Using rapid prototyping to create an ultrasonic corrosion monitoring sensor for 3M BIC business Ionix, a leading manufacturer of high temperature piezoelectric ceramics and ultrasound devices.
The 3M BIC design team was tasked by Ionix to build a rapid prototype enclosure for an ultrasonic corrosion monitoring sensor using Formlabs resin printers, which use the Stereolithography Apparatus (SLA) additive manufacturing process. SLA employs a laser to harden a UV-sensitive resin by photopolymerization, layer by layer. Fused Deposition Modeling (FDM) prototyping was inappropriate because of the fine tolerances, surface finish and detail needed. For example, to meet explosion proof standards a fine pitched threaded section across a wide diameter was required.
Four parts were manufactured for evaluation. The FormLabs software, Preform, allows the control of thickness for each layer, from 100 microns down to 25 microns, and in this case, made a big impact on the time taken to complete the build. For example, at 100 microns (0.1mm) the four parts would take approximately 2.5 hours to print, whereas setting the layer thickness to 25 microns (0.025mm) would take closer to 7 hours to print. Given creating the same components in steel using CNC manufacturing had a lead time of 2-3 weeks, it was decided that the accuracy and detail requirements would justify the longer time, and the layer thickness was set to 25 microns.
Several things were considered to ensure the part quality was suitable. Many types of 3D printing require the use of support structures to ensure the part is stable, supported and fixed securely to the build platform during the printing process. The bare minimum of support was used on external flat faces or convex faces and avoided on critical surfaces such as threads, small holes or very thin surfaces. ‘Post -processing’ was then used to remove the supports from the part, as well as any evidence that they were there.
A tolerance was set of 100 microns for the majority of the parts and reduced to 50 microns for the more critical components. This allowed them to fit together without post-machining and properly evaluate the effectiveness of the mating components. Another component required a thin front face set at 150 microns, more easily achievable in the eventual metallic construction, but for rapid prototyping required the increased resolution of the SLA process.
The build software showed where support was needed and auto generated the ‘best case’ structure. In this instance, we had to edit the support placements quite significantly taking into the account the delicateness of the parts. The fine threads were reproduced accurately and fitted well with its counterpart, satisfying the client’s requirement.
Being able to produce a prototype in such short turnaround was a major advantage to Ionix, whilst maintaining feature tolerances that would be required in the final part for rapid evaluation.
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