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Thread: Part Design Feasibility Question

  1. #1
    Join Date
    Feb 2018
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    2

    Part Design Feasibility Question

    Hello, I registered a while ago. I thought I would give this a try. Any feedback is appreciated.

    I am one of the engineers, aka sales engineer or program coordinators. My group does the job quoting off of the customers part designs, and if we are awarded the job, it stays with the engineer as we have our assigned customer base. We do our best to anticipate the challenges, both of parts meeting quality and performance expectations, and the mold operation aspects. What I have on my plate currently is quoting an assembly package with 5 molds. There is one mold that we initially threw the caution flag up on a couple of months back. In this second round of bidding, there hasn't been any changes to the design based on our first input.

    I cannot post the part drawing for confidentiality reasons. But I'll give a verbal description of the basic design and the mold concept. The principle feature of the part is a straight hollow tube. It has an inside diameter of 1.0", a tube wall thickness of 3/16", and total length is appx 20". The designer's intention is to lay the part down on parting line, and to pull two core pins, most likely with hydraulic cylinders, coming in from each end. There is some draft on the I.D.s, if you call 0.05 degrees per side some draft. They cannot allow any more draft for reasons of fit and function. The drawing does include notes for gating, two gates at one end of the tube opposite each other. The material is an impact grade polypropylene. Inside the molding cavity (impression) the cores have no side to side support, only from their root protruding from the ends. Obviously it will need some ejection too.

    I would like to hear some thoughts on what we might encounter on a part/mold of this design.

  2. #2
    Join Date
    Aug 2011
    Posts
    535

    Re: Part Design Feasibility Question

    Hello,
    First of all, thanks for registering here. We are a small community and hope to help each other out.
    So as I see it, you have to pull two 10 inch cores or 1 inch in diameter and mold a part. I do not think it is a major issue. You need a lot of space around the mold to have your core pulls and depending on the number of cavities you will need enough machine pressure to pull these cores out as the plastic will shrink on the cores. You will also need good cooling but a 1 inch dia is good enough.
    As far as the part is concerned, I think warp may be an issue. You will also need good venting.
    Let us see what others come up with to keep this discussion going.

    Suhas

  3. #3
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    119

    Re: Part Design Feasibility Question

    I'm not a mold design expert but if I gave it a shot I'd probably do it in a similar fashion.

    Two gates is certainly desired, overall much more balance. I like this design much more than a long cylindrical part machined down into the mold plates, these are never fun to process! parts stick in cavities, stick to cores, cant ejection part from core, there's a great deal of roundness problems etc. etc. No fun.

    Some things to watch out for in your processing is flash at parting line and at hydraulic core shutoff (you will have plenty of venting!), surface finish of those hydraulic cores may need some tuning so your cores don't rip your part when pulled, some type of draw polish to help allow air enter because you'll need to avoid a 'vacuum' type scenario inbetween those cores during pull, causing quality issues - unless there's a hole in your part that could help this, you will need plenty of cooling around your part and in those cores, can you afix the part into the B side real good in numerous areas along the cavity section to provide support when core pull happens?

    EDIT: Actually you'd probably be better off executing core pull while the mold is still closed, this will help provide a nice stable pull and prevent stretching in the cavity as there would be support on each side of the part and along the cavity walls

  4. #4
    Join Date
    Jul 2012
    Location
    Belle Vernon, PA
    Posts
    113

    Re: Part Design Feasibility Question

    I might have misread or understood you, but here are my thoughts;

    2 - 10" cores per part (don't forget to adjust for shrinkage!), meeting in the middle of the part, it should have some kind of socket on one end of the core that the other core locks into when mating in order to keep deflection and misalignment at a minimum. I'd gate them at the middle of the part, flowing out (I'm unsure about what the requirements are for gating). The cores will need to be polished pretty well to pull without tearing. A lot of this also depends on other features the part has as well. If you're really in a bind, find a good tool shop, have them sign a NDA and have them either build the mold or help you guys out.

  5. #5
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    119

    Re: Part Design Feasibility Question

    Quote Originally Posted by moldingppm View Post
    in order to keep deflection and misalignment at a minimum
    Good point about deflection. Some type of tapered shutoff on those long cores would help - and they'd need to be strong enough to withstand those high molding pressures.
    But 1" is pretty thick, as long as those cores shutoff on one another I think you'd be okay.

    Without compromising waterline, you could even drill a small hole through one of those cores to the center shutoff to provide air in your part during corepull, otherwise you might have low pressure or 'vacuum' scenario as I mentioned above, and part will want to concave on itself during corepull.

  6. #6
    Join Date
    Sep 2012
    Location
    Lakewood, WI
    Posts
    70

    Re: Part Design Feasibility Question

    The cores will deflect if gated in the center of the part and flowing down to both ends. With or without locks. I'd bet my lunch on it. 2 gates on one end at parting line 180 from each other should work because the flow and cavity pressure will be more equal as the plastic flows down the length of the part. The problem though is if you are using 2 cores that meet in the middle how do you get the runner over to the other side of the part? A manifold or 3-plate would need to be used. Flowing down the length should combat against some warp, but the pressure gradient from one end to the other may be very large causing warp. Using as big a gate as possible would help this. You may also want to consider running the part without gate seal. This should help reduce the overall pressure gradient.

    Additionally, the cooling of the core pins need to be really good or there will be a hot spot on the part at the center where the core pins meet. This could be very problematic. May want to consider Noren Thermal Pins or send the core pins to them and they can fill and cap them which provides even better heat transfer. Just need to be careful how much stock you remove from the core pin for cooling, because the pins will be more susceptible to deflection. https://norenproducts.com/index.php/...ext-5170-2-0-2

    Can the tube have any holes in it? If it can then you could support the cores by saddling around them from both core and cavity side of the mold. Then you could gate just about anywhere.

    I'm not sure about using just one core pin 20" long, but it would eliminate the issue of trying to get the runner to both sides of the part. You could then run air through the pin during the entire for cooling. Just not sure if 20" is too long. If the plastic if flowing evenly down the length of the part should keep it straight, but I have no experience with something that long.

    I would agree to spend the extra money on the locking core cylinder(s) so they can be pulled prior to opening the mold. Unless your machine has the capability to apply core pressure during the filling and packing of the part. Most do not.

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