Injection molding is a common method for mass production and is often preferred over other processes, given its capability to economically make complex parts to tight tolerances. Injection molding is also one of the most efficient manufacturing processes due to fast cycle times associated with high rates of cooling, low material waste, and the potential for using recycled and sustainable materials [1].
Mold design is one significant activity in a much larger product development process. Since product and mold design are interdependent, it is useful for both product and mold design engineers to understand the plastic part development process and the role of mold design and mold making. A typical product development process is presented in Figure 2.1, which includes different stages for product definition, product design, business and production development, ramp-up, and launch.
An injection mold is a complex assembly with many components. As shown in Figure 3.1, most production molds are comprised of a customized mold base, custom feed system, custom mold inserts, and other mold parts. Three alternative procurement options are indicated below each category, wherein the top set is the most common option for production molds. Generally, mold base suppliers are the preferred source for mold bases and generic mold components such as ejector pins, sprue bushings, etc. Similarly, hot runner suppliers are the most common option for highly efficient feed systems in production molds. Most mold cavities and cores are machined into mold inserts and assembled into the customized mold base along with a feed system and other components to realize a functional mold. This procurement strategy is most common as it provides reasonable trade-offs between availability of standard components, reasonable costs, and flexibility in mold design.
During the mold layout stage, the mold designer confirms the type of mold and determines the dimensions and materials for the cavity inserts, core inserts, and mold base. Mold bases are only available in discrete sizes, so iteration between the inserts’ sizing and mold base selection is normal. The goal of the mold layout design stage is to develop the physical dimensions of the inserts and mold so as to begin procurement of these materials. Mold material selection is also an important decision, since the material properties largely determine the mold-making time and cost as well as the mold’s structural and thermal performance.
For an acceptable molded part to be produced, the polymer melt must completely fill the mold cavity. Accordingly, the wall thickness of the molded part and the gating locations must be specified such that the melt is able to traverse from the gates to the edge of the cavity. Mold filling analysis is used to ensure that the melt can not only fill the mold at achievable molding pressures, but fill the mold as intended to achieve the desired quality.
The purpose of the feed system is to convey the polymer melt from the molding machine to the mold cavities. The design of feed systems can range from very simple to very complex. Increased investment in the feed system design will tend to provide for reduced cycle time and less material waste when using the mold. However, it is possible to over-design the feed system, and the “best” feed system design is a function of the production volume, availability of molding pressure, and level of allowable investment.
Gates provide the important function of connecting the runner to the mold cavity, and initiating the flow of the melt into the cavity. There are many different types of gates, with the most common types of gates being the edge and pin-point gates. Referring back to the cost estimation of Section 3.4, gating represents a small portion of the mold cost but has a significant impact on the operation of the mold. Knowing when to use what type of gate, and how to properly dimension the gate(s), can reduce the likelihood of mold re-work.
Venting is normally considered a minor aspect of mold design, and frequently neglected until molding trials indicate mold inadequacies related to venting. An understanding of the purpose and function of vents can assist the mold designer to design vents where clearly needed and ensure that the mold can accommodate additional vents if required.
The cooling system is extremely important to the economics and operation of the designed mold, and yet remains one of the most under-engineered systems in injection molds. Perhaps the reason for the lack of engineering is that the temperature distribution is not obvious when molding compared to defects related to flow.
Plastic part designers often use design for manufacturing and assembly (DFMA) guidelines to reduce the number of parts in an assembly [1]. For this reason, plastic part designs can be extremely complex with many features and tight tolerances. The delivery of plastic moldings that satisfy the dimensional requirements is a joint responsibility of the mold designer, molder, material supplier, and part designer. The part designer should provide a design with uniform thicknesses and achievable specifications. The material supplier should provide consistent polymer resin and useful guidance regarding material properties. The molder should select suitable and consistent processing conditions for operation of the mold. The mold designer should provide a mold with balanced melt filling and cooling, and for which the mold cavity dimensions were engineered for an appropriate shrinkage.
The ejection system is responsible for removing the molded part(s) from the mold after the mold opens. While this may seem a simple function, the complexity of the ejection system can vary widely depending on the requirements of the molding application. Many issues must be considered including the need for multiple axes of movement, the magnitude and distribution of the ejection forces, and others. Before beginning the analysis and design of the ejection system, an overview of its function is first provided.
Injection molds are subjected to high levels of pressure from the heated polymer melt. When this pressure is integrated across the surfaces of the mold cavities, forces result that typically range from tens to thousands of tons. The structural design of the mold must be robust enough to not only withstand these forces, but also to do so while producing high-quality molded products.
This book has sought to provide an engineering approach to mold design; the emphasis has been on the examination and modeling of fundamental mechanisms that govern the design and use of injection molds. The examples have purposefully been made as simple and clear as possible so that the practitioner can apply the design and analysis methods to more specific and advanced molding applications.
Injection molding is a preferred manufacturing process given its ability to quickly and efficiently make complex products of high quality. However, it is quite common for problems to be encountered during mold commissioning given the challenge of delivering stringent yet diverse key product characteristics (KPCs) while also managing significant uncertainty related to material properties and start-up processing conditions. When problems occur, it is important to assess the root cause and associated corrective remedy. Typically, the issues will arise from one of four sources: 1) material properties, 2) processing conditions, 3) product design, or 4) mold design.
