Open mold is a technique in which the product is made on a mold that is not covered by a second mold or vacuum foil during impregnation. Pressure manipulation is not used during fiber wetting and the emission of volatile substances is usually greater than with closed mold techniques. This technique offers less control over production than closed mold techniques.
However, the two most well-known open-mold techniques are fiber spraying and hand lamination, which are relatively simple. Open mold techniques can also be high tech, such as laser consolidated winding. In addition, a manufacturing process can be started using an open-mold technique, but before it hardens, the product can still be covered and pressure can be added to remove excess resin or reduce air pockets (cavities).
With fiber spraying, a mixture of short fibers and resin is sprayed onto a mold using a special spray gun. The fiber direction is random; the thickness is determined by the duration with which a particular location is sprayed. This is usually used to make large objects, such as ships or structures that are coated.
With hand lamination, you place individual layers in a mold and then wet them with a roller or brush. This is a labour-intensive process, in which you must take measures to prevent the layers from shifting. It is also an inexpensive way, because you can work with simple tools and few consumables, such as vacuum foil, are required.
Winding is a technique used to make cylindrical containers, such as pressure vessels. By orienting the fibers during the winding process, major weight benefits can be achieved. With this technique, the liner, which forms the liquid/gas-tight inner layer, is often used as a mold. Removing the mold can sometimes be a challenge. Collapsible molds are available for this.
Fiber placement uses pre-preg, thermoplastic tape or fiber bundles that are placed in the mold by a computer-controlled robot (without the involvement of human hands). The advantage is that fibers do not have to lie straight in the mold. Automation is necessary to apply this technique, because otherwise it is impossible.
Pultrusion is a technique in which the material is ‘pulled’ through a mold with the shape of the profile cross-section. Instead of pushing, fiber-reinforced materials are processed by pulling. Fiber-reinforced materials are difficult to push through a mold, so pultrusion pulls the raw material through the mold. As a raw material, fiber bundles and fiber mats are usually put together and passed through a resin bath before being pulled through the mold.
In this process, the profiles are hardened at high temperatures (about 130 degrees Celsius) and cut to a certain standard length. It is possible to make infinitely long profiles, but in practice the profiles are usually cut into pieces of about 6 meters.