Working moulds
Working moulds in porcelain industry
On the start page of this section (Forming) we have described the process of mould production in detail. Only from the mother mould the so-called working moulds are then produced. Working moulds in traditional porcelain production are made as plaster moulds. On the page plaster moulds we explain why plaster is so helpful in the production of porcelain. How the working moulds themselves are made, we show on our page mould making.
In traditional porcelain production, the working moulds give the wet mass its shape and produce the porcelain. They are basically designed as negative molds and are used in all classical production methods
- Jiggering
- Rolling
- Pressing
- Casting
The working form determines a considerable part of the quality of a porcelain article. The number of error sources is extremely large. The most common ones are
- Loss of contours due to wear and tear
- Cracks and bubbles due to lack of density
- Holes and edges due to solid residues
- Wilts and seams due to unclean mould production
- Glaze edges by deposition of sodium silicates
Cost factor working moulds
The work forms represent a considerable cost and performance factor in the classical porcelain production. On the one hand they have to be laboriously made by hand, on the other hand they are not durable indefinitely. If a mould is often used to produce porcelain, the customer often decides - without knowing it - by his price specification
The lower the price target, the longer a mold is used. This usually leads to the loss of the contours of an article. Particularly affected by this are
- Stacking edges of cups, bowls and mups
- The washing out of reliefs and embossings
- Different wall thicknesses
Here an example:
The Porcelain bowls "Vexus" by Holst Porzellan are very complicated to produce because of their asymmetrical shape. With this type of mould, the plaster mould already tires between 60 and 90 articles, depending on temperature and humidity. We have agreed with our partner factory that the mould will be replaced after 80 items. The weight of the casting mould of a 28 cm Vexus-Bowl is 6,3 kg.
It takes 25 molds to make 2,000 Vexus Bowls. At the end of production, the factory has 157.5 kg of waste plaster molds. This plaster waste is not suitable for further use in the production of porcelain, or the factories lack suitable technology. Up to now, this production waste has been used in the construction industry at very low cost.
2.000 Vexus Bowls result in a weight of 3.600 kg of finished products. During the production of such cast articles, 5 to 10% C-grade, i.e. defective goods, are usually produced. This is destroyed on site. In order to produce 3,600 kg of porcelain ready for shipment, the factory has about 517 kg left, i.e. 15% waste disposal.
By the way, porcelain production also generates considerable amounts of chemical waste, which must be disposed of by specialized companies. Unfortunately (status 11/2019) only the monitoring of such waste, as well as hazardous waste, is part of a BSCI qualification. Being aware of our responsibility, we make every effort with all means at our disposal to work towards an environmentally friendly disposal of all production waste. Of course, we cannot save the world, but we can make a small contribution.
The working moulds for casting
The work moulds for pressing
Large-volume articles such as plates, bowls or even large plates are produced from so-called "press moulds". Usually the so-called pressure pressing with two-part molds (see picture). In English, this process is also known as "High-Preshure-Casting", which means "high pressure casting. Technically incorrect, by the way.
The liquid porcelain mass is "pressed" into a mould tower from below through a pipe system in the base. The mass makes its way from form to form from bottom to top. Such press moulds are the most expensive, largest and heaviest plaster moulds in porcelain production. While one working mould is sufficient for the rotary moulds, two parts have to be made for the press moulds, as upper and lower part. This explains the considerable difference in price between a round plate and a square plate of the same size. Angular articles that cannot be produced by rolling must generally be pressed in this or similar ways. Learn more about the manufacturing process of pressing here.
The working mould for jiggering
Working moulds for rolling
In traditional porcelain production, round flat pieces are "rolled over", i.e. a rotating stamp distributes the mass of the "Hubel" evenly on a negative mould. In English, this technique is usually referred to as "rolling". Since there is no distinction between rolling and jiggering, the words "exterior" (for rolling over) and "interior" (for jiggering) should be used.
As a rule, flat parts in sizes from 12 to 29 cm are produced in this way.
The special feature of this production method is that the porcelain mass in the mould itself hardens until the article can be "put over" and is ready for the next production step. Depending on the geographical location of the factory and technical equipment, the porcelain hardens for up to 36 hours. In other words, and in the worst case scenario, it takes 652 production moulds to make 3,000 plates within a week. But with only 4.67 production cycles, the mold can do hundreds more.
This makes it understandable why plates from traditional porcelain production in particular hardly stand a chance against their competitors from isostatic mass production. Learn more about the manufacturing process of rolling over.