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Mould Design - Overcapacity

Like many other parameters, the overcapacity has a specific purpose for the creation of the “perfect container”. Do glassmakers understand the importance of the overcapacity or is it simply just a “cavity/hole”? What is the difference between the BB, PB and NNPB processes with regards to overcapacity?

It is only the skill of the designer who can determine the overcapacity, parison shape and position of the blank fill line. All influence the glass distribution within the bottle. If for example, the overcapacity is incorrect, there is nothing whatever production can do to fix any wall thickness problems. Not only can it be that production experience issues with glass distribution (wall thickness), it often happens that parisons collapse during invert and reheat or that Bird Swings are made – all this is because of incorrect overcapacity.

Last but not least, it could also force production to slow down the machine to gain additional blank time (to “fix” some of the previous mentioned defects).

WHAT IS IT?

Overcapacity is the cavity (air space) in the parison created during the parison formation on the blank side of a forming machine.

[ligm01]Overcapacity in the parison is the sum of the cavity (air space) created by the plug (plunger) in the finish during settle blow and the cavity (air space) between the baffle and the glass after settle blow. 

 

During counter blow these cavities (air spaces) merge together, which becomes the total parison overcapacity.

 

 

PARISON DESIGN

Overcapacity is created during the construction of the parison layout and its main purpose is to assist in the distribution of the glass in the container, especially for the Blow & Blow process.

If the overcapacity is incorrect the glass distribution will also be incorrect, especially in the lower third of a bottle made with the Blow & Blow process. During the design stage the designer must be aware that the overcapacity depends upon the type and shape of the container, and the speed of the forming machine.

Remember, if the overcapacity is…

Too large, it will cause

  • Thin glass in the heel/bottom contact area.
  • Pronounced settle wave.
  • Unstable parison that is difficult to control due to the expansion of the air within the parison.
  • A parison very susceptible to swinging on invert.
  • Defects produced with a large overcapacity are for example: Swung Baffle, Wedged Bottom...

 

Too small, it will cause

  • Too much glass in the heel/bottom contact area.

 

Defects produced with a small overcapacity are for example: Thick Bottom, Dropped Bottom...

 

In general the following can be said about the overcapacity size, which is expressed in a percentage number:

Round lightweight bottles tend to have a higher overcapacity and shorter parison.

Round heavier weight bottles tend to have shorter runs and lower overcapacity.

Shaped ware generally have an overcapacity which is in the low 50 percentage wise and short runs. This is necessary to avoid having the parison collapse during invert and parison reheat. The same principal is used on Flasks (high overcapacity & short run), the overcapacity can be as high as 55%.

SETTLE WAVE

Is caused by the glass to metal contact during settle blow at the blank fill line. The settle wave is a “Ghost” area which is seen approximately in the bottom 1/3 of the bottle. If the overcapacity is too low the settle wave will appear in the bottom 1/4 of the body. With a high overcapacity the settle wave will most likely be in the upper half of the body towards the shoulder area. There is a way to reduce the appearance of the settle wave by using an Anti Settle Wave design method, but, still one must keep the overcapacity at the correct amount.  

BLANK MODIFICATION ON BB

Before any modification is carried out, the designer must be aware of the actual overcapacity and any potential new overcapacity. It is not advisable to just open the blank cavity in a specific area without adjusting the blank cavity length. The position of the baffle match and the blank fill line can influence the overcapacity. If the position of the baffle match on the blank is not correct, then the baffle match will be seen above the mould/bottom plate match on the bottle. 

BB vs NNPB & BB

One must be aware that on all processes there is an overcapacity present. In NNPB & PB the overcapacity is the cavity (air space) created by the plunger. Under normal circumstance the overcapacity on NNPB & PB is in the range of 50%.

NNPB process overcapacity created by the plunger during parison formation.

 

PB process overcapacity created by the plunger during parison formation.

A good guide to change from standard Blow & Blow to NNPB or Light Weight BB is when the overcapacity begins to rise, which it will do during a Blow & Blow light weighting exercise, usually the overcapacity rises above the 50% mark.

Always think about the overcapacity and have a rule to follow!

12.02.2018, APEGG

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