Purpose

During post-processing of stereolithography photopolymers, the limited penetration depth of ultraviolet (UV) light can lead to inhomogeneous cross-linking. This is a major problem in part design for industrial applications as this creates uncertainty regarding the mechanical load capacity. Therefore, this paper aims to present an experimental method to measure the post-curing depth in stereolithography photopolymers.

Design/methodology/approach

Printed specimens made from urethane acrylate photopolymers are placed in a protective housing and are exposed on one side to UV light during post-processing. A depth profile of the hardness according to ASTM D2240 Shore D is determined alongside the specimens. UVA,-B and -C spectra are investigated and the dependence on exposure dose and pigmentation is studied. The results are directly linked to the mechanical properties via tensile tests and validated on an automotive trim part.

Findings

Exposure with a 405 nm light-emitting diode provides the deepest homogenous post-curing depth of 10.5 mm, which depends on the overall exposure dose and pigmentation. If the initially transparent photopolymer is colored with black pigments, post-curing depth is significantly reduced and no homogenous post-curing can be achieved. To obtain comparable mechanical properties by tensile tests, complete cross-linking of the specimen cross-section has to be ensured.

Research limitations/implications

The spatial resolution of the presented measurement method depends on the indenter size and sample hardness. As a result, the resolution of the used setup is limited in the area close to the edges of the specimen.

Originality/value

This paper shows that the spatially resolved hardness measurement provides more information on the post-curing influence than the evaluation of global mechanical properties. The presented method can be used to ensure homogenous cross-linking of stereolithography parts.

中文翻译：

---

立体光刻部件均匀交联的 UV 后固化深度评估

目的

在立体光刻光聚合物的后处理过程中，紫外线 (UV) 光的有限穿透深度会导致不均匀的交联。这是工业应用零件设计中的一个主要问题，因为这会造成机械负载能力的不确定性。因此，本文旨在提出一种测量立体光刻光聚合物后固化深度的实验方法。

设计/方法/方法

由聚氨酯丙烯酸酯光聚合物制成的印刷样品放置在保护外壳中，并在后处理过程中将一侧暴露在紫外线下。根据 ASTM D2240 Shore D 硬度深度分布与试样一起确定。研究了 UVA、-B 和 -C 光谱，并研究了对曝光剂量和色素沉着的依赖性。结果通过拉伸测试与机械性能直接相关，并在汽车装饰部件上进行了验证。

发现

使用 405 nm 发光二极管进行曝光可提供 10.5 mm 的最深均匀后固化深度，这取决于整体曝光剂量和色素沉着。如果最初透明的光聚合物用黑色颜料着色，则后固化深度会显着降低，并且无法实现均匀的后固化。为了通过拉伸试验获得可比较的机械性能，必须确保试样横截面的完全交联。

研究限制/影响

所提出的测量方法的空间分辨率取决于压头尺寸和样品硬度。因此，所用设置的分辨率仅限于靠近试样边缘的区域。

原创性/价值

本文表明，与整体机械性能的评估相比，空间分辨硬度测量提供了更多关于固化后影响的信息。所提出的方法可用于确保立体光刻零件的均匀交联。