Molds and Production

Thermoforming.

It is a molding process where a plastic sheet is first heated and then formed to a specific shape in a mold using pressure or vacuum suction.
This is highly recommended for the creation of large pieces with a plane geometry, when the creation of an injection mold would be too expensive. One of the advantages of thermoforming is the possibility to mold very thin thicknesses. The procedure is as follows: the polymer plate, which is heated at the proper temperature, is positioned on a cavity containing the mold. The sheet is stretched onto the mold by pressure, using either a countermold or an air pressure, or by vacuum, that is to say that the pre-heated plate stretches over the mold by suction. The piece is air-cooled and then taken out of the mould.
The thermoforming cycle is longer than the injection one: this technology is therefore to be applied for the production of small-medium quantities.

Rotomolding.

Rotomolding is used for the production of plastic parts with complex shapes in just one piece. This technology has been used since the 50s to create hollow objects of both small (ping-pong balls) and big dimensions (fan spirals or big tanks).
Rotomolding is made by rotating the mold thus spinning the material (usually polyethylene) which is then distributed around the inside of the mold’s cavity.
Other materials used are polyvinyl chloride (PVC), Polycarbonate, Polyester and Polyamide.
The major steps are the following:

1. Loading the resin;
2. Heating and melting the resin;
3. Spinning the material;
4. Cooling and unloading the piece.

The molds can be produced in many ways and many different materials can be used to make them: they can be made of sheet-steel, die-cast aluminum or by processing an aluminum block with CNC technology or by electro-erosion. This last working procedure is highly-recommended for high-quality surfaces.

Blow-molding.

This process is suitable for the production of tanks, bottles and liquid containers in general. As a first step, an injection mold for the creation of a “parison” must be produced. The parison is put inside the blowing machine. A pressurized gas is then injected, which inflates the parison so that it takes the shape of the mold. The plastic material stretches as it expands: for this reason the thickness of the piece produced will not be regular. This issue can be sorted out by using a parison with an increasing thickness in the same blowing direction, or changing the parison’s thickness where the piece’s diameter varies. This process makes it easier to create containers that would otherwise be more difficult and expensive to produce.

High-pressure injection molding.

Injection molding is a fusion process in which a fluid polymer mass is injected into a mold.
Injection usually happens at high pressure and in temperature conditions that allow the plastic material to flow in a dedicated space inside the press. Molds are hydraulically or mechanically locked inside the molding presses.
The machine used for this process is called injection press, and it consists of the following parts:

1. a storage hopper, which is filled up with granular polymer.
2. an extruder, (also called screw or screwdown) which changes the polymer state from granular to fluid by means of a combined heating action (supplied by heating elements and mechanical friction).
3. a special frame for hosting the storage hopper, the extruder and the molds’ moving elements. This frame must be strong enough to bear the weight of the above elements and the strain of all the forces involved.
4. a hydraulic circuit  for mold cooling.
5. a control panel that manages the machine’s functions.

Tonnage is the parameter that indicates whether the injection machine is suitable for producing pieces of a certain weight and dimension.
This parameter also affects the machine’s dimensions.

A simple formula is used to calculate the approximate tonnage needed for an injection machine to produce a certain piece, and this is based on the area of the object to be produced.
For example, a 100 tons machine can produce pieces with an approximate area of 150 cm² and weight of 180 to 250 g.
This is just an indication since the thickness and the type of injection used may affect this calculation.
Another important parameter, when talking about injection molding, is the “cycle time”. The length of a cycle affects the price of a product, according to the cost per hour of the injection press.
Hence, for large-scale production multi-figure molds are recommended.
A critical stage of the injection molding is the “melted polymer compacting”.
This is the stage when, as soon as the injection is over, the mold remains closed and under pressure, in order to enable the melted polymer to enter all the cavities and erase any air bubbles or blowing imperfections inside the mold.
If the compacting process is too short, the pieces produced will be low-quality, with missing or faulty parts and with the wrong mechanical features.
On the contrary, if the compacting process is too long, the product may have burned parts because of the over-exposure to heat.
Other parameters that determine the length of a cycle are the presence of movements inside the mold, the material’s consistency, the dimensions and the mass of the piece to be produced .

Low-pressure molding.

Low-pressure molding still involves a fusion process and it can also involve the use of injection points. It differs from the previous process for providing a little lower level of product finishing, increased thickness and reduced precision. On the other hand, it implies lower costs. Low-pressure allows using aluminum molds, which are simple and cheap, thus making this process preferable in case of large production numbers. The materials used in this process are different from the ones used in high-pressure injection. Polyurethane fits a great number of solutions in all its variations. Being versatile, it offers different levels of compactness and rigidity. It is a high-tech material particularly competitive in high-pressure molding as well.
Polyethylene foam is also commonly used in low-pressure molding, and AMV has been largely using it.

(EVA) Polyethylene foam

A granule with a modified EVA-base which was studied for the production of manufactured expanded cross-linked with injection technology:

• This polymer that approaches elastomeric materials, once finished it is a topic in softness, flexibility, and comfort too. Low density (0,11-0.35 gr/cm3), distinguishing Physical-mechanical property (abrasion DIN 53516/77, 90/200 mm3) togheter with a well tactile aspect (hardness 14-50 Sh.A).
• An important point of EVA foam is that it is coumpounded by granule closed-cells, (es. polyurethane is an open-cells foam), in order to avoid to absorb substances and prevent chemical external agents attacks. Also a good resistance to atmospherics agents at low and medium temperature is tested.
• Thanks to its closed-cell characteristic, EVA is Antibacterial foam.
• Great resistance to UV radiation is stated thus the colors are well maintained during the long term.
• Thanks to its excellent and easier manageability, this material grant good results in development of our drawing.
• This is an “Eco-Friendly” material, as it does not contains bad additives for healt. It means that in case of possible product combustion in the atmosphere only water and carbon dioxide will be released.
• It can be molded in a vast range of colors. Also new colors are available to be created for customer need
• A “fireproof” different formulation has already gone over the test for its classification on automotive industry (direct. 95/28/CE), and also in furnishing market (UNI 9175 – Class 1). Another special formulation is in complaint with EN 1621 – 1/97 normative to preserve in case of crash impact.