Common mold processes include NC milling, grinding, turning, wire cutting, EDM, heat treatment, surface treatment, welding repair, etc. This article will briefly introduce these terms for entry-level students.
NC milling
Metal components are primarily shaped using NC milling, providing not only the desired external form but also versatile functionalities such as milling teeth and drilling holes, making it the most universal machine.
Typical NC milling machines operate at varying speeds, ranging from 100 rpm to 6000 rpm, with precision typically around 0.05mm-0.1mm. For conventional molds without stringent precision or intricate aesthetics, a standard milling machine suffices unless the shape is complex. Some milling machines can achieve speeds exceeding 20,000 rpm, referred to as "high-speed machines." Optical facilities utilize high-speed machines reaching up to 60,000 rpm for milling high-precision and mirror-finish exteriors, achieving precision levels of 0.005mm-0.01mm and surface finishes below Ra 0.1um.
Grinding
Grinding includes flat surface grinding, cylindrical grinding, and internal bore grinding.
Surface grinding is a crucial process for metal components, essentially indispensable. Achieving precision in subsequent machining relies on first grinding reference surfaces. For instance, when you obtain a raw iron block, the initial step typically involves lightly grinding all six sides to ensure flatness. One side may be ground to serve as the machining reference. However, given the low cutting capacity of grinding machines, oversized materials require preliminary machining to reduce dimensions before grinding.
Cylindrical grinding pertains to grinding the outer diameter of a cylinder, while internal bore grinding, as the name suggests, involves achieving roundness in circular holes.
Grinding machines typically achieve the highest precision among all processes, with fine grinding precision reaching approximately 1 micron. Moreover, achieving a polished finish is easily attainable. This is why using a grinder for establishing reference surfaces is commonly recommended.
Turning
Turning is the process used for crafting cylindrical components and can also be employed for threading. In traditional mold factories, it is typically utilized for rough machining bars or primarily for threading. The primary shaping is generally performed using milling machines. However, in optical facilities, lathes hold significant importance. The Ultra-Precision Lathe (ULC), a type of lathe, is crucial for manufacturing high-precision optical lenses, boasting precision levels of 0.1 microns.
Wire Cutting
Wire EDM encompasses numerous common terms, including slow wire, fast wire, oil-cut machines, water-cut machines, and multiple cutting and finishing operations.
Machines are generally categorized as fast wire and slow wire, with the distinction being speed. The principles and working mediums differ. Fast wire machines are typically used for rough cutting and severing, with an accuracy of around 0.1mm, though they may encounter iron dissolution issues. Slow wire machines further classify into oil-cut and water-cut. Oil-cut machines offer higher precision, often advertised with a processing accuracy of 1 micron, but a more practical value is commonly taken as 2-3 microns.
I won't delve into the principles here; abundant information is readily available online.
The term "cut one, finish a few" commonly used by experts refers to the practice of making several refining cuts after the initial rough cut. The purpose of making more finishing cuts is to achieve a finer surface. Therefore, in the case of mechanisms in mold making, multiple finishing cuts are usually performed to ensure smooth movement surfaces.
Typically, for general injection pin hole cutting, one rough cut is followed by two to three finishing cuts. In the case of high-precision pinholes, a ratio of one rough cut to six finishing cuts is achievable. For certain highly precise moving inserts, refining up to eight cuts is acceptable.
EDM
Electrical discharge machining (EDM) is a commonly used molding method in the field of molds. As molds often require the creation of sharp corners, right angles, and other intricate features that cannot be milled, EDM becomes essential for achieving these details. EDM is also capable of shaping complex forms, contingent on the engineer's proficiency in electrode design.
The general precision of electrical discharge machining (EDM) can reach about 0.005mm-0.01mm, and the surface roughness can reach Ra 0.1um. It has the same capability as high-precision NC machining tools available in the market. However, compared to NC milling, it usually takes much longer to manufacture.
Heat Treatment
Heat treatment is a process used to enhance the mechanical properties, such as hardness, of iron materials. It is inevitable for the production of high-precision or long-lasting molds.
The typical approach involves rough machining and deep hole drilling on the raw material before sending it for heat treatment. After heat treatment, the mold is then subjected to middle to fine machining. Otherwise, working on a mold with excessively high hardness post heat treatment becomes challenging.
Mold factories often employ what's known as pre-hardened steel. This type of steel typically undergoes tempering after quenching, resulting in moderate hardness. While it doesn't achieve exceptionally high hardness, it possesses a certain level of strength and toughness, making it easy to machine. This makes pre-hardened steel an excellent choice for molds that do not have exceptionally stringent requirements.
Texturing
Surface treatment comes in various forms, commonly referred to as "texture." Texturing methods include sandblasting, chemical treatments, laser etching, embossing, and more. The need for surface treatment usually arises when clients specify particular patterns or require a certain level of roughness on the surface.
Apart from client specifications, the use of texturing in mold design is a skill. If a product, upon completion of injection molding and mold opening, tends to be taken away by the cavity side, preventing it from staying properly on the core side, a common technique is to apply some sandblasting on the core's surface. This increases the roughness of the core surface, allowing the molded product to adhere better.
Welding Repair
Welding repair, as a remedial process, involves filling or adding material. If iron material is damaged or experiences processing issues, welding can be applied to add material and facilitate reworking. Welding repair is categorized into arc welding and gas welding; gas welding includes CO2 welding, while arc welding involves laser welding. For precise and localized repairs, laser welding is preferred due to its accuracy.
While welding repair may seem convenient, it comes with numerous drawbacks.
Firstly, welding involves high-temperature processing, leading to overall deformation of the iron component. Extensive welding repairs require subsequent meticulous shaping to control dimensions accurately.
Secondly, welding repairs result in welding marks, which can transfer onto the plastic end product. Therefore, products with stringent aesthetic requirements are not suitable for welding repairs.
Thirdly, welding repair involves attaching another piece of iron to the original component, leading to differences in structural density and compromising mechanical strength. Therefore, its usage needs careful consideration.
The above comprises an introduction to the commonly employed processes in mold manufacturing.
