Molding Tricks Every Engineer Should Know

When it comes to product design, mechanical engineers will be called upon to maintain the cool look of a new product from the industrial designers, make all of the parts of the product work, and do so with the highest quality while remaining as inexpensive as possible.  The outer enclosure parts are often made of…

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When it comes to product design, mechanical engineers will be called upon to maintain the cool look of a new product from the industrial designers, make all of the parts of the product work, and do so with the highest quality while remaining as inexpensive as possible.  The outer enclosure parts are often made of plastic to achieve smooth looks at low cost with many functional features hidden inside, and for high volume production this means injection molded, engineered plastics.  To achieve this, over the years, an engineer will establish a series of tricks that work to produce high quality molded plastic parts, some strategic and some detailed.

In recent years there has emerged a belief that just getting some 3D CAD files of the outer surfaces and part break up for the molded plastic enclosure of a new product concept will suffice as the overseas tooler and molder will then take these files and incorporate a proper tolerance study for clearances between parts so they always fit up correctly, add all of the draft to the parts for clean release from the metal injection molds, and ensure a perfect finish.  Of course this presumes that the molder and tool maker can read the minds of the designers since there is no conveyance in this approach of the design intent details, key critical to function fitment features, strategy to control alignments, nor considerations for assembly sequence.  In fact, many mold shops will only make changes to ensure mold flow and minimize sink and shrink deformations, and they will do this in a way that makes it easier for them, i.e. faster and easier molding cycles to improve their profit margin.  If the CAD and drawing files they received aren’t clear they are under no obligation to provide re-engineered plastic parts, and it is more likely the molder will only provide what they were given, and then charge extra to fix the tooling to get the parts right.  We see this play out for companies that try to take a short cut right to a tooler/molder/Contract Manufacturer without proper engineering scrutiny or consideration of the industrial design intent, and in some cases this spells the end of a new start up.  It is unfortunate, as a more well thought out approach with the knowledge of some of the pitfalls that can be encountered, and techniques to produce good parts could have steered the product’s path to production from early on.

Well thought out 3D files and drawings to communicate the details and needs of the parts and product design will form a “contract” of the needs and expectations of the final parts, for all parties involved, and allows a place from which small details can be worked out right away.  This is really the first molding trick. Do not leave the detailed work to then end, but instead make sure things are thought through with the team before releasing to manufacturers.  This will ultimately save schedule and money, and build a working relationship with the molder moving forward through the first production parts and sign off.

Don’t make it shinny! This is really the trick of getting the full team involved even in the early industrial design stages of the product’s development, even before mechanical engineering may begin in detail.  Often the same product design can be achieved with minor changes that prevent big headaches or costs down the road.  Does the side panel of the part that is 18 inches in height really need to be domed by 0.080 of an inch with a radius of 628 inches?  This may be done without any thought, it may make the rendering look better in the graphics generated lighting, but does it need to be captured in the 3D CAD and tool to make the real plastic part.  This feature may go entirely unperceived on the final product but make manufacturing control of key mounting features molded in the tool more difficult to form and inspect.  On the other hand, there is a flip side; a design with the simplistic, flat, large panel with a shinny finish.  In this there may need to be a slight curvature to bow the face out to prevent the finished side from looking like it droops or oil cans inward even if it does not have an issue with warping when it comes out of the mold, or if the part may take some loading or user contact then it may benefit from this form to ensure a quality look over time.

Other industrial design features may look good enlarged on a computer screen, but the little unique character bump in the product’s bezel that will require a $4000 lifter action in the molding tool, and may never be noticed once the product is placed on shelf in the home, may not add to the value of the product.  A character feature like this may only distract from a product’s look.  Properly sizing the part and feature, looking at the fit up and over all product needs, and whether features truly add to the product, can prevent something that looks good blown up on the computer screen from causing issues in a product’s fabrication and release.  In fact, many features like this, that get added to get a varied look without other considerations, are dropped and don’t make it to production.  The industrial designers and mechanical engineers should work together to meet the goals of function, manufacturability, cost and looks, from early on.  It is OK to have a shinny finish just keep in mind it will require a bit more work through the whole process.

Hide that part to part gap!  Molding plastic parts well is not easy, nor is producing an assembly of those plastic parts that fit up tightly with ease to produce the clean looking product that was shown in the original renderings.  The promise of inexpensive part costs can disappear if the plastic part distorts or warps post injection cycle.  This can be due to several reasons including part geometry, variations in wall thickness, process procedures during molding including cycle time, and ultimately may require the cost of post mold cooling fixtures.  Many complex parts will have these issues to some extent, and some parts and design requirements will need the added cost of fixtures, but for most others, fixtures will just be an added step and cost due to poor design considerations.  Since many parts, especially thin ones or ones with varied cross sections, will struggle with warping to some degree, putting extra thought into part design and fit up between parts during the detailed industrial design and engineering cycle will go a long way to getting a good looking product finished on time.  This may require looking closely at mounting details, internal features like ribs and lips of one plastic part that will key into another to position them relative to one another, and hiding potential misalignments.

Even if the parts are molded without too many issues, how well thought out was the design to make the assembly of the parts not show variations in fit or require extra time in assembly to get the fit right will be important to produce a quality product for the target cost.  This is where not putting a possible misalignment “center stage” on your product design needs to be considered.  This is not to say to avoid all character lines across multiple parts or avoid designing smooth fitting enclosures, but effort, from the beginning, is needed to consider what will be reasonable to produce cost effectively and to have a strategy to align the part fit up between the parts with features in the mating pieces.  If after all the engineering is done, tooling fixtures designed, line to line fits agreed to, and the manufacture comes back asking for another 0.03 of an inch of slop in part fit alignment and another $5 per part, some designs may get heavily re-worked to just re-move that whole portion of the product’s design to avoid the fit issue and expense.  A change this late in the process, by the molder, often looks “added on,” and this makes the design and engineering team look bad as if they did not work together to consider the challenges, and the client no longer has that award winning look they thought they were going to get 5 months ago, and this could have been avoided with broader consideration of the whole process early on.

Don’t make it shinny, again!  Every product will have its outer skin, and modern products have become increasingly sleek with a tuned look.  However, to address the part fit, as just discussed, the inside of a part will have an array of ribs and bosses that key things together, as well as mount internal pieces and provide structure.  In each of these cases these inner features on the skin of the product usually need some strength but often cannot afford to use up much plastic volume, not so much due to material cost or weight, but due to wanting to avoid creating locally thick cross sections of injection molded material which will cool at different rates than the surrounding thinner skin.  This can cause sinks in the part’s opposite wall or outer surface, and even when the problem is not too dramatic it can cause inflections of smooth outer surfaces that can catch highlights that don’t look right, ruining the effort put into making the product sleek.  Shinny finishes are worse than flaw hiding textured finishes, but this is just a challenge that comes with the territory of product design.  Here are some mechanical engineering tricks, not always broadly known or shared among all engineers or molders:

  • For wide alignment tabs, or ribs, they can be broken into many skeletonized ribs
  • For screw bosses, these can be pocketed on the inside face of a part wall around the bottom of a boss, making a moat at the base, reducing the amount of material in that region. Additionally, having the bore of the boss hole be deeper than the boss is tall can locally thin the part’s outer wall to 30 percent of nominal wall thickness, and effectively reduce the risk of sinking or blush on the outside surface.
  • For ribs, selectively reducing draft on long ribs to keep them at a viable thickness at their top and not too fat at the bottom will reduce the likelihood of sink marks on the opposite side. Minimizing Internal ribbing can be done, but internal ribs are necessary, many times, to strengthen a part, and the ribs help with plastic flow throughout the part during molding.
  • Corners where molded housing walls meet can benefit from coring or notches that reduce the effective diagonal material thickness in this region of the part, which can cause not only blushes on the outside surface but also cause housing walls to bow inward, affecting fit to other parts.
  • Coring, in general, of any thick sections or transitions can be accomplished early on during layout planning and by exploiting 3D CAD tools to identify thick sections, and core them even before the plastic tooler does their initial mold flow analysis, which should not be put off in the development cycle.
  • Keep these injection molding tips in mind even for prototypes as cast prototypes can have similar or even worse issues in fabrication with some of the points above.
  • Even though wall thickness and coring are not show stoppers for 3D printing, if the final parts are to be molded, the entire design process will benefit if the end fabrication method is kept in mind, making the 3D printed prototype’s geometry more like the final part, and this will also conserver 3D printed material and prevent chunky under cured sections in your 3D printed parts.

These molding and part design tips are not just for the injection molding process alone, as other processes, like structural foam, which purport to alleviate concerns around sink marks, actually still have limitations on thinner and thicker sections, and it can be difficult to get a molder to commit to a structural foam part’s cosmetic quality without these same injection molding issues having full consideration.

These plastic part design considerations and tips are the sorts of things that should be discussed early with the molder and tool maker on the project.  If they don’t understand what is being asked of them, at first use the CAD tools to demonstrate examples of what is being talked about.  If they still don’t understand, then maybe it is best to select another mold shop as soon as possible.

So please feel free to use some of these dirty little tricks and team strategies to ensure smoother and more cost effective plastic parts for your next product design.  Ultimately, it is a move to more elegantly designed parts that will lead to better product results, and you and your team can build your own bag of tricks for future product developments.

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