Injection molding is one of the most widely used manufacturing processes in the world. Thermoplastics are polymers that soften and flow when heated, and solidify as they cool.
Applications
Injection molding is the most common modern method of manufacturing plastic parts; it is ideal for producing high volumes of the same object. Injection moulding is used to create many things, including wire spools, packaging, bottle caps, automotive parts and components, gaming consoles, pocket combs, musical instruments, chairs and small tables, storage containers, mechanical parts, and many other plastic products.
Mold design
After a product is designed in a CAD package, molds are created from metal, usually steel or aluminum, and precision-machined to form the features of the desired part. The mold consists of two primary components, the injection mold (A plate) and the ejector mold (B plate). Plastic resin enters the mold through a sprue, or gate, and flows into the mold cavity via channels, or runners, that are machined into the faces of the A and B plates.
Injection molding process
When thermoplastics are molded, typically pelletized raw material is fed through a hopper into a heated barrel with a reciprocating screw. The screw delivers the raw material forward, through a check valve, where it collects at the front of the screw into a volume known as a shot.
The shot is the amount of resin required to fill the sprue, runner and cavities of a mold. When enough material has gathered, the material is forced at high pressure and velocity into the part forming cavity
Injection molding cycle
- Clamping
Before injecting the material into the mold, the two halves of the mold are closed, securely, by the clamping unit. The hydraulically powered clamping unit pushes the mold halves together and exerts sufficient force to keep the mold closed while the material is injected.
- Injection
With the mold closed, the polymer shot is injected into the mold cavity.
- Cooling
When the cavity is filled, a holding pressure is applied which allows more polymer to enter the cavity to compensate for plastic shrinking as it cools. In the meantime, the screw turns and feeds the next shot to the front screw. This causes the screw to retract as the next shot is prepared.
- Ejection
When the part has cooled sufficiently, the mold opens, the part is ejected, and the cycle starts again.
Advantages
- Fast production
- Design flexibility
- Accuracy
- Low labor costs
- Low waste
Disadvantages
- Upfront cost of the tool
- Restrictions related to the design of parts
- Flaws that are unavoidable due to the nature of the process.