In injection molding, the clamping force is the force that presses the two halves of the mold together. Since the maximum clamping force determines the size of the mold and the part, it is an important parameter in the classification of injection molding machines. Clamping force is measured in kilonewtons (kN).
Many Parts of Injection Molding
Many parts and variables interact in injection molding until the perfect part is ready for mass production. It takes solid training and years of experience to internalize the interrelationship of all these variables in a way that results in an error-free process from the time the customer describes the need to the time the order is delivered. We have reported elsewhere on the basic structure of an injection molding machine.
Processes and Units of an Injection Molding Machine
For better understanding by the layman, we at TX Mold have simplified the description of these processes. The experts among our readers may forgive us.
A. Plasticizing unit
Simply put, plastic is usually injected in a granular, liquefied state into a cavity, known as a mold, by means of a screw. The part of the injection molding machine responsible is called the plasticizing unit and consists of said screw, the plasticizing drum, the feed hopper, and the heating and cooling system.
The screw is subjected to high torsional and abrasive forces in its work of scraping off the molten particles and passing them through the barrel and is therefore made, for example, of through-hardened cold-worked steel.
B. Injection unit and mold
The injection unit is accelerated to high speed by a servo motor and introduces the molten pellets into the mold with precisely calculated forces and doses. The tool forms a hollow mold with lower and upper halves, giving the part or workpiece its precise shape as it cools (or in the case of some plastics, vulcanizes).
The manufacture of the mold is the most complex and expensive process in injection molding because the design of the part, the various components, and the intended operation can be of any complexity. Therefore, the design and manufacture of the mold is a completely separate process from injection molding.
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C. Clamping unit and ejector unit
The mold is clamped in a so-called clamping unit, i.e., usually, the mold fixing plate is screwed to the clamping plate of the clamping unit. It can be said that the clamping unit is the most powerful part of the injection molding machine. Here, the clamping force builds up the pressure that presses the two halves of the mold together. This clamping force can be generated by either hydraulic or electromechanical means. Both methods have their advantages and disadvantages. Hydraulic technology achieves higher clamping forces, while electromechanical technology achieves faster processing.
On the other hand, it is more expensive to purchase and also requires an additional hydraulic unit that includes hydraulic oil for the hydraulics. After the two halves of the mold are pressed together, the clamping unit is mechanically locked by a toggle bar or hydraulically by a clamping cylinder. After the cycle is completed, the molded part is ejected from the mold by a hydraulically driven ejector unit.
D. Clamping force VS. Closing force
Although the clamping force is the force of, for example, 100 tons, i.e. approximately 980 kN, with which the clamping unit closes the mold halves, another force is required during the injection process to ensure a successful operation. Depending on the type and process, the injected plastic generates its own force, i.e. pressure on the upper and lower part of the mold. This pressure is known as the tool lift force. The tool must therefore remain closed. The so-called clamping force must always be slightly higher than the applied clamping force. The rule of thumb is 10%, but a company like TX Mold certainly does not work with rules of thumb. Clamping and clamping forces are adjustable and must be adjusted precisely to the plastic and the mold. If the clamping force is set too high, not only does the energy consumption increase dramatically, but the mold itself is at risk.
If the clamping force is set too high, it can affect components such as the maximum internal pressure of the mold, the compression flash, and the cooling rate. In addition, if the clamping force is too high, there is a risk of damaging the mold inserts or closing the passages for ventilation. The different high cavity pressures near the gate and at the end of the hollow mold must be taken into account when calculating the exact clamping force required. The weight of the molded part is also an important factor. Since all processes in injection molding always take place within certain tolerances, clamping forces always come into play when the critical dimensions of the part are within the upper or lower tolerance limits.
TX MOLD’s Contacts
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