Choosing Plastics: Case Study (LEGO toy truck)

I had been downsized out of one full-time job in 2003, and it was before being hired full-time at another company – so I was doing contract work to “put beans on the table” and was working at Mack Molding as a project manager for a Lego toy truck.  This was a fun project, and I’ll give credit to Mack Molding for letting me do the project plan the right way rather than throwing together a Gantt Chart a priori and then having that become holy writ.  (In January when I mapped out all the tasks to create the project’s critical path I said we would be building the first production pieces the first week of July; right on schedule, the first week of July, the first production pieces rolled off the assembly line.)

Since this was a consumer product both safety and cost were critical concerns and I, as the project manager, had to choose the materials with that in mind… but child safety was paramount, not only for the materials but for the colors.  So the very first criterion was to use FDA-approved resins and masterbatch colorants.  Second was the resin cost, and we went with a polypropylene material for cost and because similar applications had also used this material – so there was some prior familiarity with it; as I recall it was a co-polymer polypropylene, for better impact resistance (see #6 at the link).  Since Mack Molding had long-standing relationships with several resin suppliers, I leveraged their – and Lego’s – knowledge of prior resin and colorant use.

Plastics Lesson One: Leverage the experience of your suppliers to help you choose a suitable material.  If you view them as a partner who has a stake in the success of your product, and make sure they know they have a stake in its success, they will be vested in making sure you get what you need to be successful.

Polypropylene is a polyolefin, and one of the characteristics of this kind of resin is that it has a wide range for shrinkage.  Shrink is the term used for the fact that the part is designed to have specific dimensions; the mold then has to be cut to account for the fact that plastic – injected as a liquid, solidified, and cooled – shrinks some percentage.  This shrinkage needs to be accounted for when cutting the steel for the mold.  In the case of polypropylene, parts can shrink several percent (e.g., from 1%-3%) depending on any number of factors.  So my first responsibility in dealing with the material selection, after selecting the material itself, was to choose the shrink rate to be used to scale the mold so that the parts come out right to the design dimensions.

So what did I choose?  A low shrink rate at about 25% of the way from the low end of the window to the higher.  Why?  Because it’s easier to process a part to shrink more, rather than shrink less.  For example, to have a part shrink more, you can reduce the force packing the plastic into the mold during injection, reducing the part density slightly – which increases shrink.  One can also eject the part a little earlier from the mold, allowing it to cool in the air instead of in a hard fixture (the mold itself) which would hold it in place… in-air cooling allows the part to move more.

Conversely, to make the part shrink less, one can pack the part harder (increasing the material usage and cycle time) and hold the part in the mold longer (increasing cycle time).  Both of these increase the part cost.

Plastics Lesson Two: When faced with a shrink rate window, choose towards the lower end rather than the higher; if the shrink rate needs to be massaged during trials to meet dimensional targets, it’s easier and cheaper to get the part to shrink more than to get it to shrink less.  But don’t go hard-up against the low end of the range either as you want to leave a little wiggle room “just in case”.

But there was another layer to choosing the materials for the toy – the wheels needed to spin freely.  And they needed to not only spin freely all the time, but potentially after sitting on a shelf a potentially-extended period.  Also, the majority of drops would be onto the wheels, and while the main body components were made from a modified polypropylene, specifically for impact resistance, the wheels would undoubtedly bear the brunt of any impacts.

Something that people unfamiliar with plastics might not know is that similar-chemistry plastics sliding on each other have higher friction than two dissimilar plastics.  Another datum for consideration is that like plastics can micro-weld to each other over time, though admittedly at room-temperature this is not likely.  This immediately triggered my decision to make the wheels from a different resin type than the base product which, as stated above, was made from polypropylene.

I selected polyethylene as the material for the wheels (FDA approved, of course).  First and foremost was the fact that it, too, is an inexpensive resin.  It also has excellent impact resistance – better than polypropylene.  And although still a polyolefin, its different chemistry would mean that the wheels would rotate freely and would not micro-weld to the body axles.  The one wrinkle was that this would mildly complicate regrinding scrapped parts as the wheels would need to be segregated; however, since the colors would need to be segregated anyway, this was not an overly-burdensome hurdle.

Plastics Lesson Three: If products need to slide on each other reliably and with low friction, consider choosing dissimilar chemistries for the parts that need to move against another.

As I said early on, I did the project plan where we launched right on time.  Part-and-parcel with that was the fact that in any project where parts have to be injection-molded, there really need to be at least three mold trials:

  1. Initial shots.  Can plastic be put in, does the part come out close?
  2. Testing parts.  Parts made for initial product testing, form-and-fit checks.
  3. Final verification before shipping.  All changes made correctly?

Plastics Lesson Four: Budget time in the project plan for at least three mold trials.  If there are critical aspects of form, fit, or function that are exacting in their requirements, you may need additional trials.

A project manager should not only be skilled in project management, but have a deep and instinctive understanding of the materials being used and the tasks required to bring the product to fruition.  Just as a conductor is not just waving a wand to coordinate the orchestra, they must be a skilled musician in their own right to know the capabilities and limitations of the musicians, and instruments, under their direction.

© 2014, David Hunt, PE

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