×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Design resources for overmolding
2

Design resources for overmolding

Design resources for overmolding

(OP)
I'm the only engineer for a start-up company and have been assigned to redesign several parts. I'm having trouble with redesigning a thumb lever (1" tall, 1.25" ID, 2mm wall ring with a .5"x.25" lever protruding from one side). The owner of the company wants a rubber feel (TPE or Silicone) but neither of these will be strong enough for the force applied. This leads me to an over molded Nylon or ABS part, but I'm having trouble finding information on how to design, shut offs, flash prevention, peel prevention, etc.

Can the community point me to a book or other resources to help me learn how to make a manufacturerable overmolded part? Also does anyone have a company they would recommend to manufacture the part?

thanks in advance

ps. this is my first foray into plastic part design  

RE: Design resources for overmolding

Have a look at the Santoprene website.  Their design guides are pretty good at answering your questions.

Another good option to get some ideas is to go and purchase some plastic cooking implements with soft touch handles.  Have a good look at how they do it, cut them up to look into section sizes etc.

Once you have a handle (pun intended) on some of the parameters, go and see an injection moulder.  There is a bit of an art to over-molding.

Craig Pretty
Tru-Design Plastics

RE: Design resources for overmolding

" go and see an injection moulder"

Best advice for any molding job.  Talk to the people who do it every day, and have seen the mistakes made in design and mold tooling engineering.

RE: Design resources for overmolding

Check the handles and levers already made by Reid Tool or Fastenal.  You will be able to get ideas (to see what works) and even may be able to buy a lever directly.    

RE: Design resources for overmolding

(OP)
Thanks for the advice, I contacted two local (Sacramento, CA) injection molders and will probably be working with one of those.

racookpe, thanks for the suggestion, but I over simplified the part in my original post and won't be able to find something "over the counter".  

RE: Design resources for overmolding

(OP)
TVP, I ordered a copy of the book, and look forward to using it as a resource.

In this round of production an over molded part is going to be cost prohibitive for our start up. Does Eng-Tips have any tips on a rubbery or soft touch spray on coating? Does anyone have an idea on the longevity of adhesion for the Alsa soft touch paints?

thanks in advance.

RE: Design resources for overmolding

Not a spray on, but the stuff sold for tool handle grips (liquid PVC dip-coatings) might be worth a look.

RE: Design resources for overmolding

(OP)
Btrueblood, thanks, and I've taken a look. It seems that the pvc coat is targeted at parts where you can dip, while our part has splines on the interior. We could mask the top and bottom but the viscous nature of the PVC leaves drips and would be hard to get consistent. ... I haven't used the liquid pvc recently, are my assumptions correct?

RE: Design resources for overmolding

We've played with it recently, and yes, you can get some drippy, inconsistent coatings.  We did try thinning the stuff with mek and toluene, and it worked a bit better (more consistent thickness), but required multiple coats to build a decent thickness.  

Another possibility is to use a room-temp vulcanizing rubber, like silicone or urethane, and make your own mold tool for compression or squirt-fill molding.  Tough to do these without getting a somewhat bubbly coating, but it can be done.

Hm, just reread your op, and it looks like you want something tougher than silicone.  The urethanes can get pretty hard, but you may want to think about an epoxy too.

RE: Design resources for overmolding

(OP)
btrueblood, for prototypes that sounds like a solid idea; however we are looking at 1000 part run, and won't make sense to do the molding in house. Did you spray the thinned liquid pvc? How was the adhesion to the substrate?

In my previous post about the soft coating, I was thinking of an abs/nylon substrate that is coated to feel soft.  

RE: Design resources for overmolding

The first two links are to some general and specific indormation on certain products.  The third link is to a techical design guide.

http://www.apainfo.com/Download.asp?PDF=pdf/APA%20Overmolding%20Guide.pdf

http://www.apainfo.com/product_literature/main.asp

http://myweb.clemson.edu/~gmica/Publications/Ref_157.pdf

In the realm of the dip coating posted above here is one of many companies that makes several varieties of Plastisols or Organosols.
There are as many types as the chances of winning the lottery.

http://www.polyone.com/en-us/products/vinylplastisols/Pages/default.aspx

RE: Design resources for overmolding

Urethane moulding may be a reasonable way to acheive your goal.  Tooling can be as cheap as a machined block of MDF with some varnish on it, or CNC machined plastic.  There are also many choices of hardness and colour.  

If you choose a commercial supplier of this process they should be able to remove bubbles by placing under vacuum etc.  Throughput can be quite good through some of these manufacturers.  I would recommend our local supplier, but being in NZ it would not be of much use.

Here is a supplier of predominantly wheels that I came across when trying to find an alternative supplier for our work.  It would appear from their capabilities that they can handle custom work ins hort or long runs.  http://www.sunray-inc.com/index.php?page=capabilities

Craig Pretty
Tru-Design Plastics

RE: Design resources for overmolding

learnin,

Here are a couple of different options for soft-touch coatings:

waterborne, soft-touch coatings from Bayer
http://www.bayermaterialscience.com/internet/global_portal_cms.nsf/id/873A38499F7C9F72C125776D00468AB8/$file/11_BMS00038520_en.pdf

Makrofol® Softtouch and Bayfol® Softtouch films
http://www.anilinkompaniet.se/img/folie/Film&TrandsNo4.pdf

Alsa Soft Feel Paint coating
http://www.alsacorp.com/products/softtouch/softtouch.htm
 

RE: Design resources for overmolding

The pvc was dip and brush applied.  Adhesion was fine to tool steel, but then, we had designed the part to have a coating applied, with some retention grooves.

Looking at tvp's post for the alsa coatings, it looks like that is a two-part sprayable urethane.  Sounds like you can make that work, and it's just a matter of figuring out how to make it adhere (talk to the supplier), and what coating durometer/finish gives your boss a warm fuzzy.  Urethanes are inherently sticky, so you really shouldn't have any trouble, unless you are trying to make it stick to teflon...
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources