Design resources for overmolding 2

[learnin]

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

 

[cpretty]

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

 

[btrueblood]

" 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.

 

[racookpe1978]

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.

 

[learnin]

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".

 

[TVP]

Obviously it is beneficial to work closely with one or more injection molders in order to learn from them. In addition, you may find it useful to review a good reference book on the subject. The following is a link to one:

http://www.ismithers.net/publishing/wrr145/multi-material-injection-moulding/

 

[learnin]

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.

 

[btrueblood]

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

 

[learnin]

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?

 

[btrueblood]

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.

 

[learnin]

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.

 

[unclesyd]

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 Overmolding Guide.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

 

[cpretty]

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

 

[unclesyd]

Here is a link to Chemionics a supplier that has a very good cross section of what can be done with Plastisols.

http://www.chemionics.com/plastisol.htm

 

[TVP]

learnin,

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

waterborne, soft-touch coatings from Bayer

http://www.bayermaterialscience.com...2C125776D00468AB8/$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

 

[btrueblood]

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...