Email LinkCONVERSION FACTOR
Plastic can replace metal in many applications.
TOP TECH: Frank Horn is Director of
Engineering at RL Hudson &
Company
by Frank Horn
As Director of Engineering here at RL Hudson, all sorts of interesting designs cross my desk, including custom-molded rubber, shaft seal, and formed hose projects. I have also seen a fair number of examples of converting what had been a metal part into plastic, and with great success.
For custom projects of all types, my teammates and I within the Hudson engineering department utilize a variety of sophisticated engineering technologies, including Pro/ENGINEER Wildfire, the most versatile parametric solid modeling software available. We also make extensive use of Finite Element Analysis (FEA) software by Abaqus® to stress test how our designs will function in simulated conditions. We then typically use a Mold Flow Analysis (MFA) program – Pro/ENGINEER Plastic Advisor™ — to anticipate molding performance and spotlight any potential glitches that might arise during molding.
But as great as our design technologies are, there is also much to be
said for a basic understanding of when to use what material. Not all materials
are interchangeable, but there are many instances in which one material
can be used in place of another in order to, for example, save money or
improve performance.
Metals and plastics are a case in point. Since joining the RL Hudson
family back in 1998, I have seen many instances where a plastic part has
successfully taken the place of what had been a metal part. Making the
switch from metal to plastic can offer several advantages, and I can think
of a number of projects in which a new plastic part has actually outperformed
its old metal predecessor. Let me share a bit more information about both
the advantages and possible uses of high quality plastic components.
ADVANTAGES The use of plastic parts to replace metal parts can offer six distinct advantages:
- Increased Design Options
- Physical Flexibility
- Strength
- Cost-Effectiveness
- Weight Savings
- Corrosion Resistance
Here’s a closer look at each advantage:
First, plastics offer increased design options. A part made of metal
(say, aluminum) is always going to be just aluminum. On the other hand,
plastic materials can be endlessly modified (through the use of glass
fillers, for example) to better suit a particular application. We here
at RL Hudson have found various formulations of glass-filled nylon
to offer good chemical resistance and to be suitable for fuel and oil
applications. Both glass-filled nylon and glass-filled polypropylene
can be used to replace steel and aluminum parts. Also, there are times
where the versatility in molding a plastic part lets us do something
with a design that could not be done in metal.
Second, plastic parts can offer increased physical flexibility. Sometimes an application calls for some areas of the part to deform more than other areas. Plastics are great in such cases. Metal can be flexible, too, but it is much more costly to make a metal part stiff in some areas and flexible in others. Plus, in many applications, the reason plastic holds up well is because it is flexible.
Third, plastic parts can still be strong. There is a stigma of plastic not being as strong as metal, so some people are hesitant to make the switch for fear a plastic part just will not hold up as well as a metal part. But I always urge our customers not to compare plastic to metal in that way. Think instead of the actual requirements of the application. Sure, a part made of metal may be stronger than an identical plastic part, but the plastic part could still be more than strong enough for the application in question. Don’t forget: the strength of a material is one thing, and the strength of a designed structure is another. A carefully compounded, well-designed plastic part can be very strong and durable, more than hearty enough to withstand demanding conditions.
Fourth, plastics can be more cost-effective than metals. Everyone wants
to save money where possible, right? With the costs of metals on the
rise, plastic parts can offer a significant savings, particularly when
used in place of aluminum or stainless steel parts.
Fifth, plastics also offer a weight savings compared to metallics. Depending on the size of the product, this can be a significant factor in the eventual weight of the assembly for which the component is destined. And if that assembly needs to be lightweight (take, for example, a handheld product, such as a kitchen utensil), then switching out the metal with a plastic can really help.
Sixth, plastics are corrosion resistant. Unlike steel or aluminum, plastics will not oxidize over time. Neither will stainless steel, of course, but as already mentioned, stainless steel is significantly more expensive than plastics.
So that’s six big advantages. Plus, plastics with good high temperature resistance can be over-molded with rubber as easily as metal can. So if, for example, you have a metal part that includes a rubber seal, this can very likely still be converted to plastic and rubber.
SAMPLE SWITCHES Let me now share with you some of our more memorable “metal to plastic” success stories. These are all situations in which RL Hudson supplied injection molded plastic parts that performed as well as – or better than – their metal predecessors.
A couple of years ago, one of our customers was in search of ways to eliminate a secondary assembly operation and reduce costs associated with a shutoff valve. The existing design featured a brass stem that had to be inserted into a molded rubber (ethylene propylene, EPDM) plug. We replaced the brass stem with tough molded plastic (a 15% glass-filled polypropylene), eliminating the cost of metal machining. We also enlarged the stem slightly to add strength and reduce rubber usage. The plastic stem and rubber plug are bonded together during manufacturing (rubber over-molded to plastic), eliminating the need for any secondary assembly. Plus, with features molded together as one part, some tolerancing concerns were eliminated. The cost of the finished part was reduced, and the customer now tracks just one part instead of two.
Another success story involves a standard hose fitting of the kind you commonly see machined out of brass, aluminum, or stainless steel. We replicated the shape in a 30% glass-filled nylon, which was considerably cheaper than any of the metals. Our nylon part retained the necessary corrosion resistance, plus it met the demanding temperature needs of the under hood application. Increasing the glass content from 15% to 30% made this possible; as glass content increases, so does a plastic’s high temperature resistance.
A third application that springs to mind involved a cast aluminum engine part. It was an under hood application, so temperature resistance was important, but the part also needed to be corrosion resistant. The part had to accommodate an O-ring for sealing, so the metal incarnation was being machined to provide an O-ring groove. We designed the part to be injection-molded from a 30% glass-filled polyamide. The requisite O-ring groove was molded-in, removing the need for machining. Our part also incorporated stainless steel torque limiters molded into the design. These torque limiters accept the load on the plastic part and transfer it to a mating part, preventing the plastic itself from being overloaded.
And that brings me to a final important point. As versatile and useful as plastics are, they are not a panacea. They will not be appropriate for every application. No material – be it metal, rubber, or plastic – can function effectively in all environments. There will be instances in which loading extremes, temperature extremes, chemical extremes, and/or abrasion disqualify plastics from use. As always, use common sense and good judgment when selecting any material, plastic or otherwise. And if in doubt, request a test! Testing a material to see how it holds up to adverse environmental conditions is always a good idea before committing to production of any part.
DESIGN ASSISTANCE That’s all I have room for right now, but there are many more applications that have benefited by a switch from metal parts to plastic. If you have an application that might be a good candidate for a switch, please do not hesitate to give us a call at 1-800-722-6766. We’ll be happy to help you look for ways to convert or consolidate parts and save money, all without sacrificing usefulness or reliability.
