In Part 1, see here, we outlined that despite emerging technologies like additive Manufacturing with greater design freedoms to ensure good parts, the ongoing prevalence in industry of Plastic injection moulding requires the designer to be aware of and follow best practice design guidelines.
For moulded parts one key area of part design is ensuring uniform wall sections across the geometry of the part. Read below for impacts 6 to 10 below for the next chapter of this 2 part blog.
6. Increased Cycle Time and Costs
Parts with non-uniform wall sections typically require longer cooling times, as the thicker sections need more time to solidify. The part cycle time is driven by the thickest portions of the part, regardless if the majority of the part is thinner and uniform, in order to control part quality in the thick portion.
Extended cooling periods increase the overall cycle time. The moulder must hold the part in the controlled environment of the tool for longer periods to ensure that warping is controlled in situ and the part is stable (solidified) enough to be ejected. Effectively reducing the production efficiency of the injection moulding process.
A general rule is double the wall section, quadruple the cycle time.
Cycle time is the major contributor of total part cost. It is a fairly simple formulae - the hourly machine rate divided by how many shots/cycles can be produced in an hour (possibly increased by cavitation). An increase in cycle time reduces the hourly output and directly increases the part cost.
7. Stress Concentrations and Mechanical Weaknesses
Non-uniform wall thickness can create areas of stress concentration within the part. These areas are points where the stress is higher than in the surrounding material, often occurring at transitions between thick and thin sections. Stress concentrations can significantly reduce the mechanical strength of the part, making it more prone to cracking or breaking under load. In applications where the part is subjected to repeated or dynamic loading, such weaknesses can lead to premature failure and potentially hazardous situations.
8. Inconsistent Surface Finish
The surface finish of plastic injection moulded parts is important for both aesthetic and functional reasons. Non-uniform wall sections can result in inconsistent surface finishes due to varying cooling rates and material flow characteristics. Thicker sections may exhibit different textures or gloss levels compared to thinner sections, leading to a part that appears uneven or poorly finished. Achieving a consistent surface finish is particularly crucial for parts used in consumer products, where appearance can significantly impact perceived quality of the part or product to the end purchases and/or user.
9. Compromised Aesthetic Quality
In addition to functional drawbacks, non-uniform wall sections can compromise the aesthetic quality of the final product. Sink marks, warpage, inconsistent surface finishes and part line mismatch all detract from the visual appeal of the part. For consumer-facing products, where appearance is a key factor in marketability, these defects can negatively impact customer perception and satisfaction. A well-designed part with uniform wall thickness not only performs better but also looks more appealing, enhancing its overall value.
10. Increased Risk of Rejection and Waste
Finally, non-uniform wall sections increase the risk of part rejection and material waste. Parts that exhibit defects such as warpage, sink marks, or voids may not meet quality standards and must be discarded. This not only leads to material waste but also incurs additional costs in terms of reworking or remanufacturing the parts. By designing for uniform wall thickness, manufacturers can reduce the incidence of defects, improving yield rates and overall production efficiency.
In Summary; the 10 Impacts of Non-Uniform Wall Sections in Plastic Injection Moulded Parts are detrimental to the fit, form and function of a successful part design and part moulding.
This is most evident in medical grade moulding where high tolerances, sometimes down to +-0.05mm, are critical for part fitment and function and part quality is critical.
For moulding facilities in the medical sector, to achieve finer tolerances, greater investment in all areas of the business is required to achieve part quality. This is an essential ticket to play in the chosen sector.
Medical grade moulding facilities require a higher grade of moulding machines driving more expensive injection tooling. These are managed and monitored by more comprehensive Quality Control (QC) departments with ISO-13485 and Process Capability (CPK) controls and measures using more sophisticated measurement devices to monitor the moulding process of more expensive Engineering Grades of polymer is standard practice. This is placed even further into the spotlight with exotic polymer grades costing from $50 to $200 to $2000 per kg.
These are all overheads that are recovered through the cost of the part for a moulding business to be profitable.
All this can be undone with part design that is not optimised for the process of plastic injection moulding with Non-Uniform Wall Sections with high reject rates and a less competitive product for the customer the result.
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équipe design & consulting , with 20 years experience in design and manufacture of Medical grade moulded parts and product, including 5 years at the coal face as Operations Manager at a world class medical grade moulding facility; we are specialist in Design for Manufacture (DFM).
Please reach out if you feel you need assistance with your part design for plastic injection moulding, from Design Coaching and Guidance to Full Service Design Consulting.
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