Appunti Optimization and innovation processes in inglese (parte 4)

OTHER EXAMPLE

This is the most used approach because here ribs are 3 by 3, the accumulation phenomena is

reduced.

The cost of casting is more or less independent from the complexity of the geometry.

A similar problem arises from the meeting and intersection of several walls of material, so

appropriate solutions must be adopted to keep the thickness of the material constant as

much possible.

Similar considerations must be made for sharp edges, where there is a high risk of internal

cracks.

There are constraints also for the minimum thickness that could be achieve, depending on the

overall dimension of the part.

Create this figure is easier, but probably in the hot spots you will have some problems.

The inner walls of the workpieces must also be thinner (about 20% less) due to the more

di?icult cooling, otherwise shrinkage cavities can occur inside them. If di?erent sections are

required, su?iciently long ramps must be used.

The internal walls have to be a little smaller, even if I said the thickness has to be the same,

because the real objective is to obtain a constant cooling process: all the material must cool

simultaneously, the temperature has to be constant. Internal walls must be smaller by 20%.

It’s hard to produce it but not impossible.

It is also necessary to pay attention to the venting of gases. The presence of gas leads to local

deformations and excessive porosity in the piece. It is therefore necessary to prepare holes for

venting.

During the process the bubble of gas goes up and has all the time to exit from the wall. If this

metal start to solidify, then the bubble is trapped. So, it is better to insert some holes in the

geometry like in picture.

A particular casting process is that of die casting, usually used for aluminum and copper and

zinc alloys. This process requires more expensive machines and higher fixed costs (metal

molds) but guarantees higher productivity. This process has as its limitation the low weight of

the piece that can be produced (<30kg), the need to have very uniform and not too thin

thicknesses, the even stronger need not to have counter-drafts to increase productivity and

reduce the cost of the mold, the pieces have fairly high levels of microporosity (due both the

injection speed and the lack of gas vent). The presence of burrs requires further mechanical

processing.

The zinc allows a better thickness.

Chip removal process

Machining is used for most products, especially as the final stage of production.

It is convenient to produce parts by removal only when very small batches are involved or

when high precision is required, otherwise casting and forging are cheaper.

When a good finish is needed, when there are parts in contact, moving parts, removal

operations are mandatory within the production process. Parts of the most disparate sizes

and a wide variety of materials can be made.

1. Always avoid these processes if possible, forging or casting

2. Widen surface tolerances as much as possible

3. Think about the fixtures: a wide flat surface is always welcome

4. Avoid sharp or right angles that can lead to tool breakage

5. Avoid interrupted cutting to increase tool like and to allow the use of ceramics

6. Design the part so that it is rigid enough not to deflect or cause the tool to deflect

during machining

7. Use standard components whenever possible

8. Use surfaces that are as flat and non-conformed as possible

9. Avoid undercuts, which involve the use of special tools (this undercut is really complex

to produce)

10. Use molding whenever possible

11. Avoid the use of hard materials

12. Consider adequate overthickens (0.4 mm in general is fine) (we need 0.4 mm because

the engagement is not proper. It is suggested to remove a very small amount of

material).

13. Ensure that surfaces bound by geometric tolerances are machined with the same

setup

14. Use standard tools.

Reducing the cutting force reduce the productivity.

Turning

Turning is a widely used process thanks to the versatility of shapes that can be obtained.

Productibility quantity:

- Parallel lathe: low

- Turret lathe: low to medium

- CNC lathe: low to medium

- Single-spindle turning center: medium to high

- Twin-spindle turning center: from high to very high

The best is the last one. For a single part a manual solution is the best. Also, for maintenance,

for very small batch is ok to do manual operations.

In turning:

1. Design massive parts, not long and fine, to limit their deformation or the use of steady

rests.

2. Avoid interrupted cutting by drilling, tapping, milling tabs, etc. after turning.

3. Rake angles provide greater accessibility to the workpiece.

In turning operations, you produce very high forces.

Drilling

Drilling tools are very flexible tools.

1. Prepare surfaces orthogonal to the hole for both inlet and outlet (tool breakage)

2. When cutting interrupted, prevent the knife from coming out of the material

3. Improve through-holes than blind holes for chip removal and follow-up operations

4. Avoid holes with straight bottoms, it is cheaper to have them conical

5. Avoid deep L>3d holes for which gun drills are required (>cost due to slower speed)

6. Avoid holes with d<3mm due to di?iculty.

Reaming

1. Do not use reaming to correct hole position errors, even if they are small.

2. Avoid intersecting normal holes with bored holes to prevent bit breakage and burrs.

3. If a blind hole is to be reamed, define an overstroke.

Deep hole drilling

Deep drilling (or with gun drills) allows you to drill holes of variable diameter (from 3 to 50 mm)

with great depths. It has a very sti? tip with one or two end plates. Lubricants are introduced

into the tool. When designing this process, it is necessary to take into account the non-

negligible size of the tool and the possibility of rotating the workpiece instead of the tip (less

deflection of the tip and greater precision are obtained).

Deep hole drilling is slower, higher cost.

Milling

1. Always use standard tools with large fillet radii rather than from models

2. Tool-compatible fillet radii

3. When it is necessary to create small planar surfaces, end mills are better than face

mills, which is usually have to remove more material

4. For facing, it is convenient to create spot faces (creation of spot faces allows you to

remove a lower quantity of material)

5. It is cheaper to create a chamfer than a fillet

6. For dongles, use standard seat dimensions

7. For milling close to edges, it is necessary to consider the clearance

8. Avoid burrs increases the life of the tool

9. Stacking several workpieces allows an increased productivity (several workpieces

milled at the same time)

10. Use the largest possible tools (high speed, stock removal and sti?ness)

Plastics

Plastics are widely used in all manufacturing sectors. Characteristic to make them so used:

- Low cost

- Good chemical resistance

- Low weight (used in aerospace)

- Very good machinability (low cutting force, easy to deform)

- Possibility of surface treatments (painting)

- Wide variety of characteristic (various families of plastic)

- Multiple materials (composites) can be combined together.

In some cases, plastic is a more convenient material. The base of component materials is

plastic. Sometimes plastic is also a sustainable material. With plastic er are able to reach

high productivity.

However, plastics also have disadvantages that limit their use:

- They are flammable

- They have low operating temperature (most of plastic doesn’t arrive to temperature

over 100°C while operating)

- They can only withstand low mechanic loads (plastic are soft materials, hardness is

not so good)

- They wear very quickly when in contact with metal parts.

There is no precise and universally accepted definition of what plastics are. The name derives

from the Greek plastikos (to form/to melt).

One definition of plastics is that they are defined by large molecules (polymers) that are

created synthetically or modified from certain chemical components (monomers). A

molecule is the smallest set of atoms that has the characteristics of a group.

The characteristics of a plastic depend on:

- Characteristics of the molecule

- Modes of interaction of molecules with each other (chemical bonds, etc.)

Plastic is also called “polymeric material”: they are long chain of polymers. They are not solid,

they are very high viscosity fluid, they are like glass. Creation of the polymeric chain is called

polymerization.

The first plastics used were natural fibers, such as linen, cellulose and natural rubber. Already

in ancient times the importance of keeping fibers long, chemically reactive and bonded

together was understood.

One of the first plastic products was celluloid, derived from nitrocellulose (used as an

alternative to gunpowder) treated with solvent and then with camphor. In this way it becomes

browsable and this sheet has much better mechanical and hygroscopic characteristics than

the starting material (wood pulp).

The progenitor of modern plastics was Nylon, created by DuPont in 1934 and marketed in

1938.

Plastics are characterized by fairly low costs, especially if compared to their low density.

Even for small volumes, prices remain low.

The cost of plastics depends on the cost of crude oil, from which most polymers come, but

this dependence is not as strong as in fuels. In plastics, a large part of the cost is due to the

technological process that must be used to produce these materials. For example,

Polyethylene (PE) and PVC cost about ¼ of the cost of polycarbonate (PC) and PET, although

the amount of crude oil needed for the four materials is almost identical.

Sale of plastics on the Italian market: polyethylene is the most common plastic.

Resin

Resin is the base for plastic. In the production of plastics, there are several stages The major

resin manufacturers are: DuPont, ICI, Exxon, Mobil, Shell, BASF, Allied Signal, Hoechst, etc.

There are about 40 "standard" resins on the market, often produced by more than one

company, blending and subsequent polymerization therefore allow to obtain a wide variety of

di?erent plastic materials The production of resins is an ongoing process that must be kept

under strict control Resins are produced in granules, flocs or liquids (very viscous).

The bonds between atoms that can be found in plastics are mainly covalent (each atom

shares an electron with another atom depending on its "valence"). Valence: number of

bounding that can be created. Some materials, such as carbon, have a higher chance of

forming bonds with other atoms.

Carbon

Carbon has a tendency to form rings made up of 6 C atoms. When this ring is present in the

molecule it is called aromatic, if this is not present the molecule is aliphatic.

The base of plastics is carbon that could create a lot of connection. Carbon is common for the

valence number.

Also, silicium could create a lot