About mold tooling and materials
A casting mold (flexible mold, mold) is the core of production process and a critical element of the artificial stone shop structure because:
Judge it for yourself:
a. Artificial stone products feature a high degree of imitation of natural surfaces, which is ensured by use of mold materials perfectly reproducing the relief of natural stone, wood, stucco molding etc.
b. The natural product color largely depends on the mold inner surface being capable of holding color combinations with smearing, watercolor wash, contrast, mixing, spraying etc. effects.
c. A product must have a well-defined geometry to allow implementation of the design and easy installation. And this is hardly possible if your molds are prone to load or shrinking deformations.
· Durability of molds is immediately related to the finished production cost. Depreciation of molds may eat up as much as 35% of the product cost (for example, formoplast dies). Thus both efficiency of your business and its profitability depend on your mold performance.
· Too rigid molds, such as those made of polyvinylchloride, extend the stripping strength development time, increase reject rates and labor intensity and thus an excess amount of molds is required. Given that their surface contributes to “pilling” of diluted colorant coatings, it is impossible to create the right color schemes of products. This results in lots of cavities and voids appearing on the decorative face of the product (i.e. rejects). It takes plenty of time and effort to eliminate this effect (adjustment of settings and vibration time, use of special chemicals etc.).
The above may be readily illustrated by relatively cheap molds of PCV (polyvinylchloride).
Their disadvantages include:
A. Own weight (taking into account the mold plus concrete weight, its dimensions need to be limited for production casting).
B. The internal physico-chemical structure of the mold material (in combination with its weight it makes the mold unfit for vibratory concrete mix casting as the mix tends to disintegrate which results in low quality products.
C. Mold face (it may be used 2 to 4 times without reeking prior to casting, then reeking is required but the material surface won’t hold this coating). It also behaves in the same manner when liquid colors for concrete need to be applied according to concrete painting technologies.
D. Inadequate mould stretchability (unfit for production of castings with surface undercuts since products get broken at these places when stripped).
Now for materials used to produce flexible molds (solid and split).
There are a few basic types of mold materials that are described below:
1. Formoplasts — the cheapest type of mold materials (2 to 3 $/kg). This hazardous material is used primarily at “garage” (family) wild stone workshops and extremely hard to process (turning block material to chips and a foundry is needed). In addition, the short life of such molds (about 50 molding-stripping cycles) requires that the mold stock be monthly renewed. Depreciation of such dies is at least $3 per sq m of finished stone (less labor intensity), which makes such molds extremely inefficient.
2. Silicone compounds and resin elastics are readily made into molds. Perfect for gypsum-containing mixture casting. When cement-based silicones are used for stone casting they tend to disintegrate quickly and irreversibly (mechanically and chemically) in the alkaline medium incidental to any concrete mixture. (An exception is some imported silicones based on platinum catalyzers, but their price is too high — 27 to 54 €/kg. And even if made of such materials, molds only show durability of 100 to 400 castings (depending on the material price). Another disadvantage is an increased number of bubbles scattered all over the stone face and difficulty in mold painting to transfer color to a finished item due to the greasy surface of silicones as distinct from polyurethane resins.
3. Polyurethane resins (Russian made). Being moderately priced (about 5 to 6 $/kg), they have more shortcomings like durability of as low as 40 cycles (meaning high depreciations costs) and high shrinkage levels (meaning distorted stone geometry). However, in terms of a price-quality ratio, polyurethane resins are generally suitable for certain types of artificial stone. Yet if improved by adding some foreign-made ingredients (which will raise their price to 7 to 8 $/kg), polyurethane resins may endure 100 to 200 cycles depending on how complex a product surface texture is. This procedure, however, requires attendance by a specialist who may not be always available.
4. Polyurethane resins (USA and Western Europe). Perfectly suitable for vibratory cement casting. The average price of such materials is 15 to 25 $/kg. Their durability is up to 1,000 castings, which makes depreciation costs negligible (0.1 to 0.4 $/sq m). Readily made into moulds and are practically unshrinkable. It should be borne in mind, however, that over 200 varieties of polyurethane compounds are produced worldwide at present. Some feature excellent resilience, other show high strength etc. For our own purposes, Forstone specialists have selected an optimal set of properties, such as density, casting consistence, strength, flexibility, abrasive resistance and some other including the price and availability of materials.
What’s most important is that flexible molds made of such materials are easy to strip, reproduce the most elaborate surface textures (including items with plenty of undercuts) and provide a perfect reflection of the most complex volume differences.
All the grades of this material are easy to use. Components are mixed manually or with a mixer in a clean dry container, according to the recommended formula ratio by weight or volume. Since the blend lifetime to gel formation is 10 to 90 minutes, depending on its weight and grade, it is advisable after mixing to allow 2 to 20 minutes for entrapped air bubbles to leave the mix, or to deaerate the mix with a vacuum pump. At FORSTONE laboratory the entire mix used for mold casting is subjected to deaeration to remove air bubbles, which results in a dramatically increased strength and run length of Forstone molds. The mold and tooling are pretreated with a releasing agent to prevent material sticking. Polyurethane compounds are suitable for all the primary molding types: open pouring into a solid mold, coating and pouring under the casing.
Flexible molds may be produced using a potting method.
In this case their bottom surface will be flat (modular mould), however due to the deformation of side walls caused by the weight of poured concrete they need to be substantially increased in thickness. This leads to considerable material consumption and much higher mold costs. Open pouring is used primarily for flat items with slight relief differences. This is the simplest molding method especially suitable for beginners. The pattern is secured rigidly in a leakproof form, with a pattern-to-pattern side distance usually exceeding 2 cm. The prepared mix is applied to the pattern with a soft brush for accurate relief coating and removal of air microbubbles. Then the main body of the mix is carefully poured so as to ensure a minimum mold thickness of 6 to 10 mm.
The prime advantages of open pouring are simplicity, uncomplicated forms and a possibility of visual air pocket control (when using transparent polyurethane grades). The drawbacks include high material consumption and increased mould rigidity at the highest thickness points and the resulting risk of the mold tearing at such points or of a broken product.
Product stripping from" filling" flexible mold фото распалубки
There is also another method involving the following procedure
Coating is used to make high-relief molds or blank and short-run molds. Фото обмазки
This involves mixing a small amount of material applied to the mold surface with a brush. The material flows down from the vertical surfaces into cavities from where it is removed and recast on the mold, waiting for viscosity to start increasing. Since all the materials have a viscosity comparable with that of oil paint viscosity, one application is seldom enough to ensure a coating of more than 0.5 to 1 mm thick, so after the primary material polymerization taking 40 to 90 minutes, a new layer is applied to the still sticky layer until the requisite thickness is obtained. Normally, the average mold thickness is 3 to 7 m. The major advantage of this method is low material consumption, while its major shortcomings are labor intensity, mold thickness variations, very thin top relief layer and mould disintegration risks.
After a resilient field shaper is attained, a casing of rigid materials (polyester, fiberglass/epoxy composites, prepelleted polyurethane, concrete etc.) must be made.
Because polyester casing materials are fairly expensive and environmentally/fire hazardous, expensive if made of fiberglass/epoxy composites, make the mold much heavier (to the extent of being impossible to lift in manual production) and dump vibration produced by vibratory casting, it is advisable to use two-component prepelleted polyurethane for the purposes. Фото пены
But this method, too, has a number of shortcomings if molds are made in-house:
1. Since the casing is made under pressure, a special machine is needed with 1.5 to 3 hours devoted to each casing (until the mixture is fully set).
2. Installation of the mold and pattern in the machine is manual and takes a long time.
3. In addition, a foam agent casing needs to be made in which (more exactly, in its cover-substrate) up to a hundred holes are to be drilled manually for in its excessive foal removal.
4. Since molds differ in height and surface shape and texture, the number of foam agents has each time to be verified empirically, which does not guarantee against:
A. Excessive foam pressure that results in destruction of the standard pattern, flexible molds and casing itself, which requires not only this work to be redone but also all the previous ones (which is costly and sometimes impossible).
B. Insufficient foam pressure that leads to foam subsidence and destruction, which cannot be detected until 3 to 12 hours after a foam agent mix is first poured into the mold and the pattern.
C. And the most inconvenient thing is having to manage to pour in the foam and press down the mold cover-substrate manually within 30 seconds after mixing the foam agents, since as soon as this time expires an uncontrolled foaming reaction entailing not only a temperature rise to 90 to 120°C but also a
An essential attribute of each resilient mold is a rigid casing in which it is placed prior to pouring and from which it is removed for stripping. Фото кожуха The casing enables the mold to be fastened for the top pouring mode and its vibration conditions to be distributed. It also ensures secure attachment of fresh castings for transportation and loading into drying cells or for stacking. Molds should be provided with a flange hanging over and overlapping the casing side walls. In this case excess concrete can be easily removed with a palette knife to prevent it from smudging the casing by sticking to it and eventually rendering the casing unserviceable.
There are certain requirements specified for casings:
As a rule, the production cost of one quality casing for filling (modular) molds varies between $30 to $35 per sq m of a mold.
Incidentally, companies that offer flexible molds are often not honest enough selling modular molds without casings. What they conceal in fact is that these are only suitable for low-production garage workshops since if used for facing stone production exceeding 50 sq m daily the producers run into serious problems caused by lack of flanges and original casings.
Aware of the above factors, FORSTONE produces professional molds, employing experienced specialists and specific equipment that a mold producer particularly needs.
5. Caoutchouc rubber. Alkali-acid-resistant rubber used for making molds by hot polymerization under pressure. Rubber molds may be produced on special heated presses used at industrial rubber plants. They have a long lifetime with single daily pouring and may be cleaned with a subacid solution. In this case only complete dies of metal are used. Molds (dies) are cast against patterns that have surfaces without undercuts because their rigidity prevents a new casting from being stripped without surface damage. Their production cost is very low as the cost of 1 kg of rubber is between 60 and 90 rubles and the production time per mold is 10 to 15 minutes. Naturally, hot pressing requires a second (pressure) shape-generating plate (punch).
Since this plate is typically flat the resulting mold is rigid, semirigid or low-resilient. Such molds are suitable for castings with a rather simple (flat) surface texture, at least 4.5 cm thick.
As production of metal tooling is a very costly process requiring a great number of metal-working machines, complex documentation and skilled personnel, the above molds are usually produced for traditionally similar castings with regular or simple surface design (paving slabs, kerbstone, gutters, balusters etc.). If a NC machine is used to produce a complex metal mold, the production of a metal stone prototype alone, using the Rubble Stone facing, will cost a minimum of $5,000.
FORSTONE offers the products of the factory producing rubber molds for casting the PP “PROMYN” paving slabs. The factory has state-of-the-art equipment and features three caoutchouc rubber shops, two mold shops, four paving slab shops and two artificial stone shops.