Appunti optimization and innovation of production processes in inglese (parte 5)

Blowing

Blow molding is used to make hollow geometries, usually with thin walls such as bottles. The

piece is in fact "blown" towards the surfaces of the mold.

The workpiece is first extruded as a hollow profile and then blown. Usually this process is

continuous and implemented on appropriate carousels that allow a high level of automation.

In extrusion blow molding, a tube is (a) extruded (usually vertically), (b) clamped in a mold

that has a cavity much larger than the tube’s diameter, and (c) expanded by blowing until it

fills the mold cavity.

Compressed air is typically used, with pressures ranging between 350 and 700 kPa.

In injection blow molding, a short tubular piece (parison) is first injection molded and then

transferred into the cavity of a blow mold. The piece is first extruded as a hollow profile and

then blown to its final shape. This process is usually continuous and carried out on rotary

machines, which allow for a high degree of automation.

In the case of highly quality products, that must achieve a constant wall thickness, the first

process is injection instead of extrusion to have a very controlled starting shape for blowing.

To improve filling, a light mechanical action provided by a movable piston can also be applied.

It is important to note that this process does not produce perfectly controlled geometries: the

outer wall that comes into contact with the mold generally follows its shape, but the wall

thickness is not uniform, partly due to gravity, which causes greater thickness at the bottom.

To increase production volumes, highly automated rotary systems (with extrusion) are usually

employed.

The di?erences between extrusion and blow molding initiation are:

- Equipment is about 50% cheaper

- Produces more waste

- Faster process

- More materials that can be used

- More uniform walls

- More stable process

- Lower mechanical characteristics

With this method, it is possible to create shapes that are then separated.

Thermoforming

Thermoforming represents a family of processes used to form sheets or films of thermoplastic

material over a mold through the application of heat and pressure or vacuum. Thermoforming

is mainly used with thermoplastic materials. It consists of deforming a sheet of resin until it

slide till the shape of the mold. The process can be vacuum or pressure assisted.

A sheet (produced by extrusion) is heated in an oven (though not always) to its softening

temperature, but below the melting point. It is then removed from the oven, placed over a

mold, and forced to adhere to it by applying vacuum pressure. Since the mold is usually at

room temperature, the shape of the plastic is fixed as soon as it comes into contact with the

mold surface. Due to the low strength of the softened materials, the pressure di?erence

created by the vacuum is generally su?icient for forming, although compressed air or

mechanical means may also be used. The components produced through this process

include packaging, advertising panels, and appliance housings.

Let’s now focus on vacuum thermoforming:

The process involves heating the plastic sheet in the upper part of the machine through

radiation, and then pressing it against a metal mold so that it takes its shape. To promote

adhesion between the sheet and the mold, a vacuum is created through micro-holes

distributed over the entire mold surface.

The vacuum serves two main purposes:

1. The force applied to the sheet is low (about 0.7 bar), so the hot sheet does not tear.

2. It is necessary to remove all the air trapped between the mold and the deformed sheet,

otherwise air bubbles could form.

The micro-holes are small enough to prevent the plastic from entering, but numerous enough

to draw out all the air from the gap between the metal mold and the sheet.

This is a very flexible process: it allows the production of both thin and thick components

(e.g., plastic plates, bathtubs — in which case punches and overpressure are also used), and

the achievable dimensions can vary greatly.

With the vacuum technique, very low pressures (maximum 1 atm) are used to deform the

material, ensuring better uniformity of the process compared to other techniques, although

production speeds are lower. Before being removed, the part must cool down to maintain its

shape. The vertical walls near the grips are usually thinner than the base of the formed part.

The process can also be carried out thanks to the use of

compressed air (up to 20 atm). In this case, you have much

higher production speeds and reduced costs. if the film is thin,

this increase the risk of film breakage.

Sometimes a mechanical punch can also be used to support

vacuum processes, so as to limit the risk of breakage of the

plastic sheet.

In processes of this type, it is necessary to consider certain issues: the thickness is not

perfectly uniform because the stretching of the sheet is a somewhat chaotic (probabilistic)

process that depends on the inclination of the mold walls.

The thickness along the vertical walls is usually smaller due to the greater stretching, whereas

the bottom walls of the mold retain a thickness almost equal to that of the original sheet since

they adhere more closely. In contrast, the corners can have very thin sections.

A technique used to achieve greater control over the thickness is called reverse draw

forming, which consists of inflating the entire sheet before deforming it. In practice, the

material is initially blown away from the mold, forming a dome shape so that it reaches an

area comparable to that of the final part. This step reduces the thickness in the central region,

where the thickness usually remains unchanged. After this pre-stretching phase, the material

is then pushed toward the mold either by the e?ect of vacuum or by a mechanical punch.

Reverse draw forming is particularly used when deep parts need to be produced, as it helps

ensure a more uniform wall thickness throughout the component.

Another problem with thermoforming is that while the surface of the sheet in contact with the

mold achieves excellent surface finishes, dimensional and geometric tolerances, and detail

accuracy, the inner side has poor control. It may show surface depressions or sink marks due

to the non-uniform thickness of the material.

To further reduce the issues related to uneven wall thickness, it is possible to use two-mold

forming: two molds are employed, one being the positive and the other the negative of the

desired part. However, this process is quite critical, as the amount of material must be

perfectly calibrated; otherwise, it can lead to severe distortions and flash formation. This

method is particularly used when strict dimensional tolerances are required on both sides of

the component or when very large parts are to be produced.

A process that ensures excellent uniformity of the formed piece thickness is snap-back

forming, where the sheet is pre-stretched before being drawn into the mold.

Another variation is tap forming, which allows the material to stretch during the process,

making it suitable for deep-shaped parts, and it also features lower equipment costs. In the

case of matrixless thermoforming, components with a “bubble-like” shape can be produced.

However, the complexity of achievable geometries is very limited due to the absence of a

mold, and only the contour geometry of the gripping frame can be varied.

One of the key aspects of thermoforming is the design of air outlet holes in the vacuum

chambers.

These holes must have two fundamental characteristics:

- The initial section must be calibrated with a very small diameter (about 0.6 mm) to

prevent material from being sucked into the vacuum chamber.

- The second section of the hole must widen, to avoid excessive pressure losses, given

that the vacuum pressures are relatively low (around 1 atm).

In the presence of undercuts, removable dowels must be used; for larger dowels, it is

necessary to ensure vacuum conditions beneath them as well.

Finally, the process can be automated using carousel-type machines, which allow continuous

production and high e?iciency.

Advantages:

- Low cost of machines

- Low temperature

- Large components possible

- High speed

Detriments:

- High cost of material (sheet metal)

- Relevant waste of material

- Only one side is dimensionally accurate

- Significant thickness variations

Rotary forming

In rotary forming, molten material is required to solidify inside a mold that is rotated. In this

way the material solidifies on the inner walls of the mold only. This process is carried out for

large parts, when thermoforming would not be possible, It is also possible to make closed

profiles; In fact, essentially only closed shapes can be created.

This process was developed to overcome the limitations of blow molding, particularly when

producing large objects with significant wall thickness. Blow molding, which uses

compressed air, allows only thin-walled parts to be made — otherwise, excessive pressure

could cause the part to break. It is a process suitable only for thermoplastics (except for a few

rare cases involving thermosetting materials). In rotational molding, molten material is forced

to solidify inside a mold that is rotated continuously. In this way, the material solidifies only on

the inner walls of the mold, forming a hollow structure. This process is mainly used for large

parts, where thermoforming would not be feasible, and it also allows the production of closed

profiles. The machines used are quite simple and can make multiple pieces at the same time.

Some pieces can then be cut if they have not a “closed” geometry.

It is one of the few processes that makes it possible to create double-walled components.

Operation: the solid thermoplastic material (in granule form) is placed inside the mold, which

is then heated until the material melts. Once the melting is complete, the heating is stopped,

and the material is allowed to solidify inside the mold while it is rotating. Thanks to the

rotation, the material solidifies on the inner walls of the mold, forming the desired structure.

The inner surfaces of the part do not have a uniform thickness, whereas the outer surfaces,

which are in contact with the inner walls of the mold, are well defined. With this process,

there is a significant accumulation of material at the corners, since they can retain more

molten material and cool down faster. This is actually an advantage, as corners are usually

critical areas in molded products. In rotational molding, reinforcing elements can also be

added to the virgin material (for example, glass fibers embedded in polyethylene), which

increase the mechanical strength of the product. The part is usually rotated around two

perpendicular axes to ensure that the inner walls of the mold are properly coated, achieving a

uniform and complete formation. In general, rotational molding machines use a cluster of

molds that rotate simultaneously, allowing multiple parts to be produced at once. The molds

are me