Gating System Design Optimization for Injection Moulding

Abstract

Injection moulding process is a vast and advantageous process in the field of science and technology. Injection moulding process gives birth to vast application which replaces not only industrial products and parts but also capture home appliances and the products of our day to day life. But from the technology point of view we are still using hit and trial methods. In industries, workmen and craftsmen who are engaged in these operations are only expertise but to a certain extent they are also dependent on hit and trial methods which sometimes increases the manufacturing cost of the product. There is a need to develop a technology which guides the designer that what methods he has to follow to design a particular product. There are basically three step in the injection moulding cycle. Firstly, design of sprue which are tapered to increase the rate of flow. Secondly, design of runner whose efficiency is pre determined and fixed. Efficiency here refer to smoothness or least obstruction provided to the flow material. Thirdly, selection of the gate which pour the flow material finally into the cavity. Selection of gate is quite a difficult task for the designer; only skilled crafts-men can handle easily. Now again the problem arrives to assemble these the three steps according to the type of cavity. In this paper i tried to develop a system with the help of CAD/CAM which facilitates the user to work on injection moulding operations starting from the cavity to the selection of gates and then to the efficiency of runner and finally to sprue. This results time reduction in designing process and to a certain extent reduces manufacturing cost also.

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International Journal of Mechanical and Industrial Technology ISSN 2348-7593 (Online)
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Gating System Design Optimization for
Injection Moulding
1Dr. M P Singh, 2Akhilesh Jaguri
1Department of Mechanical Engineering, Jagannath University, Jaipur, India
2Department of Mechanical Engineering, Jagannath University, Jaipur, India
Abstract: Injection moulding process is a vast and advantageous process in the field of science and technology.
Injection moulding process gives birth to vast application which replaces not only industrial products and parts
but also capture home appliances and the products of our day to day life. But from the technology point of view we
are still using hit and trial methods. In industries, workmen and craftsmen who are engaged in these operations
are only expertise but to a certain extent they are also dependent on hit and trial methods which sometimes
increases the manufacturing cost of the product. There is a need to develop a technology which guides the designer
that what methods he has to follow to design a particular product. There are basically three step in the injection
moulding cycle. Firstly, design of sprue which are tapered to increase the rate of flow. Secondly, design of runner
whose efficiency is pre determined and fixed. Efficiency here refer to smoothness or least obstruction provided to
the flow material. Thirdly, selection of the gate which pour the flow material finally into the cavity. Selection of
gate is quite a difficult task for the designer; only skilled crafts-men can handle easily. Now again the problem
arrives to assemble these the three steps according to the type of cavity. In this paper i tried to develop a system
with the help of CAD/CAM which facilitates the user to work on injection moulding operations starting from the
cavity to the selection of gates and then to the efficiency of runner and finally to sprue. This results time reduction
in designing process and to a certain extent reduces manufacturing cost also.
Keywords: Moulding, Sprue, Lead time.
1. INTRODUCTION
It is a manufacturing process for producing parts in which thermoplastic and thermosetting plastic materials are used. Raw
material is feed into a heated barrel, where it cools and hardens to get desired configuration and shape of the cavity. Mold
maker (or toolmaker) is used to make molds from metal (usually either steel or aluminum) and precision-machined to
form the desired part. For manufacturing a variety of parts injection molding is used.
Fig:-1 Injection moulding machine

International Journal of Mechanical and Industrial Technology ISSN 2348-7593 (Online)
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Process characteristics:
 A ram or screw-type plunger to force molten plastic material into a mould cavity.
 Uses thermoplastic or thermo set materials.
 Produces a parting line, spruce, and gate marks.
 Ejector pin marks are usually present.
 Produces a solid or open-ended shape.
Applications:
Injection moulding is the most common method and best method of part manufacturing. It is ideal for producing large
volumes of the same object. Injection moulding is used to make many
things such as pocket combs, wire spools, bottle caps, packaging, automotive dashboards, mechanical parts, and most
other plastic products available in a variety of forms. Some advantages of injection moulding are high production rates,
repeatable high tolerances, and the ability to use a wide range of materials, low labour cost, and minimal scrap losses.
Injection Process:
Fig:-2 Hopper, nozzle and die area shown in Small injection molder
In process of injection molding, from a hopper granular plastic is fed by gravity into a heated barrel. By a screw plunger
the granules are moved forward, the plastic is forced into a heated chamber, where it melts completely. When the plunger
moves forward, through a nozzle the melted plastic is forced in, letting it to enter the cavity through a gate and runner
system. As the plastic solidifies the mold remains cold almost as it is completely filled.
Cycle of Injection Molding:
The injection molding cycle is the sequence of events that occur during the injection mould of a plastic part. With the
mould closing the cycle begins, followed by injecting the polymer into the cavity. A holding pressure is maintained when
the cavity is filled, to compensate for material shrinkage operation. The screw turns, feeding the next shot to the front
screw- in the next step of the cycle. The mould open and the part are ejected, once the part is cooled.
II. RESEARCH METHODOLOGY
Methods of gating system design are categorized under followings steps which are numerated as follows:
STEP -1
Firstly-Surface Area, Volume and Type of material is calculated. This data is provided by the customer to the designer
that what are the geometric properties of the product.
Secondly- velocity of flow is decided by the designer which is fixed for every material.

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For Example Gate velocity = 5m/s to 30m/s
(Minimum for house hold product and max for ABS material)
Thirdly-No. of cavities are selected either it must be single cavity or multi cavity according to the demand of customer.
Fourthly, de-gating of gates are selected by the designer.
Material constant (n):
Table .1 Material types
Group1
Polythene, polystyrene:
DENSITY = 1.05 gm/cm3
n=0.6
Group2
Polyacetal, polycarbonate:
DENSITY = 1.21 gm/cm3
n=0.7
Group3
Cellulose acetate, polymethyl methaacrylate,
nylon
DENSITY = 0.941 gm/cm3
n=0.8
Group4
PVC:
DENSITY = 1.38 gm/cm3
n=0.9
STEP-2
Firstly gate area is calculated by:
Gate area:
Width of gate:
Secondly, gate thickness is calculated by the relation
Gate thickness:
Now gate thickness determines the type of gate should be used for a particular type of cavity.
Flow chart for single cavity
Fig:-3
Single Cavity
Degating
Type
Enter gate thi
0.1-0.9(in mm)
Result
Enter Gate Thickness
< 1.5
Result
0.1 to 1.49(mm)
Result

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Flow chart of multiple cavities:
Fig:-4
STEP-3
In this step Runner is selected according to the performance efficiency.
It is being observed by designer that
Table .2.Material types
Type
Priority ( Acc. To efficiency)
Trapezoidal (D=W)
2nd
Round (D)
3rd
Modified Trapezoidal (R=D/2)
1st
Hexagonal
4th
Diameter of runner: =
Start
Enter no of Cavities
Degating
Type
Enter gate thickness (in mm)
1.6-5.0
0.1 to 0.89
1.0-1.59
Result
Result
Result
Enter gate thickness (in
mm)
Result
Result
Result
0.1 to 0.59
0.6 to 0.99
1.0 to 5.0
Film gate/flash
gate
Fan
gate
Side/edge gate
Ring
gate/Trapezoidal
Submarine
gate
Ring gate

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Graph b/w diameter of runner and length of runner.
Fig:-5
Hence length, height and diameter of the runner are determined by the designer according to the type of gate selected
STEP-4
In this step standard sprue is selected by the designer and its diameter is adjusted according to the diameter of runner at
one end and other diameter is derived from the relation,
D2 = D1+ 2L tanA
Where, D1=diameter of runner/diameter of one end of sprue.
L= length of sprue selected by the designer
A= tapered angle (20 to 50)
Hence sprue is selected with desired diameter, length and tapered angle.
Example
Volume = 214836.42 cubic millimeters
Surface area = 88624.93 millimeters^2
Example Part Model

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III. CONCLUSION
When designing a gating system, it is very important to fill the molten metal to fill the cavity properly. Therefore, the
design of gating system is very difficult. A proper runner and gating system is very important to secure good quality die
castings through providing a homogenous mould filling pattern. For mouldings with multiple cavities, adjusting injection
parameters of each cavity to the same level is particularly difficult. The resultant rate of failed product is untenable. The
aim is to gain high levels of productivity and to reach a level of accuracy that meets the required conditions.
To achieve a good gating system design, the following aspect should be taken into consideration:
Firstly, total volume of the cavity, to be injected. In regular practice, a single cavity is chosen frequently. But by using a
multi-cavity die will save labour cost and improve the production efficiency. The designer should consider the economic
and technical issues in order to select an acceptable cavity number that meets the overall requirements.

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Secondly, degating type selection which means to separate mouldings from the runner or the feeding system.
Thirdly, by increasing the gate thickness or area will result characteristics curve with reduced slope, so that the filling
time and gate velocity vary accordingly. A larger gate area will decrease both the filling time and gate velocity. A low
gate velocity will lead to high air entrapment and porosity. On the other hand low flow rate will result in poor casting
surface; a short filling time gives good surface finish.
Modern injection moulding–especially in the 3C industry–needs less time to fabricate accurate products such as the cell
phone with digital camera, the camera lens and the cell phone shell. To achieve the above mentioned goals in the current
product development, the CAD/CAM is preferred method to provide essential part of solution. It provides the technology
for the representation of design intent, design schemes and solution. CAD/CAM technology greatly enhances product
design quality and shortens design and manufacturing lead time.
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