is a simplified version of thermoforming, where a sheet of plastic is heated to a forming temperature, stretched onto a single-surface mold, and forced against the mold by a vacuum. This process can be used to form plastic into permanent objects such as turnpike signs and protective covers. Normally draft angles are present in the design of the mold (a recommended minimum of 3°) to ease removal of the formed plastic part from the mold.
Relatively deep parts can be formed if the formable sheet is mechanically or pneumatically stretched prior to bringing it into contact with the mold surface and applying the vacuum.
Suitable materials for use in vacuum forming are conventionally thermoplastics. The most common and easiest to use thermoplastic is high impact polystyrene sheeting(HIPS). This is molded around a wood, structural foam or cast or machined aluminium mold, and can form to almost any shape. This high impact material is hygienic and capable of retaining heat and its shape when warm water is applied and is commonly used to package taste and odor sensitive products. Vacuum forming is also appropriate for transparent materials such as acrylic, which are widely used in applications for aerospace such as passenger cabin window canopies for military fixed wing aircraft and compartments for rotary wing aircraft. Vacuum forming is often used in low-level technology classes for an easy way to mold.
Original equipment manufacturers (OEMs) utilize heavy gauge vacuum formed components for production quantities in the range of 250–3000 units per year. Vacuum-formed components can be used in place of complex fabricated sheet metal, fiberglass, or plastic injection molding. Typical industry examples besides product packaging include: fascias for outdoor kiosks and automated teller machines, enclosures for medical imaging and diagnostic equipment, engine covers in a truck cab or for construction equipment, and railcar interior trim and seat components. Vacuum formers are also often used by hobbyists, for applications such as masks and remote control cars.
Types of Molds
Machined aluminium molds are like cast aluminium, but are cut out of a solid block of aluminium using a CNC machine and a CAD program. Typically, machined aluminium is used for shallow draw parts out of thin gauge material. Applications may include packaging and trays. Cost is a significant factor with this type of tooling.
Composite molds are a lower cost alternative to cast or machined aluminium molds. Composite molds are typically made from filled resins that start as a liquid and harden with time. Depending on the application, composite molds can last a very long time and produce high quality parts.
Once a vacuum forming has been created out of a sheet of plastic, a finishing operation will be needed in most cases to turn it into a usable product. Common vacuum forming finishing methods include:
Guillotining: The product is cut out of the sheet by pressing a blade through the product into a die underneath. This is a clean way of removing vacuum formed parts from the material sheet. It doesn’t require a special cutting tool to be made for an individual product and is therefore suitable for low volumes of parts where straight lines are no problem. Cutting only straight lines and being a fairly slow approach compared to other finishing methods, guillotining can be expensive for projects with larger, more complex quantities.
Drilling: If simple round holes are the required finish, manually drilling them is a good solution for small quantities. Drilling guides can be used to ensure holes can be drilled quickly in the right place. As this is a labour-intensive method, it is only suitable for small production quantities.
Roller cutting: Process whereby the vacuum formed product is placed on a custom made cutter and pushed through a roller cutter machine. An efficient way of cutting vacuum formed items from the original sheet of vacuum forming material. The cutter can also cut any necessary holes, such as cable or access holes, at the same time. Roller cutting is suitable for fairly large items, where precision alignment is not required. As the vacuum formings and the cutter are rolled sideways through the roller cutter machine, some misalignment can occur. Roller cutting can’t be used for making holes or features in the sides of vacuum formings, as the cutter tool always cuts vertically from the bottom.
Press cutting: This is a very precise method of cutting which uses a press and a custom precision made cutting tool. This method is very suitable for items where the roller cutter process wouldn’t achieve the precision required. It’s often used for punching very small finished items out of a plastic sheet, rather than punching features into a product. Press cutting can’t be used to make features in the sides of vacuum formings.
Cutting with a CNC machine: Machining is a very precise method of creating holes & features. The real advantage is that it can be used to create features in the sides of vacuum forming, e.g. guide rails for a tray that needs to slide onto shelving. It can also cut sidewalls of a different depth than the pockets—something which cannot be done with roller or press cutting.