Friedrich-Alexander-Universität Erlangen-Nürnberg

Pressure Induced Solidification – A novel fabrication process for optical polymer parts

Pressure Induced Solidification – A novel fabrication process for optical polymer parts

From the general challenges of primary shaping processing technologies for thermoplastic polymers the following assumption can be derived: If temporal and local temperature and pressure gradients can be avoided during cooling and phase transition from liquid to solid state of the polymer, more homogenous parts with excellent size accuracy can be fabricated.

This can be achieved by a pressure induced phase transition which occurs simultaneously in the whole part. In conventional processing, e.g. injection molding, phase transition during cooling is temperature induced and time shifted. Especially for thick walled parts like optical lenses this time shifted solidification leads to unfavorable dimensional deviations and high residual stresses. After pressure induced solidification (PIS) at high temperatures cooling of the solid part leads to a more homogenous shrinkage and less residual stresses (Fig. 1).

Within the first project phase an improved understanding of the behavior of thermoplastics during compression and expansion was generated in test setups. A central finding was the verification that specific volume of a thermoplastic can be precisely defined by pressure, temperature and volume control during cooling of the solid part in the PIS process.

Figure 1: Photoelastic study of stresses in thick-walled (12 mm) polycarbonate parts; the amount of chromatic lines is a measure for stresses and orientations in the parts. left: Part produced with temperature and pressure gradients similar to conventional injection molding (max. principal stress difference approx. 16 N/mm2)
right: Part produced by pressure induced solidification (max. principal stress difference approx. 2.5 N/mm2)

A new type of injection molding process

The primary objective of the current second project phase is to transfer the basic knowledge of PIS into a novel variant of injection molding. The main task is the conception and construction of an appropriate machine and mold technology based on the basic findings of the first project phase. Special attention was paid to sensor integration, temperature management as well as thermal and pressure induced deformations of the innovative injection mold (Fig. 2). Processing trails using the PIS mold are conducted in order to increase understanding of the process and to compare the PIS process to conventional injection molding.

Fig. 2: Injection mold for the pressure induced solidification process, left: Ejector side with driven temperature controlled piston for pressure application
right: Nozzle side with form-fitting heating and cooling

Publications

  1. Ehrenstein. G, Drummer. D, Rudolph. N, Schmid. A, Druckverfestigung – ein neues Verfahren zur Herstellung hochpräziser Kunststoffoptiken In: Tagungsband 1. VDI-Fachkonferenz Kunststoffe in optischen Systemen, Erlangen, 2010
  2. Jungmeier. A, Drummer. D, Kühnert. I, Dickwandige Kunststoffoptiken -Druckverfestigung als neues Verfahren für präzise Bauteile In: Tagungsband 2. VDI-Fachkonferenz Kunststoffe in optischen Systemen, Erlangen, 2011

Mission

SAOT provides an interdisciplinary research and education program of excellence within a broad international network of distinguished experts to promote innovation and leadership in the areas

Optical Metrology
Optical Material Processing
Optics in Medicine
Optics in Communication and Information Technology
Optical Materials and Systems
and Computational Optics.