Material Selection
Micro molding often involves exotic or highly engineered compounds called thermoplastic resins. Accumold is accustomed to working with micro mold material in many diverse forms such as PEEK, PEI (Ultem®), carbon-filled LCP, or glass-filled nylon. Soft durometer or elastomeric resins also are common.
Selecting the correct micro mold material, resin and thermoplastic is absolutely critical. Micro molding is part science and part art form. True micro molding DFM engineers are constantly navigating the complicated labyrinth that connects OEM demands, with available micro molding resins to accomplish the goals of the project.
Choosing Micro Mold Material
Choosing the correct micro mold material or plastics seems simple at the project start. You’d think a project leader simply picks the desired material that fits their project goal and injects liquid plastic into a mold. The assumption that a process for larger molds applies to micro molding, however, doesn’t play out in real-life scenarios. Thermoplastics and resins behave in unusual ways when micron or half-micron tolerances are required.
While some projects require micro mold material that are rigid and strong, others require resins that can maintain an ultra-thin profile. Some resins can withstand heat, while others must follow strict guidelines due to extended contact with the human body. Obviously, there are many cases where the desired material simply can’t be used due to some other design restrictions.
PEEK for example, a popular choice in medical micro molding, is 30% glass fiber reinforced and pelletized for injection molding, yet it cannot fill ultra-thin areas. Ultem, also an extremely popular option for micro-optics applications, also can’t fill ultra-thin micro molding projects. Consequently, to achieve OEM’s demanding project goals, balanced with various material limitations at micron sizes, our DFM engineers face a Rubix cube of micro mold materials selection.
Our extensive experience working with various thermoplastics makes us experts on exactly how to match up the proper material to the appropriate part geometry, robustness and performance.
When pushing the limits of molded plastic parts the material selection process often goes beyond the datasheet. Knowing what materials will work in any given situation is an invaluable part of our 30-year history molding thermoplastics in micro spaces.
Complications With Micro Molding Resins
It’s of vital importance to understand the full set of variables that can affect the performance of a molded part. Most available thermoplastics have various grades or versions produced to achieve unique results. Additives that include, but are not limited to glass, carbon, tungsten and other fibers are routinely added to resins. These additives dynamically change how micro mold material melt, flow and fill in part geometry.
Examples of Micro Molding Materials
There is no one micro mold material that’s best for any particular project. Like paint on a canvas, selecting the correct material, plastic, or resin heavily depends on the project scope and goals. In some cases your objective could be rigidity, in other flexibility. In some cases you may need a heat-resistant material or one that stands up to the cold. With tens of thousands of options, here are a few more popular choices.
Polyethylene (PE)
Polyethylene, first accidentally developed in 1898 by Hans von Pechmann[5], is considered the most “widely used” of the plastic resins available. It is estimated that 80 million metric tons of material are used annually.[6] It’s most common use is for packaging, specifically plastic bags.
Polypropylene (PP)
Polypropylene was first developed in the mid-1950s by scientists Paul Hogan and Robert L. Banks (the same two from Phillips Petroleum that discovered the HDPE). They were working with ethylene and propylene separately for a project and “accidentally” combined the two materials forming both crystalline polypropylene and linear polyethylene.
Nylon (Polyamide)
Nylon, or polyamide, is a highly engineered thermoplastic synthesized from ethylenediamine.[12] The material was first introduced in 1938 by DuPont as a fiber and then as an injection moldable grade in 1941.
Polycarbonate (PC)
Polycarbonate was first sold commercially in 1958 when both Bayer and GE scientists from across the globe independently “discovered” similar processes for producing the material earlier that decade. GE’s material, Lexan, and Bayer’s Makrolon are still very common brands of PC today.
Delrin (Acetal / Polyoxymethylene / POM)
Polyoxymethylene, or Acetal, or its more common trade name, Delrin, was first developed in the 1920s but because it was not considered thermally stable it wasn’t initially commercialized. It wasn’t until 1952 when DuPont scientist stabilized the process, a patent was filed in 1956, and commercial production began in 1960.
Polysulfone (PSU)
Polysulfone, known for its “toughness and stability at high temps,” was first introduced to the market in 1965 by Union Carbide. It is considered the “highest service temperature of all melt-processable thermoplastics.” It’s also used as a high-end replacement to Polycarbonate for specialty applications.
Polybutylene terephthalate (PBT)
Polybutylene Terephthalate was first marketed in 1970 by the company now known as Ticona.[24] PBT is a semi-crystalline resin with “excellent” mechanical and electrical properties. It is also considered highly resistant to chemicals.[25] The material tends to shrink very little during forming and is mechanically very strong.
Acrylic (Polymethylmethacrylate / PMMA)
Acrylic, developed in the 1930’s as a coating, commercialized in 1937 as a moldable resin. Although it comes in many other variations it is probably best known by the name of Plexiglas and has played an important role in safety and glass replacements.[27] Its extremely high durability and transparency make it a perfect candidate for long-life applications
PEEK (Polyether ether ketone)
British chemical company, Imperial Chemical Industries (ICI), first patented the PEEK formulation in 1978 under the trade name Vitrex.[30] The PEEK polymerization is considered an “organic polymer thermoplastic” and is resistant to thermal breakdown and is mechanically and chemically very stable.
Ultem (Polyetherimide / PEI)
Polyetherimide, otherwise known by its trade name Ultem, was introduced by General Electric (now SABIC) in 1982.[33] PEI is considered to be a relative to PEEK. It’s typically cheaper than PEEK, it’s also clear (or amber) but is less resistant to heat and strength when compared to its cousin. It can withstand continuous use temperatures of 340°F (170°C).
LCP (Liquid Crystal Polymer)
LCP, or Liquid Crystal Polymer, as a moldable resin is fairly new even though the components and research that ultimately lead to the resin we know today happened as early as 1888.[38] It wasn’t till 1980, almost 100 years later, that the injection moldable version was available to the market.
