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Answers to Questions about Glass Manufacturing, Molding, & Shaping

By: Berlin Packaging Specialist
Date: January 26, 2020

Glass Manufacturing, Molding, & Shaping


How long does it take to manufacture a bottle and at what temperature is the glass processed? It takes 10 to 15 seconds to transform the glass gob into a glass bottle that is immediately sent to the annealing lehr in order to eliminate the surface tensions in a process that lasts 1 to 2 hours. Manufacturing glass is both an art and science.

What is "sampling inspection?" A production batch can be satisfactorily evaluated using a statistical control method (internationally regulated by a specific reference standard), which consists in the random sampling of a certain number of pieces in proportion to the overall quantity of the production batch. The results of this sampling inspection should be representative of the quality of the whole batch.

Why can a glass furnace never be turned off? This is a technical feature of continuous production that is common to blast furnaces as well. The melting temperature of 1500°C takes around 12 days to reach, with very high energy consumption. Thus, the furnace cannot be turned on and off at will. Moreover, the glass material in its cooling phase could cause damage to the delicate parts of the furnace such as the “throat”, and the refractory material that the furnace is made of goes through some highly critical points during certain changes in the temperature range (approximately between 1200°C and 1100°C and between 900°C and 800°C).

Since it is the result of an industrial operation, can glass have internal impurities? If so, how can it be considered to be the most hygienic material? It is important to distinguish between glass purity (the raw material melts at about 1500°C, while the shaping of the container takes place at about 900°C and these temperatures “purify” everything) and its overall hygienic quality, which depends firstly on how the container is packed at the glass factory and then on how it is stored and used later by the food packing company. Packaging processes — if properly carried out — normally ensure the maintenance of the same hygiene conditions that apply during the production process itself. So if we are talking about “glass” as a material, it is hygienic by definition, but if we are talking about glass containers (bottles or jars), the hygienic qualities depend on how the container is used.


How is the mold changed? This is one of the most complex operations in a glass factory and it must be carried out by one or more teams of qualified technicians.

When the required quantity of an article has been reached, production is stopped by deviating the glass gobs from the feeders into special cooling water tanks instead of into the molds. Some technicians start by working on the feeder mechanisms to set the new glass weight, if necessary replacing the cuvettes through which the glass gob passes and setting the new parameters for the entire feeder pipe. Others work on the machine sections, replacing the molds and preparing the new production process, including the transfer of the new articles to the annealing lehr. These two phases are carried out at very high, uncomfortable temperatures, with the additional psychological pressure of the “time factor”, since factories work on a continuous production cycle.

The change of molds can take 2 to 6 hours, or even 1 full working day, depending on the production, to finish setting it up. While these operations are being carried out in the so-called “hot” area, another team is setting up the new parameters on electronically controlled machines in the “cold” area, where products are selected and packed, using — where possible — a range of defective samples of the new article which have been kept from previous productions.

Which parts of the molds are interchangeable and what changes can be made without having to remake the entire series of molds?

  • Finish equipment group (neck-ring, guide-ring, and plunger), provided that it fits the diameter of the neck.
  • The bottom, with or without a punt or to allow different capacities.
  • The blow mold allowing a neutral or customized version of the same bottle.

What is the difference between a single gob and a double gob? A double gob is when two gobs are collected at the same time from a feeder using a cuvette with a double orifice.

In some cases it is possible to collect 3 or 4 gobs at the same time, that all go into the same forming section of the machine (obviously into different molds). The machine can have 6, 8, 10, or even more sections. With the use of more glass gobs, it is possible to produce more articles in the same production time unit.

Are bottles made by welding two halves together? No. The vertical lines that are visible to a greater or lesser extent on bottles and jars correspond to the joining points of the two half-molds.

What are the small dots on the bottom of a bottle for? The sequence of space and dots represents a mold code number and allow electronic control of the production process and of that particular mold.


Is it possible to create any kind of container with glass? Theoretically, there are no size or shape limitations. It is generally the customer who decides, based on the production process (manual or industrial), or economic aspects, etc.

What is the reason for the stippling on the bottle base? The stippling helps to keep the bottle stable on the conveyor belt during production and to keep the bottle punt (which is still at 650°C) separate from the metal belt, preventing cuts to the bottle base through contact with different temperatures.

What are the engravings (letters and numbers) on bottles for? They represent a sort of identity document for the bottle: they contain the name of the producer with its factory trademark, the brim capacity and the filling level, the number of mold cavities used to produce the bottle, and the symbol which indicates that the bottle/jar is suitable for food products.

  1. Nominal capacity in cl or ml
  2. Symbol for measuring container bottle*
  3. Filling level
  4. bottom with dots
  5. Symbol indicating it is suitable for food
  6. Manufacturer’s trademark
  7. Reference cavity number
  8. Bottom with crescents
Engraving Chart

* The reversed epsilon similar to a number “3” is the symbol established by EC regulations to identify measuring container bottles. This refers to containers which have the necessary metrological features guaranteed by the glass manufacturer, which make it possible to measure the actual contents with the appropriate accuracy when filled to a certain level from the brim (EC directive 106/75).

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