OPEN AREA Is it an important technical detail or a new issue when negotiating?

THIS ARTICLE APPEARS IN THE EDITION OF MARCH 2015 OF TECNICA MOLITORIA INTERNATIONAL  

Regardless of other factors such as the quality of workmanship, guarantee of raw materials, heat treatments, finishes, delivery times and prices, is a larger open area a decisive factor when choosing which die to purchase?

First, let’s start with a clear definition of an open area. An open area is expressed as the percentage of open space of the die (holes) in relation to the total space of the working area, namely: number of holes X area of each hole

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the width of the area with holes 

X internal diameter of the die X Π

The higher the percentage of the open area, the higher productivity will be according to the formula: percentage of open area

X the specific weight of the product to be processed = productivity index 

Therefore, is productivity directly related to the “open” space of a die? To produce more, does a die need to have the minimum possible amount of material between one hole and another? 

In general, a larger open area means higher productivity but, unfortunately, the answer is not as simple and obvious as it seems. 

During production, a series of factors need to be considered, which are not to be underestimated:

· Type of product to be pelletized 

· Preparation of raw materials

· Mechanical factors

· Hole pattern

1. Type of product to be pelletized: some animal feed is “easy” to pelletize such as chicken feed. For this type of feed, which has a low fibre content, extra rows of holes and less space between the holes are often requested: for example, a high-density die, where the effort required is low and the die with a larger open area is resistant, without any major risk of breakage. 

Other animal feed is “difficult” to pelletize, such as straw or alfalfa, which have a high fiber content. In this case, low-density dies are used, which have a larger space between the holes and therefore a smaller open area. In many cases, the working area of the die is even reduced in order to decrease the effort and therefore the risk of breakage. Finally, standard-density dies are used with “generic” materials without a high percentage of fibre and afford a good compromise between the open area and die resistance.

2. Preparation of raw materials: the preparation of raw materials should not be underestimated. In some cases, we have found that coarsely prepared raw materials, i.e. not finely ground, benefited from dies with a smaller open area. On analysing the reasons for this, we noted that the steel between the holes somehow allowed the materials to be slightly ground, preventing very large pieces of materials from entering the holes and blocking the passage, and thus increasing productivity. 

3. Mechanical factors of resistance: this is perhaps the most delicate of the factors to bear in mind when deciding on the open area of a die. We always need to remember that a die is a piece of steel that has undergone processing (turning and drilling) that has “weakened” it and heat treatments (such as vacuum tempering) that have hardened it. At the end of all production processes, a die is obtained with the right balance of hardness (resistance to deformation of its original shape) and fragility (resistance to breakage once its original shape has been deformed).

This good compromise, which we consider as a hardness of 52-54HRC after heat treatments, is likely to fail when more holes are made than those prescribed by the standard, which weakens the material and increases the risk of breakage.

The only way to avoid this would be to make a softer die, with a value of less than 52HRC, however, the risk of breakage, in this case, would be reduced but the speed of wear would increase.

Having analysed all these factors, can we say that a larger open area leads to higher productivity? The answer is “yes”, BUT...

· only if the raw material has been correctly prepared, otherwise there is a risk of reduced productivity. 

· only if the fiber content of the formula is low, otherwise there is a risk of higher productivity but only for a short time because the die will break. 

Moreover, a discerning customer will need to take into account not only the greater risk of breakage of the die but also the faster wear of the die and calculate whether the additional production that might be obtained with a larger open area will cover the extra costs due to faster wear of the die and the downtime of the machine required for its replacement.

Therefore, when a customer says that he has chosen a competitor because it offered a larger open area, we are very perplexed by the customer’s reasoning. 

Furthermore, considering that what our competitors are able to do in terms of die production La Meccanica is able to do just the same, we very often prefer to lose an order rather than sell a customer a machine that, in our experience, will involve a high risk of breakage. To us, customer loyalty is everything, and we want to share our experience with our customers.

At La Meccanica, we have been producing dies on a world scale for over 50 years. We have produced over 50,000 dies and sold them to 60 countries worldwide for use with all types of materials, with claims amounting to less than 2% per year.

In conclusion, if we imagine the pellet-mill press as the heart of the plant and the die as the heart of the press, would you ever ask a surgeon who is about to operate on your heart to do the operation in another way because you think it is more beneficial?