Rapid parts for the long haul

Stereolithography (SL) remains one of the most widely used ways of rapidly prototyping complex parts. SL can build prototypes with high accuracy and a wider range of physical and mechanical properties than selective laser sintering, laminated object manufacturing, 3D printing, and fused-deposition modeling.

Industry trends, however, are pushing the development of stronger, more durable SL resins good for more than just prototype formandfit evaluations. New SL grades are now going into parts for full functional testing, short production runs, and mass custom manufacturing, as well as for niche applications.

For rapid manufacturing, parts made from SL resins must be dimensionally accurate and have long-term durability. SL resin datasheets, however, highlight typical properties of recently built parts. Tests take place within a few days of part fabrication according to various standards. For most rapid-prototype applications, datasheets give enough information to gage how well the prototype will perform short-term. But for rapid production and mass custom-manufacturing purposes data-sheet information is often incomplete. It gives no indication as to how well the resin will perform over the product's expected life.

Recently, many commercial acrylate-epoxy hybrid or dual-curing photopolymers have begun being formulated with both free radical and cationic photoinitiators. During the SL build, these photopolymer resins form interpenetrating polymer networks (IPNs). The polymer networks and mechanical properties of the finished part evolve over time due predominantly to the dark reactions of epoxies. Furthermore, water absorbed from the environment often affects the acid-catalyzed cationic curing. This can cause dimensions and properties to change over time. As a result, the finished part may become fragile in weeks or months, making it unsuitable for production.

There was a recent 66-week investigation on how aging affects SL parts made from an acrylate-epoxy hybrid resin. The study helps shed light on how these cross-linked IPN systems stand up to thermal and mechanical stress over the long haul. It evaluated how the tensile properties of the ProtoTherm 12120 photopolymer changed under a variety of scenarios as well as the amount of water it absorbed at ambient conditions.

EFFECTS OF AGING ON TENSILE PROPERTIES
One group of UV-postcured ProtoTherm 12120 samples were held at ambient while another (UV plus thermally postcured) saw 100% RH for up to 66 weeks. The Young's modulus and tensile strength of UVpostcured parts rose slowly over time, making the aged parts very rigid and strong. Meanwhile, the elongation at break trended higher initially and then fell somewhat to about 2 to 3% on average after aging at ambient for 66 weeks. The results indicate that aged parts may give a high modulus and tensile strength, thus letting the SL materials serve as a replacement for some unfilled, clear production grade polymers. Next, the aged, UV-postcured samples were thermally cycled to 160°C for 2 hr one day before testing. Both the tensile modulus and strength of the samples fell into a relatively narrow range regardless of how they had been aged. Tensile properties of samples compared to a similar group that was subsequently aged at ambient showed that the 160°C postcuring nearly eradicated the effects of aging.

Furthermore, thermally postcured samples maintained essentially the same modulus and strength over time. Aging in ambient had little effect on tensile properties. The above results confirmed an earlier theory that the properties of fully cured acrylate-epoxy IPNs should not change during use. Thermal analysis revealed that 160°C heattreated parts saw no further thermally induced, exothermic polymerization. Such parts would remain dimensionally stable despite long-term aging. In contrast, thermal treatment at about 80°C (up to 24 hr) produced different results on SL parts made from another commercial resin. These parts did not completely cure. This likely would lead to further part brittleness with aging.

Aging for 66 weeks in 100% humidity at room temperature barely degraded the tensile properties of another group of UV and thermally postcured parts. The Young's modulus and tensile strength of the initially UV and thermallypostcured parts dropped slightly after 66 weeks in 100% RH. Elongation-at-break rose slightly. Water may act as a plasticizer that reduces the interactions of polymer chains and allows for greater extension.

WATER ABSORPTION
Researchers watched how the various postcured samples absorbed water at room temperature for up to one year. The water uptake of the ProtoTherm 12120 material was extremely low. The UV and thermally postcured samples had slightly lower water absorption than those UV postcured.

The bound water was likely responsible for the slight drop in tensile modulus and higher elongation for the aged, wet samples. These effects are due to how water plasticizes the IPN matrix. The absorbed water, surprisingly, did not embrittle the parts as they aged, indicating good resistance of the fully cured material to hydrolytic breakdown of the polymer networks.

All in all, the long-term results were good. These along with high build accuracy, fast photo speed, and insensitivity to moisture makes the Proto-Therm 12120 photopolymer a prime candidate for rapid prototyping and functional testing at elevated temperature and high humidity. And its outstanding resistance to aging without deterioration in mechanical strength makes it suitable for rapid production applications.