Three-dimensional (3D) printing with high-resolution stereolithography (SLA) has grown in popularity for creating personalized medical devices, e.g. Invisalign braces, bone scaffolding. 3D printing is now starting to expand to weight-bearing components, e.g. prosthetic feet, as data on the dynamic properties impact and fatigue is published in the literature. The next step towards using 3D printing in impact applications is to assess the capability of the high-resolution SLA process to manufacture components of uniform impact resistance. A uniform process results in lower defect rates and part rejection. The first hypothesis of this study is the impact strength of 3D printed parts from a SLA printer is uniform, which is defined as a manufacturing process capability index (C~pk~) greater than 1. Because impact testing is destructive, a surrogate measure to check a part’s viability for resisting an impact load also needs to be established. The second hypothesis of this study is the impact strength and hardness of a SLA part are positively correlated.

 

Thirteen notched Izod specimens were printed on a Form2 SLA printer using the manufacturer’s clear V4, photocurable resin. Once all the specimens were printed, washed in isopropyl alcohol, and cured with ultraviolet light, the impact resistance was quantified using a pendulum impact tester in a notched Izod configuration. Then, the hardness of the specimens was quantified using a HBW 10/250 scale. This Brinell scale consists of applying 250 kgf to the specimen with a 10 mm ball.

 

The impact data was processed by computing the mean and standard deviation. The manufacturing process capability was then computed using the traditional equation C~pk~ = (upper standard limit – impact data mean)/(3\*standard deviation of impact data). The upper standard limit (USL) was set as 17.5% of the manufacturer’s specification for what the impact resistance should be for this polymer. This 17.5% was chosen as current prosthetic materials have an impact resistance with a standard deviation of 1% - 34%. The correlation between impact resistance and hardness was quantified using Spearman’s correlation coefficient since the data was not normally distributed.

 

The impact resistance of the clear, SLA polymer was 0.59 ± 0.14 ft-lb/in. With an upper standard limit of 0.53 ft-lb/in, the process capability index was 0.133. Impact resistance and Brinell hardness were not correlated with a Spearman coefficient of r = -0.108, p = 0.73.

 

Since the process capability index was less than one, 3D printing with SLA polymers is not a viable manufacturing process for creating parts of consistent impact resistance. The current technology would lead to too many rejected parts. Also, Brinell hardness and impact strength were not related. There is no non-destructive method to spot-check these components before use. Therefore, future work is needed to reduce the variation in 3D printing with SLA and establishing surrogate measures to checking components.


Manufacturing Capability Index of 3D Printing Parts for Impact Applications

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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