Injection molding is an efficient means of manufacturing parts in high volumes. Most often used for thermoplastics and thermosetting polymers, it can be applied in the design and production of metal parts as well. Parts are produced by injection molten material into a mold that is developed per the specifications of each individual project.
How Does Injection Molding Work?
Regardless of the size or complexity of parts, injection molding involves heating material to very high temperatures, injecting it into the mold, and allowing it to cool and solidify.
The process first requires a mold to be produced. This requires the design and engineering expertise of a toolmaker who can ensure the mold has all the features of the desired part. Molds are usually made of steel or aluminum and can be used to form anything from tiny electronics components to food storage containers, to car body panels, oil and gas equipment, and aerospace parts.
To optimize a mold design, the most suitable materials must be selected and a full-scale prototype made. Initial mold production tooling or 3D printing can be used to complete this stage. With 3D-printed tooling, low-volume production runs can be managed. Once the overall product design is approved and mold design completed, manufacturing can begin.
Injection Molding Process
The process by which plastic parts are injection molded is fairly straightforward and constant. It is generally carried out in the following steps:
- Clamping and Injection: Material, such as thermoplastic resin pellets, is fed through a hopper and heated and melted in a barrel within the injection molding The clamping unit closes the die via a toggle or hydraulic mechanism. Once its viscosity is reduced, the plastic is forced through a hopper into a mold cavity by a ram or screw-type plunger.
The material is drawn past the screw through a check valve. It forms a shot, or collection of material in front of the screw, that is used to fill the mold cavity to the desired amount. This step also compensates for shrinkage and provides a cushion to safely transfer pressure to the cavity.
When the proper volume is achieved, the molten plastic is forced into the part forming chamber at high pressures.
Pressure Control: By adjusting the speed of the screw, the machine can manage injection speed. This helps control pressure as the molten plastic fills the mold cavity. The switch from constant velocity to constant pressure control occurs at a point the screw shifts, which is normally between 95% and 98% of capacity. Thus, the system can compensate for thermal shrinkage in the typically less-than-one-second injection time.
Packing pressure is maintained until the cavity entrance, or gate, solidifies. At this point, material can no longer re-enter. The screw will then reciprocate and start gathering material for the next cycle. Meanwhile, the material in the mold continues to cool. Once dimensionally stable, or hardened, the material is demolded by a variety of components (i.e., pins, strippers, and sleeves) and driven forward in the process.
Molding: Before entering the cavity, molten plastic flows through a sprue. Runners and gates deliver the material into the molds. Once the mold hardens, the machine’s ejector rod pushes a plate that aids in ejecting it. In some cases, multiple cavities can be connected by a runner so multiple products can be formed simultaneously.
Due to the nature of the process, final parts may have parting line, sprue, gate, and ejector pin marks. Skilled designers can make adjustments that can place these anomalies in hidden locations.
There are some variations of injection molding. A two-shot mold can be created by injecting two separate materials. Both are used to make one unique part. Examples of two-shot molded products include multi-color items or knobs with a soft touch.
Two chemical components may be injected for thermosets. This initiates a chemical reaction that crosslinks the material, or combines its molecules to create a viscoelastic solid. Since it can solidify rapidly in the injection barrel/screw, adjustments to barrel volume, cycle time, and use of a thermally isolated hot mold can reduce the required time to solidify the component.
Molded products can have a range of mechanical, dimensional, and aesthetic properties, which can be controlled by adjusting a variety of conditions. These include injection speed, cylinder temperature, and mold temperature, just to name a few. The types of conditions are so numerous it takes a good deal of experience to select not only materials, but the best-suited molding conditions for an application.
How Long Do Molds Last?
A steel mold, over its lifetime, can often process a million parts or more. While steel molds can be costly to make, they can meet a wide range of production needs. Aluminum molds may be used as well, depending on the application. However, these are often not suited for high volume production or when dimensional tolerances are narrow. Compared to steel, aluminum is more prone to wear, deformation, and damage during the injection molding process.
Laszeray Technology’s Injection Molding Capabilities
Ensuring injection molding meets your needs requires working with a manufacturing specialist such as Laszeray Technology. Our capabilities include mold design with many types of materials and for parts of any geometry or complexity. We can produce single or multiple injection capable molds and offer machine tooling, rapid prototyping (including CNC milling and 3D printing), and manufacturing of injection mold dies.
At our facility, we use injection molding presses from 65 to 1,000 tons that can process over 250 different materials. Molds are maintained here, as is customer inventory; we can warehouse high-volume items whether they are bottle caps, vacuum cleaner parts, medical components, or parts of airplane engines.
If you have additional questions about the injection molding process or are interested in our injection molding services, contact Laszeray Technology at 440-582-8430.