PRODUCTION OF ENAMELLED STEEL BATHTUBS AND HOWER TRAYS BY THE METHOD OF APPLYING ENAMEL POWDERS IN ELECTRIC FIELD
Alfonz Moravcík, Martin Moravcík
Festap s.r.o., Hattalova 2, 83103 Bratislava, Slovak Republic - Rienoesslgasse 12/3, A -1040 Vienna, Austria

Introduction
As we have published several times, this unique technology has been successfully applied in Slovakia for the past 15 years. Such an anniversary is a good reason for us to assess this process in broader context again.
As the professional public knows it, it was in the eighties, when the first applications of this new physical principle of enamelling steel products appeared worldwide. The most frequent applications were observed in the enamelling of white sanitary equipment. The shape of the enamelled product was one of the criteria for successful application. In this respect, the key products to be used for application of this method regarding the size were the sanitary products like bathtubs and shower trays.
The Austrian company LEGAT introduced the first industrial attempt for application of this method of enamelling bathtubs. However, it is true, that this production was closed shortly after its introduction, mainly due to high production costs. However, the quality of enamelled coatings was remarkable.
As it is generally well known, it was in the eighties when in the former Czechoslovakia, a broad research program has taken place, focusing on the research, development, and application of the enamelling technology based on physical principle of application of enamel powders on steel products in the electric field. The research resulted in the development of enamelling frits, suitable for preparation of required powders, production technology for powders, special technology for surface pre-treatment, and coating technologies for various product types. Design and building of the first automated line for production of enamel powders followed by the line for enamelling of flat elements has confirmed the great potential of this method while solving complex problems related to enamelling technology in general.
In the second half of the eighties, the program for product research and development in Slovakia has started, resulting in the construction of the production plant for enamelling of steel bathtubs with the target capacity of 100 pcs/hour. The research and development within this program was finished in 1989 by putting the installed production capacities into operation.
The entire technological process was divided into three parts. In the first part, the development of frits for preparation of powders was completed, namely frits for the preparation of the basic enamel, frits for preparation of white enamel, and frits for preparation of colour enamels. A fully automated line for production of powder was installed with the capacity of 200 kg/h. This has become the material basis for successful utilisation of this method. As we will mention later, this was the most important attribute of the economic effectiveness of the entire process.

Fig. 1 The organization chart of bathtubs production.

This scheme shows the material flow through different production operations. The production process starts with pressing of the steel sheets with thickness up to 2 mm. Pressing takes place in the automated line from Müller, a well known company in Europe. After the completion of all operations, the pressings are placed into pickling baskets, which are part of the automated line for surface pre-treatment before enamelling.
This is a non-continuous line consisting of fourteen positions of separate baths and automatic insertion of individual baskets into relevant media, with computer controlled time and temperature. The last operation is drying, followed by hanging to the suspension conveyor belt.
The chemical process of the surface pre-treatment consists of alkali degreasing through hot and cold flushing, followed by sulphuric acid flushing, neutralisation, passivation, and thorough flushing in demineralised water. This simple surface pre-treatment is highly reliable. It is important that there are no residues of alkaline salts on the surface.

As it is shown in Fig. 1, the process for application of enamel powders on the product surface is designed in such way that it is possible to make two layers in the first line, i.e. the basic and topcoat powder followed by baking. Line 2 with reduced number of coating positions is available for cases requiring the third layer and the second baking. This means that the entire production passes through the Line 1 where all products get the two layers of enamels followed by baking.
Those products, which do not meet the quality criteria after the first baking, are treated on the Line 2 with the layer of top coating enamel. The colour enamel layer is applied in the same mode.
Application of colour enamels on the white basis offers several advantages, which improve quality and reduce costs. To explain the entire process closer, we provide the video recording, which confirms the great potential of this method for application on products of such size and such demanding shape, as are the bathtubs. The great flexibility of the process is visible from the broad range of products enamelled on this line, ranging from enamelled bathtubs up to lots of shape modifications of bathtubs and shower trays.
On the occasion of the 15 Anniversary of this production technology, I consider it is appropriate to mention some confidential information about the production. As this paper was written in November of 2003, to present the effectiveness of this process we have used the results from October of the same year, recorded in the production files.
The capacity of all production operation, i.e. pressing, surface pre-treatment, coating, and baking,
are synchronised for the capacity of 100 pcs/h. For powder production, we have two automatic lines available with capacity of 200 kg/h, also shown in the videotape.
The operation of depositing the enamel powder seems to be the most important to assure the quality of enamelled products. Because of varying sizes and shapes, it is difficult to achieve uniform thickness of the deposit.
Fig. 2 shows the measured average values of enamel thickness after the first baking, the primer and topcoat together, and enamel thickness after the second deposit, i.e. three layers, two bakings. This thickness tolerance accomplished within one month cannot be achieved in any other way. More precisely, the thickness of the enamel deposit on such difficult product like a bathtub cannot be absolutely even.

Fig. 2 Enamel thickness / October 2003.

Fig. 3 The thickness variation in different measuring points on the product (a). The measurements were taken every half hour during one month from 32 measuring points (b) and (c).

In the time period from 01.10. till 30.10.2003 we have produced:

  33 911 pcs / bathtubs
  1 447 pcs / shower trays
  45 000 pcs / enamelled bathtub feet
Total 35 388 pieces + feet

From this production volume, 164 pcs products were out of conformity, i.e. 0.46 %, thereof 55 pieces, i.e. 0.15% due to steel defects. 3.6 % of all produced bathtubs and shower trays were corrected by second baking.
There is one important fact, that this process is a closed material cycle with nearly 100 % utilisation of the enamelling materials. This efficiency of material utilisation has been made possible by high level of automated controls. The quality of products is a separate issue. We carefully follow all parameters having impact on the final quality. As shown in Table 1, we perform the quality

Aassessment in accordance with the relevant standards applicable for this type of products. Assessed parameters correspond with European standards for these products.
Testing methods used for quality assessment are shown in the Annex.

Table 1. Quality assessment in accordance with the relevant standards

The main intention of this paper was to inform and to point out the areas where this enamelling method is not established yet. We know there were several trials to apply this method on these products, which have failed due to various reasons. This issue can be discussed from the point of technical complexity of the application process on these products, from the point of production facilities, economical effectiveness, and other criteria. In times, when the enamelling industry is in recession, it is essential to look for solutions in the modernisation of production technology. We believe that the enamelling technology using this new physical principle has a broader potential. As an example we have in this paper presented its application for enamelling of bathtubs and shower trays. In principle, the excellent quality of enamelled coatings achieved by this method should give sufficient reason for its faster expansion. In our opinion one of the main obstacles in broader use of this method is the high price of enamel powders, which is maintained at an unrealistic level. As the production volume of enamelled products for sanitary use, such as bathtubs and shower trays becomes stabilised, our attention should be focused on its development. We see the modernisation of production technology and quality improvement as a way to strengthen the position of this industry in the marketplace. This technology presents itself as optimal means to that objective.
In conclusion we would like to inform you that as presented in this paper, our company has owned the production facilities for manufacturing 300 000 bathtubs annually since 1992 and that this method has been applied to more than 2 million units manufactured since 1989. Our company is working in accordance with ISO 9001:2000 and ISO 14001 and owns the complete know-how of this process.

Bathtubs testing
Testing of enamelled surfaces of bathtubs pressed from one piece pursuant to STN 91 41 01 with reference to British Standard BS 1390:1990.

Test description:
1. Enamel thickness measurement
The enamel thickness measurement takes place at certain points described in Annex 1, using thickness gauge ELCOMETER 345.

2. Enamel shine inspection
The enamel shine grade is determined by visual inspection and comparison with reference sample.
Several lines using 4B pencil are applied on both the tested and the reference samples. Then we try to scrub them off by ten scrubs using dry cloth. When it fails, we try to remove the line with a wet cloth. When it fails in this case, we observe the mirror reflection of the lighted 40 W bulb fibre located in the distance of 15 cm from the surface under 45 º angle.

Test evaluation:
:: Level 1 – dry abrasion
:: Level 2 – wet abrasion
:: Level 3 – bulb fibre reflection is visible
:: Level 4 – bulb fibre reflection is not visible
Our enamel No. 1512 corresponds to the Level 1 – dry abrasion.

3. Enamel shock resistance test
The test is performed according to STN ISO 4532.
The enamel must not crack away or crack after the strike of the steel ball with force of 40N (Wagner hammer).
The resistance of enamel No. 1512 conforms with the standard.

4. Enamel resistance to hot water test
We clean the surface of the tested sample using solution agent and let dry at temperature of 110ºC for 2 hours. After cooling we do the weighing with accuracy of 0,2 mg. The sample is then boiled in distilled water for 48 hours. After boiling we flush the sample with distilled water and again dry at 110 ºC for 2 hours. After cooling we weigh the sample again with accuracy of 0,2 mg.
The weight loss is given by the difference between the initial and final weight. The corrosion rate is given by the equation:

VK(2) = Δm /10 /g/m2.d/ and it must by less than 3g /m2.d
Δm - weight loss
VK(2) – corrosion rate
d – 48 hours

The results obtained in tests of enamel No. 512:
2,61 g/ m2
  2,71 g/ m2
  3,125 g/ m2
  2,889 g/ m2
  3,015 g/ m2

5. Enamel resistance to alkalies test
The tested surface shall be cleaned with the solution agent, then the filtration paper is placed on the tested surface and several drops of 10 % Na2CO3 are applied. The etched area is covered with clock glass. After 20 minutes, the glass is removed, the enamel surface is washed under running water and dried with the filtration paper. For test evaluation, we make several lines on the etched area using 4B pencil and we attempt to remove the lines by ten scrubs using dry cloth. If this fails, we use wet cloth.
Enamel No. 1512 shows resistance to 10 % Na2CO3 in level AA – the effect of 10 % Na2CO3 is unrecognizable.

6. Enamel resistance to acids test
The test procedure is similar to the alkali test using 10% citric acid applied for 15 minutes on the tested surface.
Enamel No. 1512 shows resistance to 10% citric acid in level AA – the effect of the acid is unrecognizable.

7. Testing of surface porosity
FESTAP also performs testing of enamelled products for porosity using POROTEST apparatus.
We use the head HV-3 PK. The testing voltage is 0,3-3 kV.
The purpose of the test is to find defects in the enameled surface. In contact of the tested surface with high-voltage electrode, the electric charge shall occur in the defects or points with thin enamel layer.
The tests are performed in accordance with the standard CSN ISO 2746. The enameled surfaces of bathtubs must be free from defects in the functional area.
Enamelled bathtubs estap ® are free from defects after the deposit of the second layer.

 

 

 

 


 

 

The International Enamellers Institute
+39 02 3264283   +39 348 8003263
The International Enamellers Institute (IEI) Viale Vincenzo Lancetti, 43 20158 Milano - Italy
All rights reserved - Best View in 800x600 pixels - Powered by l lova