This document provides an overview of the plastic injection molding process. It describes the main components of an injection molding machine, including the hopper, heating barrel, injection screw, and nozzle. It also discusses the molding unit, material selection factors, key process parameters like temperature and pressure, and common post-molding finishing operations. The document concludes that injection molding is widely used for high-volume plastic production due to its ability to make complex parts at low cost and high efficiency.

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Process Study
By
Ahmad Alkhathami
ETLS 502 Manufacturing Processes
Instructor: John Walker
November 5, 2009

2. 1
Introduction
Although plastic is considered a new industry, its share is two-third of the manufacturing
industry. There are two major types of the plastic: thermoplastic and thermoset. Thermoplastic
materials can go through the processe of melting and solidification many times over and over,
while thermoset once it is molted and solidified it can’t go through this process again. Recently,
there are over 17,000 types of plastics and about 750 new types discovered every year. The
classification of these types is based on many factors such as the molecular structures of the
material, the type of bonds in the molecules, and the types of atoms that shaped the molecule.
Recently, plastics industry is expanding rapidly and plastics materials applications are increased.
This is a result of many advantages the plastics have over other materials. These advantages, for
instance, due to the light weight comparing with all metallic materials, high resistivity against
corrosion and electric conductivity, low process cost, and low thermal conductivity. This paper
addresses in detail the plastic injection molding process, which is one of the plastic forming
methods. Moreover, it is the most widely used process of making plastic parts in high volumes,
with relative low capital investment in machines and labors. This paper explains the Plastic
Injection Molding process including the machines, the materials used, the parameters that affect
the process, cost estimate, and the secondary operations needed after molding the parts.

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The Injection Molding Machine
The injection molding machine receives the plastic raw material as pellet or powder and
makes the final part mostly in one operation. In fact, it is rarely for the plastic products to go
through secondary operations due to the good finishing of parts’ dimensions out of molding. The
injection molding machine consists of two main parts, the first part is the Injection Unit, and the
second part is the Molding Unit. Fig.1 belows is a diagram of the Plastic Injection Molding
Machine.
Fig1: Plastic Injection Molding Machine
The Injection Unit has four major parts which are
1- The hopper, which is the first part of the injection unit that takes the hallow shape. The
hopper is where the plastic raw materials are charged before injection. The hopper is set
vertical to allow the material to flow toward the gravity. The design of the hopper size
depends on the amount of the material and the duration needed for each cycle. Also, in
many designs the hopper works as a dryer to dry the moisture from the plastic materials
before injection.

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2- The heating cylinder (barrel). After the material exits the hopper, it gets into the heating
barrel, where it gets heated, molted, and prepared for injection into the mold. The barrel
has three zones: the rear zone, the center zone, and the front zone. The injection cylinder
size should be designed to be double the amount of the material for each cycle of the
production. For example, if the amount of the material needed to form the part is 50 g,
then; then, the cylinder of the Injection Unit should be at the size of 100g.
3- Injection screw, which is located inside the heating barrel, and it has two main functions.
First, it generates heat through rotation which leads to molten and homogenizing the
plastic material. Second, it moves the material toward the nozzle and injects them into the
mold. The Injection Screw does not have the same diameter throughout the channel, yet
its diameter gets bigger closer to the nozzle. This design helps in creating the frictional
heat. At the end if the Injection Screw there is a screw tip that allows the molten material
to flow only toward the nozzle. The angle and length of the screw tip is determined based
on the viscosity of the material. Furthermore, there is a non-return valve that is installed
on the injection screw to prevent the molten material from going backward while the
screw is pushing the molten material toward the nozzle.
4- The Nozzle, the molten material exits the heating barrel to the mold through the nozzle.
The nozzle is a two-piece that takes the tube-shape.
The second part of the Injection machine is the Molding Unit, which is the part that receives,
cools, and forms the molten plastic in a cavity/ cavities. After that, the mold ejects the formed
part. The mold unit consists of two parts: the injection half and the ejection half. In the injection
half, the molten material comes from the nozzle through a sprue bushing, and then from the
sprue bushing to the mold cavity through a runner. The runner channel cross section must take a

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uniform circular shape to create equal pressure over the molten plastic to ensure the uniformity
of the plastic molecules. Then, the molten goes to the cavity where it gets cooled and solidified.
After that, the formed part is ejected by the ejector half. The ejector half consists of three parts
1- The ejector pins, which pushes the finished part from the cavity.
2- The ejector plats, their function is to hold the pins.
3- The ejector rod, its function is to hold the plats from the backside of the mold.
Material Selection
Injection Molding process can use both thermoplastic and thermosetting materials.
However, how can we decide which material to choose for our process among over 17,000
types of plastic material? The answer to this question is based on many factors the designers
must keep in mind:
1- The properties of the material needed to be formed, the mechanical properties such as
the strengths, hardness, and fatigues behavior, as well as the thermal and electrical
properties.
2- The availability of the material as a matter of the existence as well as the matter of cost.
3- The customer needs for specific characteristics of the part to be made.
The designers use the assistance of the computer programs to get the material types that fits
well with the characterestics they look for. However, there are other materials that are used
as additional materials just to change the properties or to reduce the cost of the products.
These materials called fillers, one example of the filler materials is the clay. Another
material called reinforcements that help in increasing the strength of the plastic material.

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An important factor that must be taken in consideration during the selection of the
material is the Melt Flow Index, which considers the flowability of each material. This
factor helps in understanding the material behavior during the flow of the process.
Process Design
In general, the injection molding process goes through the following three phases:
1- Heating phase, in this phase the material is heated to shape the polymerization
molecules. Also, it melts the material in order to be shaped in the mold.
2- Pressure phase, pressure is applied in the barrel to molten the material and to inject them
into the mold.
3- Cooling phase, cooling occurs in the mold to solidify the part while it is taking the shape
of the cavity. Water is the most popular coolant used in this process.
The efficient results out of good designing and controlling of these three phases come from the
good consideration of their parameters. In fact, there are over 200 parameters that affect the
molding process, yet the most crucial parameters are
1- Temperature
In each stage of the injection molding, the temperature should be maintained to the required
degrees. In fact, in the heating cylinder the temperature increases by 10° to 20° F while the
material flows from zone to zone. Two factors play the role of producing heat to the material.
The first one is the heater bands around the heating the cylinder and the nozzle. The second
factor is through the rotation of the injection screw. However, the design if the heating

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system and the mold temperature control vary based on the molten temperature and
characteristics of the material used.
2- Pressure
We have two system of pressure in the Injection Molding. The first one is for the Injection
Unit, to melt and move the material by the screw. The injection screw can produce pressure
up to 20,000 psi while the average pressure that is applied in the injection unit is around
10000 psi. this pressure is determined by the ratio of the length of the screw to its diameter
𝐿
𝐷
, the typical ratio is 20:1.
Some parameters taken in consideration for pressure design such as the shear rate which is
the surface velocity of the plastic at the wall of the barrel, and it is calculated using the
following formula:
SR=
𝐷𝑁
ℎ
ft/min where, D: is the screw diameter
N: is the rate of screw rotation
H: depth of the channel
The average SR is approximately 150 ft/min, but again this value depends on the properties
of the material such as the melt flow index.
The amount produced per hour which is determined by how much horse power needed to
turn the screw. In addition, the HP amount is determined by the diameter of the screw to
avoid any breakage. The average amount is from 5-15 Ib/hr for each HP, which is mean for a
2 in diameter screw that provide 15 HP, the amount produced is between 75-225 Ib/hr.

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The second system of pressure is for the clamp unit, where it is related by the maximum
force needed to keep the mold closed during the injection. This force must be higher than the
pressure that the flow of the molten material produces.
Finishing and Machining
For most of the injection molding parts, there are no need for any secondary operation,
where most of the features can be considered during the designing of the mold. However, in
some cases it is necessary for the parts to go through further operations, an example of this
necessity is when the volume of the production is small, something around 25,000 part/year.
In this case, It is more efficient to do secondary operation rather than investing high capital in
buying a high-cost complex mold. Moreover, when, there is need to add additional features
to the part, which is difficult to be made by the injection mold machine. As a result, the part
after the ejection from the mold goes through one or more of the three secondary operations
shown below:
1- Assembly operation
Plastic parts may go through simple assembly such as snap fitting, or complex assembly such as
welding or machining. As the metallic many of the welding process can be used to join plastic
parts. Most common process, for instance, are the Ultrasonic Welding, Induction Welding, and
Friction Welding.
2- Machining Operation
Drilling, tapping, lathe turning, grinding, sanding, and milling all are machining process used
on both metallic and plastic material. Furthermore, the process designs are quite similar in

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both types of materials regarding the speed of cut, feed rate, depth of cut, and the using of
coolant.
For plastic cutting Water-jet Cutting is the most popular application. However, when there is
a need for fine polished finish surface Laser Cutting machine is the most efficient method.
3- Surface Finishes and decorating
There are many methods for surface finishing of the plastics such as Flame treatment,
Plasma process, and Acid Etch. Also, painting has variety of applications by using spray
machines, brushing, or rolling.
Injection Molding Cost
Although Injection Molding Machine is not a high cost capital investment comparing with
other process, the cost estimate must go through accurate analysis. In fact, it could be
difficult to estimate the real cost of building the Injection Molding process, yet there are three
main factors that can determine approximately the actual cost to make the products out of the
Injection Molding processes. These factors are the labor cost, materials cost, and machines
cost. The material cost is determine by the cost per 𝑚3
, considering that there is about 10%
scrap. Labor cost is determined by measuring the labor rate per hour, the benefit, and
overhead cost. The benefit and overhead cost are about the same as the labor rate. On the
other hand, the machine cost depends on the size of the machine, the level automation, and
the mold cost, as well as the setup cost. In addition, the cost of the annual maintenance is
estimated to be about 8% of the machine cost.

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Conclusion
In summary, the injection molding process is the most widely process used in the plastic
industry for high volume production. This paper addressed the Injection Molding Machine’s
parts and the general functions of each part. Also, it gave a brief explanation about the major
parameters that must be considered while the machine being designed, such as the
temperature and pressure control. Furthermore, it showed a general guideline for the
selection of the plastic material in the process among high number of plastic materials in the
market. Though most of the Injection Molding applications are one operation process,
sometimes there is need for secondary operation of the parts to add some features. Finally,
the Injection Molding process is one of the manufacturing process that make huge impact on
the manufacturing industry according to its capability in making useful products in low cost
and high efficiency.

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Bibliography
- Brayce, Douglas. Plastic Injection Molding. Society of Manufacturing Engineers,1996.
- Degramo, Paul. Black, J. Kohser, Ronald. Materials and Processes in Manufacturing.
John Wiley and Sons, Inc. 2003.
- Malloy, Robert. Plastic Part Desing for Injection Molding. Hanser Publisher, Munich
Vienna New York, 1994