3M has developed a technology that uses ultrasonic energy applied to a plastic-filled mold to relieve flow-induced stresses in a still-forming precision optical part.
The technology addresses the growing use of molded plastics in precision optical
applications as manufacturers move away from heavier and more expensive glass components. Another issue is that complexity of the parts has grown substantially.
Previously used practices to mold newly designed precision optical parts are proving insufficient, according to 3M. These methods include coining, injection compression, variothermal response molding, and slow servo-driven injection for simple parts such as screens, diffraction gratings, and some lenses or mirrors.
New parts feature significant compound curvature with thick and thin wall sections. Dimensional tolerances have also tightened.
Here’s how the 3M systems works: 1) The mold is heated to a temperature of 104-116ºC; 2) Plastic is injected into the mold cavity; 3) When plastic at the gate has solidified, ultrasonic energy is applied to the mold without remelting the solidified plastic in the gate until the temperature increases to 116-122ºC; and 4) Then the mold is cooled to a temperature of 101-107ºC and subsequently reheated to 116-122ºC. The mold is then cooled and the part is ejected.
The source of ultrasonic energy is an ultrasonic transducer, optionally including at least one of a resonant horn, a booster, and a waveguide.
The types of parts that 3M is targeting include lenses, prisms, mirrors, light pipes, beam splitters, polarizers and diffraction gratings used in several industrial and consumer products. One key application is electronic display devices, such as liquid crystal displays and projectors, televisions, computer monitors, e-readers, cellular phones, and MP3 players.
Specific goals of the 3M ultrasonic-assisted molding process include substantial removal of birefringence, residual stresses, sink marks, knit marks, weld lines, and voids in acrylic plastics.
There are interesting discussions in the 3M patent describing the causes of optical problems in parts and the role of ultrasonics in easing the stresses and reducing the cooling time. One example: “Because ultrasonic energy helps relieve the thermal shock associated in going from a heated barrel into a cold mold cavity, it is, in some exemplary embodiments, possible to reduce the cooling time requirement needed for before ejection of a substantially defect-free precision molded article from the mold.”
The lead inventor is Stan Rendon, who is the Global Technical Service and R&D Operations Manager at 3M. Three years ago he was listed as one of the inventors of a process to ultrasonic injection mold on both sides of a carrier web.
Optical Systems, which includes film and molded products, is a major business group at 3M, which is based in St. Paul, Minnesota. 3M Optical Systems Manufacturing is based in Shanghai.