Injection molding is a popular manufacturing method for many reasons. It has proven especially valuable to those in the consumer product development sector, since plastics are a primary component of many consumer products, and injection molding is one of the best ways to manufacture plastics. Let’s take a quick look at the three major phases of the injection molding process, and then discuss the advantages and disadvantages of the process.
Injection Molding Process, Basic Step 1: Product Design
Design is one of the most important facets of the production process because it’s the earliest opportunity to prevent expensive mistakes later on. There are many objectives to design for: function, aesthetics, manufacturability, assembly, etc. The right design is one that accomplishes the required objectives to a satisfactory level, but it may take a lot of creativity to get there. Product design is most often accomplished with computer aided design (CAD) software, like SolidWorks. Proficiency with CAD software is vital because it allows for quicker iterations and more accurate prototyping if necessary.
Some specific ways to avoid costly mistakes during the product design process are to plan for uniform wall thickness whenever possible, and to gradually transition from one thickness to another when changes in thickness are not avoidable. It is also important to avoid building stress into the design, such as corners that are 90 degrees or less. (Read more about Injection Molding Defects here.)
A skilled team of design engineers will be able to brainstorm, design, and improve upon a variety of solutions to meet the particular complexities of a specific project.
Injection Molding Process, Basic Step 2: Mold Design
After a looks-like, feels-like design has been tested and slated for further production, the mold (or die) needs to be designed for injection mold manufacturing. Molds are commonly made from these types of metals:
Hardened steel: Typically the most expensive material to use for a mold, and generally the longest-lasting (which can drive down price per unit). This makes hardened steel a good material choice for products where multiple hundreds of thousands are to be produced.
Prehardened steel: Does not last as many cycles as hardened steel, and is less expensive to create.
Aluminum: Most commonly used for single cavity "Prototype Tooling" when a relatively low number of parts are needed for testing. Once the injection molded parts from this tool are tested and approved, then a multi cavity steel production tool is produced. It is possible to get many thousands of parts from an aluminum tool but typically it is used for lower quantities.
Beryllium-Copper alloy: Typically used in areas of the mold that need fast heat removal or where shear heat is concentrated.
Just as with overall product design, mold design is another opportunity to prevent defects during the injection molding process. We have previously written blogs on the Top 10 Injection Molding Defects and Avoiding Mistakes in Injection Molding, but here are some examples of how poor mold design can be a costly mistake:
Not designing the proper draft: This refers to the angle at which the finished product is ejected from the mold. An insufficient draft can lead to ejection problems, costing significant time and money.
Improperly placed or sized gates: Gates are the openings in a mold through which thermoset or thermoplastic material is injected. Each will leave a vestige (scar), which can create aesthetic or functional problems if not properly placed.
The number of parts (cycles) required, as well as the material they will be made of will help drive decision-making as to how and with what materials to create the mold.
Injection Molding Process, Basic Step 3: The Manufacturing Process
When a product has been properly designed, approved, and die cast, it’s time to start the actual manufacturing! Here are the basics of the injection molding process…
Thermoset or thermoplastic material in granular form is fed through a hopper into a heating barrel. (Learn more about the differences between plastics in our PLASTICS course.) The plastic is heated to a predetermined temperature and driven by a large screw through the gate(s) and into the mold. Once the mold is filled, the screw will remain in place to apply appropriate pressure for the duration of a predetermined cooling time. Upon reaching this point, the screw is withdrawn, the mold opened, and the part ejected. Gates will either shear off automatically or be manually removed. This cycle will repeat over and over, and can be used to create hundreds of thousands of parts in a relatively short amount of time.