Craft and Design
Application of Injection
Moulding (Mobile Phone)
5700
Summer 1999
HIGHER STILL
Craft and Design
Application of Injection
Moulding (Mobile Phone)
Support Materials
This Support Material was written by teachers in West Lothian Council. The work was substantially
funded by West Lothian Council and the final production was carried out by the Higher Still
Development Unit.
CONTENTS
Preface
Notes on injection moulding
Annotated photographs showing key features of injection moulding
PREFACE
These notes describe the processes used in the manufacture of the Motorola
D160/170 Mobile Phone
This support material has been produced to assist in the teaching of the unit Designing
for Manufacture at Intermediate 2 and Higher. It can serve as a detailed example of
the application of injection moulding.
Where centres have access to this or another similar phone casing the features of
injection moulding described in this pack can be shown to students in detail. To assist
where this is not possible annotated photographs of the Motorola phone have been
provided in this pack to highlight key features.
DET Craft and Design Support Materials:
Application of Injection Moulding (Mobile Phone)
1
NOTES ON INJECTION MOULDING
Injection moulding requires the creation of a mould in a block of metal. Molten
plastic is injected into this mould under pressure. The following notes refer to features
of the process which can be identified on the casing of the mobile phone
Spark Erosion
This is a process used to make the mould. It gives the surface of the plastic a textured
finish. This can vary from fine to rough depending on the customer’s specification.
Split Lines
This often appears as an indent or groove on the side of the product.
It is where the two parts of the casing fit together, one inside the other.
Tapered Sides.
All items that are injected moulded have tapered sides. This allows them to be
removed from the mould. The taper can be as little as 1:200 on a smooth surface but it
has to be much greater on one that has a rough texture.
Radius Corners.
There should be no sharp corners, either internal or external, on an item that has to be
injection moulded. Sharp corners are weak and may break easily. Radius corners
provide strength.
Injection Point
This is the point at which the plastic has entered the mould.
It is usually designed to be on a concealed surface so as to prevent any blemishes
appearing on more visible surfaces.
Two methods of injection can be used. These are the hot runner method where the
plastic is injected directly onto the surface of the product or the more conventional
method of using a sprue which is trimmed off manually when removed from the
mould.
Ejection Points.
There are numerous injection points which are usually visible on the inside surface.
These show where ejection pins have pushed the set plastic to release the shape from
the mould. It is important to have enough pins to ensure that the plastic is ejected
square to the mould and to prevent stresses and distortion building up.
Location Points
Location points or bosses are used to locate parts together. These can be clearance
holes and tapping holes to allow the use of small self-tapping screws.
Wall Thickness
These can be as little as 1mm. Most products have a uniform wall thickness to
prevent sinkage from occurring. Although the plastic is very thin, injection moulded
plastic products can be very strong. The strength can come from the external shape of
the casing, the mix of materials used and the internal structure of webs and ribs.
DET Craft and Design Support Materials:
Application of Injection Moulding (Mobile Phone)
2
External Ribs and Break Lines.
On external surfaces these are often used as aesthetic features to break up a plain
surface or divide up an area. On larger flat surfaces they can be used to give the case
more strength and rigidity and to disguise any blemishes that can often occur.
Injection moulded flat surfaces are liable to bow and distort as the plastic cools and
shrinks. Ribs are sometimes used on the inside of the product to help alleviate this.
Sinkage
This is caused by non-uniform wall thickness and usually appears as a mark or
distortion where two surfaces or edges of non-uniform thickness meet.
Material Information
Most plastics are now able to be recycled. The recycle symbol is usually visible along
with some information about the type of plastic that has been used. ‘PC + ABS’
means that the plastic used on that product is Poly Carbonate with ABS.
Material Selection
There is now a vast range of plastics available. Most are mixed to the specification of
the customer. This can range from a cheap general-purpose plastic to one that is super
tough. ABS (Acrylonitrile Butadiene Styrene) is the most commonly used plastic for
injection moulding. Other materials or additives are mixed with it to meet the desired
specification.
Secondary Operations
There are many secondary operations carried out during the assembly of the product.
Rubber seals, brass inserts and small metal contact plates are just a few of the things
fitted to the injection moulded case of the mobile phone.
Date Symbol and Part Number
Quality assurance plays a big part in any manufacturing industry today. Each injection
moulded part will have a date symbol giving at least the month and year it was
produced. It will also show the part number to assist with communications and reordering.
DET Craft and Design Support Materials:
Application of Injection Moulding (Mobile Phone)
3
DET Craft and Design Support Materials:
Application of Injection Moulding (Mobile Phone)
4
DET Craft and Design Support Materials:
Application of Injection Moulding (Mobile Phone)
5