<<  <  Page 4/4
Rob Spiegel
User Rank
Re: Is it the plastic?
Rob Spiegel   8/17/2012 11:37:56 AM
That certainly helps with clarity, Dave. That's a major change in stiffness with just a relatively small change in temperature. Also seems odd that stiffness alone would affect the latch.

Dave Palmer
User Rank
Re: Is it the plastic?
Dave Palmer   8/17/2012 11:31:31 AM
@Rob: It sounds like the problem isn't expansion of the plastic but, as Brooks says in the article, the change in stiffness with temperature.

I'm not sure what material this door latch is made from, but often plastic parts that are designed to flex in operation are made from polypropylene.  The elastic modulus of polypropylene changes by a factor of five between room temperature and 100°C, i.e. it is five times less stiff at 100°C than at room temperature.  I'd guess that, even from room temperature to 90°F, it might change by as much as 50%.

The mechanical properties of plastics are highly dependent on temperature and strain rate.  Design engineers need to keep these effects in mind.  When designing a steel part, it's usually safe to assume that the elastic modulus, yield strength, tensile strength, and ductility are the same from room temperature to at least 300°F.  With plastics, you can't assume that the properties listed on the datasheet are the same properties the material will have in your application.

Several years ago, Joseph Ogando wrote a great article for Design News regarding the use and abuse of plastics datasheets.  It points out many of the parameters that affect the properties of plastics.  I've sent it to a lot of people over the years as a reference.

Rob Spiegel
User Rank
Is it the plastic?
Rob Spiegel   8/17/2012 10:43:43 AM
Sounds like the hook should have been designed to be free of the plastic even as the plastic expanded. Even so, should the plastic on this microwave -- which is expected to get hot -- expand so easily? At merely 80 degrees? Seems the plastic may be part of the culprit.

<<  <  Page 4/4

Partner Zone
Latest Analysis
Design News previews some of the solutions exhibitors that will be at the huge UBM-organized design, engineering, and manufacturing trade show in Anaheim in February 2016.
The common denominator in just about every electromechanical system is the use of a motor. If sensors will be the eyes and ears of the IoT, motors will be the arms and legs.
The IRB 8700 is aimed at material-handling applications in the automotive, transportation, and other heavy industries.
Governmental policies and mandates in Europe are leading the way in creating low-carbon manufacturing.
Digital healthcare devices and wearable electronic products need to be thoroughly tested, lest they live short, ignominious lives, an expert will tell attendees at UBM’s upcoming Designers of Things conference in San Jose, Calif.
Design News Webinar Series
11/10/2015 11:00 a.m. California / 2:00 p.m. New York
10/29/2015 11:00 a.m. California / 2:00 p.m. New York
10/20/2015 8:00 a.m. California / 11:00 a.m. New York
12/2/2015 11:00 a.m. California / 2:00 p.m. New York
Quick Poll
The Continuing Education Center offers engineers an entirely new way to get the education they need to formulate next-generation solutions.
Jul 6 - 10, Building Raspberry Pi Controllers with Python
SEMESTERS: 1  |  2  |  3  |  4  |  5  |  6 |  7

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.
Learn More   |   Login   |   Archived Classes
Twitter Feed
Design News Twitter Feed
Like Us on Facebook

Sponsored Content

Technology Marketplace

Copyright © 2015 UBM Canon, A UBM company, All rights reserved. Privacy Policy | Terms of Service