Plastic Injection Molding – All You Need To Know

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.

Process called “decoupling” can be used 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.

Source : reblingplastics.com

Thermoset injection molding is an automated process transforming granular material into molded shapes that Rebling Plastics has been performing since the company’s inception. It typically is the most economical way to mold thermoset materials.
Thermosets are materials that cure through a chemical reaction activated by heat and pressure, thus forming a strong molecular bond. This molecular bonding enables thermoset materials to maintain their physical properties at elevated temperatures which is why they are used in applications such as electrical connector housings, automotive ashtrays, and cookware appliance handles and knobs.

The thermoset injection molding process begins by adding granular or pelletized material to a hopper. The material is gravity fed into a barrel and screw, which heats and moves the material toward the mold. The material is injected into a hot mold which has been clamped shut under enough force to keep the mold together while the material is forced into the cavity under high pressure. The plastic flows through a runner system in the mold to reach all the cavities. When the mold cavities are filled, the parts cure to a solid form. The mold then opens for part removal, and some parts simply drop into a box, while others are removed by hand or automated equipment.

Thermoset vs. Thermoplastic Injection Molding

Unlike thermoplastic injection molding, cured thermosets cannot be reground and mixed with virgin material for reprocessing. In general, thermoset materials have a low viscosity, so when the material fills the mold cavity under pressure, some of the material will leak between the two halves of the mold typically at an area of the cavity where vents have been added to allow for gas and air escapement. This is called flash. While this problem can normally be prevented in thermoplastic injection molding by proper mold construction and processing parameters, flashing cannot be avoided in thermoset injection molding. Therefore, flash must be removed as a secondary operation. When possible, the parts are de-flashed in an automatic tumbling operation rather than hand de-flashing to save costs.

Cycle times are longer for thermoset materials than thermoplastic materials due to the chemical reaction and cure that takes place in the mold. Since thermosets are more abrasive than thermoplastics, Rebling Plastics considerable expertise in mold design (including choice of tool steels, heat treating and plating requirements) as well as mold maintenance will insure a long lasting productive mold life.

Thermoset injection molding equipment

Thermoset injection molding presses ranging from 80 to 200 ton clamping pressure. Having these types of presses allows  to make different sized parts with the option of making multi-cavity molds. In addition to the presses, we also use a variety of auxiliary equipment to keep molding costs down. Robots and sprue pickers are used to remove parts and runners from the mold. Removing flash from the parts by sandblasting or automated feeding and tumbling is only an advantage.

Source : reblingplastics.com