Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • 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!

Join Eng-Tips
*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.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

carlbud (Mechanical) (OP)
31 Aug 09 14:00
I am designing a 4 cavity mold. The parts are rings that are 1.5" OD, 1.25 ID by .375 thick molded of 325 valox (PBT). My customer would like to see the gates on the ID of this part. He machines these and from his experience if we were to subgate on the OD there can be a visual mark where the gate was after machining.
I am not sure what the best approach is to solving this problem.
I am wondering if I could use a 3 plate mold and drop down to the center of each cavity with a sprue gate and then from there use a small runner to a subgate into the ID of the part but I have never seen this approach used and I am afraid it would give me problems because of the 3 required drops in size to get to the part (nozzle, sprue gate, and subgate). Is this a legitimate concern?
I could use a Hot Runner Mold, but am afraid the cost would be too much.
The other option that I see is a conventional runner with an edge gate on the OD of the part. This gate could be bigger and thus possibly would not show the marks after machining. I am guessing that the material is compacting at the gate and this could be reduced by adjusting the molding process.
Does anyone have any thoughts or suggestions?

Dell M70
SW 2008 SP3.0

cpretty (Mechanical)
31 Aug 09 16:23
I came across exactly the problem you describe and looked at a number of options also.  Even the older hands here didn't really know how to tackle it.  We couldn't justify hot runner either.

We spend a bit of time trying to get a tunnel gate to go into the centre of the ring and connect to a tab on the inside.  You may be able to acheive this, as your ring has a larger ID than ours.  Look into I-Mold tunnel gates as a possibility.

The end result for us was to build a 3 plate tool to drop into the centre of each ring.  See attached pic.

Tool arrived yesterday, so hopefully should get it in the machine this week.  Will let you know how it goes.

dpackard (Mechanical)
31 Aug 09 18:09
Could you use an internal ring gate on the part or does part have to be degated when it is ejected?
carlbud (Mechanical) (OP)
31 Aug 09 19:17
We would like the part to degate. If there is some vestige that would be ok because the customer has to cut a groove on the ID anyway, but we don't want a separate operation for just the gate.

Dell M70
SW 2008 SP3.0

carlbud (Mechanical) (OP)
31 Aug 09 19:40
That is what I was envisioning with the three plate mold. We should have room for subgates there. As I said, I am a little concerned about dropping sizes in the flow of the plastic. I would be very interested to hear if you have any problems filling the parts.

Dell M70
SW 2008 SP3.0

patprimmer (Publican)
31 Aug 09 21:14
Use a straight shot to the intermediate plate in a three plate design, put runners in the intermediate plate, then hot shots or sprus to a tab on the inside rad. Clean up the tab when machining the grove. It would be better if the tab was a full internal ring but that might require extra machining. Is the weld line an issue that would be best removed.

I am talking a ring inside the cylinder with a thin land going to the cavity, not a full diaphragm gate.


See FAQ731-376: Forum Policies for tips on use of eng-tips by professional engineers &
for site rules

carlbud (Mechanical) (OP)
1 Sep 09 11:03
I don't know what you mean by straight shot and hot shots. I like the idea of a tab or a ring gate as long as it can be broken off close before machining. We currently mold this part with a diaghram gate which makes for an extra machining step to cut it out.
Thanks for the advice,

Dell M70
SW 2008 SP3.0

Pud (Mechanical)
1 Sep 09 11:38

I would have thought that if the volumes are large enough to warrant a 4 cavity tool, the additional cost of a runner system would soon be recouped against the cost of Valox runners with a 3 plate tool - or even the cost of regrinding/redrying the runners.

I would be tempted to go for a single impression, using a multi drop hot tip fed onto the inner bore as sub gates in at least 4 places. You may be able to dispense with the machining then, which will effectively reduce the whole combined process time, with zero waste.



patprimmer (Publican)
1 Sep 09 11:45

I posted in a hurry.

Straight shots or hot shots are different names for the same thing which is simply a hot sprue bush.

They are simpler and less restrictive than a hot tip so they suit the more difficult to mould materials.

On reflection, they cannot be used from the runner on a three plate mould. I have used them on hot runners in place of hot tips for several engineering plastics like glass filled nylon.

The hot runners were made by using a split plate and a ball tip cutter and cutting the runner into each half then bolting it together. This way it can be streamlined with no blind holes.

PBT will work in such a system.

See FAQ731-376: Forum Policies for tips on use of eng-tips by professional engineers &
for site rules

spider007 (Mechanical)
2 Sep 09 9:46
one quick and dirty way is to have an H stile runner, with each branch subgated under the part into a sucker pin that is in the center of the ring.  From there the material would go up the flat on the pin into the runner in center....
dpackard (Mechanical)
2 Sep 09 21:05
Do you have experience with using this material (325 valox) with a sub gate?  If you do does it tend to break in the runner and not exit the sub gate?
cpretty (Mechanical)
3 Sep 09 0:15
Got my tool up and running, with no real problems with drops in flow.  We were running Polypropylene with a PTFE filler though, so flow is pretty good, and of course no submarine gate into the part.
carlbud (Mechanical) (OP)
3 Sep 09 8:36
We run this material a lot with subgates. The only mold we have trouble with is one where the gate comes in on a fairly shallow compound angle. I can definitely avoid this on this part since there is plenty of depth to gate into.

Good to hear. I have gotten advice that this approach should work fine as well. I will approach my customer today with the different options and see which way they want to go.

Thanks all,

Dell M70
SW 2008 SP3.0

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!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close