MANUFACTURING PROCESSMANUFACTURING PROCESS(BMFG 2323)(BMFG 2323)

LECTURE 7LECTURE 7

~ INJECTION MOLDING PROCESS~

Prepared and presented by: Masjuri Bin Musa @ Othman

Faculty of Mechanical Engineering(Department of Innovation & Engineering Design)

Universiti Teknikal Malaysia Melaka

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INTRODUCTIONINTRODUCTION

- The process is similar to hot-chamber die casting.

-The pellets or granules are fed into the heated cylinder, and the melt is forced into the mold either

by a hydraulic plunger or by rotating screw system of an extruder.

-Rotating screw type is considered to be of better design because of their improved mixing action,

maintain a uniform melt temperature, and better blending of the materials of any coloring agents.

- The barrel (cylinder) is heated externally to promote melting of the polymer.

- Nowadays, modern machines using the reciprocating or plasticating screw type.

- Produce products such as cups, containers, housings, tool handles, knobs, toys, plumbing fixtures,

electrical and communication equipment components.

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Injection-Molding MachineInjection-Molding Machine

Figure: A 2.2-MN (250-ton) injection-molding machine. The tonnage is the force applied to keep the dies closed during injection of molten plastic into the mold cavities.

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Examples of Injection Examples of Injection MoldingMolding

Figure: Typical products made by injection molding, including examples of insert molding.

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Schematic illustration of injection molding with (a)plunger and (b) reciprocating rotating screw

Process involved:-As the pressure builds up at the mold entrance, the rotating screw begins to move backwards

under pressure to a predetermined distance.

- This movement actually controls the volume of material to be injected.

-The screw then stops rotating and is pushed forward hydraulically, forcing the molten plastic into

the mold cavity and the pressure supplied is usually range from 70 to 200MPa.

-Once when the part has cooled sufficiently (for thermoplastics) or cured (for thermosets), the

molds are opened and the parts is removed from the mold by using ejectors.

-The molds then closed back, and the process is repeated automatically.

-Since the material is molten when injected into the mold, complex shapes with good dimensional

accuracy can be obtained.

-Disadvantage: Residual stress happened in the part due to uneven cooling of the part in the mold.

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Figure shows the sequence of operations in the injection molding of a part with a reciprocating screw

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- Molding of parts also can have multiple cavities and accommodate this particular part design,

molds may have several components, including runners, cores, cavities, cooling channels, inserts,

ejectors just like those used in metal casting dies.

- There are three basic types of molds:

1.Cold-runner, two-plate mold – this design is the simplest and most common used in industries.

2.Cold-runner, three-plate mold – the runner system is separated from the part when the mold is

opened.

3.Hot-runner mold – also known as runnerless mold; most expensive process; the molten plastic is

kept hot in a heated runner plate.

- In the cold-runner molds, the solidified plastic remaining in the channels connecting the mold cavity

to the end of the barrel must be removed as a scrap, which usually is done by trimming.

- This scrap later, can be chopped and recycled.

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Basic types of molds

Types of molds used in injection molding: (a) two plate mold; (b) three plate mold; and (c) hot runner mold

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- In hot-runner molds, there are no gating system, runners, or sprue attached to the molded part;

cycle times are shorter because only the molded pat must be cooled and ejected.

Multicomponent injection molding (coinjection/sandwich molding)

- Allow the forming of parts with a combination of various colors and shapes; eg: the molding of rear-light covers for automobiles which made of different materials and colors.

Insert molding

- Involves with metallic components such as screws, pins, and strips, where by they are placed in the mold cavity prior o injection and then become an integral part of the molded product.

- Common ex: Electrical and automotive components.

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BLOW MOLDINGBLOW MOLDING- Modification of extrusion-and injection-molding process.

Extrusion blow molding

- A tube or preform is first extruded.

-Then it is clamped into a mold with a cavity much larger than the tube diameter and blown outward to

fill the mold cavity.

- Blowing usually is done by using a hot-air blast at a pressure ranging from 350 to 700kPa.

- Typical die materials are steel, aluminum, and beryllium copper.

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Injection blow molding

- A short tubular piece (parison) first is injection-molded into cool dies.

- Next, the dies open, and the parison is transferred to a blow-molding die by an indexing mechanism.

- Hot air is injected into the parison, expanding it to the walls of the mold cavity.

-Further related process is called sretch blow molding where by the parison is expanded and elongated

simultaneously.

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Schematic illustrations of (a) the extrusion blow-molding process for making plastic beverage bottles; (b) the injection blow molding process

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Schematic illustration of a three station injection blow molding machine for making plastic bottles

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ROTATIONAL MOLDINGROTATIONAL MOLDING

- Produce parts such as storage tanks of various

sizes, trash can, buckets, housings, large hollow

toys, carrying cases.

- Most thermoplastics and some thermosets can be

formed into larger, hollow parts by using this

process.

- In this process, a thin-walled metal mold is made

in two pieces (split-female mold), and it is designed

to be rotated about two perpendicular axes.

- The mold is heated usually in a large oven, and is

rotated continuously about the two principal axes.

- This action tumbles the powder against the mold,

where the heat fuses the powder w/out melting it.

Figure shows the rotational molding (rotomolding or rotocasting) process. Trash cans, buckets, and plastic footballs can be made by this process.

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THERMOFORMINGTHERMOFORMING

Figure shows various thermoforming processes for thermoplastic sheet. These processes are commonly used in making advertising signs, cookie and candy trays, panels for shower stalls, and packaging.

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- A process for forming thermoplastic sheets or films over a mold by the assistance of the application

of heat and pressure.

- In this process, a sheet is clamped and heated, and forced against the mold surfaces through the

application of vacuum or air pressure.

- The sheets used in thermoforming usually are made by sheet extrusion in a form of coil.

- The mold is generally at room temperature, and because of the low strength of the materials

formed, the pressure differential caused by a vacuum usually is sufficient for forming.

- On the other hand, for thicker and more complex parts require air pressure ranging from 100 to

2000kPa, depending on the type of material and thickness of the sheet.

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COMPRESSION MOLDINGCOMPRESSION MOLDING

Figure shows types of compression molding, a process similar to forging: (a) positive, (b) semipositive, and (c) flash. The flash in part (c) has to be trimmed off. (d) Die design for making a compression- molded part with undercuts.

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- Used mainly with thermosetting plastics.

- A preshaped charge of material, premeasured volume of powder or resin is placed directly into

a heated mold cavity (~2000C).

- Forming is done under pressure (10 to 150 MPa) from a plug or from the upper half of the die.

- Typical products: dishes, handles, container caps, electrical components, etc.

- There are three types of compression mold:

i)Flash type: for shallow or flat parts.

ii)Positive type: for high-density parts.

iii)Semipositive type: for quality production.

- If compared to injection molding:

• The complexity of parts produced is less.

• Dimensional control is better.

• Cost of the dies are less.

- Dies are made from tool steels and may chrome plated or polished for an improved surface finish.

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TRANSFER MOLDINGTRANSFER MOLDING

Figure shows the sequence of operations in transfer molding for thermosetting plastics. This process is particularly suitable for intricate parts with varying wall thickness.

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- Enhancement of the process of compression molding.

-The uncured thermosetting resin is placed in a heated transfer chamber, and once when the

material is heated, it is injected into heated closed molds.

-The material will flow (by forces) through the narrow channels into the mold cavity at pressure up

to 300MPa.

-Since the resin is in a molten state as it enters the molds, the complexity of the parts and

dimensional control is much more better compared to injection molding process.