craft and design - earlston high school · det craft and design support materials: application of...

Craft and DesignApplication of Injection

Moulding (Mobile Phone)

5700

Summer 1999

Craft and DesignApplication of Injection

Moulding (Mobile Phone)

Support Materials

HIGHER STILL

This Support Material was written by teachers in West Lothian Council. The work was substantiallyfunded by West Lothian Council and the final production was carried out by the Higher StillDevelopment Unit.

CONTENTS

Preface

Notes on injection moulding

Annotated photographs showing key features of injection moulding

DET Craft and Design Support Materials:Application of Injection Moulding (Mobile Phone)

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PREFACE

These notes describe the processes used in the manufacture of the MotorolaD160/170 Mobile Phone

This support material has been produced to assist in the teaching of the unit Designingfor Manufacture at Intermediate 2 and Higher. It can serve as a detailed example ofthe application of injection moulding.

Where centres have access to this or another similar phone casing the features ofinjection moulding described in this pack can be shown to students in detail. To assistwhere this is not possible annotated photographs of the Motorola phone have beenprovided in this pack to highlight key features.


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NOTES ON INJECTION MOULDING

Injection moulding requires the creation of a mould in a block of metal. Moltenplastic is injected into this mould under pressure. The following notes refer to featuresof the process which can be identified on the casing of the mobile phone

Spark ErosionThis is a process used to make the mould. It gives the surface of the plastic a texturedfinish. This can vary from fine to rough depending on the customer’s specification.

Split LinesThis 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 beremoved from the mould. The taper can be as little as 1:200 on a smooth surface but ithas 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 beinjection moulded. Sharp corners are weak and may break easily. Radius cornersprovide strength.

Injection PointThis 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 blemishesappearing on more visible surfaces.

Two methods of injection can be used. These are the hot runner method where theplastic is injected directly onto the surface of the product or the more conventionalmethod of using a sprue which is trimmed off manually when removed from themould.

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 fromthe mould. It is important to have enough pins to ensure that the plastic is ejectedsquare to the mould and to prevent stresses and distortion building up.

Location PointsLocation points or bosses are used to locate parts together. These can be clearanceholes and tapping holes to allow the use of small self-tapping screws.

Wall ThicknessThese can be as little as 1mm. Most products have a uniform wall thickness toprevent sinkage from occurring. Although the plastic is very thin, injection mouldedplastic products can be very strong. The strength can come from the external shape ofthe casing, the mix of materials used and the internal structure of webs and ribs.


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External Ribs and Break Lines.On external surfaces these are often used as aesthetic features to break up a plainsurface or divide up an area. On larger flat surfaces they can be used to give the casemore 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 andshrinks. Ribs are sometimes used on the inside of the product to help alleviate this.

SinkageThis is caused by non-uniform wall thickness and usually appears as a mark ordistortion where two surfaces or edges of non-uniform thickness meet.

Material InformationMost plastics are now able to be recycled. The recycle symbol is usually visible alongwith 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 SelectionThere is now a vast range of plastics available. Most are mixed to the specification ofthe customer. This can range from a cheap general-purpose plastic to one that is supertough. ABS (Acrylonitrile Butadiene Styrene) is the most commonly used plastic forinjection moulding. Other materials or additives are mixed with it to meet the desiredspecification.

Secondary OperationsThere 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 thingsfitted to the injection moulded case of the mobile phone.

Date Symbol and Part NumberQuality assurance plays a big part in any manufacturing industry today. Each injectionmoulded part will have a date symbol giving at least the month and year it wasproduced. It will also show the part number to assist with communications and re-ordering.


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