Injection Molding - Tubular Design

You should be fine with one degree.

But if 1 deg is used on the id, if extrapolated over 6.75" the material between the two ID's will be very minimal. Is smaller than 1 deg acceptable?

I've used as little as 1/2 degree on square features, but I don't remember the material we were molding, Nylon maybe. Polypro is pretty slick, so give it a go.

If you really want a straight wall, use a collapsible core. More $$, but even undercuts can be molded into the part.

Good luck.

Your local injection mold company owner/designer is usually your best source of information... If you already had one lined up for production.

Finish your design to meet your engineering requirements. If you like the sounds of recommendations you get here on CR-4 and the additional design efforts required to test those alternatives are small then go ahead and check their practicality. But if hours of design layout and computational time are involved in looking at the alternative better you wait for what the injection molder tells you. Make sure you keep a detailed chronicle of your calculations and design decisions because you may have to do them over.

Prepare your detailed part drawing to a sufficient preliminary state that an injection molder will understand what is required but not so complete that a company superior or perhaps the purchasing department can proceed to contract without your approval. One legitimate trick here is to not specify the material. This has the bonus of letting an injection molder recommend a better material. (There are many variants of polypropylene) If someone's name is on the drawing (not just initials) there is a good chance a the shop will call before taking any real action like cutting mold metal.

Send the preliminary drawing to your preferred supplier. When they see a good drawing demonstrating serious work on your part it is very likely, especially in today's economy, they will be willing to spend time helping you and making recommendations. Direct contacts from design engineers accompanied by a good drawing are usually good motivators for a busy sales department. They are even better motivators for a sales department that isn't busy.

Once you have the supplier's recommendations you can redo your calculations as needed and revise your drawing. Only at this point is it reasonable to ask for a part and tooling quotation. If your company practice is to write a formal request for quotation that will be processed by another company department like Purchasing make sure the name of the supplier (and salesman contact info.) who saw your drawing and helped with your design (assuming that you still favor them) is on the requisition paperwork.

Ed Weldon

mellesy -- Some additional thoughts:

Maybe what I say in my last post is nothing new to you; but I thought it would be worth saying for the benefit of other readers. Shops that manufacture parts from expensive hard volume production tooling really dislike having to build bad manufacturing practices into the tools for a lot of reasons. To them it is very important that the design engineer know what their process and tooling can do. They will likely want to spend some time with you discussing alternatives of tooling and materials. To prepare for that discussion you should have some rough idea what the annual production volume of the part will be.

An important thing to keep in mind if the tubes you are dealing have closed ends is that the mold cores for the tubes will not be supported on the ends and will be more subject to deflection from molding pressures. Deflection can result in thinner walls.

Reducing mold pressures can be done by increasing the temperature of the melted polymer when it enters the mold. Polypropylene is a crystalline polymer whose long chain molecules begin to break a few degrees above the ideal pressure for molding. The result is increasing brittleness of the plastic in the part. This can only be detected in an opaque plastic by destructive test. You don't want to have your supplier be forced to resort to this kind of practice to meet tolerance requirements in the parts.

BTW, I'm no expert in plastics or injection molding. It's just that I worked for 4 years as a manufacturing and quality engineer for a company that made centrifuge bottles and tubes. Many of these components were made out of polyolefins including PE, PP and intermediaries that were sort of like mixes. The quality problems I dealt with were frequently those described above.

If your supplier can demonstrate ability and experience to make the tubes with zero draft angles to your satisfaction you will likely have made a good choice. Your L/D ratio of 2.6 and wall thickness of 0.10 to 0.15 does not look all that difficult in polypropylene.

Ed Weldon

I give Ed Weldon a GA.

A draft of .5 to 1 degree per side should be adequate. At one degree, the wall thickness will increase (or decrease, depending on where the draft is started) by .118 inch. Therefore, the outside wall needs to be drafted to maintain an even wall thickness. Even wall thickness is one of the cardinal rules of injection molding, and one that is often broken. Uneven walls cause differential shrinkge, molded-in stresses, and long cycle times.

The cores MUST be cooled, either with baffles (blades that force cooling water up into the ends of the cores) or with heat pipes. Without cooling, the cores will continue to heat up until they approach the temperature of the molten plastic, the parts will have tremendous size variations, and the cooling time will have to be continually adjusted as the cores heat up. The part should be filled with either a single gate located at the top end between the two cylinders, or (if the ends are closed) with one gate in the top middle of each cylinder. (A secondary gating scheme would be to have several gates around the bottom of each cylinder. The top would need to be well vented.) The part needs to be removed from the mold using a stripper plate, which pushs on the entire bottom end of the part at the same time. Ejector pins will just distort the plastic and may push entirely through the part without moving it.

Cylindrical parts always shrink onto round cores and away from the outside walls. Unfilled polypropylene has a high mold shrinkage rate, which means that it will grip onto the core very tightly. The core needs to be highly polished. If the ends of the cylinders are closed, you may also need to get air up into the end of each cylinder during ejection to keep the vacuum from collapsing the part as it's ejected. (I've seen this happen. It's not pretty.)

Unfilled polypropylene is also very soft, making a cylinder difficult to eject without distortion. If your design will allow it, a polypropylene material filled with talc or mineral will stiffen the part and reduce shrinkage, making it easier to eject.

Dennis

Thanks, Dennis. Very interesting and useful comments. I am not a judge here, just a student; but it sounds like you know what you the subject well. My contact with this technology goes back some thirty years; but I recall the injection molder, a very competent outfit, who made thin wall moldings for my employer describing similar mold features and practices. Point is that you seem to be describing time proven plastic mold design practice.

Ed Weldon

Hi Millesy:

Add the draft angle to the inside and the outside to keep the material thickness uniform, if your overall design allows it, the connection area may be the only exception to the taper. As there are two material thicknesses in that area it should be ok.

I don't think it's necessary to add draft to the OD. Just increases the tool cost. Maybe I missed something, but, the difference top to bottom won't be enough to bother with, and the outside will be straight.

Just my opinion.

Lynlynch,

That is interesting, you suggested a collapsable inner core, then in a later post you comment about tool cost on my post, with CNC machining, which likely is the method this mold will be made the tool cost straight vs tapered will hardly be an issue, likely with the same amount of machining time.

Of course production volume may be a factor on overall cost. If the outside and inside is tapered, would it be possible not to have a split mold, as the part can be ejected easily.

Right on. Cost is not much different.

Still not sure what the finished part looks like. Since the plastic will shrink away from the OD of the tool, why would you need a taper on the outside?

On the other hand, if your rendering is correct, why not just extrude the shape and finish the end/ends with a solvent bonded cap, or whatever?

I don't know enough about the requirements to give a better answer. In fact, even if I knew everything about the requirements I might not have brains to give a better answer.

This opinion may only be worth what it cost.

Hi Lynlynch,

I counted extrusion out, since we were all talking molding, I agree, likely cost effective, if there was talk about a cap, I missed it.

The rendering is correct to the sizes posted, however it did not show taper.

We need to know anticipated volume to decide on the process.

No cap, just speculating, since we don't know what this thing does. Does it hold cans or was that just an example.

Yes, we need to know volume, too.

What about the production volume?

Go for it.. and hope you don't have to edm the mold when it's done.

EDM, that is a good thought actually.

If manufacturing allows to taper EDM the inner core and then remove some material from the end and offset it, the wall thickness will be uniform and all of the base material, the inner and outer can be used.

If there is only a taper on one side, with the sizes you specified, there will be a very thin .040" wall thickness on one side, or during production the process will use excess material if offsetting to the + side.

The double taper could be a good thing all around, material usage, strength and part ejection, while material saving when mold making.

The question of draft or no draft also depends on whether one end of these tubes is partially closed or fully open.

I'm imagining a tool similar to the 2 part epoxy dosing syringe systems (like araldite) where there is possibility of venting into the cavity behind the core.

Mould cooling should also be STRONGLY considered. For example if the core is "warm" relative to the cavity, then with more cooling happnening at the outside durface, shrinkage would be away from the core, enhancing part release (with the related reduction in cored surface finish), while if the core is chilled, then greater draft may be needed for that length of draw.

If it's open ended, then extrusion would seem a preference, though it would be possible to then core from both ends with tuned drag on the moving half of the tool to make sure it always comes off the fixed half. Even if the core was fully on the fixed half, having the external parting line at halfway would allow easier part ejection and require significantly less draft on the core.

Whatever you decide, start with the minimum draft condition and have the tooling such that both cores are able to be removed from the tool independantly. If zero draft doesn't release, then the cores can be removed, machined/polished and replaced for next condition. It also means the cores can be polished for ongoing tool maintenance.

A warning: Polypropylene is nicknamed "polywarpalene" and for good reason.

David Hunt, PE

davidhunt@outdrs.net