When it comes to prototyping, the key details include speed, cost-effectiveness, and constant iteration to achieve the required end results. Overmolding, on the contrary, is quite a slow and expensive process. Most times, it is reserved for scaling up production parts.
Creating prototypes using the overmolding process presents engineers and product designers a unique challenge. Overmolding in prototype CNC machining is often an insurmountable obstacle to development. Therefore, engineers tend to avoid it as much as possible.
The Several Steps of the Overmolding Process
The overmolded assembly consists of a substrate and an overmold, with the overmold partially or completely encapsulating the substrate. Typically, the substrate is made of metal or a rigid polymer, while the overmold is composed of a softer material.
1.Substrate Preparation
The first few steps involve the preparation of the substrate. It encompasses cleaning the surface and preparing it for the application of overmolding material. At this stage, it is essential to ensure the substrate is free from dirt, dust, or debris because they could interfere with the adhesion properties of the material.
2.Mold Design and Creation
The next step involves the design and creation of the mold. Engineers ensure that the design allows proper fixturing to prevent substrate movement during molding. The mold is typically made of metal but can sometimes be plastic. Mold creation is done with CAD software, allowing for accurate and precise specifications.
3.Material Injection
After carefully placing the substrate into the mold using the designated fixturing features, the mold cycle begins by closing the mold tool. With the mold tool fully closed, the press enters the injection phase. Plastic is injected into the tool, creating the overmolded part around the substrate.
4.Ejection of Finished Project
The overmold is first cooled to a temperature that maintains the material’s rigidity. Then, the mold opens, revealing a fully overmolded part. Ejector pins or similar components are employed to remove the part from the “B” half of the mold. Machinists trim the excess material off the overmolded part, and apply any necessary finishes to the final product.
Material Compatibility
One of the most prominent challenges in prototype overmolding is material compatibility. The materials used in pre-manufactured components and the overmold material must be compatible with each other. Failure to achieve compatibility can result in issues like warping, cracking, and delamination. Resolving this issue involves selecting the appropriate material grade and increasing both process and mold temperature. It is also essential to examine the compatibility of color concentrates and consider using lubricated grades.
Engineers now prioritize adhesion requirements during material selection, considering the end-user environment and application. However, not all overmold materials can adequately bond with all substrates. Compatibility between the overmold material and the substrate should be carefully assessed, considering factors like exposure to oil or organic chemicals and temperature. Special grades of overmold material are necessary for automotive under-hood environments, microwave-safe products, and dishwasher applications. The expected service life of the end product also significantly influences the adhesion requirements.
Expensive Metal Tooling
Metal injection mold tooling can be very expensive. Simple tools alone can cost thousands of dollars. More intricate ones can easily reach tens of thousands of dollars. Injection molds endure tremendous pressure and extreme heat during each cycle, necessitating the use of robust and durable materials to withstand these harsh operating conditions without deformation.
The creation of injection molds involves the fabrication and assembly of intricate components through a combination of manual and automated processes. This meticulous approach ensures the production of reliable and repeatable injection molding processes. To achieve precise and tight tolerances, specialized technologies such as CNC machining and electrical discharge machining (EDM) is employed, contributing to the overall cost.
Additionally, various design considerations including side pull actions, cavity count, texturing, multi-material shots, tight tolerances, engraving, undercut features, and inserts can significantly impact the tooling expenses. Designing, constructing, and assembling injection molds is a labor-intensive and intricate undertaking that demands time, expertise, and skill. Injection mold builders and designers possess specialized training and proficiency, making their services invaluable but requiring a considerable investment.
In prototype overmolding, the cost is only spread over a smaller quantity of parts, typically ranging in the tens or hundreds. Such a limited scale often fails to justify the significant investment required.
Time Consuming Nature of Metal Tooling
The lead times for both mold tooling and molded parts are excessively long. The manufacturing process typically requires around 5 to 7 weeks for tooling production and an additional 2 to 4 weeks for parts manufacturing and shipping.
The extended duration can be attributed, in part, to the intricate nature of the molds used. These molds encompass the exact shape of the part and also incorporate elaborate systems for material flow and rapid cooling, including runner and water-cooling systems. Furthermore, the tooling process involves multiple stages, including mold design, prototyping, testing, and refinement.
Each stage requires careful attention to detail and iterative adjustments to achieve optimal mold performance. The design and testing phase alone can span several months before the final mold is deemed suitable for production, and any alterations to the design can further prolong the overall turnaround time. Any modifications or design changes to the molds can significantly extend the tooling time. Waiting for months for a machined prototype is not a viable option.
Inability to Replicate Overmolding with Direct 3D Printing
During CNC prototype machining, current printing technologies generally work with a single material at a time. They do not provide the capability to simultaneously print multiple materials. Furthermore, they cannot pause mid-print to allow the introduction of another component then continue the print around that component (overmolding).
While 3D printing has improved significantly, it still has limitations in terms of material compatibility, resolution, and the ability to create strong interfacial bonds between layers. No currently available 3D printer can effectively replicate the adhesion between two materials. And this adhesion is integral to the functionality of many overmolded products. Therefore, direct 3D printing is currently unable to replicate overmolding due to the technical constraints.
Conclusion
Overmolding has proven to be an efficient method for creating high-quality production parts. However, prototyping with overmolding has been associated with many challenges. Most of these challenges are in terms of CNC prototyping costs and lead time. With new, improved technologies, they can save thousands of dollars while also reducing the lead time significantly.
