Today’s plastic manufacturing plants are highly automated, which allows companies like LEGO, Mattel, and Playmobil to produce millions of high-precision parts every day. Such an immense scale-up would not have been possible without the advent of CNC machining. It can even be utilized for the production of customized end-use parts.
Read on to learn more about the impact CNC has had on automation and how it holds up to other manufacturing processes.
The Early Days.
The first cast items stem from Mesopotamia over five millennia ago, but they were restricted to metals such as iron and copper. Skipping forward to 1856, the first cellulose-based thermoplastic was invented. It could be poured, molten, into a mold.
It took over 15 years for the first injection molding machine to arrive, yet the wait was worthwhile since it led to the automated production of items such as buttons, billiard balls, combs, and piano keys. True synthetics soon took over after the introduction of Bakelite in 1912, but products like eyeglass frames, toys, playing cards, cigarette filters, and diapers are still made of cellulose acetate.
Molds had to be machined or even carved by hand. As a result, no two parts were the same. Left and right car fenders were always asymmetrical, as any World War II-era assembly worker can recall.
The war marked the start of industrialization as we know it and the first experimental milling machine sprouted from U.S. Air Force research at MIT in 1949. They were able to computer-calculate airfoil coordinates for the semi-automated manufacturing of helicopter blades.
In those bygone days, tool paths had to be hardcoded into paper punch cards, which impeded precision and required extensive post-finishing.
An alternative for mold making was to use a tracing mill. It reverse-carved the target object out of a block of solid material by following the contours of a full-scale master model. Car manufacturers could now take plaster molds off of their 1:1 clay models to reproduce them in steel for mass production.
In the 1960s and 1970s, computers were the harbingers of Industry 3.0 as we know it. CNC machines achieved micron-precision based on 3D CAD solid models in molds for injection molding, thermoforming, rotational molding, or blow molding.
Moreover, the enhanced accuracy gave rise to the invention of complex features such as multi-cavity molds, master unit die molds (MUDs), multi-material over molding, thin-wall molding, insert molding, co-extrusion, collapsible cores, and side-action cams for undercuts.
Not only that, but most precision-engineered metal components across the automated assembly line also come from, you guessed it, a CNC mill. Think assembly jigs and fixtures for post-machining and testing. This provides the ecosystem needed for fully robotized automation. And to ensure tolerance limits aren’t exceeded, CNC machines can be outfitted with a laser scanner for quality control.
Data-driven machining with repeatable precision also meant that molds could be produced to high-quality levels with regard to appearance, durability, and systems for ejection, insertion, picking, cooling, heating, ventilation, hot runners, hydraulics, and pneumatics.
It empowered mold makers to invent standards such as DME, JIS, and BS EN 201. HASCO molds consist of standardized modules that can be produced offshore and shipped for production with a local HASCO-compatible supplier.
The effect on the industry has been nothing short of revolutionary.
In the 1990s, aluminum molds were introduced. They are much better machinable when compared to steel, improving cost and lead times while still lasting for hundreds of thousands of cycles.
Aluminum molds are repairable and can be made by smaller CNC work cells. They also cool quicker and more evenly, reducing cycle time and minimizing part distortion. While a surface coat improves quality, glossy surfaces and fiber-reinforced plastics remain challenging.
CNC Steps Up.
A third category of plastics manufacturing besides mold-based processes and extrusion is CNC itself. There are laser cutters, plasma cutters, water jet cutters, and 3D printers which are essentially CNC extruders.
As 3D printers grow objects in layers, the weakest link in their mechanical strength is interlayer adhesion and not the material itself. CNC machines cut directly into the target material, so the end product remains homogeneous.
To bypass initial startup investments, CNC-based production is viable up to 100-200 parts. When fully automated by robots for machine tending, loading, and unloading, as well as part finishing, it is the ideal technology to bring about mass-customized production lines.
Where 3D-printed polymers fall short in part strength and tolerancing, CNC-machined items for end users know their own drawbacks:
- The abrasive process renders soft polymers such as PS, LDPE, TPE, and silicones impossible. Using carbon tooling does enable composites next to common plastics such as polypropylene (PP), ABS, nylon, and acrylic (PMMA), as well as advanced plastics such as polycarbonate (PC), Delrin (POM), high-temperature PPS, PEEK, machinable wax for lost-wax casting, and Garolite PCB laminate.
- Multi-axis machines, all the way up to nine-axis CNC, enable complex spindle rotations, even multiple tool heads to work on the same part. But seemingly simple features such as deep pockets and flat-bottomed or non-perpendicular holes remain challenging.
- Tooling houses usually choose a finite toolset to improve throughput speed and reduce process complexity. It is common wisdom that manufacturing costs can be greatly reduced when the product design meets the capabilities of available cutters. For example, choose bevels over fillets for outer corners, and use radii or dogbone fillets instead of sharp internal corners.
- There is a limit to detail. All internal features will have a radius due to the CNC tool. The maximum detail for embossed text is 0.5mm, and it’s 0.6mm for hole diameters. Even though 1mm wall thicknesses are possible, 1.5mm is ideal to prevent warping.
- CNC machining of brittle polymers such as acrylic is an art on its own, and parts need to be carefully designed to prevent stress cracking and crazing (the formation of fine cracks on the surface).
But this doesn’t take away the fact that CNC is in the process of taking up a prime spot in the manufacturing arsenal of the future.
When artificial intelligence-enhanced CAD/CAM software can automatically convert 3D models created by the end-customer in a web-based product configurator into efficient and quality-optimized CNC tool paths, there is no stop to popping out custom on-demand manufactured items such as jewelry, artwork, medical orthoses, car parts, phone cases, or fashion accessories.
To give rise to an automated and flexible Industry 4.0, and uplift humans toward higher-skilled creative and managerial work, it’s essential that CNC becomes one of the core pillars in STEM education right next to robot programming, 3D printing, and CAD modeling.