Plastic injection molding can be an ideal method for producing parts across a wide spectrum of industries and end use applications. As a reliable manufacturing process, plastic molding has been utilized for more than 100 years. Suitable for low cost commodity parts, as well as those that require blends of high complexity and tight tolerances, the injection molding process offers a high degree of design freedom and cost competitiveness.
The Basics of Plastic Injection Molding
The basic steps of the injection molding process are the same for any part. First a custom mold tool is fabricated which has a negative cavity space to be filled with melted plastic resin to form the desired part. This tool is then placed into an injection molding machine, also called a press, which controls the process of making parts with the tool.
During each cycle of the process the press and tool do a few essential steps. Heat is added to the barrel of the press which melts a solid thermoplastic resin into a liquid. The liquid resin is then injected from the barrel into the part cavity of the mold.
Once full, the press continues to apply pressure to the part cavity to fully pack out the part with resin. Finally, the tool is cooled at a temperature that solidifies the resin back into a solid plastic. The tool can then be opened to eject and retrieve the finished part prior to the next molding cycle.
Once the tool and process have been established to make acceptable parts this process can run repeatedly to make hundreds, thousands or even millions of parts in a production run.
In most applications each molding cycle is producing parts in a near finished condition, meaning additional manufacturing steps are few or nonexistent when compared to other manufacturing methods. This combined with the fact the process operates with relatively short cycle times makes plastic injection molding one of the most cost effective means of making parts at production volume.
There are several factors that determine the cost of an injection molded part. The primary ones being the cost and amount of resin needed for each part, the size of the press required to mold the part and the length of the cycle in the press to complete each part.
Material specifications are typically based on a list of functional requirements that can include mechanical properties, as well as chemical and environmental compatibility needs. Based on these requirements designers can choose from a multitude of resins from low cost commodity solutions to high performance engineering grades designed for specific applications.
The press size and cycle time required to make a part are directly related to parts size and geometry, in general bigger parts require larger presses. A key advantage is that once these basic criteria are set, a significant amount of geometric complexity can be added with little impact to part cost. It is common for new molded parts to be able to incorporate multiple pre-existing components into a single molded part at a significant cost savings.
Part designers are able to utilize a tried and true set of criteria to successfully design injection molded parts. These traditional rules provide guidance on design factors like wall thickness, feature size and draft for part release out of the tool. Generally, injection molders will provide design for manufacturability (DFM) feedback on part designs to ensure they are suitable for the specific molding application.
It is critical to consider several factors such as material selection, part design, tooling and the selected press since they all need to work together to effectively mold parts. Understanding how these factors are interrelated and managing them to make part production as straight forward as possible is an important step to complete prior to moving forward with a new mold build. Moldflow simulation is another critical tool used to analyze potential part risks. With this predictive computer analysis designers can review part and tool design factors to understand if a part will fill and pack out successfully.
Plastic Molding Processes
While the traditional design guidelines should be followed whenever possible, new parts are constantly challenging the limitations of molded part design and traditional injection molding in general. To broaden the scope of parts that injection molding is suitable for, several variations on the process have been developed for niche applications. These techniques are often the difference in being able to make a non-traditional part successfully or not.
Structural Foam Molding:
- In this molding process variation a blowing agent is mixed directly into the resin stream feeding the press. The blowing agent creates a chemical reaction resulting in the formation of small gas bubbles inside the molten resin which expand once injected into the tool cavity. This has the effect of dramatically reducing the pressure required to fill and pack out a part because the expanding gas does the work instead of the press.
- Lower press tonnages
- Part weight reduction
- Large, extremely thick, and/or hard to fill parts can be molded successfully
External Gas Assist (EGA):
- In this process Nitrogen gas is introduced on the non-aesthetic side of the part after the tool is 100% filled with resin but not yet fully packed out. The gas is held at a high pressure which forces the molten resin onto the opposite side of the tool. The aesthetic side of the tool ends up with a clean and uniform appearance.
- Lower press tonnages
- Less molded in stress and part distortion
- Thicker sections in part geometry can be molded
Internal Gas Assist (IGA):
- This process is used specifically to mold very thick sections in parts. The process is similar to EGA in concept with one distinction. In IGA the Nitrogen gas is introduced inside of the part geometry essentially blowing the molten resin up like a balloon from the inside. The gas creates a hollowed out channel through the thick region of the part and forces the resin against the external walls to create a packed out appearance in the thick section.
- Extremely thick sections can be molded with high aesthetics
Insert Molding/ Overmolding:
- This technique makes a single component by molding a new part over an existing part that is placed inside the mold tool prior to the molding cycle. It is especially useful for molding soft touch geometry like handles over top of hard functional components and also for capturing inserts or hardware that would otherwise have to be installed through secondary operations like ultrasonic welding.
- Reduced assembly cost
- Combined material applications
Spotlight on Injection Molding Tooling
Selecting the correct part design and molding process are paramount in determining the commercial viability of a new part. Consideration also needs to be given to the proper specification and construction of a mold tool to make each new part.
Tools are most often fabricated out of specific tooling steels but can also be made from aluminum for lower cost prototyping applications. Quality tool design and fabrication is as critical to a successful part launch as the rest of the part design process. Mold tools are built by specialty tooling shops who have the requisite trade skills and equipment required to successfully complete a tool build.
It is important for a molder to have a close relationship with a tool builder to ensure that the design of the tool satisfies requirements for the production cell and the resulting part quality. Part designs often include specific callouts regarding how a tool must be constructed including where parting lines, gates and ejector marks can be.
Toolmakers exist all over the globe and can offer specific technical competencies or cost advantages that may be required for a particular project. A molder may quote building a new tool with several different toolmakers to best identify the toolmaker to partner with. As an up front and often significant capital investment to get a molded part project launched, it is important that part designs are mature and unlikely to change when creating a production tool.
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Finishing, Decorating and Other Secondary Processes
When required, injection molded components are suitable to combine with a number of secondary processes that may be required to assemble or decorate a part to completion. This can include installing threaded inserts via thermal or ultrasonic equipment, or other specialty equipment utilized to weld or bond plastics together. Parts can also be painted, silk screened, pad printed or laser engraved to provide a desired aesthetic appearance or mark a part with critical identification information.
Mack Molding has taken pride in being a world class plastic injection molder for the past 100 years. Founded as a molder in 1920, the company has honed decades of experience to grow into one of the top 10 non-automotive custom injection molders in North America. Mack currently operates 6 molding facilities on the east coast in Vermont, Massachusetts and North and South Carolina that service a worldwide customer base in applications that range from first shot prototypes to full scale automated production. Mack offers 120+ injection molding machines that range from 28 to 4000 tons and shot sizes from 0.6 to 800 ounces, allowing the company to service a wide variety of parts that range from commodity to validated medical components.