COMPRESSION MOLDING OF REAR PROJECTION TV SCREEN
PROJECT BACKGROUND
Large area plastic screens are manufactured with different intricate patterns and surface textures on both sides in order to achieve the optical properties required for rear projection television systems.
THE PROBLEM
A manufacturer of plastic screens was experiencing an 80+% product rejection rate due to the appearance of ‘bright spots’ on the screen when viewed in transmission. The screens were compression molded from acrylic resin between a micro-textured aluminum surface with a lenticular pattern and a relatively smooth nickel coated hardened steel surface. After repeated pressings, the plastic screen became permanently adhered to the aluminum tool surface initially causing optical defects on the plastic screen and ultimately resulting in the loss of the expensive tool.
Based on the information provided by the client, a testable hypothesis (‘plausible explanation’) was established to guide the problem solving approach:
- One or more of the components in the acrylic resin formulation caused localized areas of the resin to stick to the relatively reactive aluminum oxide tool surface
- The “bonded” resin filled in the micro-texture on the aluminum tool surface and caused a smooth area (an optical defect, a ‘bright spot’) to be replicated on the next screen which transmitted more light than the surrounding area
- When a sufficient number of sites were bonded, the entire part stuck to the tool
APPROACH TO RESOLVING THE PROBLEM
A combination of materials characterization techniques were employed on the mating aluminum tool/plastic screen surfaces and acrylic resins to test this hypothesis, understand the material basis of the problem and provide a solution.
THE RESULT
Optical microscopy (OM), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), Electron Spectroscopy for Chemical Analysis (ESCA) and Auger Electron Spectroscopy (AES) were used to compare ‘good’ and ‘defect’ areas on the ‘as received’ aluminum tool that had been used for a number of plastic screen pressings. The micro-etch pits of the textured surface in the defect areas showed the presence of acrylic resin coating the walls of the etch pit (see acrylic filament arching out over pit in SEM image) with a significantly higher sulfur content than the sulfur content detected on the good areas (AES spectra).
The source of the sulfur is thiol and disulfide compounds/functional groups present in the acrylic polymer as chain transfer agents to control the molecular weight. The tendency to produce defects varied with resins from different manufacturers. The ‘worst’ acrylic resin had 728 ppm total sulfur as determined by bulk x-ray fluorescence analysis while the ‘best’ resin had 572 ppm total sulfur. It was further determined by elemental analysis and gas chromatography-mass spectroscopy (GC-MS) that the ‘worst’ resin had a much higher fraction of its sulfur content in volatile (see comparative GC-MS traces below) and low molecular weight acrylic polymer fraction than did the ‘best’ resin.
Based on this information, a laboratory test was established to screen the ability of compounds thought to be present in the acrylic resin formulations to produce a ‘stain’ on small aluminum tool test coupons which would transfer to the acrylic part as a smooth area upon pressing with resin pellets at elevated temperature. Of the 30 materials selected for screening based upon what was known about the composition of the various resin formulations, only thiols produced a stain on the aluminum surface which caused optical defects in the mating acrylic surfaces (see following photograph).
A process was developed to protect the aluminum surface from chemical reactions with organosulfur compounds by bonding an ultra-thin coating to the micro-textured metal surface.
CONCLUSION AND BENEFIT TO THE CLIENT
The manufacturer was able to develop a commercially viable manufacturing process that produced TV screens in high yield and still use its metal working technology based on aluminum dies to achieve the optical properties required in the end use application.
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