**4. Forms and cores for cold setting materials**

In addition to metal casting, several important casting processes exist for the production of molds. For example, thermoplastic resins are injected into molds under high pressure. Commodities thermosets usually have very low viscosity and can be poured just like liquid metals. Casting of concrete is also relevant. Here, large volumes and tonnage can be cast. These molds can be built as forms and also be constructed using PBJ-printing technology (see [23]).

In concrete casting, carpenters are often involved in preparing the formwork. The castings are mostly simple geometries. Pillars, walls, and ceilings are predominately made by this casting process. But more complex forms such as curved stairways are poured into wooden molds (see **Figure 14** and compare [24]).

In prototyping, even large components are cast with reactive resins. Here as well, large molds are necessary to give the casting the correct geometry. The casting process is essentially analogous to the conventional concrete pouring.

**Figure 14.** (a) Conventional procedure in construction: (i) pouring concrete into formwork, (ii) finished de-cored wall and (b) construction made by a carpenter: curved wall structure.

Concrete casting usually consist of smooth walls, which can be used repeatedly, and handcrafted wooden construction, which is often used only for one part. After the mold is set up, the reinforcement is usually built from iron. Then, the previously mixed concrete is poured. After curing, the formwork is cut off and the concrete can be surface treated.

In concrete construction high complexity is seldom realized due to the formwork technology. In contrast to metal casting, the heat of the cast material cannot be used for de-molding.

A high degree of complexity can be achieved with printed forms. Molds may be used once or several times depending on their geometries, for example, whether forms have undercuts or not is a relevant criterion.

The printed forms have to be surface treated irrespective of the base material used (**Figure 15**). There are two essential points that need to be considered. Firstly, it is important to prevent cement from entering the pore space of the mold. This would also lead to the mold sticking to the form and to cement leaking around the peripheral regions. Both these factors mean that the outer surface of the casting will be damaged.

Secondly, the separation of casting and form must be made easier if the form is to be used several times. In this case, the adhesion of various materials to each other is to be considered as well as the exact arrangement as lack of air circulation results in high adhesive forces during the separation process.

Hydrophobic substances such as fats are usually used as release agents. These can be applied directly onto the forms. A free-flowing greasy substance thereby allows through-flow of air from the pore space and enables easy removal from the mold.

In case strong forces are created during unmolding, the forms can be strengthened prior to use by infiltration. For example, this can be done by epoxy.

In some cases, the integration of ejectors into the mold is advisable. These can be implemented as jacking screws. Targeted stresses can be built up in the area of the form in order to reduced bending stresses while separating the mold.

In prototyping, even large components are cast with reactive resins. Here as well, large molds are necessary to give the casting the correct geometry. The casting process is essentially

**Figure 14.** (a) Conventional procedure in construction: (i) pouring concrete into formwork, (ii) finished de-cored wall

Concrete casting usually consist of smooth walls, which can be used repeatedly, and handcrafted wooden construction, which is often used only for one part. After the mold is set up, the reinforcement is usually built from iron. Then, the previously mixed concrete is poured.

In concrete construction high complexity is seldom realized due to the formwork technology. In contrast to metal casting, the heat of the cast material cannot be used for de-molding.

A high degree of complexity can be achieved with printed forms. Molds may be used once or several times depending on their geometries, for example, whether forms have undercuts or

The printed forms have to be surface treated irrespective of the base material used (**Figure 15**). There are two essential points that need to be considered. Firstly, it is important to prevent cement from entering the pore space of the mold. This would also lead to the mold sticking to the form and to cement leaking around the peripheral regions. Both these factors mean that

Secondly, the separation of casting and form must be made easier if the form is to be used several times. In this case, the adhesion of various materials to each other is to be considered as well as the exact arrangement as lack of air circulation results in high adhesive forces during

Hydrophobic substances such as fats are usually used as release agents. These can be applied directly onto the forms. A free-flowing greasy substance thereby allows through-flow of air

In case strong forces are created during unmolding, the forms can be strengthened prior to use

After curing, the formwork is cut off and the concrete can be surface treated.

analogous to the conventional concrete pouring.

74 New Trends in 3D Printing

and (b) construction made by a carpenter: curved wall structure.

the outer surface of the casting will be damaged.

from the pore space and enables easy removal from the mold.

by infiltration. For example, this can be done by epoxy.

not is a relevant criterion.

the separation process.

**Figure 15.** (a) Printed form mounted (form for casting upside down) and (b) designer sink made from concrete.

The mechanical properties are relevant for multiple phases in the application of these proc‐ esses. Strength is desirable during the handling phase and while filling the mold. While demolding of disposable forms, low strengths are desired. Particularly suitable are shapes and forms whose strength can be influenced after casting. These can be water-soluble forms with a hydrophobic coating or forms whose binder loses strength even at very low temperatures.

The thermal behavior of the forms plays a subordinate role in this process. Similarly, loss on ignition or residual ash is not relevant.

In this process, accuracy and surface quality are just as relevant as in the metal casting. However, the proportions of the surface features of the component dimensions are often so extreme that the surface finish plays only a subordinate role. The achievable accuracies usually do not greatly exceed the general requirements in the construction industry.

See **Figure 16** for an experimental casting of a large-scale structure.

**Figure 16.** (a) Composite mold from 3D printed formwork elements and (b) resulting large-scale concrete member.
