Question

Q5 (a) Compare the Direct and Indirect extrusion works with the aids of diagram.

(b) In metal forming, there were temperature ranges used including cold, warm and hot working.

(i) Compare between Hot and Cold working of metals.

(ii) Discuss the advantages and disadvantages for both processes.

(c) In designing and developing new products, engineers need to be aware the design consideration factors for plastic components. Give TWO (2) general design consideration factors, ONE (1) design consideration for extrusion process and FOUR (4) design consideration factors for moulding process.

Answer 1

Answer 5.a-

What is Extrusion?

Extrusion is defined as a process that creates objects of a fixed cross-sectional profile by pushing material, aluminum for example, through a pre-made mold. The process results in uniform products of a particular shape. By forcing the aluminum through the mold or die, it conforms to the shape of the die and it’s possible to produce even complicated cross-sections that might not be available through other means.

Comparison between Direct and Indirect extrusion

Direct Extrusion

a.The metal billet is placed in a container and driven through the die by the ram.

b.A dummy block is placed between the ram and the billet in order to prevent them from touching.

c.Friction is at the die and container wall → requires higher pressure than indirect extrusion.

d.Called forward extrusion.

Indirect Extrusion

a.Thehollow ram containing the die is kept stationary and the container with the billet is caused to move.

b.Friction at the die only (no relative movement at the container wall) → requires roughly constant pressure.

c. Hollow ram limits the applied load.

d.Called backwards extrusion.

Diagram

Answer 5. (b)

Metal forming

Forming, metal forming, is the metalworking process of fashioning metal parts and objects through mechanical deformation; the workpiece is reshaped without adding or removing material, and its mass remains unchanged.

Types

a.Cold forming

b.Warm forming

c.Hot forming

Temperature range for these processes

Cold working. <=0.3Tm

Warm working. 0.3Tm-0.5Tm

Hot working. 0.5tm-0.75Tm

Where Tm is melting of the metal being Formed

Answer 5-(i)

Comparison between Hot working and Cold working of metals

Hot working./Cold working.

[a.Hot working is carried out above the recrystallization temperature but below the melting point, hence deformation of metal and recovery takes place.

simultaneously Cold working is carried out below the recrystallization temperature and as such, there is not the appreciable recovery of metal.]

[b.During the process, residual stresses are not developed in the metal.

During the process, residual stresses are developed in the metal.]

[c.Because of higher deformation the temperature used the stress required for deformation is less.

The stress required to cause deformation is much higher.]

[d.Hot working refines metal grains, resulting

properties.

Cold working leads to distortion of grains.]

[e.No hardening of metal takes place.

Metal gets work hardened.]

[f.If the process is properly performed, it does not affect ultimate tensile strength, hardness, corrosion and fatigue resistance of the metal

It improves ultimate tensile strength, yield and fatigue strength but reduces the corrosion resistance of the metal.]

Answer 5 (ii)

Advantage and disadvantage of Hot working

Advantage

a.Due to hot working, no residual stresses are introduced in the metal.
b.Hot working refines the grain structure and improves the physical properties of the metal.
c.Any impurities in the metal are disintegrated and distributed throughout the metal.
d.The porosity of the metal is minimized by the hot working.

Disadvantage

a.Due to the loss of carbon from the surface of the steel piece being worked, the surface layer loses its strength.
b.This weakening of the surface layer may give rise to fatigue crack which results in failure of the part.
c.Close tolerances cannot be obtained.
Hot working involves excessive expenditure on account of high tooling costs.

Advantage and Disadvantage of Cold Working

Advantage

a.Better dimensional control is possible because there is not much reduction in size.
b.The surface finish of the component is better because no oxidation takes place during the process.
c.Strength and hardness of metal are increased.
d.It is an ideal method for increasing the hardness of those metals which do not respond to heat treatment.

Disadvantage

a.The ductility of the metal is decreased during the process.
b.Only ductile metals can be shaped through cold working.
c.Over-working of metal results in brittleness and it has to be annealed to remove this brittleness.
d.To remove the residual stresses set up during the process, subsequent heat treatment is mostly required.

Answer 5.(c)

Two genral design considerations factor for Plastic components

Material Considerations

Manufacturers often select a familiar grade of plastic from a similar application or rely on recommendations from suppliers. Resins chosen this way may be adequate, but are rarely optimal. Plastic selection is a complex task that involves many considerations, such as

a.Temperature

b.Assembly

c.Finish

d.Cost

e.Availbility

Radius

Radius should always be a consideration in regard to the part’s thickness – eliminating the likelihood of areas of high stress and possible breakage of the part. A general rule of thumb is that the thickness at a corner should be in the range of 0.9 times the nominal thickness to 1.2 the nominal thickness of the part.

One design considerations factor for Extrusion process

Regular wall thickness

Always try to achieve an even wall thickness in your extrusion design. Variations in thickness can make the flow of plastic material through the tool difficult to regulate, causing cooling at different rates and distorting the finished profile. Irregular extrusion wall thicknesses can also lead to difficulties in manufacturing and increased production costs.

Four Design consideration factor for moulding process

Whenever you begin a design project involving plastic parts, where your end goal is to have the component injection molded, you'll want to consider some things on the front end of your design process that will allow for time and money to be saved during the production phase of your project. This foresight at the beginning stages of product design will be useful in understanding the limitations and challenges faced by the manufacturers of your product.

These 10 considerations are by no means exhaustive when considering the best plastic injected part design, but they will for sure get you very close to where you need to be and at the very least, point you in the right direction. So let's check out these helpful tips:

1. Consider the Parting Line

​Try and imagine your part being molded. It can be difficult for some to think in the "negative", and by that I mean visualizing the empty cavity space that the liquid plastic will fill in order to mold into a plastic part. Inevitably, once that cavity space is filled, your part will have to be ejected from that space. In order for that to happen, the two mold halves will have to separate, leaving behind your plastic injected part in one side of the mold (preferably the side where you plan on ejecting from). You need to visualize and plan for where this "parting line" will be so as to ensure that your part does not get trapped in the mold.

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2. Avoid Undercuts (If Possible)

Undercuts on your part wont necessarily make it more difficult to mold your part, but rather more difficult to demold. The undercut portion of the plastic part will get trapped inside your mold once the part is cooled and hardened, and in turn making it impossible to eject from the mold without other mold actions. Many times, undercuts are a necessity for part function. Side actions and lifting mechanisms will have to be introduced to your tool in order to deal with the ejection of your part. Sometimes this is not avoidable, but if you are looking to save on tooling cost, it might be worth trying to redesign in order to eliminate necessary tooling costs.

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​​3. Uniform Wall Thickness

You want to make sure that you try and make the wall thickness of your part as consistent as possible. It's alright if you don't have uniform wall thickness to some extent, but uneven wall thickness greatly increases the likelihood of sink marks, warpage, voids, molded-in stress, longer cooling times, and even material flow restrictions. If wall thickness must be uneven, it is best to have smooth transitions that taper over some distance. The size of the part and the ability of the material to fill will determine the minimum wall thickness allowed for your plastic injected part.

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4. Shrinkage and Warpage Considerations

All materials shrink at different rates and at varying degrees within the cooling process of your injection molding operation. Shrinkage and warpage are two different phenomena that can occur. Shrinkage occurs where there is a difference between corresponding linear dimension of the mold and the molded part. Warpage is a dimensional distortion in a molded plastic caused by excessive residual stress in the part. There are various things to consider when trying to control either one of these occurrences. Material considerations, part geometry considerations, tooling considerations, and processing considerations all play a part.