Summary
This article highlights the innovative development approach taken by Round Peg engineers to create a durable, injection-moulded face plate for a bollard suitable for outdoor use in a public environment.
The injection moulded design needed to meet the demanding IK10 impact protection rating, but the manufacturing technique meant the design had to be right first-time.
Through a combination of in-house prototyping, testing and simulation, the engineers successfully developed a robust design. This passed an externally run verification test on the first attempt, resulting in an efficient development timeframe.
The Challenge
Designing an injection-moulded part correctly the first time is important because of the high costs associated with making changes after the mould is created. In other words, a classic ‘measure twice, cut once’ scenario!
The challenge with this design was to ensure the face plate could support a radio antenna while still complying with an IK10 impact protection rating, which is a necessary standard for installations in public areas.
Our Strategy
To achieve rapid design iterations, engineers put together a simple in-house impact test. This involved dropping a steel ball onto the prototype, to replicate the test as specified in the IEC 62262 standard. The drop height was calculated to achieve equivalent impact energy, and multiple repeats were performed.
Figure 2: Performing a drop test.
Tests were performed on prototype faceplates, which were 3D printed in-house using fused-deposition modelling (FDM). The rationale for using 3D printed prototypes was that injection moulded components typically perform better due to having a homogenous structure which is stronger than the interlayer bonds formed during printing. If a 3D printed structure passed the test, so should an injection moulded piece.
Figure 1: A schematic of FDM 3D printing showing the weak bonds and voids between layers. (Taken from https://www.3dbeginners.com/what-is-fdm-3d-printing/)
The Development Process
An initial design was developed using classical strength calculations and injection moulding design knowledge.
The first prototype failed at the point of impact, so static FEA analyses were used to identify weak points in the design. This confirmed the weakness at the centre of the face plate, as observed in the initial drop test.
Figure 3: The failed prototype from the initial impact test
Figure 4: A FEA static analysis result of the initial concept. The dark blue regions show acceptable stress and the green regions excessive stress.
Strength is typically added to injection moulded components by changing the geometry to influence the load paths through the component. For example, adding support ribs to non-cosmetic faces avoids moulding defects like sink marks. This is also more cost-effective than increasing the part’s thickness, which would lengthen the moulding cycle time and raise production costs.
The subsequent redesigns and final design incorporated additional supports removing the reliance on bulk material thickness at the impact point. The simulation re-run showed much-reduced stress concentrations and increased part stiffness.
Figure 5: A FEA static analysis result of the final concept. The dark blue regions show acceptable stress and the green regions excessive stress.
The prototype of the final design successfully passed the in-house drop test. This meant the design was ready for manufacture, subject to final DFM (design for manufacture) checks. The material selected for the production version was a UV-stabilised Delrin, an “engineering” plastic known for its higher strength and stiffness properties. This helped achieve improved sealing performance.
Conclusion
The injection moulded production version passed the externally run verification test on it’s first attempt and was awarded an IK10 impact rating. This development serves as a compelling example of the role strategic planning plays in ensuring optimal results in engineering projects.
Figure 6: The final injection moulded production version installed on a post.