Process variants
Vacuum thermoforming
It is one of the most widespread processes, because it is enormously versatile and more economical than other pressure or mechanical processes. It consists of holding the semi-finished product in a structure and heating it until it reaches the rubber-elastic state to place it on the mold cavity and adapt it to its geometry. The air is removed by vacuum (10 KPa), which pushes the sheet against the walls and contours of the mold. Once it has cooled, the piece is removed. The equipment and dies are relatively inexpensive, and are limited to simple superficial designs.
Made up of male
Also called mechanical forming. It is similar to the previous process with the difference that once the sheet is placed on the structure and heated, it is mechanically stretched over a male mold and vacuum is applied through a pressure difference, which pushes the plastic onto the mold surfaces. Objects can be formed that have a depth-diameter ratio close to 4:1. Male molds are readily available and generally cost less than female molds, although they are also more prone to deterioration and require more space.
Formed by matching mold
It consists of attaching the sheet to a structure and heating it to form it between male and female dies. It allows manufacturing very exact pieces with minimal tolerances, also achieving great precision in dimensions and details (the dies must be protected since any defect would be reproduced in the piece). The cycle usually lasts between 10 and 20 seconds.
Vacuum forming with help core and pressure bubble
It is the most used process for thermoforming very deep geometries, since the pressure bubble and the assist core make it possible to control the thickness of the formed object, which can be uniform or variable. The process consists of fixing the sheet and heating it so that a bubble is then created using controlled air pressure, which stretches the material to a predetermined height, normally controlled by a photoelectric cell. The support core (which is normally heated to prevent premature cooling) then descends and stretches the material as close as possible to its final shape. The pin penetration should advance to 70 to 80% of the depth of the cavity. Finally, air pressure is applied from the side of the pin at the same time that a vacuum is formed over the cavity so that the forming of the sheet is completed.
Vacuum forming with helper core
Process used to prevent the thinning of the edges and corners in glass or box-shaped products, as it allows the plastic material to be mechanically extended and stretched to the mold cavity. The plug is usually preheated and is typically 10-20% smaller than the cavity. Once it has been introduced, the air is extracted from the mold, thus completing the formation of the part using a vacuum.
Core-assisted pressure forming
It is very similar to the previous process with the difference that air pressure is applied to force the plastic sheet to adapt to the walls of the mold.
Solid phase pressure forming
This process is very similar to core forming, however the starting material is a solid flat part that generally consists of synthetic powders molded by compression or extrusion, which are heated below their melting point and compressed into the shape of a sheet of desired thickness. The hot material is stretched by a pin and air pressure forces the material against the walls of the mold. The process allows the material molecules to be oriented, improving firmness, tenacity and resistance to cracking.
Vacuum deep relief forming
It consists of placing a sheet of hot plastic over a box and creating a vacuum, which creates a bubble inside the box. Subsequently, the male mold is lowered and the vacuum is released from the box and is made on the male mold so that the plastic adapts to the walls of the mold. It is possible to obtain complex pieces with incoming and outgoing parts.
Trapped Sheet Contact Thermal Pressure Forming
It is a very similar process to direct forming with the exception that both pressure and vacuum can be used to adapt the material to the walls of the mold.
Made with air cushion
Similar to relief forming but with the prior creation of a bubble by stretching the material.
Free forming
It consists of blowing a hot plastic sheet onto the silhouette of a female mold using pressures of up to 2.7 MPa.
Mechanical forming
It is a process in which neither air pressure nor vacuum are used to shape the part. The technique is similar to coincident molding, although coupled male and female molds are not used, but rather mechanical forces to bend, stretch or hold the hot sheet. Wooden templates are usually used to obtain the desired shape and ovens, ribbon heaters and heat guns as heat sources. Typically a flat material is heated and rolled around cylinders, bent at angles, or mechanically formed into tubes, rods, and other profiles. This process has a variant called ring and core forming, which consists of a form of male mold and female countermold with a similar shape between which the hot plastic is introduced, adapting to its shape as it cools. It also does not use vacuum or air pressure.