Presses

How does rubber or silicone actually get its shape? One of the main molding processes for rubber, natural rubber or silicone is pressing. Pressing is used to form moldings (as well as elastomer parts). The process is carried out using special machines known as presses. Here, the moldings are pressed in into form using individual tools in elastomer presses. In addition, the moldings are also simultaneously vulcanized during the pressing process and thus made dimensionally stable. Presses are categorized into vulcanization presses, also known as compression presses, and injection molding presses.

Types of presses and their construction

Vulcanization/compression presses

Daylight presses

Daylight presses are hydraulically operated vulcanization presses that have multiple levels. The levels refer to the working space between the heating plates, on which are the molds are placed. The heating plates are held by a frame (frame press) or columns (column press). The plates are heated electrically, by oil or by steam. During the pressing process, the heating plates along with the molds are pressed together. The resulting pressure forms the parts (compression method, transfer molding). Daylight presses are also often used for pressing friction lining materials, even if they are not made of elastomers.

Application: Molding simple rubber parts such as rings, stud plates, buffer plates or plugs

C-Frame presses

With this type of vulcanization press, three sides remain open, which enables access from three sides.

Application: C-frame presses are suitable for molded products as well as endless products such as conveyor belts and plates.

Common press dimensions

Pressing force: approx. 5 – 10 N/mm²
Plate size:  250, 300, 500, 600, 800 mm²
Temperatures: 150 – 200°C

Injection molding presses

With injection molding presses, a plasticized, preheated, vulcanizable rubber compound is injected into a heated mold/tool and subsequently vulcanized. Injection presses can be constructed horizontally or vertically. Furthermore, a screw can be used for filling, which automatically actuates the injection process with an injection piston or by moving the cylinder.

Frame corner injection machines

These injection molding machines, also referred to as corner molding, are used to connect moldings that have already been vulcanized, such as rubber composite profiles, in order to form a closed frame, for example. For this, the parts to be connected are inserted into the mold, the rubber compound is injected using piston pressure, and then vulcanized.

Application: Frames, window guides, sealing profiles (for the automotive industry)

Automatic mat vulcanizing system (AUMA)

Conventional presses are not suitable for pressing and vulcanizing rubber plates and sheets. Therefore, the continuously operating automatic mat vulcanizing system (AUMA) was developed. The AUMA consists of two strong pillars, between which a heating cylinder/heating drum is located. In front of the heating cylinder, there are deflector rolls and behind it a tensioning roll. An unwinding stand is in front of the machine with the material to be vulcanized and at the rear is the winding stand for the vulcanized product. An endless pressure belt winds around all rolls and cylinders. The pressure generated by the belt presses the material. The material to be vulcanized runs through the various zones of the AUMA in one piece. As the material passes along the heating drum, it is heated and vulcanized.

Application: Vulcanizing and doubling endless rubber webs, conveyor belts, rubber plates etc. with a thickness of 0.5 to 20 mm.

Conveyor belt presses

Belt presses, conveyor belt presses or double belt presses are made for pressing plates, mats, and webs of rubber. Here, the rubber is pressed and vulcanized between two continuously running steel cords.

Application: Production, for example, of profile strips for car tires, handrail profiles, conveyor belts

Transfer presses

With transfer presses, long or endless profile strips or strips of material such as tire tread wear for retreading or conveyor belts for the mining industry are gradually pressed into form and vulcanized in sections. Transfer presses can produce form strips with relatively small tools, which is otherwise only possible with costly tools or sometimes impossible to produce with other processes.

Application: Long or endless profile strips or strips of material such as tire tread wear for retreading or conveyor belts for the mining industry.

V-belt presses

V-belt presses are not presses in the traditional sense but rather comparable with a mold. Here, a so-called unvulcanized V-belt blank is used, on which the V-belt structure is produced. For vulcanization, the unvulcanized V-belt blank is placed in a mold and enclosed from the outside. After vulcanization in a steam boiler, the V-belt blank is removed from the mold and placed on two rotating rolls, on which the actual V-belts are cut using sharp blades.

Application: Producing V-belts

 

Pressing methods

Transfer molding process

In compression molding processes, rubber mixtures/compounds are placed in a mold and injected through channels into a mold upon closing and then subsequently vulcanized (also called transfer molding process).

Compression molding

This method is used to form rubber parts. For this, (preformed) mixed blanks are placed in the open mold. The mold is then closed, whereby the blanks are pressed into form and then vulcanized. To avoid air pockets, the form is overfilled by 5% and vented during vulcanization by opening and closing. Once the moldings have their final shape, they still need to be deburred.

Application: The compression method is mostly used for production of simple products, which can be easily removed from the mold, for small-scale production or large and/or heavy parts that require long heating times.

Injection transfer molding process (ITM)

In this molding process, a plasticized, preheated, vulcanizable rubber compound is injected into heated molds and then vulcanized.

Application: Small to medium-sized parts, large quantities with consistently high quality and short production cycles

 

Manufacturers of hydraulic presses and injection molding presses

Battenfeld, Berges, Berstorff, Desma, Dieffenbacher, Draftex, Fontijne, Gumix, KraussMaffei, Landshuter Werkzeugbau (LWB), Lauffer, Luigart, Miura Press, Saspol, Schwabenthan/Servitec, Siempelkamp, Wickert

Presses for rubber/natural rubber processing have a long service life and can therefore be purchased as pre-owned. Used presses can also be modernized. When modernizing presses, the following components can be replaced: control systems, heating plates, hydraulics.

Related articles

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Mould carriers (polyurethane molding)

Vulcanization

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