Domande in inglese Optimization and innovation of production processes

HAZ).

This zone causes several problems:

- Oxidation of the surface: due to prolonged exposure to high temperatures in the

presence of oxygen.

- Localized hardening of the cut edge: the high thermal gradient and rapid cooling

can cause unintended quenching of the material, creating a harder microstructure.

- Increased brittleness: the harder microstructure may also be more brittle and

prone to cracking or defect propagation, especially in dynamic or fatigue-loaded

applications.

- Residual stresses: thermal gradients can introduce internal mechanical stresses

that reduce the integrity of the part.

37. Plasma

Plasma is the fourth state of matter, in which electrons are separated from atoms, and

temperatures can reach up to 2000 °C. Plasma is generated by creating a potential

diEerence between the electrode and the workpiece to be cut; the electrons emitted

by thermionic eEect collide with gas molecules, stripping additional electrons and

creating an avalanche eEect that ionizes the gas.

The ionized gas is then directed toward the workpiece, which sublimates at the point of

contact with the plasma jet. A second assist gas may be used to protect the

workpiece.

The surface finish of plasma cutting is poorer compared to laser cutting, but its cost is

lower.

38. How to create perfectly vertical surfaces with plasma cutting

The only way is to orient the cut so that the good-quality cut surface is on the side of

the part you want to keep, and the slanted cut surface is on the scrap side. It is not

possible to obtain two vertical surfaces with plasma cutting, since HDP cutting always

produces tapered surfaces.

39. Which gases are used in plasma cutting?

DiEerent assist gases are used depending on the material and the size of the

workpiece:

- For thick carbon steel cutting, oxygen is used, taking advantage of the oxidation

heat.

- For stainless steel cutting, oxygen is not used to avoid decarburization and

chromium depletion.

- For light alloys, air or nitrogen is used to prevent violent reactions between oxygen

and aluminum.

- For titanium, air or nitrogen is used.

40. HDP method

The HDP method is a technique developed to make the plasma jet thinner and more

precise, improving cutting quality compared to conventional plasma sources.

HDP focuses the plasma jet more tightly, resulting in a more defined cut. An initial

pierce hole is required, and at the end of the cut an accumulation of material can still

be observed. To reduce noise and the dispersion of debris into the air, the process is

often carried out in a tank with 4–5 cm of water above the workpiece. As assist gases,

inert gases or oxygen are used; oxygen, similarly to laser cutting, causes surface

striations.

41. Comparison of the fibers used in composites

Composite materials are made of a matrix, usually epoxy resin, and fibers. The most

commonly used fibers are Kevlar, glass, and carbon fibers. Carbon fiber is the most

expensive and has the highest elastic modulus of all, but it has the lowest elongation

and tensile strength. Kevlar and glass fibers are similar in all characteristics except for

cost: glass is much less expensive.

Although Kevlar is more costly, it is used when higher resilience is required—that is, the

ability to withstand an impact. An example of application is bulletproof vests.

42. Stages of the autoclave cycle for composite materials and cycle time

An autoclave is essentially a pressurized vessel in which temperature, pressure, and

the vacuum inside the bag can be controlled. With this type of system, the laminate

undergoes a curing cycle at elevated temperature while simultaneously being

subjected to a hydrostatic pressure state. The strength of the autoclave process lies in

the fact that the laminate is loaded with a stress state that, at every point, acts along

the normal direction to the laminate surface.

The autoclave cycle is divided into several stages:

- A vacuum is drawn inside the bag down to about 15 kPa.

- The material is heated up to 250 °C and the temperature is held constant for 30

minutes.

- The autoclave pressure is increased up to 15 bar and the temperature is raised to

310–340 °C.

- The temperature is maintained for three hours.

- Cooling is performed under pressure down to 65 °C.

- The autoclave is opened, and the part is allowed to cool in air for about six hours.

43. Selective assembly and related cases. When is it convenient to use it?

Selective assembly consists of measuring the components and creating optimal

pairings. It is used when it is not economically convenient to reduce component

variability (tolerances are already very tight, or reducing variability is diEicult). Cases:

- Equal process capability (same range in which most of the production lies) for

components A and B: all components are measured and divided into classes a5,

a4, …, b2, b1. Components in class a5 are then assembled with those in class b5,

and so on, ensuring that the pairs satisfy the tolerance required for assembly.

- DiEerent process capabilities, with one equal to the required tolerance: classes

and their sizes must be defined based on the worse process, and then several

classes of the better-performing component must be produced in the

corresponding proportions.

- DiEerent process capabilities, both worse than required: in this case, multiple

batches must be produced for both components so that the combined output

results in two distributions with the same variability. Component A is produced,

measured, and divided into classes. Multiple batches of component B are

produced, with enough pieces to approximate the variability of A. This requires

producing many batches and often results in significant scrap.

44. Characteristics of PE and PP

- Polyethylene (PE) is the simplest of all polymers. It is composed only of hydrogen

atoms as external elements, which gives it good resistance to chemical agents,

high electrical resistance, and good thermal resistance. Its mechanical properties

depend on its branching and density, which are determined by the manufacturing

process used in its production.

- Polypropylene (PP) has better mechanical properties than polyethylene. It is also

much stiEer. Therefore, this material is used for structural mechanical parts,

whereas PE is preferred where a certain degree of flexibility is required. PP can also

operate at higher temperatures compared to PE.

45. On what principles is the black-box method based, and how does it work?

In the DFM optimization process of a product, one of the steps consists of purchasing

certain non-standard components from outside the company. These components are

produced by specialized manufacturers whose high production volumes (for that

category of product) allow them to oEer lower overall prices.

The black-box approach involves imposing on the supplier only the minimum number

of constraints strictly necessary to meet the required technical specifications—

without “entering into” the supplier’s production process. This leaves the

manufacturer free to use any technologies available in their facility and thus provide

the lowest possible price for the chosen component.

The common mistake made in past years was imposing unnecessary constraints that

were irrelevant to the product’s functionality, ultimately increasing the component’s

cost.

46. Tolerance synthesis with quality costs

In tolerance synthesis, it is also possible to take into account the quality costs that a

product may incur due to diEerent values of functional tolerance. The functional

tolerance is not chosen solely on the basis of design requirements but also as a

compromise between production costs and quality costs. These costs naturally have

opposite trends: as one increases, the other decreases.

A curve of production costs is created for various plausible values of the product’s

functional tolerance. Several values are selected, and for each of them the

optimization from the earlier step is repeated, allowing the construction of an almost

continuous cost–functional tolerance curve.

Calculating quality costs requires several steps:

1. Definition of failure/breakage modes for high and low values of functional

tolerance.

2. Definition of the failure/breakage cost.

3. Evaluation of the probability that such an event occurs for each functional

tolerance value.

4. Calculation of quality costs, obtained as the product of the event cost and its

probability.

5. Summing the two cost curves (production cost + quality cost).

47. Plastic extrusion

Extrusion is a process used for both thermoplastic and thermosetting materials (the

latter at temperatures below their curing point).

Raw plastic materials in the form of granules or powders are poured into a hopper and

fed into the extruder barrel. The extrusion screw is shaped in such a way as to reduce

the cross-sectional area available to the material while simultaneously melting it with

appropriate heaters. The screw geometry forces the material to be pushed forward.

The extruder head is made of metal and has the negative shape of the profile to be

produced. It is usually equipped with pressure and temperature sensors that control

the screw rotation speed to keep these values within a precise range. The material is

cooled with air or water as soon as it exits the die.

48. Calculation of assembly times

Calculating assembly times is a complex activity because it depends on numerous

factors related both to the operator and to the characteristics of the component. For

this reason, arbitrary estimates are not used; instead, standardized methods based on

tables of average times are employed.

The total assembly time is divided into two fundamental phases:

- Handling time (picking up the component, moving it to the mounting position),

- Insertion time (inserting and fastening the component).

There are also methods that define tables indicating the potential assembly times of

components. In Italy, the actual times are always discussed with labor unions. Among

these methods are:

- Boothroyd method (mentioned earlier)

- Hitachi method: the Hitachi method assigns a score from 0 to 100 to each

operation; the more complex the operation, the higher the score assigned. The sum

of the scores indicates the overall complexity of assembling the product. Twenty

classes of operations have been defined.

- Westinghouse method

49. Describe and draw a diagram of the sinker EDM cutting process.

Sinker Electrical Discharge Machining (EDM) is a non-conventional machining process

that removes material through electrical discharges between:

- A shaped tool (electrode)

- The workpiece

Both are immersed in a dielectric fluid. In the sinker EDM process, the electrode has

the negative shape of the cavity that must be produced in the workpiece; the final

cavity is the positive copy of the electrode.

The electrode and the workpiece are connected to a DC power source and immersed

in a d