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The technical questions every plastic product designer should be able to answer

“This is beautiful! It must be good design.” Product design is often evaluated from the aesthetic perspective. Visual appearance can make a product a commercial success or a failure, but it is still only one part of the bigger picture. For physical products, positive user experience requires technical functionality. Technology and aesthetics are not in contradiction - ideally they strengthen each other. Below, I have listed a few topics that I believe every plastic product designer should be familiar with regardless of their background, be it in industrial design or in engineering.

How do polymers behave in general?

Rather oddly, polymers are both solid and liquid at the same time. They behave unlike any other material. The career of a designer lasts hardly long enough to gain thorough understanding of the polymer nature, but in order to avoid the most obvious pit falls of plastic design it useful to have knowledge of the reasons and mechanisms behind them. Mechanical loads, environmental conditions and time - especially in combination - challenge every plastic product. If they have not been properly considered in design, the product is unlikely to last to the end of its expected lifetime .

What are the strengths and limitations of the plastics I use?

The spectrum of different plastic grades seems not only confusing but also indefinite. If you look at the plastic components produced by an average company, however, it is likely that 95% of them are made of roughly 8-12 different plastic grades. The understanding of different materials begins with comprehending the difference between amorphous and semi-crystalline plastics. Knowing the strengths and limitations of 5-8 different of each will take you quite far. Which ones they are depends on the business area you are in. Seen from this angle, the world of plastics is not as complicated as it may seem.

How does my design behave in the process?

Producing good and constant quality at reasonable cost is what injection moulding is all about. Plastic components must be designed with the process in mind. It is therefore important for the designer to understand what happens in the mould during the production cycle. A few basic principles and some imagination are all that is needed for elementary flow and cooling analyses. How does the melted plastic flow, where does it (prefer to) go first? What happens when the material cools? How do the effects manifest themselves in the product and how should they be considered in the design? There is always some deviation in the process.

What type of tooling is needed around my design?

Years ago, a customer told me to “just deliver the 3D-files and the tooling people will figure out how to wrap the steel around it - if not I’ll bang the table with my fist until they do”. That is not how it works. At the very least the parting line, draft directions and moving sliders should be considered in the 3D-geometry, ideally ejection and gating as well. It also helps if the dialogue with the tooling designer can be started at an early phase, something that is unfortunately seldom the case today. The understanding of the typical structure of an injection moulding tool, the ability to draw a cross-section of a part on a piece of paper, and some thinking is already a good start. Furthermore, if you have an idea of how the tooling is machined and built you can make the life of the tooling manufacturer much easier.

What are the differences between hardness, strength and stiffness?

Understanding mechanical loads and being able to choose the most suitable materials and design structures in response requires the theoretical and practical knowledge of physics. If you are planning to outsource design services, you can test candidates by asking them to define the difference between strength, stiffness and hardness. If they are unable to do that it is best not to proceed no matter how simple the product is. Technical understanding is the backbone of good plastic design, but it is worthless without the ability to apply it in practice. The article What makes some product desginer so successful focuses on some of the properties I have found successful product designers tend to have.  

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