Injection molding for dummies?

In the past I've designed parts that made their way through several
manufacturing processes, ultimately being injection molded as the numbers
justified. We are working on a new part that we know will require injection
molding for both its production rate and the fact that it would be cost
prohibitive to machine ect..

I am sure that once the initial design is done and a molder looks at the
part, he is going to find some items requiring changes. I would, however,
like to minimize the amount of rework that needs be performed.

Is there a book/web guide that may prevent me from making idiot mistakes
designing the part?

Brian Hokanson
Starting Line Products

 

My best bet is to review the GE, Bayer and other plastics manufacturer
Design Guides in pdf format that they make available for over view
issues. I found them with Google. Someone asked for that info in this
group a few months back and put them in as links on his web site.

Your closest college engineering department library will have a lot of
books no doubt as well as the Modern Plastics and other magazines with
their articles.

The other way to refine a new design that is costly to produce, is to
do what everyone does at one time or another and ask a design
consultant at an industrial design firm to analyze your part for
moldability and a flow analysis to recommend gate size and positions,
along with other issues, like optimal choices in wall thickness, draft
and parting lines for a specific plastic.

Bo

 

Is there a book/web guide that may prevent me from making idiot mistakes

Plastic Part Design for Injection Molding by Robert Malloy is the
classic for the field and would be a good book to pick up if you are
going to be doing a lot of designing for plastic.

http://www.amazon.com/gp/product/15...348737/ref=sr_1_1/002-3866277-2198404?ie=UTF8

Larry

 

Found it:

Mark Bannister's site:

http://www.injection-moldings.com/faqlinks.html

Bo

 

Brian
After many years designing plastic parts I learned the hard way i.e it
ended up costing money. I would recommend looking at protomolds web
site it gives a lot of good advice and I wish I had access to that
advice back then.
You state that you you have designed a lot of parts through many
manufacturing processes I can only assume that you learnt a lot by
using those processes. Plastic mouldings are no different.
Being a long term Soldworks user this much I can advice you on.
1.Solve the customers problem and come up with your design.
2.Put that design to one side and then think about the manufacturing
process that you are going to use. It will have a different set of
problems you'll need to overcome.
3.Start from scratch with your model (I'm thinking injection molding
here) think about split lines, then think about drarft angles
(especially if you want rough surfaces, the rougher it is the larger
the draft). No sharp corners (helps plastic flow). Use the
CosmosExpress its suprisingly not bad. Play with the MouldFlow Express
its quite an eye opener. And add your fillets last to your model. Get
SLS prototypes made, they are generally 60 to 80 % of the strength of
the end moulded part for functional testing. (FDM parts are ok SLS is
just for looks)
4.Communicate with your tool designer as to what your trying to
achieve as an assembly of parts, dont just send one part through send
him an Edrg of the complete assembly so he understands what your trying
to achieve.
5.If you think a particular area of your design is going to change or
your worried about it get the tool designer to make that area an insert
in the tool. Its easier to remove that small part & rework it rather
than doing the whole tool and the tool designer can ensure that cooling
lines are not under bits you may want to rip out and replace.
6.All ways get the tool designer to send a mold tool GA to you before
they start to cut metal, you want to check that the ejection pins are
not on a presentation face of your part and that the injection point
for the platstic is not in a similar position.
7. I always insist that the moulder sends me a first article inspection
report of the part and that i have a preproduction batch of parts to
check (approx 200 parts). This is especially important if your usinng
materials like acetal. Ensure that any parts you recieve have had at
least 24 hours in air before you inspect them. Some plastics suffer
from post moulding shrinkage.
8.Avoid darker coloured plastics (black). They can appear cheaper but
they show flow marks which can be a nightmare to get rid off. Thats why
most car trim is actually painted.
9.The tool designer is an engineer too, all he wants to do is solve
problems and have an easy life. There is some part of moulding thats a
black art but if you work with the guy (or girl) it will get resolved
quickly and hopefully cheaply.

To put things into perspective I joined a mainly sheetmetal firm some
years back (they took me on because of my plastics background) Now I've
learnt a lot from the rest of the guys about sheetmetal and i've passed
on my knowledge as best i can about plastic but all I can really
recomend is communicate. The thing I love about Engineering is that you
dont stop learning.

Other places to look for info are the other major plastics
manufacturers like DuPont, a lot of them have on line guides you can
down load.

Hope this helps
Ken




The point I'm trying to get at is that we're Engineers and solving
problems is what we do

 

Ken has a great set of recommendations.

In addition, when you get to the actual molding, you need a real good
guy who has learned all the wrong ways things get done to make the best
parts in your initial setup runs. The newer closed loop injection
molding machine control systems make far more stable parts, & they are
needed for close tolerance work.

If it is a touchy item with critical tolerances, as most of my parts
are, then you need to develop gages to check parts quickly, as
measuring with plastic parts for close tolerances doesn't work so well.
Weighing parts, is also a way to understand wether the molding process
remains very stable, or is causing size changes. Even different lot #s
of plastic can cause size changes.

Lots of things to look at, but they are manageable.

Bo

 

I'll second that. I took Bob Malloy's summer course at UMass Lowell a few years ago using his
book as the text. Learned more in a week than I thought possible!

Art

 

Ken,

Just to add to your exellent advise,,,

Keep copies of models of the molded parts (the models actually used by the
tool maker) isolated, and in there original SW version format, as well as
parasolid.

I had an instance of a complex surfaced model that changed shape when
brought from 2001+ to 2003. About a year after the original tool was made,
we had to add features that were required to blend with existing cavity
features. If we hadn't caught the discrepency (just in time) the toolpath
would have gouged the hell out of the existing cavity. I ended up having to
re-install 2001+, and make the changes there.

You never know what the programmers, at Solidworks, are going to change from
one release to the next. In this case it was the surfacing algorithms. If
the part is constructed with analytic shapes, you're probably OK.

Regards

Mark

 

To add to the excellent advice you have already gotten, give a lot of
thought up front to what material you need to make your part in. The design
rules are different (in the details) for each material. What kind of loads
will it see, what kind of deflections can you allow? What kind of tolerances
can you live with? What will be the environment it lives (or dies) in?
Temperature, sunlight, chemicals, water, lubricants, ozone and other factors
I can't think of right now can all affect your material. What kind of
cosmetic requirements do you need to meet? What kind of assembly processes
will it see?

Jerry Steiger
Tripod Data Systems
"take the garbage out, dear"

 

Mark
thanks for that I've had a very similar problem in the past, i
completely forgot that one. I always supply my mould makers with
parasolid files where possible, they seem to be more stable than iges &
sat and are small making them easy to email.

One last thing if you we want to get one book on plastics get
Product Design with plastic by Joseph B Dym
It has a very good section on the properties of plastics. Actual
plastics, Design guides, Process etc. Best of all it costs about £20,
superb book and I use it frequently.
regards
ken

 

So does the Modern Plastics Encyclopedia IIRC.
(Subscribe to the magazine & get a copy each year IIRC).
Might be online now too: http://dev.modplas.scd.net/worldencyclopedia/search

Nor should the better suppliers be forgotten.
HTH

 

And to add to what excellent advise Jerry gave, I note, that then when
the design requires a type of use and a material that doesn't tend to
fit but must...then is when you need a really clever solution. There
are grades of plastics "tuned" to achieve certain levels of performance
with additives or copolymers that are just not commonly known, unless
you work with a lot of different plastics all the time.

Sometimes it may be a job that really does take specalist designer in
the field of what is being attempted, even though that costs more
money.

Other times it may be as simple as calling the technical centers at
Dow, GE, Huntsman, Bayer and such, and just asking for their
recommendations, and they give their best answers for free.

Bo

 

Thanks all for the tons of advice, specially the reading materials list.
I don't anticipate being able to design the part fully due to some of the
specialized knowledge that only comes with experience. My hope is that I'll
get the model close enough that whomever we chose as the molder will have
minimal model rework, and the changes necessary won't require scraping and
starting over the design process. I'm also hoping to avoid a situation that
requires a very special machine ( read expensive ) to mold the part.

I work much cheaper than a specialist. The more work I do, the less the
project is going to cost overall.

 

Brian
not a problem more than glad to help. This group have helped me out
loads over the years so what comes around goes around. If you get
really stuck send me a edrg and I'll have a look (no charge lol).
regards
Ken