The vacuum production process is a thermoformic process, in which a plastic sheet is heated to a high temperature, stretched on a mold with a configured surface and impacted onto the mold as a result of a vacuum. This process can be used to create solutions and products from plastic for fixed objects such as illumination signs and protective covers on the one hand and a variety of solutions for industry on the other. Relatively deep parts can be created by combining pneumatic support systems that are integrated into the process in contact with the surface of the mold during the vacuum process.
Suitable materials for use in the vacuum process are conventional thermoplastic materials. The most common and easiest material for thermoplastic use is polystyrene sheets (HIPS). A variety of molds from different materials can be used with this material such as wood, structural or cast foam or aluminum. Almost any shape can be manufactured with this material.
This high-impact material is hygienic and able to retain heat and shape even when hot water is used. It is usually used for packaging food and odor-sensitive products. Use of the vacuum forming process is also suitable for transparent materials such as acrylic, which are widely used in aerospace applications such as cabin canopies for military fixed wing and rotary wing aircrafts.
Original Equipment Manufacturers (OEMs) use components created by vacuum forming for production quantities in the range of 5-5,000 units per year. Components created in a vacuum can be used instead of complex metal, fiberglass or plastic injection. Typical examples of the industry besides product packaging include: display signs for kiosks and ATM machines, housings for medical imaging and diagnostic equipment, a variety of covers for motorized vehicles and interior sectioning and seat components of a train car, all this, as well as construction equipment.
There are a number of issues that may be encountered in the vacuum forming process. Moisture absorbed in the thermoplastic sheets before production can expand and form bubbles within the inner layers of the plastic. This significantly weakens the material and the final product, on top of the visual aspect problem.
This problem can be solved by drying the plastic for a long time at a high enough temperature. Cracks and breakages can form around and inside the mold, as a result of overheating of the plastic and therefore the temperature must be carefully monitored in the process of heating the material. Material residues can accumulate on the mold if the process is not carefully controlled.
On the other hand, deformations deviating from the design can occur in the material when the part is too large or parts on the mold are too close to each other. Finally, it is of great importance that the product angles are designed to ensure easy retrieval of the material from the mold without causing distortions.
Types of Molds
Molds for vacuum forming can be made from many types of materials.
Wood paneling is the most common material for vacuum processes because it is relatively inexpensive and allows the customer to make design changes easily. In the process of manufacturing the mold, adhesives are combined with the wood to give it its finish. The number of samples that the user is able to obtain from each mold depends on the size of the part and the thickness of the material. Nevertheless, this type of mold is suitable for small quantities, up to a few dozen only. Beyond that, the mold tends to wear and will need to be fixed. Therefore, it is common that after the product is approved using the wood mold and before the transition to serial production, a stiffer mold is designed suitable for the production quantities of hundreds and thousands per year. These molds are made of ceramic composite materials, or cast aluminum.
Cast aluminum molds are the most accurate and durable and usually have temperature control lines that pass through them so that the heat of the resulting plastic can be set, while the manufacturing process can also be accelerated. The main disadvantage of this type of mold is its cost and costs of changes in the development process.
Composite molds are a lower cost alternative to aluminum molds. These molds are usually made of solid resin that starts as a liquid and hardens over time. Depending on the application, molds from composite materials can be used for a very long time as well as for production of high-quality parts.
The simplest way to create a mold for a large quantity of identical items is to duplicate a single item mold many times and then combine all the parts into one larger production mold.
It is necessary to take into account the extraction angles at the mold design stage. A minimum angle of 3° is recommended to facilitate the extraction of the formed plastic part from the mold.
After pulling the sheet / plastic board from the vacuum machine, it is necessary to perform an additional step of final cutting of the product from the sheet residue and spatial processing of the product in the required areas.
Common methods of cutting:
Cutting with a CNC machine
Machine cutting is a very precise method for creating holes / machining / final cutting from the sheet residue. The real advantage is that it can be used to create texturing on the sides of the product, such as rails for a tray that needs to slide on shelves. The process can also be used to cut side walls at a depth different from the pockets – something that cannot be done with a roller cutter or press. This process ensures accuracy and speed but requires very expensive machines that need periodic calibration.
This is a very precise method of cutting that uses a press and a specific precision cutting tool. This method is very suitable for items where the roller cutter process will not achieve the required accuracy. It is often used for very small finished items produced from plastic sheets, instead of punching into the product. The press cannot be used to create elaborations on the sides of the product.
A process in which the vacuum formed product is placed on a custom cutter and pushed through a cutting machine / band saw. Cutting with a saw is suitable for fairly large items, where precise alignment is not required. Saw cutting cannot be used to create holes or to add finishing on the sides of the product, since the cutting tool always cuts vertically from below.
If the required finish includes simple round holes, then manual drilling is a good solution for small quantities. “Jig” tools can be used to ensure that holes are drilled quickly in the right place. Since this is a labor-intensive method, it is suitable only for small production quantities.
The product is cut out of the sheet by pressing a blade on the product. This process does not require a special cutting tool for a single item and is therefore suitable for low volumes of parts where the lines are straight. Since cutting is done in straight lines only and the process is rather slow as compared to other finishing methods, guillotine cutting makes it expensive for projects with larger and more complex quantities.