Appunti Optimization and innovation of production processes (parte 1)

Battery 12,5 s Yes

O-ring 9 s Yes

Electronics 5 s No

Case

Clip 1 6,5 s Yes

Clip 2 15 s Yes

For O-ring integration is technically feasible but not convenient because it’s a standard

component, and you are substituting a cheap material with rubber that is more expensive.

Snap fits

A snap-fit is a mechanical joining technique that enables two components, typically plastic,

to be assembled without screws, adhesives, or other fasteners. The principle relies on elastic

deformation during insertion, followed by a secure lock achieved through friction or geometry.

Key characteristics:

- Temporary elastic deformation

- Stable retention after engagement

Snap-fits o?er low-cost assembly and production, as they are generally molded in plastic and

require no special surface finishing. Nonetheless, their design is not trivial: developing a

reliable snap-fit often demands additional engineering e?ort. Due to their simplicity, low cost,

and ease of use, snap-fits have become increasingly widespread in modern manufacturing.

Annular/circular Snap-Fits

The annular or circular snap-fit is the most basic form of snap connection. Engagement takes

place symmetrically around an axis—for instance, in a pen cap. Such joints are characterized

by low engagement force, quick assembly without tools, self-retention, and reversibility (they

can be repeatedly assembled and disassembled).

Their main limitation, such as a tooth, that increase interference between the outer ring and

the inner cylinder. The larger the tooth, the higher the force required for both engagement and

disengagement. A common example is the closure of a bottle cap. These features are easily

manufactured through molding.

Advantages: provides uniform holding strength and allows disassembly through simple

pulling.

Cantilever snap-fits

Among all types, cantilever snap-fits are the most common. Deigning them requires greater

engineering e?ort to achieve reliable performance, which makes them comparatively more

costly. The principle involves a flexible tounge that elastically deforms during insertion, snaps

into place, and then returns to its original shape. Disassembly requires re-flexing the tongue. It

is critical to ensure that the tongue operates within the material’s elastic limit to prevent

failure. Cantilever snap-fits are characterized by asymmetric forces: the disengagement force

is significantly higher than the engagement force, making them suitable for robust yet

reversible connections.

U-Shaped Snap-Fit

The U-shaped snap-fit, one of the most commonly used, increases flexibility and limits

deformation, thus reducing the risk of breakage. In this case, the lever located near the

engagement point makes disengagement easy, though this can sometimes be a drawback

because it may occur accidentally.

Advantages: greater durability, less concentrated stress, higher tolerance to dimensional

variations.

All snap-fit mechanisms are designed for a limited number of cycles; after a certain number,

depending on the mechanism, they break. This is because the imposed deformation durning

flexing initiates a low-cycle fatigue phenomenon, which eventually leads to material failure.

Snap-fits are therefore an ideal solution especially when disassembly is not intended, i.e.,

when the joint remains in place. They are meant for connections to be disassembled only a

few times, or not at all.

Snap-fits maintain retention only if they are preloaded (slightly compressed), where friction

prevents the joint from opening by itself. Achieving this requires very tight tolerances to ensure

proper engagement. When the manufacturing process cannot guarantee su?icient precision

(e.g. in low-cost processes), intermediate low-sti?ness shims (elastic layers to compensate)

are used.

External snap-fits are straightforward to produce, as they do not require additional mold

complexity. Internal snap-fits, on the other hand, often demand motorized tooling, such as

ejectors and sliders, and longer cycle times to ensure adequate material sti?ness.

External snap-fits are simpler to manufacture, whereas internal snap-fits require more

complex tooling.

Lifter

Lifters are movable mold components used to extract parts with internal undercuts. During

mold opening, the tilted lifter rises and shifts laterally, thus releasing the snap-fit. An

alternative to lifters is to introduce a slot in the design, enabling direct demolding and

eliminating the undercut.

4) Reduction of support costs

Support costs can also be decreased as a consequence of earlier optimization measures:

better resource management reduces personnel expenses, while minimizing the number of

parts to assemble lower both inventory and purchasing costs, in addition to reducing

assembly workforce requirements. Beyond these indirect e?ects, specific actions can directly

target support cost reduction:

1. Simplifying the production system (e.g. fewer suppliers, fewer quality checks).

2. Minimizing the likelihood of errors (scrap and rework) through training, clear

documentation, and standardized references (such as color codes).

Unlike direct costs, organizational costs are less visible and, as a result, often

underestimated, particularly within small enterprises.

5) DFM Impact

The final phase is the assessment of the impact that the modifications made through the DFM

(Design for Manufacturing) approach have had on the product, such as changes in material,

assembly, strategy, number of suppliers, and so on. Therefore, at the end of the process, a

performance verification phase of the current product is necessary, along with an evaluation

of how DFM a?ects:

- Development time (critical in some sectors such as automotive),

- Development costs (e?ort/result balance),

- Product quality (performance and number of inspections, durability),

- Component reuse costs (if the component can be used in other products),

- Lifecycle costs (if it contains toxic materials, disposal procedures, etc.).

In addition, it is important to assess whether the product changes have led to an economic

advantage. Development time can be extremely valuable. For example, in car design, time can

be so crucial that it may be worth thousands of dollars per day. For this reason, decisions

resulting from DFM approaches must be evaluated in terms of their impact on development

time.

The relationship between development time and DFM is not easy to establish, but it may

happen that excessively extending development time leads to disadvantages, despite savings

in production costs. Furthermore, development costs are closely linked to development time,

so excessively increasing development time might reduce production costs but require

greater overall expenditure. Manufacturing cost models

Selling cost

For each € spent on direct material, I have to spend an additional % related to the general

costs (how the comply is organized etc.). These general costs are multiples of the direct costs.

Primary costs consist of the material cost, labor cost (workforce), and tooling cost.

Production costs are obtained by adding overhead costs related to production support to the

primary costs. By further adding the general overhead costs of the company – such as

administration, sales, and other expenses – we obtain the selling price of the product. Finally,

by adding the profit margin, we determine the selling price of the product. We saw that, by

using costs drivers, it is possible to estimate overhead costs starting from direct costs.

Profit margins are lower now (this picture belongs approximately to the year 2000) and the

manufacturing has a bigger impact on costs today. It’s so high because I also include the

buying of standard components (as well as that of raw materials).

That chart shows the cost distribution in the automotive sector; for other, less technology-

intensive industries, the engineering cost is significantly lower.

Cost calculation

Correct calculation based on your manufacturing dimension.

If variable and fixed costs are determined, the total costs for production can be calculated

with the following:

(The total cost is always composed of a fixed cost and a variable cost, it does not depend on

the type of organization implemented).

In the variable costs it is also necessary to include the cost of the material, which can be

calculated considering its use and the cost of the waste material (scraps). Scrap material is

often sold, but at a very low price, especially when considering possible cleaning or

processing costs (scraps have a residual value). Therefore, when evaluating the material cost,

it is also necessary to take into account the repurchase cost.

The chart shows a simple example of how to find the Break-Even Point, which is the point

where sales and total costs are equal (the base of every strategic decision). Fixed costs and

variable costs are summed to obtain the total cost curve. In general, this curve is

representative in the case of small-scale production. For larger production volumes, the curve

tends to flatten slightly. The marginal cost for large-scale production decreases somewhat

thanks to economies of scale. Sales behavior also depends on the sales volume itself, and

certain commercial strategies can cause the sales curve to flatten as well. This type of chart is

useful for evaluating alternative production processes, but it is not suitable for assessing

profit. However, such an analysis is important for estimating the Break-Even Point.

Toyota discovered that giving a selling price, giving a manufacturing plan, there is a break-even

point. It can be defined based on time or based on number of sales/products.

Process selection

In choosing production processes, it is essential to consider:

- Fixed costs: the costs that do not change with the level of production

- Variable costs: the costs that change proportionally with the number of units produced

- Number of units to be produced: because the production volume directly a?ects the

convenience of one process over another.

Cost estimation methods

In general, it is possible to use alternative methods at di?erent development phases of the

production process. In a first phase, qualitative methods are used (e.g. similar product

weight), where the cost is estimated on the basis of similarity. In later stages it is more

convenient to have a precise cost estimate and analytical methods are used (can be used

only ex-post, when you have to modify a product process that already exists).

Cost estimation in product development depends on the project stage. With a well-defined

plan, an analytical approach calculates costs component by component (materials, labor,

machine time), given precise numerical results. In early design stages, simpler qualitative

methods are used:

- Analogy-based methods rely on similar past components and company historical

data. You

- Non-structured reasoning compares processes using rough fixed/variable cost graphs

to choose the most cost-e?ective option.

Qualitative methods help iden