The correct shape of glass gobs has an important role in the distribution of the glass in the container during its forming with IS machines. If we want to have the correct composition and good thermal conductivity of the glass in the forehearth and inside the feeder mechanism, the gob must be created with the correct shape, weight and length that are suitable for the container being produced. Taking a small 50 ml., 100-gram, rectangular perfume bottle as an example, we can see the blank mould in Figure 1, which shows a cylindrical gob of glass, which, passing through the funnel, takes on an oval shape, adapting to the inside of the mould. It is clear to see how the glass gob changes shape because of the impact with the mould.
The tip, created by the cylindrical-shaped glass gob and its large dimensions, hits the shoulder of the blank mould on which it stops. The end of the gob bends sideways because of gravity, taking some of the glass with it, facilitated by the empty area that forms between the walls of the mould and the end of the glass gob.
After the settle blow of the baffle, the counterblow takes place inside the parison, enabling the glass to swell and come into contact with the internal surfaces of the mould. Keeping in mind that cylindrical glass gobs are not suitable for this type of article, diverse thicknesses can easily be seen in the upper part of the parison in Figure 2. Consequently, after the final blowing in the blow mould, the small perfume bottle formed...

The correct shape of glass gobs has an important role in the distribution of the glass in the container during its forming with IS machines. If we want to have the correct composition and good thermal conductivity of the glass in the forehearth and inside the feeder mechanism, the gob must be created with the correct shape, weight and length that are suitable for the container being produced. Taking a small 50 ml., 100-gram, rectangular perfume bottle as an example, we can see the blank mould in Figure 1, which shows a cylindrical gob of glass, which, passing through the funnel, takes on an oval shape, adapting to the inside of the mould. It is clear to see how the glass gob changes shape because of the impact with the mould.
The tip, created by the cylindrical-shaped glass gob and its large dimensions, hits the shoulder of the blank mould on which it stops. The end of the gob bends sideways because of gravity, taking some of the glass with it, facilitated by the empty area that forms between the walls of the mould and the end of the glass gob.
After the settle blow of the baffle, the counterblow takes place inside the parison, enabling the glass to swell and come into contact with the internal surfaces of the mould. Keeping in mind that cylindrical glass gobs are not suitable for this type of article, diverse thicknesses can easily be seen in the upper part of the parison in Figure 2. Consequently, after the final blowing in the blow mould, the small perfume bottle formed has an irregular distribution of glass at the bottle of the container, as can be seen in Figure 3. One of the causes of this defect can be the plunger profile inside the feeder. In fact, a plunger with a semi-spherical tip creates a cylindrical shaped gob, not, as mentioned previously, suitable for this kind of article.
Forming homogeneous, vertical and straight gobs Well-conditioned glass in the forehearth forms a homogeneous, vertical and straight gob, which enters the blank mould with the requested weight and shape. The features that affect the weight and the shape of the gob areas follows:
– the temperature of the glass;
– the orifice diameter of the couvette;
– the shape of the tip of the plunger;
– the stroke of the plunger;
– the height of the tip of the plunger above the couvette, is its lowest position;
– the height of the cylinder above the spout;
– the ratio of time between the reciprocal strokes of the plunger and the shears;
– the distance between the shears and the couvette.
The weight of the gob varies by changing the stroke of the plunger and the height of the cylinder above the spout, while its shape can be modified by changing the ratio of time between the reciprocal strokes of the plunger and the shears. A plunger with large diameter and semi-spherical tip creates a short, large, cylindrical-shaped gob suitable for a bottle or vase with large mouth. To manufacture this type of bottle correctly we must use a plunger with a conical tip in the feeder mechanism, to obtain a conical structured glass gob, so as to go through the blank mould perfectly vertical, without any curling and sideways movements of its final part (Figure 4), enabling it to stay inside the walls of the mould, taking up all the area of the neck ring.
In the case of smaller and hotter gobs, we must use a plunger with a smaller diameter, since the glass is more fluid. The glass flows along the plunger thanks to its weight, and moves faster during plunger movement.
During the rising stroke of the plunger the flowing of the glass slows down, the gob tightens in its upper part and the narrow section is cut by the shears (Figure 5).
Feeders require numerous adjustments and considerable technician experience.

Fundamental features to improve the shape of the glass gob
We have already seen thatin order to modify the gob, change the weight of the glass, its shape and size, lengthen or shorten the tip of the gob, we must work on the mechanisms of the plunger, cylinder and shears. With the above-mentioned indications as examples, we can now define the fundamental features to improve the shape of the glass gob.
If the tip of the glass gob is too long (Figure 7), the actions to be taken are as follows:
– lower the plunger and lift up the cylinder so that the weight of the gob remains unchanged;
– lift up the position of the shears;
– adjust the shears for faster cutting;
– lift the stroke of the plunger higher and slow down the lower stroke.
If the upper extremity of the gob is too thin (Figure 8), the actions to be carried out are as follows:
– decrease the approach of the plunger;
– increase the stroke of the plunger and lower the cylinder so that the weight of the glass gob is not changed;
– speed up plunger strokes;
– if necessary, use a plunger with a larger diameter.
If the glass gob is squeezed in at mid-length (Figure 9), the following actions may be taken:
– lower the zero point of the plunger and increasing its upward stroke;
– use a plunger with a larger diameter;
– increase the speed of the plunger stroke;
– raise the position of the shears.
Other than these mechanical adjustments, there are other important facts that must be controlled in order to obtain a constant gob, the most important of which is the temperature of the glass, which must be maintained constant.
We rarely consider that the forming of a conical glass gob requires a plunger with a conical tip without which it would be difficult to obtain good distribution of the glass in the containers. In Figure 6 we can see the parison inside the blank mould with correct distribution of the glass by means of the counterblow, which can be obtained if all criterion discussed in this article are used. We must also bear in mind that thermal homogeneity and the correct composition of the glass also help improve the quality of the finished items. It is clear that these indications are of fundamental importance for the optimization of the entire productive cycle.