design considerations for molded plastic components

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Design Consideration for Moulded Plastic Components Rajender Thakur M.E (Manufacturing Technology) Punjab University

Transcript of design considerations for molded plastic components

Page 1: design  considerations for  molded plastic components

Design Consideration for Moulded Plastic Components

Rajender ThakurM.E (Manufacturing Technology)Punjab University

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A synthetic material made from a wide range of organic polymers such as polyethylene, PVC, nylon, etc., that can be moulded into shape while soft, and then set into a rigid or slightly elastic form. They are usually synthetic, most commonly derived from  petrochemicals, but many are partially natural

The strength of plastics can be improved with reinforcement of glass fiber, mica, talk etc

Plastic

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1.Light in weight, unbreakable, odourless and can be easily moulded2. Low conductivity of heat and electricity - insulating properties.

3.Excellent finishing; possess good shock absorption capacity, high strength as well as toughness. 

4. Possible to recycle plastic

5.Corrosion resistant 

Benefits

Limitations

1. Embrittlement at low temperature and deformation at high pressure2.Plastic materials have low heat resistant and poor ductility3.  Nonbiodegradable.4.Recycling of plastic is not cost effective process

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Design consideration for molded plastic Components

Wall ThicknessCooling time is proportional to square of wall thickness. Large cooling time for solid will defeat the economy of mass production. (poor conductor of heat)

Thicker section shrink more than thinner section, thereby introduce differential shrinkage resulting in warpage or sink mark etc.

As far as possible wall thickness should be uniform or constant through out the part.If there is any solid section in the part, it should be made hollow by introducing core. This should ensure uniform wall thickness around the core

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Thick and stiff enough for the job. Wall thickness could be 0.5 to 5mm.Thin enough to cool faster, resulting lower part weight and higher productivity.Any variation in wall thickness should be kept as minimum as possible. With varying wall thickness - achieving close tolerance becomes very difficult and many times impossible. Where wall thickness variation is essential, the transition between the two should be gradual.

What are the considerations for deciding wall thickness?

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At corner, wall thickness increases to 1.4 times the nominal wall thickness. This results in differential shrinkage and moulded-in stress and longer cooling time. Therefore, risk of failure in service increases at sharp corners. Corners should be smoothened with radius. Radius should be provided externally as well as internally

It is preferable to have radius of 0.6 to 0.75 times wall thickness at the corners

Corners

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Temperature dependent change in volume - 29% in crystalline and 8% in amorphous-.

Compressibility of melt under pressure is 10-15%. On falling temperature of melt in the mould, decrease in volume is more Therefore void can not be perfectly filled in. Sink mark which appears objectionable can be hidden by providing a groove of sufficient depth and width

SINK MARK IS INEVITABLE

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RIBS FOR STIFNESS CONSIDERATION1. Improve stiffness and increases rigidity.2. Enhances mouldability as they hasten melt flow in the direction of the rib.3.Ribs are placed along the direction of maximum stress and deflection on non-appearance surfaces of the part4. Mould filling, shrinkage and ejection also influence rib placement decisions.

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Draft Required for easy ejection of molded parts from cavity.

A taper of 1º to 4º is ususal for polymers but tapers of less than 1º can be used for deep articles.

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1.A feature raised above a surface

2.As a boss rises from a surface, it thickens the surface at that point raising the risk of sink or development of voids as the part cools

3.Boss thickness between 40 and 60 percent of the thickness of the wall from which the boss rises 

Bosses in Plastic Parts

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  To strengthen the boss without thickening its walls. The most common of these is to surround the boss with gussets to support and strengthen its walls (see Figure 1b)

If a boss is part of a vertical wall, it should not create a thick area in the wall. There should not be any increase in thickness due to mass between boss and wall

Extra points

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Holes The presence of holes disturbs the material flow during molding This results in a potentially weak point and some form of bosses may be necessary. Through holes are preferred to blind holes

Inserts Inserts are used in molded parts to strengthen them. Inserts are usually made up of steel ,brass, aluminium etc. To anchor the inserts in parts , a mechanical or friction lock must be provided Inserts with sharp corners cause stress concentration in surrounding plastic

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