Injection molding is a manufacturing process that allows the production of large quantities of parts by injecting molten material into a mold. Injection molding can be performed using a variety of materials, such as metals, glass, elastomers, and confectionery, but it is most commonly used with thermoplastic and thermoset polymers.
The injection molding process can be divided into several steps. First, a mold needs to be prepared with the desired part shape and size. The molten material is then heated to its melting point and injected into the mold using an injection machine. In the mold, the molten material cools and solidifies into the desired part shape. Finally, the mold is opened and the part is removed.
Injection molding has many advantages. First, it can produce parts with complex shapes and structures that may be difficult to achieve with other manufacturing processes. Secondly, injection molding can quickly produce large quantities of parts, which is very important for mass production. In addition, injection molding enables high precision and surface quality.
Table of Contents
- Introduction
- Cold Runner Moulds
- Hot Runner Moulds
- Two-Shot Moulds
- Unscrewing Moulds
- Family Moulds
- Conclusion
Introduction
Injection moulding is a widely used technique for mass production of identical or similar parts. The process involves feeding the material for the part into a heated barrel, where it is mixed and melted by a helical screw. The molten material is then injected into a mould cavity, where it cools and hardens to the shape of the mould. The mould is usually made of metal, such as steel or aluminium, and precision machined to match the features of the desired part.
Injection moulding has many advantages, such as high efficiency, low waste, high quality and versatility. However, it also has some challenges, such as high initial cost, complex design and maintenance requirements, and limited material options. One of the key factors that affects the performance and cost of injection moulding is the type of mould used. There are different types of injection moulds that have different characteristics and applications. In this article, we will discuss some of the most common types of injection moulds and their advantages and disadvantages.
Cold Runner Moulds
Cold runner moulds are the simplest and most common type of injection moulds. They consist of two plates: a fixed plate and a moving plate. The fixed plate contains the sprue (the channel through which the material enters the mould) and one or more runners (the channels that distribute the material to the cavities). The moving plate contains the cavities (the spaces where the parts are formed) and the ejector pins (the devices that push out the parts from the mould). The plates are clamped together by a hydraulic or mechanical system.
The main advantage of cold runner moulds is their low cost and ease of use. They are suitable for low to medium volume production and simple part geometries. They can also accommodate multiple cavities in one mould, which increases productivity and reduces cycle time.
The main disadvantage of cold runner moulds is their material waste and energy consumption. Since the runners are not heated, they also solidify along with the parts, resulting in excess material that has to be trimmed off and recycled or discarded. This increases material cost and environmental impact. Moreover, since the runners have to be cooled down before opening the mould, they increase cycle time and energy consumption.
Hot Runner Moulds
Hot runner moulds are a type of injection moulds that use heated components to keep the material molten in the runners. They consist of two plates: a fixed plate and a moving plate. The fixed plate contains the sprue bushing (the device that connects the nozzle of the injection machine to the mould) and one or more hot runners (the heated channels that distribute the material to the nozzles). The moving plate contains the nozzles (the devices that inject the material into the cavities), the cavities and the ejector pins.
The main advantage of hot runner moulds is their material efficiency and energy saving. Since the runners are heated, they do not solidify along with the parts, eliminating material waste and reducing cycle time. This also improves part quality by avoiding cold slugs (solidified material that can cause defects) and pressure loss (reduced injection pressure due to friction in cold runners). Hot runner moulds can also accommodate multiple cavities in one mould, increasing productivity.
The main disadvantage of hot runner moulds is their high cost and complexity. They require additional components, such as heaters, thermocouples, controllers and wiring, which increase initial investment and maintenance costs. They also require precise temperature control and balance to ensure uniform flow and avoid thermal expansion or contraction issues. Moreover, they are limited by material compatibility, as some materials may degrade or decompose at high temperatures.
Two-Shot Moulds
Two-shot moulds are a type of injection moulds that allow for producing parts with two different materials or colours in one cycle. They consist of two sets of cavities: a primary cavity and a secondary cavity. The primary cavity forms the base layer of the part, while the secondary cavity forms the overmoulded layer. The two cavities are rotated or transferred by a rotary platen or a shuttle system.
The main advantage of two-shot moulds is their design flexibility and functionality. They can create parts with different properties, such as soft touch, colour contrast, biocompatibility or electrical conductivity, in one process. This eliminates the need for secondary operations, such as assembly, painting or bonding, which reduces cost and time. Two-shot moulds can also improve part quality by avoiding mismatch or misalignment issues.
The main disadvantage of two-shot moulds is their high cost and complexity. They require special injection machines with two injection units and two barrels, which increase initial investment and floor space. They also require careful material selection and process optimization to ensure compatibility and adhesion between the two materials. Moreover, they are limited by part geometry and size, as some shapes or features may not be suitable for overmoulding.
Unscrewing Moulds
Unscrewing moulds are a type of injection moulds that allow for producing parts with internal or external threads. They consist of a core (the part that forms the internal threads) and a cavity (the part that forms the external threads). The core and the cavity are connected by a rack and pinion system or a hydraulic or electric motor. The core rotates during injection to create the threads, and then reverses to unscrew the part from the mould.
The main advantage of unscrewing moulds is their capability to produce complex threaded parts in one cycle. They can create parts with different types of threads, such as metric, imperial, tapered or customised, in one process. This eliminates the need for secondary operations, such as machining, tapping or threading, which reduces cost and time. Unscrewing moulds can also improve part quality by avoiding stress or deformation issues.
The main disadvantage of unscrewing moulds is their high cost and maintenance. They require additional components, such as gears, motors, sensors and wiring, which increase initial investment and maintenance costs. They also require precise alignment and synchronization to ensure smooth rotation and unscrewing. Moreover, they are limited by part geometry and size, as some shapes or features may interfere with the core rotation or unscrewing.
Family Moulds
Family moulds are a type of injection moulds that allow for producing parts with different geometries in one cycle. They consist of multiple cavities with different shapes and sizes in one mould. The cavities are fed by a common sprue and runner system.
The main advantage of family moulds is their material efficiency and productivity. They can produce multiple parts with different designs in one shot, reducing material waste and cycle time. This also reduces tooling cost and inventory space.
The main disadvantage of family moulds is their quality compromise and balance challenge. Since the cavities have different shapes and sizes, they may have different filling, cooling and shrinkage rates, resulting in dimensional variations or defects among the parts. Moreover, since the cavities share a common runner system, they may have different pressure drops or flow rates, resulting in uneven filling or packing among the parts.
Conclusion
Injection moulding is a versatile and efficient manufacturing process that can produce parts with various materials, shapes and sizes. However, it also requires careful consideration of the type of mould used, as it affects the performance and cost of the process. There are different types of injection moulds that have different characteristics and applications, such as cold runner moulds, hot runner moulds, two-shot moulds, unscrewing moulds and family moulds. Each type has its own advantages and disadvantages that should be weighed according to the specific requirements of the product.
