Injection Molding Press

Plastic injection molding presses are classified or rated based on tonnage, or more specifically, the clamping pressure or force. Presses can run in size from less than 5 tons of clamping pressure to over 6000 tons. The higher the press ton rating, the larger the machine.

A machine rated for 60 tons can deliver 60 tons of clamping pressure. This pressure keeps the mold closed during the injection process. Too much or too little pressure can cause quality issues. Too much or too little pressure can also cause flashing, where excess material appears on the part edge.

How to know the required press tonnage?

There are many factors that are taken into consideration when determining the size of the press. The size of the part, the polymer being used and something called the safety factor. The safety factor is an additional numerical percentage buffer that is added to the calculation to help avoid defects in the final part. Some recommend adding 10% to allow for the safety factor. As mentioned earlier, the MFI (Melt Flow Index) of the plastic compound will also impact the pressure needed to produce the part. Many calculations include the platen size as well as the mold and part size, however, to get an estimate of the press size your project will need, we have simplified it even further.

Reference :

Thermal Stability

The first purpose of the injection unit for molding a crystalline material is to deliver to the mold the necessary amount of a homogeneous melt ( with no unmelt and no degraded material ). The rules of construction of the injection unit are then dependent on the molding material requirements in term of thermal behavior and heat needed. The first point to take into account for a crystalline material is the thermal stability at melt temperature, to avoid degradation. Then, screw, nozzle, backflow valve, adaptor should be designed to provide efficient melting of crystalline material and delivery of molten polymer to the mold.

Melting behavior is different between plastics are different. The amorphous polymer starts softening just after Tg and presents a continuous change in viscosity. This gives a very large temperature range to operate ( but a large variation of viscosity with temperature). In contrast, the crystalline polymer stays solid up to the melting point and suddenly melts to the liquid phase at a high temperature. This limits the processing range of temperature between unmelt and thermal degradation.

The second factor is the time the material stays at that temperature. For all polymers, the molecules can withstand a certain temperature before degradation can start. Obviously, the acceptable time limit becomes shorter when the temperature is higher. Degradation will result in the generation of gases which cause bubbles in the melt, splays on parts, mold deposit, yellow and brown marks on the parts.

The average residence time (or Hold-Up Time, HUT) in the injection unit is linked to the amount of polymer in the cylinder, the shot weight and the cycle time and can be calculated with the following:

HUT= (Weight of Resin in Cylinder) x Cycle Time / Shot Weight


HUT= (Maximum Screw Stroke x 2) x Cycle Time / Current Screw Stroke

Note: Effective screw stroke = distance the screw travels during rotation only