Materials fit for micromolding

To choose the appropriate material, engineers should know how common resins perform in tiny molds.

Oct. 20, 2011

9 min read

Authored by:
Aaron Johnson
Accumold LLC
Ankeny, Iowa
Edited by Jessica Shapiro
[email protected]
Key points:
• Liquid-crystal polymers, polypropylene, high-density polyethylene, and acetal all completely filled the test part.
• Acrylic, glass-filled polybutylene terephthalate, and glass-filled nylon all filled about half the test part.
• Experienced molders can get more performance out of borderline resins by optimizing molds and processing parameters.
Resources:
Accumold LLC
“Precision Plastic Components: Micromolding is More Than Small Parts,” Machine Design Webinar, Oct. 8, 2009
“Precision Plastic Parts: Developing Components to Meet Project Requirements,” Machine Design Webinar, Dec. 14, 2010
“Micromolding Small Parts,” Machine Design video, Feb. 28, 2011
“How to Plastic Injection Mold Microscopic Parts,” Machine Design video, Dec. 15, 2008

Today’s engineers are often unaware of the chemical or biological properties of resins used in injection molding. But asking several questions can help with resin selection.
• What’s the part’s expected operating environment?
• Does it need to withstand solder reflow temperatures or other high-heat situations?
• Does it touch the human body or other biological materials?
• Is lubricity important?
• How about hygroscopic properties?
• How much should it cost?

After getting the answers to these and other questions, the next step is to review some commonly used resins. The basics on a few are described in the accompanying box, “Common molding materials.”

In addition to material properties, many other variables affect molded-part performance. For example, most thermoplastics come in a variety of grades that mold differently from each other. And additives such as glass, carbon, and other fibers change molded-resin properties and affect how the resin melts, flows, and fills a given geometry.

Resin data sheets generally show information on larger sample bars. Micromolded parts — typically those with features measured in microns and total part size 0.25 in. or less — may require different processing parameters, such as gate size, than manufacturers suggest.

That’s why it’s a good idea to consult an experienced micromolder when in doubt about a part or material. However, knowing what resins have the best chance for success can help when matching material properties to project requirements.

Flow studies
To compare the performance of common engineered resins in thin-wall applications, engineers at Accumold, Ankeny, Iowa, molded a range of resins in the same mold using standard processing parameters specified for each resin. The mold had a 0.003-in.-thick wall meant to test thin-wall molding capabilities and a thick-to-thin transition to help resin fill the mold.

To keep variables consistent across the tested resins, the same mold was used to form 100 parts of each resin without any modifications. In real-world applications, molds are often adjusted to minimize shrinkage or match other resin characteristics. For this study, the goal was to maintain a 0.003-in.-thick wall for the longest fill length possible, not to get the longest overall fill.

Technicians measured and averaged the dimensions of the 100 parts molded for each resin.

The tested resins were acetal, acrylic, high-density polyethylene (HDPE), liquid-crystal polymer, nylon, polybutylene terephthalate (PBT), polycarbonate (PC), polyetheretherketone (PEEK), polyetherimide (PEI), polypropylene (PP), and polysulfone (PSU). The results are summarized in the accompanying table, “Micromolding results.”

The tested polymers that completely filled the mold’s 42:1 aspect ratio were LCP, HDPE, PP, and acetal. In all four cases, shrinkage after cooling brought the average final length below the 0.1267-in. mold length, but molds optimized for specific resins would have prevented shrinkage.

The LCP tested was reinforced with 30% glass fiber and colored black. The HDPE was white and designed for high melt flow. Engineers described the final product as “very flexible,” a characteristic which could be a benefit or a drawback, depending on the application. The acetal resin was designed for multicavity and thin-wall molding.

Acrylic, PBT, and nylon resins filled about half the mold. Engineers felt that better molds, molding parameters, or a narrower thin-wall section could have let the resins push further but doubted they could have filled the entire 0.1267-in. mold.

The average aspect ratio for acrylic parts was 26:1, with the best part achieving a 32:1 aspect ratio and all PMMA parts warping. The PBT resin contained 30% glass reinforcement and averaged a 25:1 aspect ratio. The center of the PBT flow tended to push further than the edges, and these parts also warped. The 50%-glass-fiber-filled black nylon 6/6 reached an 18:1 aspect ratio. However, the final fill line was extremely uneven.

PSU and PC resins both averaged aspect ratios of 14:1 with average fill lengths of 0.0418 in. and 0.0411 in., respectively. PSU is not noted for its long-aspect, thin-molding capabilities, and despite its bulk rigidity and toughness, the parts were flexible to finger pressure at the tested thickness.

Transparent PEI and a PEEK grade reinforced with 30% glass fibers brought up the rear with 10:1 and 3:1 average aspect ratios, respectively. The PEI was touted as “enhanced flow” by its manufacturer and did have an even fill edge despite traveling only 0.0294 in. The PEEK supplier classified its product as “easy flow,” but it only traveled 0.009 in., and the flows’ edges traveled further than the centers.

Common molding materials
Low-density polyethylene (LDPE)
Developed in 1898 by Hans von Pechmann and commercialized in the 1930s, LDPE is considered the most widely used plastic. About 80 million metric tons are used globally every year. LDPE film is the basis of plastic bags or coatings for paper items such as milk cartons. Other common uses include blow-molded containers and injection-molded consumer products like mop buckets and kitchen containers.

High-density polyethylene (HDPE)
Paul Hogan and Robert L. Banks at Phillips Petroleum discovered HDPE, which hit the market in 1953. It is used for injection molding bottles, shipping containers, and other distribution and storage devices. It can also be processed as a film or via extrusion.

Polypropylene (PP)
PP was developed in the mid-1950s by Hogan and Banks. It is durable and inexpensive with high tensile and compressive strengths. It is normally tough and flexible, especially when copolymerized with ethylene. The PP-ethylene copolymer competes with other engineering plastics like acrylonitrile butadiene styrene. It resists many solvents, chemicals, and acids, and is widely used in the medical industry.

Nylon or polyamide
This thermoplastic synthesized from ethylenediamine was introduced by DuPont, Wilmington, Del., in 1938 as a fiber and in 1941 as an injection-molding resin. Nylon 6/6 is the most widely used in the U. S.; nylon 6 and nylon 12 are also common. Nylons resist weather and friction wear, especially when blended with reinforcing glass fibers, but tend to be hygroscopic. Common applications include gears, bearings, and mountings.

Polycarbonate (PC)
PC was first sold commercially in 1958 after Bayer and GE scientists independently developed similar processes. The amorphous polymer has outstanding impact resistance and works in applications up to 125°C. It often replaces glass due to its toughness and clarity, but it also comes in opaque and translucent grades.

Acetal or polyoxymethylene (POM)
Acetal, more commonly referred to by its trade name, Delrin, was developed in the 1920s and commercialized in the 1950s. It is chemically resistant and hydrophobic, but its heat resistance and strength are lower than other engineering polymers. Its inertness and wear resistance make it well suited for bearing, wheel, caster, and food-industry applications.

Polysulfone (PSU)
PSU was introduced to the market in 1965 by Union Carbide. It withstands temperatures up to 345°F, among the highest temperature limits of all thermoplastics. It is transparent, absorbs little moisture, and resists chemicals and solvents. Despite its higher cost, it is often used in applications where flame retardancy and resterilization are required.

Polybutylene terephthalate (PBT)
This semicrystalline resin was first marketed in 1970 by the company now known as Ticona. It has good mechanical strength, is electrically insulating, resists heat and chemicals, absorbs little moisture, and has minimal shrinkage during processing. It is commonly sold in a flame-retardant grade. PBT is used in automotive, industrial, consumer, and medical applications.

Acrylic or polymethyl methacrylate (PMMA)
Acrylic was developed in the 1930s as a coating and commercialized in 1937 as a moldable resin. Its durability and transparency make it a good candidate for long-life applications, especially those where transparency is required. The polymer is slow-burning or self-extinguishing and, when it does burn, it doesn’t generate toxic by-products.

Polyetheretherketone (PEEK)
Imperial Chemical Industries (ICI), patented PEEK in 1978 under the trade name Victrex. It resists chemicals and thermal breakdown and is mechanically stable. Its inertness makes it a natural material for medical devices and implants, but it’s also used in aerospace, automotive, and chemistry applications.

Polyetherimide (PEI)
Better known by its trade name, Ultem, amorphous thermoplastic PEI was introduced in 1982 by a business unit of General Electric that is now SABIC. It has less heat resistance and strength than more-expensive PEEK, but still withstands continuous use at up to 340°F and has a low coefficient of thermal expansion. It comes in transparent, opaque, and glass-filled grades as well as in copolymers. It’s used in medical, automotive, electronics, fiber-optic, and aerospace applications.

Liquid-crystal polymer (LCP)
LCPs were invented in 1888 but not commercialized in moldable grades until the 1980s. The polymers form highly ordered regions that give them strength and the ability to withstand high temperatures, chemicals, radiation, and weathering. They are good candidates for thin-wall moldings.