Insert molding is an injection molding process related to overmolding, but differs in several key ways. Overmolding involves injecting a second resin into a more robust plastic substrate. Overmolding allows product designs to mold ergonomic devices, power tool handles, and rubber gaskets directly onto pre-existing parts without adding additional labor. Insert molding, on the other hand, involves placing a preformed part (usually metal) into the mold before injecting the plastic material.
Thus, the use of inserts in injection molding allows injection mold design engineers to efficiently add threaded metal inserts to plastic parts, embed metal electronic components within protective plastic housings, or create common hand tools such as scissors and screwdrivers. As an injection molding production process, insert molding is easier to control and can provide better results than other methods of encapsulating metal parts in plastic parts. Elimold has been in the injection molding service industry for many years and has a wealth of experience. We have summarized the following knowledge and guidelines related to insert molding.
Guidelines for Optimizing Insert Mold Design
When designing custom insert molds, mold design engineers should follow common DFAM best practices, incorporating rounded knurls, avoiding sharp corners, and optimizing draft angles. While undercuts (also known as mechanical locks) add complexity and cost to any part, incorporating undercuts into the insert mold design provides greater pull-out strength to the part.
And, a good rule of thumb when designing inserts is to ensure they are kept small relative to the plastic component they are embedded in. Generally speaking, inserts should extend at least 0.016 inches (0.4 mm into the mold cavity), and to prevent sink marks in the plastic, the molded part under the insert should be at least one-sixth the diameter of the insert.
A common production defect associated with this technique is delayed cracking of the plastic around the metal insert. If the in-mold hoop stress of the insert is not accounted for, the two halves of the part can easily separate and fail as the injected resin shrinks and cools (operating under the assumption that the plastic around the insert is subject to strain equivalent to the mold shrinkage). Toughened, more durable resins are recommended because these materials have better elongation and fracture resistance.
Additionally, adhesion issues between dissimilar materials are a common problem for many insert molding applications. While overmolded parts can take advantage of the chemical bonds formed between different thermoplastic resin layers, metal inserts do not form chemical bonds with the overmolded plastic. Therefore, the metal and plastic parts must be designed to mechanically bond to each other.
The key advantage of embedding threaded components into plastic parts is the improved mechanical quality of the component. Such metal components increase the strength and durability of the entire product, which is useful for device housings and electronics enclosures, for example. Mating components can also be equipped with metal bushings or sleeves to improve wear resistance and extend the life of the component.
Insert Molding Production Process Optimization Guide
Good prototyping habits are key to cost-effective insert molding. Product design teams should test their designs early. Prototyping is essential to identifying design and performance issues, as well as predicting potential defects that may appear over time. A well-designed end-use testing program includes, for example, testing the performance of the part’s components under the expected temperature and environmental conditions of its end use. Detecting and resolving errors before production begins also helps reduce production cycles and speed time to market. It is critical that the tooling used to make the mold is held to precise tolerances, which helps ensure that the insert is accurately placed or suspended within the mold as the molten material enters.
Also, because most insert molding is done in a vertical press, it is easier to load the hardware without it falling out of place. If production volumes are high enough, it may be worth investing in a robot or automated system to load the inserts into the mold, but most low- to medium-volume work requires inserts to be placed into the mold manually. This increases unit cost and labor time, but is generally a more economical option for production runs of these sizes.
Summary
One of the problems in insert molding product quality control is the close fit between the metal surface and the resin. The surface ion activity of metal and polymer materials is different, which is related not only to their chemical state but also to their physical properties such as surface roughness. It is a complex issue. In the development of practical insert molding products, these complex characteristics must be thoroughly considered. Even in extremely small gaps, water droplets and chemicals will increasingly penetrate into the gaps due to capillary action, which may cause unexpected problems in the future. Insert molding has unique characteristics of resin and metal composites. If you need to manufacture insert molding molds, you can contact Elimold to serve you.