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Lecture 5.8: Injection molding process Injection molding process is a closed molding process. The process is suitable for both thermoplastic and thermosetting polymer based short fiber reinforced composites. The schematic diagram of injection molding process is shown in figure 1. Fibers and polymer are either preblended or mixed manually and fed into the hooper. The mixture goes into the heated barrel where softening of the material takes place due to heating from the barrel. As the screw rotates, mixing of fiber and resin takes place and at the same time the blend is pushed towards the converging section of barrel where it is injected through nozzle into the mold cavity with high pressure. If natural fibers are used as reinforcement instead of synthetic fibers, sometimes, a separate hooper to feed the natural fibers into the barrel may be provided near the injection end of screw. Because, if natural fibers are mixed with resin and fed to the barrel, fibers have to travel the whole distance of the barrel and extensive fiber damage may take place due to high heating and shearing action for long time. Providing separate hooper, natural fibers will mix with soft resin and readily injected into the mold cavity. The raw materials used in injection molding are given in the table 1. Figure 1 Injection molding process. Table 1: Raw materials used in injection molding process Materials used Matrix Thermosetting: Epoxy, polyester, polyvinyl ester, phenolic resin, Unsaturated polyester, polyurethane resin, Urea formaldehyde, Thermoplastic: polypropylene (PP), polyethylene (PE), nylon, polycarbonate (PC), polyvinyl chloride (PVC), cellulose acetate, polyether- ether ketone (PEEK), Acrylonitrile-butadiene-styrene (ABS), polystyrene (PS) biodegradable polymers such as poly lactic acid (PLA), poly vinyl

Lecture 5.8: Injection molding process - NPTEL · Lecture 5.8: Injection molding process Injection molding process is a closed molding process. The process is suitable for both thermoplastic

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Page 1: Lecture 5.8: Injection molding process - NPTEL · Lecture 5.8: Injection molding process Injection molding process is a closed molding process. The process is suitable for both thermoplastic

Lecture 5.8: Injection molding process

Injection molding process is a closed molding process. The process is suitable for both

thermoplastic and thermosetting polymer based short fiber reinforced composites. The

schematic diagram of injection molding process is shown in figure 1. Fibers and polymer are

either preblended or mixed manually and fed into the hooper. The mixture goes into the

heated barrel where softening of the material takes place due to heating from the barrel. As

the screw rotates, mixing of fiber and resin takes place and at the same time the blend is

pushed towards the converging section of barrel where it is injected through nozzle into the

mold cavity with high pressure. If natural fibers are used as reinforcement instead of

synthetic fibers, sometimes, a separate hooper to feed the natural fibers into the barrel may be

provided near the injection end of screw. Because, if natural fibers are mixed with resin and

fed to the barrel, fibers have to travel the whole distance of the barrel and extensive fiber

damage may take place due to high heating and shearing action for long time. Providing

separate hooper, natural fibers will mix with soft resin and readily injected into the mold

cavity. The raw materials used in injection molding are given in the table 1.

Figure 1 Injection molding process.

Table 1: Raw materials used in injection molding process

Materials used

Matrix Thermosetting: Epoxy, polyester, polyvinyl ester, phenolic resin,

Unsaturated polyester, polyurethane resin, Urea formaldehyde,

Thermoplastic: polypropylene (PP), polyethylene (PE), nylon,

polycarbonate (PC), polyvinyl chloride (PVC), cellulose acetate, polyether-

ether ketone (PEEK), Acrylonitrile-butadiene-styrene (ABS), polystyrene

(PS) biodegradable polymers such as poly lactic acid (PLA), poly vinyl

Page 2: Lecture 5.8: Injection molding process - NPTEL · Lecture 5.8: Injection molding process Injection molding process is a closed molding process. The process is suitable for both thermoplastic

alcohol (PVA), soy based plastic, starch based polymers etc.

Reinforcement Glass fiber, carbon fiber, aramid fiber, Mica, natural plant fibers (sisal,

banana, nettle, hemp, flax etc.)

(all these fibers are used in the form of short fibers, flacks, chopped fibers

and fillers)

Reaction injection molding

Reaction injection molding is one of the advanced processing methods for composite

manufacturing. In this method, two reacting ingredients are mixed by impingement and

injected into the mold cavity. Chemical reaction starts immediately after mixing the materials

and polymerization takes place. Short fibers and fillers can be used as reinforcement which

are added in the containers of reacting ingredients. Polymerization process completes in few

seconds (5-30 seconds). Major portion of reaction injection molding process acts as a high

pressure pump. This process is suitable only for thermosetting resin where a curing reaction

takes place within the mold cavity. Another variation of reaction injection molding is

structural reaction injection molding. In this method, instead of using chopped, short fibers

and fillers, continuous woven fiber mat or chopped strand mat can be used. These fabrics are

already placed into the mold cavity similar to resin transfer molding and resin mixture is

injected over it where resin rapidly reacts, polymerizes and cures to process composite

product. Reacting ingredients such as isocyanates, polyurethanes, and polyamides do fast

polymerization process and cycle time of the process is reduced and production rate is

substantially enhanced. The schematic diagram of reaction injection molding is shown in

figure 2.

Page 3: Lecture 5.8: Injection molding process - NPTEL · Lecture 5.8: Injection molding process Injection molding process is a closed molding process. The process is suitable for both thermoplastic

Figure 2 Reaction injection molding process.

Application

The process is well suited for high production volume at low cost especially for automotive

parts. Typical examples are: Automotive panels, air spoilers, fenders, body panels, fascia, and

pickup truck boxes.

Advantages

1. All types of polymers (thermosetting resin and thermoplastics) can be processed with

injection molding process.

2. Fibers tend to become aligned during injection into mold cavity as they pass the nozzle,

this characteristic can used in the composite part design to optimize directional

properties.

3. In reaction injection molding, no heat energy is required and mold cost is also low.

4. Structural reaction molding is suitable for producing structural parts in high volume at

low cost. Small to large complex geometries of composite parts can be produced with

this technique.

Page 4: Lecture 5.8: Injection molding process - NPTEL · Lecture 5.8: Injection molding process Injection molding process is a closed molding process. The process is suitable for both thermoplastic

Disadvantage

1. Mainly suitable for chopped fiber reinforcement.

2. The initial capital investment is high.

3. Due to high shearing action into the barrel and nozzle, extensive damage to the fibers

may take place.

4. Due to high tooling cost and operation cost, it is not preferred for small production

run.

5. Polymer burning may take place onto the cylinder walls and then peeling into the melt

which appears as black spots onto the surface of composite part.

6. If a little moisture is present in the fiber or polymer, it will appear in the form of

bubble onto the finished part.

7. High fiber reinforcement can not be achieved in this process.