Speaking the language of Plastic:  A guide to Injection Mold Part Design

In the universe of manufacturing, part design is the celestial body around which the orbits of product functionality, manufacturability, and market success revolve. Particularly in injection molding, part design isn't just about aesthetics or dimensions; it's about envisioning a component's journey from granular plastic pellets to a durable, functional part integral to a complex assembly.

Designing parts for injection molding, like other manufacturing processes, is a delicate balance of mechanical properties, product design, cost efficiency, and performance. Unlike other manufacturing processes, however, injection molding necessitates a design approach that considers the behavior of plastic as it transforms from a molten state to a solid state within the confines of a precisely engineered mold. The designer must anticipate the flow of plastic, the impact of cooling rates, and the interplay between the part structure and the physical forces exerted by the molding machine.

The significance of part design in product development is paramount. It is the blueprint that dictates the feasibility of manufacturing and sets the stage for the product's ultimate performance. In injection molding, the part design dictates the mold design, which in turn, influences cycle times, production costs, and the longevity of the mold itself.

A well-conceived part design can lead to a robust product with optimal functionality, longevity, and aesthetic appeal. Conversely, poor part design can result in manufacturing challenges, increased costs, and potential product failures. It's this high-stakes environment that elevates the role of the part designer from a mere contributor to a principal architect of a product's success.

This guide is aimed at helping to speak the language of plastics more fluently. At the core of injection molding part design is a profound understanding of how plastics behave. This knowledge serves as the foundation upon which all design elements are constructed. But why is material behavior so critical in part design? The answer lies in the unique properties of plastics that can be both advantageous and challenging for designers.

Plastics are not a monolith; their properties vary immensely. From the tensile strength of nylon to the impact resistance of polycarbonates, the choice of material influences aspects of part design, and more significantly, the performance of the finished part. As compared to metal, wood and other materials, plastics have a low modulus and tend to creep. They have a viscoelastic nature, meaning they respond differently to a force applied at a high rate rather than a low. When looking at the stress-strain performance of plastic, an often omitted part of the conversation is to ask, “But at what strain rate does this apply to?”

Fun fact: Did you know mechanical thermostats in cars use a substance that is basically candlewax to detect the cars temperature and open and close valves to regulate coolant flow? Like candlewax, plastics expand and contract greatly upon melting and solidifying respectively. These expansions and contractions are the chief driving elements of good part design.

At its heart, DFM involves designing parts with the manufacturing process in mind. It is about understanding the capabilities and limits of the injection molding process and leveraging this knowledge from the earliest stages of part conception. DFM aims to minimize the complexities in manufacturing, reduce material waste, and avoid costly design modifications down the line.

The integration of DFM principles into part design begins with simplification. By simplifying geometries, reducing part counts, and optimizing the part for the molding process, designers can significantly reduce the risks of defects and streamline the production process.

Varying wall thickness can also prevent the mold from being filled adequately. If thinner and thicker regions are an absolute necessity, make sure that the flow starts in the thick region and fills the thin regions last. Make sure that flow does not go from thick regions to thin regions and then through to thick regions again, as this will cause significant pressure drops that effect the integrity of the mold capabilities. Sometimes this needs to happen, like in the case of molding living hinges, but should be avoided. The transition from a thick to a thin region should be as gradual and smooth as possible.

Strength in injection molded parts does not necessarily come from bulk. The strategic use of ribs and gussets can significantly increase part stiffness without compromising the uniformity of wall thickness. However, like any feature, they must be designed with care to prevent sink marks and ensure they do not overshadow the core geometric design principles.

Typically, ribs should be about 60% of the adjoining wall to avoid shrinkage issues. To be more precise, however, it is recommended to use .5 wall thickness for high amorphous and 0.75 wall thickness for semi-crystalline.

Place gussets at strategic locations to support load-bearing areas and to enhance the overall rigidity of the part without affecting the material flow.

When designing for holes, through holes are preferred over bottoming holes, but with some setbacks as well.

Through holes have more options to be able to make in the tooling and feature more dimensional stability, as tooling can meet from both sides of the hole. However, when the flow front of the molding moves around a bottoming hole, it must split into two fronts as it moves around the hole. The two fronts then meet again on the other side of the hole. The place where the fronts meet is known as a weld line and creates a local area of weakness. Keep this in mind any time a feature splits a flow front into two parts. A bottoming hole will not do this.

A common feature in molding is known as the snap fit. These allow for small ‘arms’ of plastic to snap over other pieces to hold parts together. How do we design these? A few important points:

• The snap fit should be sized so that the snap fit does not exceed 70% of the yield strain of the material at any point

• The snap fit arm should not be in a stressed state after it has been snapped

• It is common for the cross sectional dimensions of snap fit arms to diminish down their length- this saves material and makes for a more ideal stress profile for the arm.

If your stress value is 70% or less of yield strain, you should be good to go.

For simplicity, this was not shown with draft. You will notice the other part of the tooling will have to mold the top and sides of the snap fit feature, and come into contact with the pass through shutoff that creates the overhang on the snap fit. In areas where the two tools make contact, higher draft values will preserve the tooling life. Aim for 5° or more draft where tooling contacts other tooling.

Technological advancements in simulation software have empowered designers to predict outcomes more accurately than before. However, mold simulation technology has its limitations. As a rule of thumb, simulation is accurate in showing where distortions such as warp, sink, or shrink will occur, but the magnitude of such distortions can vary widely from the predictions of the simulation. To date, there is no replacement for understanding and following good injection mold design principles.

Injection molding is a highly efficient process for mass-producing plastic parts and components. One of its primary advantages is the ability to produce large volumes of parts with a high degree of consistency and repeatability. This uniformity is crucial for industries where precision is paramount, such as automotive and medical device manufacturing.

Another significant benefit is the ability to design and produce complex geometries that would be difficult or impossible to achieve with other manufacturing methods. Injection molding allows for intricate designs with precise detail, undercuts, and tight tolerances, opening a vast array of design possibilities.

Additionally, injection molding is cost-effective at scale. Although the initial cost of mold design and creation can be high, the per-part production cost is significantly reduced for large runs due to the speed of the manufacturing process. This makes injection molding an excellent choice for long-term production where the initial investment can be amortized over a high volume of parts.

Material selection is another advantage. A wide variety of plastics and polymers can be used in injection molding, allowing for material properties to be tailored to specific application needs such as flexibility, strength, or resistance to heat or chemicals.

Despite its many benefits, injection molding also has drawbacks. The upfront investment for tooling is substantial and can be prohibitive for small businesses or for products with a low production volume. This initial cost also makes it less ideal for prototyping compared to additive manufacturing processes like 3D printing.

The need for high-volume production to justify the initial costs means that injection molding is less flexible when it comes to scaling production up or down quickly. Design changes can be costly and time-consuming since they often require modifications to the mold, which can be a significant setback in the fast-paced product development cycles of today's market.

Environmental concerns also come into play. While plastics are versatile, they are derived from non-renewable resources, and the end products can be challenging to recycle. This has led to a push for more sustainable practices within the industry.

Balancing the pros and cons of injection molding is crucial. It remains a dominant manufacturing method in numerous industries due to its efficiency and scalability. However, businesses must carefully consider the environmental impact, upfront costs, and design requirements before committing to this process.

In conclusion, the success of injection molded parts lies in a design philosophy that interlaces functionality with aesthetic appeal, guided by the principles of design for manufacturability (DFM). Uniform wall thickness, appropriate material selection, thoughtful surface finishes, and a symbiotic relationship between part and mold design are foundational elements that contribute to the part's integrity, cost-effectiveness, and market viability. By navigating these design tenets with precision and creativity, manufacturers can ensure each part not only meets the rigorous demands of production but also fulfills the expectations of end-users, embodying a blend of innovation, quality, and practicality that stands at the heart of the injection molding industry.