The global plastic injection parts market is currently valued at US$325 billion, and this number is expected to increase by 5.7% by 2025. Stimulated by the rapid development of the Asian economy, the use of process efficiency and new resin chemistry, the use of plastic parts in increasingly complex applications has brought tremendous opportunities and challenges to product designers and industrial engineers.
Of course, a significant advantage of plastic injection molding is that it can produce very high output at a low unit price relative to the development cost (mold).However, these same high volumes demand the use of good design practices. Even small, incremental improvements to mold design, which might be insignificant in small batch production, can have a major financial effect when considering millions of finished parts. That’s why it makes good sense to apply sound design logic at the outset of any mold project.
While designing two multi-cavity molds for a product that will be used in a high-temperature environment, I found four key considerations that universally apply to any project for any application. The following is my experience designing the mold for this part.
Design for the Material
Every thermoforming resin has unique chemical and mechanical properties that need to be mated to the tool steel that’s going to be used to mold it. For example, when using PEEK, a high-temperature engineering plastic used in automotive, aerospace, and medical applications, the mold needs to withstand higher molding temperatures while maintaining dimensional stability.
Design for Draft Angles
Resins have unique shrink rates and percentages which can cause the molded part to stick inside the cavity. The manufacturer’s spec sheet can help determine minimum draft angles, but this is also affected by the part’s surface texture. To a certain point, more texture requires more draft.
It’s very common for product designers to specify parting lines at the right-angle intersection of two perpendicular faces. If one of those faces is a cosmetic surface, it may be damaged if there is any flash in the mold.
To avoid the possibility of this damage it’s a good idea to move the parting line away to the adjacent, non-cosmetic face. That parting line should be moved along a draft angle of a few degrees, not 90 deg. If flash should occur it can then be trimmed off without damaging the look of the finished part.
Design for Wall Thickness
Managing wall thickness is important for controlling stress marks and to ensure the design is meeting minimal wall thickness while maintaining as much consistency as possible in t The gate represents an area of high initial injection pressure. A narrow wall thickness also implies a restriction which increases injection pressure. These two forces, if not balanced out, can create shear, flashing, and even damage to the mold. Therefore, it’s good design practice to increase wall thickness near the gate, decrease injection pressure, or both.
Second, there is an increased likelihood of flash because the pressure is exerted precisely on the split line. If a flash occurs, it will take significant time and effort to remove, leaving a corresponding mark on the finished product. Lastly, adjacent areas of unequal mass will cause sink marks. These are not only unsightly but can also compromise the part’s structural integrity.
Design for Ejection
The application of ejection force should be balanced out over the surface area of the part—taking into account thickness and mass—to keep the part from warping or breaking. Additionally, in the area surrounding the gate, stripper plates or additional ejector pins will be needed. These pins are necessary to clear the gate of plastic in the event of a short shot.
Since the area near the gate will be under stress, it’s good practice to make the wall thicker if possible or create pads or other flat areas to provide a push-off for the force of the pin to act against.
Although there are many things to consider when trying to improve your plastic injection molding design, if you can take these four tips into consideration, you will manufacture high-quality products on your path. As designers and product engineers continue along the journey of new product development, it is crucial to have a keen understanding of the nuances of plastic injection molds and how to mitigate them.