. Super critical CO2 or N2 is termed as super critical fluid (SCF), which is mixed with molten polymer in the machine barrel and injected into the mould. Super critical fluid is continuously fed to the barrel of injection molding machine with the help of two stage high pressure rotary compressor and high capacity gas accumulator. The screw design is also optimized for the single phase polymer/ gas solution. Shut off nozzle is also incorporated to hold the polymer/ gas solution after plasticization and to avoid any unwanted pressure quench causing phase separation.

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The microcellular injection molding process involves four steps, they are as follows;


A supercritical fluid (SCF) of an atmospheric gas (CO2 or N2) is injected into the polymer through injectors installed in the barrel to form a single-phase solution. The SCF delivery system, screw and injectors are specifically designed to facilitate the rapid dissolution of the SCF in the polymer.


A large number of nucleation sites (orders of magnitude more than with conventional foaming processes) are formed throughout the polymer during the molding process. A substantial and rapid pressure drop is necessary to create the large number of uniform sites.


Cell growth is controlled by processing conditions. Precise control of pressure and temperature is achieved through specifically-designed machine controlled hardware and software.


After injection into the mould, part shape is controlled by the mould design. Although mould modifications are not required, in some cases modifications will optimize the benefits of microcellular technology.

1.3 Types of Microcellular Foam

There are two types of microcellular thermoplastic foam; close cell and open cell thermoplastic foam.


Close cell foam morphology is characterized by uniform bubble size with thin wall separating the bubbles. Scanning electron microscope (SEM) micrographs of a typical closed microcellular polymer cross section is shown below.


Open cell microcellular thermoplastic foam offers a range of properties well suited for many micro porous material applications. The open cell structure allows particles and fluid to flow through the material, resulting in a product that can be used as a membrane. The structure of micro cell foam in polymers displays the formation of thin walls between cells. Where the bubble impinges they flatten to form thin walls of material in which they are growing. When these films between adjacent cells becomes unstable and break randomly, an open cell structure develops. Scanning electron microscope (SEM) micrographs of typical open cell microcellular foam cross section is shown below.


2.1 Historical aspects

Historically, microcellular polymers are not new to us. It can be observed mainly in thin sections and the high shearing regions of structural foam. Dr. Nam Suh and his team of students at the Massachusetts Institute of Technology invented microcellular processing in the early 1980s. The advantageous of this technology includes reduced material cost and to promote the material toughness incorporation of micron sized near spherical cells that act as crack arrestors by blunting the crack tip [1]. The solid state batch processing - microcellular technology was discovered at the Massachusetts Institute of Technology (MIT) from 1980 to 1984 [1] , and in 1984, the first U.S. patent on microcellular technology was granted [2].

The commercial application of this technology started in 1995 by Axiomatics Corp. (later renamed Trexel Inc.). Trexel Inc. started extrusion based microcellular processing. Then, the injection molding system with plunger for injection and screw for plasticizing and gas dosing was developed in Trexel Inc. in collaboration with the Engel Canada around mid - 1997. After the success of plunger based microcellular injection molding, the first reciprocating screw injection microcellular molding machine was developed by Trexel and Engel in 1998 [3]. This machine was a milestone in the commercialization of microcellular injection molding systems. This type of machine is still number one choice among the processors around the globe. Trexel developed first microcellular blow molding machine in 2000.

MuCell® is the most well known trade name for this technology, licensed by Trexel Inc. Many research groups and injection molding companies were involved in the development of this technology, prior to Trexel's announcement of MuCell®. However, they did not finish the commercialization of their technologies for real applications. The MuCell ® technology uses a reciprocating screw as the SCF dosing element, and the SCF is injected into the reciprocating screw through the barrel. It makes full use of the shearing and mixing functions of the screw to quickly finish the SCF dosing and to maintain the minimum dosing pressure in the barrel and screw for the possible continuing process of microcellular injection molding.

In addition, two other trade names of this technology were found later on; Optifoam® licensed by Sulzer Chemtech [4] and Ergocell® licensed by Demag (now Sumitomo - Demag in 2008) [5]. Optifoam® is a microcellular technology that uses a nozzle as the SCF dosing element. It is a revolutionary change to the traditional SCF dosing method, which adds gas into the barrel. This unique, innovative idea has a special nozzle sleeve made of sintered metal with many ports to let gas go through as tiny droplets. On the other hand, the melt flow through the nozzle is divided into a thin film between the nozzle channel and the sintered metal sleeve. As a result, the gas can diffuse into the melt in a short amount of time. The gas - rich melt is then further mixed in a static blender channel that is located in the downstream of the nozzle dosing sleeve. The advantage of this technology is that the regular injection screw and barrel do not need to be changed. The regular injection molding machine in existence can be easily be changed to use Optifoam® process.

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TABLE 1: Summarizes the technologies of commercial microcellular injection molding processes.

However, only some of these applications have been successful [4]. At K2001, Demag Ergotech introduced its Ergocell® cellular foam system [5]. Ergocell® technology has reached an agreement with Trexel to have their customers pay a reduced price to the MuCell® license when using Ergocell ® technology legally. The Ergocell® system is essentially an assembly of an accumulator, a mixer, a gas supply, and a special injection system that is mechanically integrated between the end of the barrel and the mold to put gas into the polymer and create the foam upon injection into the mold. A special assembly needs to be created for each screw diameter. Additional hydraulic pumps and motor capacity must be added to operate the mixer and accumulator injection system. The system only uses carbon dioxide as the blowing agent.

The latest developing foam technology from IKV is the ProFoam® process [6]. It is a new and cheap means of physically foaming injection molding technology. The gas, either carbon dioxide or nitrogen, as the blowing agent is directly added into the hopper and diffuses into the polymer during the normal plasticizing process. The plasticizing unit of the molding machine is sealed off in the feeding section of screw for gas adding at pressure, but feeding of pellets of material occurs at normal conditions without pressure. With this ProFoam® process the part can reduce up to 30% weight via the foaming.

Trexel continues to develop and support the microcellular injection molding process worldwide. There are already over 300 MuCell® injection microcellular molding machines in the world. Through the efforts of many more organizations, more and more advances are being made for the microcellular injection molding process. These organizations include not only original equipment manufacturers (OEMs) licensed from Trexel but also numerous unlicensed organizations, such as universities. All of them are contributing to further advances in microcellular technology.

2.2 Solid state batch foaming

The microcellular batch processing technology was invented at the Massachusetts Institute of Technology (MIT) from 1980 to 1984 [1], and the first U.S. patent on microcellular technology was issued in 1984 [2]. Jonathan Colton showed a heterogeneous nucleation mechanism from the effects of additives in the polymers at certain levels of solubility [7]. Jonathan Colton also investigated the methodology of foaming for semi-crystalline polymers such as polypropylene (PP) [8]. The gas can be dissolved into the amorphous structure because raising the temperature beyond its melting point eliminates the crystalline phase of PP. This heterogeneous nucleation is now dominating today's industry processing. On the other hand, the crystalline material, such as PP, has been used for microcellular foam by Jonathan's method in the industry practice now. Chul Park and Dan Baldwin studied the continuous extrusion of microcellular foam. Chul Park investigated both, the dissolution of gas at the acceptable production rate and the application of a rapid pressure drop nozzle as the nucleation device [9]. Dan Baldwin studied the microcellular structure in both crystalline and amorphous materials [10]. Sung Cha investigated the application of supercritical fluid, such as CO2, to dissolve the gas faster and to create more cells [11, 12]. With supercritical fluid, the cell density was increased from 109 cells/cm3 to 1015 cells/cm3.Vipin Kumar also used thermoforming supersaturated plastic sheets to study the issues of shaping three - dimensional parts [13]. Sung Cha also found that the large volume of gas in polymers decreases significantly with the glass transition temperature of plastics. Therefore, simultaneous room temperature foaming is possible. All of these pioneer contributions are fundamental to microcellular foam technologies. Through many people's creative research, this technology has completed the laboratory stage and transitioned to industry application.

2.3 Advantageous of microcellular foam

The microscopic cell size and large number of cells in microcellular material can reduce material consumption as well as improve the molding thermodynamics, which results in a quicker cycle time. Additionally, the process is a low - pressure molding process and produces stress - free and less warped injection molding products. The major differences between conventional foam and microcellular foam are cell density and cell size. The typical conventional polystyrene foam will have an average cell size of about 250 microns, and a typical cell density in the range of 104 - 105 cells/cm3. Microcellular plastic is ideally defined with a uniform cell size of about 10 μm and with a cell density as high as 109 cells/cm3 [1]. It is possible to make this kind of microstructure cell density with microcellular injection molding if material and processing are controlled very well. The scanning electron microscope (SEM) morphology of glass - fiber - filled PBT is an excellent example of microcellular injection molding that almost matches the ideal definition of microcellular plastics made by batch process. It is made by using 30% glass fiber and reinforced polybutylene terephthalate (PBT) with a 15% weight reduction. The cell density is about 8Ã-108 cells/cm3, with an average of 15 μm of uniform cell distribution. However, this microstructure is not always the result of microcellular injection molding.

The microstructures of industrial parts from microcellular injection molding are characterized by an average cell size on the order of 100 μm, although the real cell size can be varied from 1 μm to 100 μm. However, the cell structure of the microcellular part with microcellular injection molding might not necessarily be defined as the cell density of 109 cells/cm3. The microstructure of ABS has a cell density of about 106 cells/cm3, and it definitely shows a microcellular structure with an average cell size of about 45 μm.

Microcellular foam overcomes the major disadvantages of conventional foam, such as a long cycle time and a thick wall. The most important advantages of microcellular foam can be summarized as under;

1. The main advantage of structural foam molding (one of the conventional foams) is to increase stiffness without increasing the weight of the component. Microcellular foam can be made for this target as well, by redesigning thin wall structures and by creating a nice cell structure to save material (weight reduction by a thin wall) and cost (shorter cycle time).

2. The microcellular process can be used for thin - wall solid parts that are difficult to make full mold filling from flow restrictions, which results in either clamp tonnage shortage or injection pressure limit.

3. Microcellular technology allows mold filling without foaming because the gas - rich melt reduces viscosity significantly.

4. The microcellular process almost eliminates all dimension stability problems, such as sink mark, flatness defects, warp, and residual stress after molding due to the elimination of pack and hold phases during molding.

5. The microcellular process dramatically reduces cycle time if the part is designed properly.

6. Microcellular processing equipment can be designed to save more energy since the peak of injection pressure is not necessary and also saves up to 50% of clamp tonnage.

The disadvantages of microcellular foam are the same as conventional foam, such as poor surface finish, strictly balanced runner system for multi-cavity mold, nontransparent application only, and complicated processing technology.

2.4 Morphology of microcellular foam

2.4.1 Batch process vs. injection molding

Figure 1 (a & b) shows a typical SEM photograph of a microcellular HIPS part produced by the solid state batch process and microcellular injection molding process.

Figure 1: (a). Typical SEM of HIPS microcellular of batch process (white bar indicates 10 μ m), saturated by N2 gas. Average cell size: 7 μm. (b). Typical SEM of center core for a HIPS microcellular injection molding process (white bar indicates 100 μ m). Weight percentage of N2 gas: 0.8. Average cell size: 90 μm.

Although the same material is used for the samples shown in Figure 1 (a & b), two different processes have many differences as under;

1. The cell density from the injection molding process is much less than the cell density from the batch process.

2. The shapes of cells are different because of different processes. The part from the batch process has irregular shape and uniform thin wall thickness among the cells. However, the part made by the injection molding process has the cells with spherical shape and much thicker and non-uniform wall thickness among cells.

3. The average cell size of injection molding part in Figure 1(b) is about 12 times larger than the cell size of the batch process part.

4. The micrograph of the sample from the batch process shows some partial open cell structure, whereas the micrograph of the sample from injection molding process displays entirely close cell structures.

5. The microstructures of the overall cell structure between the batch process and the injection process are also distinctly different. The cell structure of the sample from the batch process is uniform across the whole thickness as shown in Figure 1(a). However, the sample from injection molding has obvious skin - core architecture, as shown in figure 2. There are possibly three different layers existing in the microcellular part made by injection molding, center core layer, skin layer, and transition layer between skin.

Figure 2: Typical SEM with skin - core structure of a microcellular part, a cross - section view for PC [3], N2 gas. Average cell size: 45 μm.

6. The injection molding parts may have some big cells surrounded by nice small cells in the center core (see Figure 2). It is because the gas mixing and diffusion may not be uniform in the melt. On the other hand, the quality of nucleation and the number of cells in the part are determined finally by molding conditions as well. The batch process does not have this kind of void in the center since the gas diffuses uniformly from both sides of surfaces to the center.

2.4.2 Amorphous vs. crystalline materials

Typical amorphous materials for microcellular of injection molding are general purpose polystyrene (GPPS), polycarbonate (PC), acrylonitrile/butadiene/styrene (ABS), and high - impact polystyrene (HIPS). Usually, amorphous material will have a wide processing window for microcellular injection molding, along with excellent cell architecture.

GPPS is also the easy material to work with to create an excellent microcellular structure. Different cell structures with different gases in the GPPS microcellular foam with 16 weight percent of CO2 gas as blowing agent create larger cells and thin wall thickness among the cells.

PC is also an easy amorphous material for microcellular injection molding. It exhibit a clear skin - core structure of the microcellular part. The processing conditions are mold temperature 160 °F, melt temperature 580 °F, pressure drop rate (dp/dt) 1.7Ã-1011 Pa/sec, and weight reduction 13%.

Another material, ABS, is an excellent material for the microcellular part since it is the kind of blend among three different components. The blend of different materials creates a heterogeneous nucleation so that high density of cells becomes possible. However, similar to the HIPS is that the size and distribution of rubber phase in the ABS will be the factor to determine the final morphology of the ABS microcellular part. The processing conditions for this ABS microcellular part are 127 rpm of screw (30 - mm diameter, 26:1 L/D) speed, 0.5 weight percent of N2 gas, 13.8 MPa of back pressure of screw recovery, and 480 °F of melt temperature. With 0.102 - m/sec injection speed the calculated pressure drop rate through the nozzle orifice is about 3.4Ã-109 Pa/sec.

Crystalline and semi-crystalline materials have important microcellular injection molding applications and have become more popular since they are widely used for different industries. The typical materials are polypropylene (PP), polyethylene terephthalate (PET), and polyamide (PA). The crystallization during cooling may expel the gas near the crystalloid so that the cell structure of crystalline material may not be as uniform as the cell structure of amorphous material [14]. On the other hand, crystalline materials will have a nice microstructure with fine cells in the core area but will have no clear boundary between skin and core. PP is the semi-crystalline material. For unfilled PP, it is difficult to make a good part of microcellular injection molding. Figure 3 is the morphology of unfilled PP microcellular made by injection molding. The cell sizes vary from 5 microns to 80 microns. The distribution of cells showing in Figure 3 is not uniform, either. However, this is a typical morphology for unfilled PP with microcellular structure. One of the reasons for this cell structure is the different generations of nucleation of cells. Some of the big cells may be from first generation of nucleation that will grow in time and subtract gas from its surroundings. Then, a second generation of nuclei also creates more cells but cannot grow as much as the first generation of cells because of less available gas in the polymer matrix left. Another reason for this cell structure is that the heterogeneous nucleation exists even in unfilled materials since no pure polymer exists in the real processing materials.

Figure 3 Morphology of unfilled PP (white bar indicates 100 μ m), CO2 gas.

2.4.3 Carbon dioxide vs. nitrogen

The same material will have different cell architectures if the foam is made from different gases. Figure 3.20 shows the different morphologies of cell structures in HDPE. Figure 4(a) is an HDPE microcellular sample made by CO2 gas that has large cells. N2 gas samples have the finest cell structure, as shown in Figure 4(b). Although argon (Ar) is an inert gas seldom used for microcellular foam, it makes a nice cell structure in HDPE in Figure 4(c), and may make the largest cell size among all three samples.

Figure 4 Morphology of HDPE with different gases. (a ) CO2 , ( b ) N2 , ( c ) Ar.

(Courtesy of Trexel Inc.)

Similar results of rigid PVC with three different gases were also tested. The sample made by N2 gas shows the finest cell size in a rigid PVC microcellular sample, as shown in Figure 5(b). Argon gas as blowing agent creates a fine cell size that is as small as the cell size made by N2 gas. However, in Figure 5(c) the rigid PVC sample made by argon has a better small size cell structure than the CO2 sample shown in Figure 5(a).

Figure 5 Morphology of RPVC with different gases. (a) CO2, (b) N2, (c) Ar.

(Courtesy of Trexel Inc.)

2.4.4 Closed vs. open cell structure

Open-celled microcellular thermoplastic foam offers a range of properties well suited for many microporous applications. The open-celled structure allows particles and fluids to flow through the material, resulting in a product that can be used as a filter or membrane. In addition, the thermoplastic base material offers many desirable properties, such as high strain to failure. High fracture toughness, low thermal conductivity, low electrical conductivity and good chemical resistance. These properties offer many advantages over other microporous materials.

In microporous materials, small pore sizes allow only small molecules or ions to flow through the media. Microcellular thermoplastic foam contains bubbles, or cells, with average diameters of 0.1-10 microns, and is characterized by bubble densities between 109 and 1015 cells/cm3. By accurately controlling the structure and pore size of the open-celled microcellular foam, it is possible to specify the particle size that is able to pass through the material. Thus, the material can be tailored to individual microporous applications. Currently, there are techniques that produce microporous foam in thermoplastics. Unfortunately, the bulk of these techniques requires complicated processing conditions and use environmentally undesirable materials. As an alternative, a simple process exists that uses an environmentally benign blowing gas to generate microcellular foam. This technique has been well established for the production of closed-celled foam (15,16).

Figure 6: (a) Close celled (b) Open Celled morphology

Typically, closed-celled foam morphology is characterized by uniform bubble sizes with thin walls separating the bubbles. To create an open-celled morphology in this foam, the thin walls between cells must be broken. If these cell walls rupture spontaneously, the open-celled structure can be generated without additional processing steps, equipment, or additives. Recently, open-celled foam has been produced spontaneously with an extrusion technique (17), but the theoretical mechanism for open cell production and the predicted effects of processing conditions on foam morphology were not explained or presented.

The microcellular foaming technique first developed by Suh et al. (15, 16), used nitrogen or carbon dioxide gas as a blowing gas to create a microcellular structure. Unlike other foaming processes that use environmentally detrimental chemicals, the gases used in this process are environmentally benign and require no hazardous material precautions or disposal. The microcellular foaming technique developed by Suh is the basis for the work in this paper. Two methods of production were developed from Suh's technique-batch processing and continuous extrusion processing. Martini (18), Waldman (19), and Colton (20) performed the preliminary work on batch processing. Baldwin (21) and Park (22) performed initial work on continuous extrusion processing.

The structure of microcellular foam in polymers displays the formation of thin walls between cells. Where the bubbles impinge they flatten to form thin films of the material in which they are growing. A two-dimensional representation of this progression is depicted in figure 7. When these films become unstable and break, an open celled structure develops.

Figure 7: Progression to cell impingement

The development mechanism for an open-celled structure indicates that two stages exist bubble growth to impingement and then cell wall thinning to rupture. First, bubbles nucleate and grow. According to Martini (18), bubble growth can be modeled as a diffusion growth mechanism. When cells impinge, the polymer regions between cells become small, and these short distances allow excess gas to quickly diffuse into the bubble. Therefore, cell impingement stops diffusion bubble growth. Second, the walls between the cells thin and rupture owing to the bubble pressure acting perpendicular to the film. A different model is developed for each stage of the process. They are a cell impingement model and a bubble coalescence model. By combining the two, an inclusive open-celled theory is developed. This open-celled theory determines the range for the processing conditions that promoted open-celled microcellular foam formation in polystyrene. The organization of the components used for this open-celled theory is shown in figure 8. To maintain the microcellular structure, cells must nucleate close enough for cell impingement to occur when cell diameters are less than 10 micron (the definition of microcellular foam). This indicates that a critical nucleation density exists. From classical nucleation theory, nucleation density is a function of nucleation time and nucleation rate. Using the equations for nucleation time and rate, the ranges are determined for two processing variables-saturation pressure and foaming temperature. The free film stability criterion is used to determine that the thin walls between impinging cells are unstable and will rupture. The rupture time is determined using the free film kinetics equations. This rupture time corresponds to the foaming time required to develop open-cells.

Figure 8: Open-celled theory flowchart

2.5 Research gap

Some of the important observations from the above literature review till date are as follows:

1. The batch process is capable of producing well controlled cell morphology for closed as well as open celled structure but suffers from following drawbacks

(i). It is a laboratory scale process. It is not suitable for industrial production demand.

(ii). Extended gas saturation time (usually 3-10 days).

(iii). Distortion in 3D geometry of components.

2. Microcellular injection molding process is well developed in terms of process control and productivity yet it has following limitations;

(i). Poor cell morphology as compared to batch process.

(ii). Difficult to control the uniform cell growth in complex 3 dimensional components.

(iii). Open celled morphology via injection molding process is not reported in the available literatures.

3. Effect of nanofillers on foaming have been studied by various researchers but still there is need for a well accepted theory for the mechanism of bubble nucleation and growth in polymer nanocomposites.

4. There is gap in research in the area of the development of nano-mechanics for microcellular polymer nanocomposites.

5. Open celled microcellular polymers have various high end applications such as tissue engineering, scaffolding, drug delivery, filtration, membrane separation, fuel cells etc. etc. There is great demand for development for commercially economical process for the fabrication for open celled microcellular foam.


3.1 Research Problem

The development of open celled microcellular morphology via injection molding technology.

3.1.1 Objective of the research

The main objectives of this research work are the following:

Objective 1: The design and fabrication of a new concept mold i.e. "Expandable Cavity Mold".

Objective 2: The development of data acquisition and control system (DAQ) for process automation at mold level.

Objective 3: The synchronization of IMM controller with DAQ controller for mold.

Objective 4: The development of process window for open cell morphology for various polymeric and composite materials.

3.2 Methodology

The process of injection molding can be explained thermodynamically with the help of pvT diagram of polymeric materials. A typical pvT diagram is shown in figure 9.

Figure 9: pvT diagram for injection molding process.

1 - 2: Injection phase.

2 - 3: Packing at almost constant pressure.

3 - 4: Cooling

4 - 5: Shrinkage

The proposed pvT diagram for expandable cavity mold is illustrated in figure 10.

Figure 10: pvT diagram for expandable cavity mold (injection molding).

In proposed equipment, the gas rich polymer melt will further gain the heat from the hot surface expandable cavity - concept mold. This phase is referred as socking phase (2 - 3) followed by foaming phase (3 - 4) caused by sudden expansion of cavity, resulting in quick pressure quench, to cause the cell nucleation and growth. The line diagram of proposed experimental set up is illustrated in figure 11.

Figure 11: Experimental setup

3.3 Time Management - Gantt chart

A tentative Gantt chart for the proposed research work is shown in Table 2.

Table 2: Gantt chart for the proposed research work


Year 2009 - 10

Year 2010 - 11

Year 2011 - 12







Literature Review

Course work

Mold Design

Mold Fabrication

Process automation

Experimental work

Thesis writing


4.1 Course work

• Registration: July, 2009

S. No.

Course Title

Course Code





TTL 714





PTL 712





HUL 810





CGPA = 7.5

• Literature Review in progress.

• Identification of Problem completed.

• Conceptual Framework developed.

4.2 Expandable cavity - concept mold design

In order to obtain the thermodynamically favorable condition for cell nucleation and growth, the idea of expandable cavity mold was conceptualized first time ever. No such mold is reported yet, in available literatures and internet searches. This mold design is capable to generate the thermodynamic condition as predicted in figure 10 for the development of open celled microcellular foam morphology. The assembly of mold is illustrated in figure 12, while individual mold parts are listed in table 3 along with their isometric view.

Figure 12: Expandable cavity mold.

Table 3: Individual mold parts


Part details

Isometric view


Sprue Puller



Bush Cavity

Back Plate

Cavity back plate


Cavity Plate

Sprue puller assembly

Servo motor

Cavity Block

Sprue Bush

Locating Ring

Guide pillar

Core plate