The Deterioration Mechanism of Silicone Molds in Polyurethane Vacuum Casting

Abstract

The industrial production of prototypes made of polyurethane via silicone molds in the vacuum casting process is one of the most widespread applications of rapid tooling. The silicone molds show progressive deterioration, as the isocyanate component of the polyurethane resin diffuses into the mold cavity surface during the casting process, thus limiting their durability. Here, we present the first comprehensive description of the underlying chemical and physical mechanisms on a molecular level. It is shown that the isocyanate polymerizes inside the polydimethylsiloxane matrix with moisture to polyurea. Polyurea clusters, which emerge from the resulting interpenetrating polymer network with continuing isocyanate exposure, promote fissure formation under mechanical demolding stresses. The mechanism was investigated with a wide variety of characterization methods, and qualitative variations were demonstrated using different commercial materials. Influencing factors such as mold geometry, process flow, and different aspects of the material composition were examined experimentally. A thorough understanding of the deterioration mechanism paves the way for the development of durable molds and an economical midseries technology in plastics processing.

Industrial‐scale vacuum casting with silicone molds: A review

“…For the production of any 3D, non-axisymmetric plastic product-that is, a large fraction of the global plastics production volume-there is no versatile process for cost-effective midseries production between additive manufacturing and injection molding that can meet the market demand. There is no doubt that VC with durable silicone molds would be a promising candidate to overcome this economic shortfall [12]. Some studies emphasized the importance of the mold service life for the BEP by comparing the economic feasibility of VC to other production methods [2,70].…”

“…However, as we will see, the service life depends on so many parameters that it is hardly useful for a scientific evaluation of the mold deterioration. Although the mold deterioration occurs with all common resins [81,111], detailed studies on the underlying mechanisms have so far only been available for PUR resins [12,[112][113][114][115][116].…”

“…As shown in (f) photos of a mold cavity before and after 19 casting cycles with PUR resin. The geometry was used to investigate the effect of specific surface area on the mold deterioration, as seen in (i) [12]; (g) polar and disperse fraction of the surface energy of silicone as a function of exposure time to MDI-based isocyanate with photos of the water contact angle after 0 and 8 h [12]; (h) X-ray photoelectron spectroscopy measurement of a silicone surface after 24 h of exposure to MDI-based isocyanate [12]; (i) number of casting cycles at the first occurrence of visible surface damage as a function of the specific surface area of the mold features shown in (f) ((f-i) adapted with permission from Reference [12], copyright 2020 American Chemical Society)…”

“…As illustrated in Figure 5a, the absorbed diisocyanate then reacts with residual moisture to polyurea (Figure 5b) within the silicone medium. Molecular phase separation of polyurea and PDMS leads to the formation of polyurea clusters within a sequential semi-interpenetrating polymer network [12,114]. The resulting spatial distribution of MDI and polyurea was investigated both on micrometer and millimeter scale [115].…”

“…The emergence of polyurea clusters explains in many respects the manifestations of the mold deterioration. The topographic elevations on the silicone surface caused by the clusters, as visualized by scanning electron microscopy [113], helium ion microscopy (Figure 5i), fluorescence microscopy, and atomic force microscopy (Figure 5h) [12], lead to an increase in surface roughness (Figure 4d) [111]. Charge-compensated helium ion microscopy has been shown to be advantageous over scanning electron microscopy because the topography can be imaged without the veil of a conductive coating [12,117,118].…”