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Ahmad Alkhathami
ETLS 502, Manufacturing Processes
John Walker
October 8, 2009
Plastic Forming
Plastic materials originally is an organic material, and most of plastics today are petroleum
based products. Plastic is a new industry comparing with the steel, where the first human made
of the plastic was in 1868. In general, plastic is divided in two types: Thermoplastic and
Thermoset. Where thermoplastic is a reversible material that can be changed from solid to liquid
and vice versa many times. This type is the most common in the plastic industry. On the other
hand, Thermoset is the plastic material that can be moltened and solidified one time and it is not
reusable. In fact, plastic industry has grown rapidly in many ways regarding the processes of
forming plastics and their applications. There are seven types of plastic forming which are
Extrusion Molding, Injection Molding, Blow Molding, Compression Molding, Transfer Molding,
Rotational Molding, and Thermoforming. All of them will be discussed in this paper except the
Injection Molding which was addressed last week.
Extrusion Molding
Extrusion Molding is much similar to the injection molding where the material is charged as
granules into the hopper from the top toward the gravity to the screw chamber. The screw is
rotating which increases the heat and the pressure until the material melts around 400° F.
However, instead of injecting the molten plastic into the mold through the nozzle as in the
injection molding method, a die is installed to allow the molten plastic to pass through the die

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taking the shape of the die. After that, the extrusion is cooled while it is moving through water
channel and moving plates. It takes long time for the plastic to be cooled. Then, the product is
cut or rolled based on its characteristics. This kind of plastic forming is most common for
products such as pipes and tubes, as well as plastic sheets, and fiber optics.
Blow Molding
The process of the Blow Molding starts with the same process of the Injection and Extrusion
molding. However, after extrusion or injection, the plastic is formed to a parison (preform). The
parison is surrounded by a mold cavity. Then, compressed air passes through the parison and
expands the plastic to the shape of the mold. After that, the plastic product gets cold and the
mold opens to eject the product and allow a new material to coat the parison again. This type is
very popular for making the hollow products such as plastic bottles and cans.
There are two kinds of Blow Molding: Extrusion Blow Molding and Injection Blow Molding,
based on the way the material is received to the mold and the forming process. Furthermore,
while the mold in EBM opens and closes to allow the income of the material and the outcome of
the product, IBM mold stays closed until the product is finished. EBM is less expensive than
IBM due to the low tooling cost; it also has a high rate of production. Some of the EBM
disadvantages are the high amount of scarp, and the limited control over the wall thickness;
However, IBM produces low scrap and is used for complex products with accurate dimensions
such as the medical industry products.
Compression Molding
In Compression Molding the molded material is placed in an open mold as an uncured preheated
plastic. After that, the mold is closed and applies a high pressure that allows the material to be in

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full contact with all the mold cavity surfaces. Heat is maintained with the pressure until the
plastic material is cured. This type of molding is used for high volume thermoset material with
high strength. The advantages of Compression Molding are the low cost high rate production,
low production time, and low labor. However, this type of forming can produce high scrap, and
the mold needs to be cleaned after each process by compressed air.
Transfer Molding
Transfer Molding is similar to the Compression Molding, yet there is some differences. While in
the Compression Molding the uncured plastic or rubber is put directly into the mold cavity, in
Transfer Molding, the material is charged and preheated into a transfer pot. Then, the material is
pressed by a plunger (ram), and it flows to the mold cavity through spur. The mold is held closed
until the plastic is cured. After that, the transfer mold is opened to remove the final part by
ejector pins. Transfer Molding comparing with Compression Molding produces better
dimensional quality, less material waste, and faster production rate due to the ability to have
many cavities. However, Transfer Molding is more expensive but the unit cost can be lower in
high production. Transfer Molding is widely used in electrical industries such as the Integrated
circuits, plugs, and connectors.
Rotational Molding (Rotomolding)
There are three main stages the Rotational Molding process goes through to make the final
products: heating stage, cooling stage, and loading/unloading stage. The raw material is charged
into the mold. Then, it gets heated while the mold is rotating in bi-axial directions under low
pressure, with no significant effect from centrifugal force. By heating and rotating, the material
flows and adheres to the mold cavity’s walls. During the cooling stage, the mold is cooled by

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either fan or water spray to solidify the material. The third stage is removing the final product
from the mold after it gets cold and solidified, then loading new material. Rotational Molding is
not a high cost process due to the using of few machines which include a mold, oven, cooling
chamber, mold spindle, and motor. The most common use of Rotational Molding is to produce
big size hollow products such as oil and water tanks, as well as small products like toys.
Thermoforming
Thermoforming is a process in which a thermoplastic sheet is heated until it gets soft, and then
deformed by a mold from one side. Then, plastic is cooled and hardened, while it is taking the
shape of the mold. After that, the product is removed and trimmed. Thermoforming is a rapid
economical method to make products from plastic sheets. It is used for mass production of the
packaging industry, bathtubs, and plastic covers. There are three types of Thermoforming:
Vacuum Thermoforming, pressure Thermoforming, and Mechanical Thermoforming based on
the way of forming the sheet by the mold.
References
- http://en.wikipedia.org/wiki/Main_Page
- UW-Stout Teleproduction Center, Understanding Plastics. Video show.
- http://www.petmachine.in/type_of_blow_moulding.htm
- http://www.oshore.com/products/archived/thermoforming.html
- United Nations Industrial Development Organization, “ Plastic Forming industry” article,
1995.