Negative mold

Room temperature hardenable vulcanizable two-component silicone rubbers are usually formulated as flowable materials. They contain reinforcing and extending fillers. Their flowability means that they are able to flow into the finest details of the to be reproduced item. After vulcanization a negative mold is produced, which is complete reproduction of the original in all its detail. This technology is utilized for the restoring and duplication of... 

Keywords RTV-2 / Silicone Rubber / Molding / Mold Making / Negative Molds / Copies / Reproduetion Materials... 

The term molding used within the seope of this paper means making faithful copies of models (patterns) out of reproduetion materials with the intermediate step of making a negative mold . 

If the sheet is formed over the male or positive mold and the inner surface of the thermoformed part touches and replicates, the mold is called positive forming (Figure 10.2a). In other way, if the thermoformed part is in the cavities of a female or negative mold and the outer surface of the part replicates, the mold is called negative forming (Figure 10.2b)... 

Fig. 20. Finite element analysis of local wall thickness for drawdown into negative mold (39). Used with permission of Institut fur Kunststoffeverarbeitung, Aachen, Germany.

A negative molded drawn part shrinks away from the wall of the mold. 

Negative molds are needed when manufacturing such casting molds. These correspond to the required mold shells and already have the necessary mold flanges and mold thickness (mostly between 7 and 12 mm). 

These negative molds are either produced according to the data or using a positive mold that is manufactured before. Structures added to the positive model, like timber graining, terracotta, or stone structures, can be replicated in the casting mold and the aluminum casting. 

Epoxy resins are used when duplicating models. A casting resin or silicone negative mold is manufactured based on a master model. The casting resin-positive models are now casted in these molds. This procedure is also used when the material of the model is unstable towards the bath solution, e.g., wood, gypsum, leather, and others. 

Figure 16.1 Traditional sheet draw-down sequence into female or negative mold.

Figure 16.6 Sequence showing prestretching with draw box prior to sheet contact with plug prior to vacuum drawing of prestretched sheet against wall of female or negative mold.

Contact forming is restricted to roll-fed thin sheet. The sheet to be heated is brought in contact with a heated plate by vacuum. When the sheet reaches its forming temperature, which is the same temperature as the heated plate, the plate and sheet are pressed against the female or negative mold. A combination of vacuum and air pressure pushes the sheet from the plate to the cool mold cavity (Fig. 16.10). When the sheet is cool, the mold drops away and the sheet moves to a trimming fixture. 

Wheel machines are designed specifically for dedicated products, such as picnic plates, dual-ovenable food trays, and takeaway coffee lids [8]. The wheel is a horizontal cylinder that contains dozens to hundreds of female or negative molds. The sheet is continuously extruded through the sheet die directly onto the slowly turning wheel. Vacuum pulls the sheet into the cool molds. As the wheel turns, the formed products are cooled. When the products are sufficiently cooled, the sheet containing the products is stripped from the wheel and fed to a conventional in-hne trimmer. 

RTV-2 silicone rubbers were invented in the 1950s as useM materials for forming flexible negative molds. The characteristic features of RTV-2 rubbers are their excellent release properties, short-term thermal resistance to 350°C, minimal environmental effects, easy processing, and high elasticity. Scheme 9 represents the CAChe representation of a model RTV-2 silicone rubber segment. 

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