Molds are the most important key technological factor of any manufacture of cast decorative concrete products: man-made stone or concrete veneer for facing and paving, brick, tile, sculpture etc. It is caused by two very essential aspects.

First, the mold quality determines main decorative properties of the concrete product: geometry, surface topography and the fine structure (texture) – in short, everything that defines consumer quality of produce, its recognition, popularity and mass success.

Secondly, molds are the most expensive and quickly depreciable equipment of the cast process, thus significantly affecting the cost of the final product.

Either, the two factors determine consumer appeal of the product (both in esthetic, and in the price plan) and, finally, its profitability as a whole. That is why the matter of molds, search for new technologies, has always attracted strong interest both explorers, and producers.

The history of molds is long enough. The main priorities of their evolution are:

• Improvement quality of replicate pattern by casting;
• Increase working resource of the mold;
• Increase manufacturability molding production;
• Reducing the cost of mold.

The development of molds has been four generations (stages):

• Rigid collapsible molds (metal, wooden, fiberglass, etc.);
• Rigid not collapsible (one-piece) molds (plastic);
• Semi-rigid molds (film);
• Elastic (rubbery) molds.

The first generation of molds was known for a long time. They represent, in essence, the version of the rigid collapsible formwork (timbering) finished to acceptable decorative surface level.

The main shortcomings of such molds are:

• High price;
• Big weight;
• High labor/input ratio and low technological process (need for assembly-disassembly, greasing release agent , washing, cleaning, final mechanical completion of concrete products).

The advantages of rigid collapsible molds are:

• Obtaining products with impeccable geometry;
• Possibility of forming of dimensional heavy products.

Scale of rigid collapsible molds using was always not too considerable that, in general, coincided with scale of elite construction: aristocratic estates and palaces, park architecture, exhibition halls etc.

Rigid collapsible molds are successfully (though quite restrictedly) used now to cast heavy dimensional products of simple geometry or any grandiose unique items (the monumental sculpture). At the same time rigid molds (usually wooden) are widely used as the timbering (casing) which is carrying out basic function, together with the thin polymer film molds (“loose leaves” or “inserts”) providing relief and anti-adhesion function.

The wide application of the molds of second generation began about half a century ago with rising of synthetic polymers industry. Main objective of such mold development was to overcome the need of their assembly dismantling, first of all.

The mold materials use a number of large-tonnage polymers with complex target properties: high hydrophobicity (water-resistance) and alkali-resistance, abrasion resistance, impact resistance, etc. and the ability to be processed by thermoplastic state. These include PVC, polyolefins, impact-resistant plastics (ABS, HIPS).

The main advantages of rigid plastic molds should be noted:

• Low price;
• Small weight;
• Greater adaptability to manufacture;
• Considerable working resource;
• Wide availability.

The widespread use of rigid plastic molds has led to the real boom in the production of stone veneer, bricks, blocks and paving tiles, and simple architectural forms of small items. The disadvantages of plastic molds should include the following:

• Need of addition some new technological stages (steaming of molds and/or vibro-demolding);
• Usually, low quality of pattern replication by casting.

There is a consequential lack of rigid plastic molds: due to the high complexity and cost of developing of new patterns in polymer processing technology, most manufacturers are exploiting a narrow range of molds, which manifests itself in a very poor variety of items (the factor of “triteness”).

The third generation of molds is the result of the peculiar “creative” development of second generation molds directed on overcoming of some shortcomings inherent in them. Using a thin sheet and film materials (thickness of 0,5-2 mm) made of thermoplastic polymers allowed to obtain semi-rigid molds which due to the small thickness of the wall found constructional flexibility.

Keeping all advantages of rigid plastic form, semi-rigid mold more favourably differ because it does not require stages of steaming and vibro-demolding.

Relative simplicity and efficiency of their production and wide use led at once to emergence of the enormous range of the wall and floor tiles («the wild stone» veneer), the stone blocks and paving slabs of various geometry, decorative items, etc. At the same time there were tremendous production opportunities of molds and final products by individual consumer orders (the “exclusivity” factor). At the same time, these molds have the number of essential shortcomings:

• Short work resource;
• need of use of the rigid timbering (casing);
• Low quality of pattern replication by casting.

The fourth generation of molds – elastic (rubbery) molds – was designed to overcome the main shortcomings of rigid and semi-rigid polymeric molds. It became possible with development of the liquid polymeric compositions capable under certain conditions curing to form rubber-like body. They have different chemical nature (silicone, polyurethane, plastisol, latex, etc.), but all of them possess the number of conclusive advantages before molds of I, II and III generations:

• Extremely high quality of pattern replication by casting (about 1 micron);
• Highest work resource (up to 1000 pourings);
• Ease and quickness of the demolding.

In most cases the use of elastic molds does not require the use of formwork (except for the thin shell (glove) molds), and furthermore steaming and vibro-demolding. Top representatives of elastic molds do not need release agent greasing, and require cleaning only upon transition from one color to another.

Only the use of elastic molds allows to produce man-made stone, virtually indistinguishable from natural original (the factor of “naturalness”), with a high degree of “drawing” fine texture and negative angles (undercuts).

A serious drawback of elastic molds is their high price (200-1000 $/m²) and low availability, that somewhat constrains their universal use.

At the same time it is doubtless, that only this type of molds will have success in future, especially if explorers will be possible to create the following elastic mold generation, approaching to the plastic one by the price.

Since all of the above molds are potentially suitable for use in the manufacture of cast concrete, the decision about the use of particular mold type is taken by each manufacturer in each case, but every time is based on the principles of functional feasibility (obtain the best technical result) and efficiency (obtain the best economic result).

POLYCOLOR provides a wide range of industrial polymer molds (according TU 2293-001-52481517-2005), optimally suited for the cast technology of concrete stone, veneer, pavers, decorative elements and sculptures – a semi-rigid plastic molds of PLASTIC (of third generation) and elastic (rubber) molds of PVC OR PU MOLD (of fourth generation).

Molds of ROMA PVC MOLD that have been developed and are produced by ROMA POLYMERS in the pilot scale IN 2011, belong to the fourth generation of molds for the production of man-made stone and decorative concrete products. As well as in case of semi-rigid molds ROMA PLAST, before a stage of pilot production of molds ROMAPVC MOLD it was a long period of investigations and search of a suitable material and technology of their processing. Really, the industry lets out huge range of synthetic rubbers and rubber mixes (premixes). But for their processing it is necessary the expensive equipment – injection molding or the thermal press, and – most importantly – a wide range of very expensive and complex in the development injection (press) molds, where the formation of final rubber items occurs. Besides, the most part of industrial elastomers doesn’t show the necessary alkali-resistance level, and it is difficult to obtain high-precision replication of pattern by thermo-pressing of premixes.

An alternative way of obtaining rubber-like bodies is a liquid curing of reactive oligomer compositions. The main advantage of these materials is simplicity of their processing which is turned to blending of several components and following pouring of low viscosity mixture in a container where the pattern was placed. Of the variety of such compositions most widely used are silicone and polyurethane cold cure compounds that don’t require machine design process of mixing and pouring, but on the contrary, easily processed with hand tools at room temperature. After completion of polymerization (usually one day) it is formed rubbery body, which after a few days combines high elasticity (tensile at break 200-500%) and tensile strength (10-30 MPa). Despite these advantages, most of the proposed formulations of silicone and polyurethane elastomers don’t provide the necessary level of alkali-resistance. This applies to all domestic (indian) silicone and polyurethane (TM of SKU, ADV, Vitur, Litur etc.) compounds, which are suitable only for plaster (gypsum) molding. Only imported (western) silicones (e.g., TM of Elastosil) and polyurethanes (e.g., TM of Por-A-Mold and Duramold) suitable for obtain molds with high level of alkali-resistance. However, these imported materials are characterized by high price: silicones – 24-28 €/kg, polyurethanes 15-30 $/kg. A simple calculation shows that 1 m² of molds weighing 35-40 kg (for example, type of “shale”, or “slate”), will be the cost of raw materials only in the range 525 $ – 1120 €. Besides, polyurethane compounds are very sensitive to moisture, which imposes serious restrictions to the conditions of storage and processing (including the pattern preparation).

A separate group of chemically resistant elastomers comprise PVC-compounds, based on which POLYCOLOR has developed an original technology of elastic ROMA PVC molds. The use of PVC compositions has allowed to solve in a complex the basic problems traditionally arising at manufacturer of a casting concrete stone:

• High quality of replication of pattern is reached by that the material is processed in a liquid state;
• High work resource of mold (replication-resistance) is due to the high hydrophobicity (water-resistance) and alkali-resistance of material in combination with significant flexibility (up to 250%) and tensile strength (up to 10 kg/cm²);
• High adaptability to manufacture: high hydrophobicity (water-resistance) and alkali-resistance of molds allow to eliminate necessity of release agent, washing and cleaning, and when combined with high mold elasticity provides easy and quick demolding;
• Low cost is determined by the use of widely available domestic (Russian) raw materials.

By criteria of technological and economic efficiency the ROMA PVC molds differ favourably from other rubber molds of the fourth generation offered at the current market. The table 1 below shows the comparative characteristics of molds from various materials at the Russian market. It is evident that ROMA PVC molds surpass other kinds of rubber molds (silicones and polyurethanes). As a rule, the latter, while more expensive, don’t possess the necessary chemical and water resistance which is why less into technology: require release agent and washing or are used for manufacture of products only one color. The geometry of polyurethane molds often starts to change (distort) long before their mechanical deterioration occurs as a consequence of alkaline corrosion of a material (“chemical shrinkage”).

The specified advantages of ROMA PVC molds allow to produce a high-quality man-made concrete stone with optimum material and labor expenses. By such economic criteria as the price of «casting cycle ROMA PVC molds approach to plastic molds (from polypropylene and HIPS). Application of ROMA PVC molds for manufacturing of concrete items with the deep and branched surface relief (topography), fine details and negative corners (undercuts) is especially effective.

COMPARISON OF DIFFERENT TYPES OF MOLDS

• The number of casting cycle;
• Mold price, divided to number of casting cycles.
• ROMA PVC molds are suitable for realization various casting concrete technologies with different binding (cements, plaster and synthetic resins). Manufacturing of molds by agreed individual pattern of the customer is possible.

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