Versatility through feeder colouring

The colour of packaging glass plays an important role for manufacturers because it fulfills two functions at once: On the one hand, colours attract attention and thus stimulate end customers' interest in purchasing. And some colours are strongly associated with certain products – such as brown beer bottles or green olive oil bottles – so breaking with these norms can be a unique selling point. But this colour association is not merely a tradition. The second important function of colour is its UV protection for nutrients and flavor. In most cases, the darker the bottle, the better the protection for the contents from UV rays. However, even almost colourless glass can now offer effective UV protection. Glass packaging manufacturer Vetropack produces such glass at the Group's Croatian site in Hum na Sutli.

It all started with dark blue

Vetropack Straža is now the plant with the widest range of glass colours within the Group and produces the standard colours white, green, brown, olive green, cuvée, and light green in larger batches. Smaller quantities of unusual colours have been produced for over 20 years using a colouring feeder. 

In the early 2000s, Vetropack Straža received an enquiry from a fruit juice producer, who wanted dark blue bottles for one of its products. However, the quantity of bottles ordered was not large enough to make furnace colouring economically viable. Instead, Vetropack Straža implemented a colouring feeder on one of its production lines. 

Vetropack has now relocated production of dark blue bottles to its Slovakian site in Nemšová. The rich blue colour is very popular today, making tank colouring worthwhile. As part of the modernisation of the flint glass furnace at the beginning of 2025, Vetropack Straža replaced the previous colour feeder with a new one that is designed for the furnace's higher production volume and offers new opportunities for customers in the beverage and food industry. 

Feeder colouring as a driver of innovation

Feeder colouring has proven to be a real driver of innovation in recent years – especially for mineral water producers and breweries that require flexible colour concepts and functional UV protection. Vetropack Straža offers smaller batches – starting from a minimum production run of two weeks – in individual colours and with short lead times. “The process does not start in the furnace, but directly in the feeder: the preheating and feed channel that transports the glass to the forming machine. There, colouring frits – pre-glass particles that produce the desired colour when melted – are specifically added to the colourless base glass,” explains Robert Zdolc, Head of Technical Customer Service at Vetropack Straža. However, in order to produce a perfectly coloured glass product using the feeder, the preparation process starts much earlier.

Preparing the line for the colour campaign

In the first step, sales and production planning schedule the start of a colour campaign. The technicians then prepare for the campaign. They first determine the required dye consumption. Two days before the campaign begins, the frits are brought to the relevant line, the mixing and dosing technology is installed, and the scales for dye dosing are calibrated. 

A decisive factor in the entire campaign is the correct handling of own cullet: reject glass from the line – i.e., glass that is separated out and later returned to the production process as raw material – must not be mixed with glass from other lines during the colour campaign in order not to distort the colour cycle. Therefore, own cullet must be clearly separated for temporary storage and transport. 

Before the actual colour change, the technicians from the glass melting department check the glass temperature and the settings of the working end and adjust the feeder to the technological requirements of the new colour. Meanwhile, optical pyrometers ensure seamless temperature control – a basic prerequisite for stable colouring results. 

From shape change to stable colour

This is followed by a classic mould change. The previous set of moulds is removed and replaced with the set required for the new production. In one specific case, preparations were made for a new beer bottle with integrated UV protection. “Appropriate frits are added to the feeder for UV protection. The glass is then not completely white, but has a slight greenish tinge. Nevertheless, the colour of the lemon beer in the bottle is almost completely unaltered – and with the same level of protection,” says Robert Zdolc. A mould change must always be carried out as quickly as possible, as valuable resources are lost during this time. The colouring frits are then fed into the feeder in precisely defined doses per hour, where they mix with the colourless base glass. This high-precision interaction is controlled centrally by a process control system.

Real-time quality assurance

During the recolouring phase, sample bottles are taken every hour and tested in the laboratory for colour values and optical properties. As soon as the measured values are within the target range, the production meets the quality requirements, so that the bottles can be tested, packaged, and delivered. 

Feeder colouring thus opens up new design possibilities for brands and manufacturers time and again – while also providing functional advantages such as improved UV protection. Currently, light blue and turquoise are the most popular colours among customers for water bottles. “We have also produced other beautiful colours in the past, but unfortunately they did not catch on in the market,” adds Robert Zdolc. With this broad portfolio, individual consulting, and recently increased capacities, Vetropack Straža will continue to support the realisation of the right design and branding for each product in the future.