What is thermoplastic injection molding?

Thermoplastic injection molding is the most common way to manufacture plastic parts. Thermoplastics are polymers that can be heated to soften or melt, and cooled to solidify as a physical change, rather than a chemical change that takes place during molding of thermoset materials.

The thermoplastic injection molding process begins by adding pelletized material to a hopper. In most cases, the material must is dried prior to molding, and frequently requires the addition of a color concentrate before loading. The material is gravity fed into a heated barrel and screw. Rotation of the screw results in shearing action on the raw pellets causing them to melt.  The screw rotation also pushes the molten plastic forward in the barrel toward the mold. The material is then injected into the closed mold at high pressure through a runner system to fill all the cavities. The mold is clamped shut under enough force to keep the mold halves together while the molten plastic is flowing. On a cold runner system, the plastic in the runner solidifies and must be discarded or ground into pellets to be reused, which we refer to as “regrind.” If a hot runner system is used, the plastic in the runner stays molten, and no material is wasted. When the mold cavities are filled, the part cools until rigid enough to be ejected. Part cooling within the mold is accomplished through water lines cut into the mold. At the completion of the cooling cycle, the mold opens and the part(s) are ejected for part removal.

Rebling Plastics uses a process called “decoupling” for thermoplastic injection molding.  In this process, the cavities are filled to approximately 95 percent of their capacity using high injection pressures. At a specific programmed position of the injection screw, the pressure is reduced and the 5 percent remaining portion of the mold cavity is filled at a lower pressure.  This process eliminates over packing of the molded part and the resulting high internal stresses caused by over packing. 

Thermoplastic injection molding equipment

Rebling Plastics uses the latest equipment for the thermoplastic injection molding process. A current list of this equipment can be found at our facilities list page. We have thermoplastic injection molding presses ranging from 28 ton to 200 ton clamping pressure. Programmable controllers are used to fine tune the molding process. The flexibility of having different press sizes allows us to mold parts from the smallest plastic injection moldable part to one that has a footprint of over one square foot.  At Rebling Plastics we always use an appropriate press size, because using a large tonnage press uses a large capacity barrel and screw. If the plastic is left inside the barrel at high temperatures for too long, the material can degrade.

In addition to the wide range of thermoplastic injection molding presses, we also use a variety of auxiliary equipment to keep molding costs down. We often use robots and sprue pickers to remove parts and runners from the mold. The runners are dropped into a grinder where they can be chopped into regrind. We also use automatic proportional vacuum loaders, which pull the correct ratio of virgin material and regrind from containers and load it into the hopper. In many cases, we have a fully automatic thermoplastic injection molding process, where the only labor costs are inspection, packaging, and the occasional gathering of material.