The basics of plastic injection molding process includes creating the product design, making a tooling a mold to fit the product design, melting the plastic resin pellets, and using pressure to inject the melted pellets into the mold.
See a breakdown of each step below:
Designers (engineers, mold maker businesses, etc.) create a part (in the form of a CAD file or other transferrable format), following fundamental design guidelines specific to the injection molding process. Designers should try to include the following features in their designs to help increase the success of a plastic injection mold:
Additionally, designers should minimize the following features to reduce defects in their designs:
Highly skilled machinists and toolmakers, using the product design, fabricate a tooling mold for the injection molding machine. A tooling mold (also known as simply a tool) is the heart and soul of the injection molding machine. They are carefully designed to contain the negative cavity for the product design and additional features such as sprues, runners, gates, vents, ejector systems, cooling channels, and moving components. Tooling molds are made out of specific grades of steel and aluminum that can withstand tens of thousands (and sometimes hundreds of thousands) of heating and cooling cycles, such as 6063 aluminum, P20 steel, H13 steel, and 420 stainless steel. The mold fabrication process takes upwards of 20 weeks to complete, including both fabrication and approval, making this step the most extended aspect of injection molding. It is also the most expensive part of injection molding, and once a tooling mold is fabricated, it cannot be drastically changed without incurring additional costs.
After operators obtain the finished mold, it is inserted into the injection molding machine, and the mold closes, starting the injection molding cycle.
Plastic granules are fed into the hopper and into the barrel. The reciprocating screw is drawn back, allowing materials to slip into the space between the screw and the barrel. The screw then plunges forward, forcing the material into the barrel and closer to the heater bands where it melts into molten plastic. The melting temperature is kept constant as per the material specifications so that no degradation occurs in the barrel or in the mold itself.
The reciprocating screw forces this melted plastic through the nozzle, which is seated within a depression in the mold known as a mold sprue bushing. The moving platen pressure fits the mold and the nozzle together tightly, ensuring no plastic can escape. The melted plastic is pressurized by this process, causing it to enter all parts of the mold cavity and displacing cavity air out through the mold vents.
Choosing the right plastic for plastic injection molding can be difficult—there are thousands of options in the market from which to choose, many of which will not work for a given goal. Luckily, an in-depth understanding of the desired material properties and intended application will help narrow the list of potential options into something more manageable. When considering the application, it is important to keep in mind the following questions:
Similarly, the questions below are useful when determining the desired material properties:
A good pointer to keep in mind is material selection also depends on the product design, both in the application and in complexity. For example, designs calling for bendable sections/living hinges will require a strong yet flexible plastic like polypropylene. Similarly, structural parts that must be resistant to chemicals, abrasions, etc. should prioritize plastics like PEEK, Nylon, and others within these families.
Included below is a table of the common injection molding plastics, each with its own set of advantages and general industry applications:
3D printing is a separate set of processes from injection molding. With 3D printing, the material is deposited in a layer-by-layer process guided by a 3D model of the desired part. With injection molding, molten plastic is injected into a mold cavity under high pressure, creating a part all at once. Both processes are considered to be "Additive Manufacturing".
In 3D Printing, Supports for overhangs and voids are also printed with the part, as well as additional features to reduce warping and other defects. 3D printing is a lengthy process, taking at minimum several hours, and can take a few days depending upon the size and complexity of the printed object. 3D printed parts are commonly produced out of thermoplastics, but other possible materials include thermosets, impregnated plastics, ceramic powder, and metal/alloy powder.
In Plastic Injection Molding, plastic injection molds come out with suitable surface quality and are essentially ready for application (with only sprues, runners, gates, and minor defects to manage). Thermoplastics are generally the preferred material used in injection molding, though newer injection molding processes can accept thermosets, impregnated materials, some metal powders, and some types of ceramics.
With 3D printing, the material is deposited in a layer-by-layer process guided by a 3D model of the desired part, while with injection molding, molten plastic is injected into a mold cavity under high pressure, creating a part all at once. To learn more, see our guide on 3D Printing vs. Plastic Injection Molding.
Below is a list of comparisons between 3D Printing and Injection molding using key metrics:
Want more information on Auto Parts Molds, Daily Necessities Molds, Medical Molds? Click the link below to contact us.