Help Troubleshooting DIY Injection Mold for Small Card

[David S]

Stupid question, but do you have enough melt for the shot size?

I am wondering the same thing.

David

 

[Mike28303]

Stupid question, but do you have enough melt for the shot size?

Not a stupid question at all! I'm sure I had enough plastic to fill the cavity, probably 3 times at least before refilling.

As an experiment, if you can pull the mold out easily, I'd throw it in an oven set to 400F, let it warm up and then put it back in the molder as quickly as you can and take a shot.

I would prefer to keep things within the limitations of my mold heater, if possible - The max recommended mold temperature for this ABS is 159F, and I have maxed out the mold heater at 240F with surprisingly no difference in the outcome. I tend to think my mold design is flawed, so if I can change something there and achieve success, I would prefer to.

I'd extend the main runner at least another 1/4 to 3/8" in order to increase retained melt temperature at the transition to the gate runner.

While I don't think your issue stems from venting, I would probably gamble in this case and increase vent depth to .003" and make them a full 1/2" wide with at least 30% of parting line being vented. It looks like you have plenty of venting BUT those unvented corners are a red flag to me. I can see your flow trapping air in each of the four corners as the flow passes the edges of the nearest vents. If I received this as a new tool, it would be immediately disassembled and those corners vented before ever going out to the press.

If your fan gate isn't ramped, consider doing so. If it's only .022" deep across the full length of the gate, you're going to need significant pressure to drive the material through that narrow slot.

Thank you for all the advice ghostdncr. I currently have 1/4" wide vents taking up a little more than 30% of the parting line. I'll be modifying the mold today to have a tiny gate to see if that gives me the pressure I need. While I'm at it, I'll go ahead and add vents to the corners.

You mention increasing the depth to .003 - would that not cause flashing in the vents if the recommended vent depth is .002"? I'm quite new to all this, and vent tolerance is something I've been thinking about in the recent weeks.

I think you're spot on concerning the fan gate being too thin for a hand operated injection. If the small pin gate doesn't get me there, I'll likely go back to using the fan gate with a ramp as you've recommended.

 

[dennys502]

You are trying to force your material around a corner. Can you inject it straight in with a short sprue?

 

[Mike28303]

You are trying to force your material around a corner. Can you inject it straight in with a short sprue?

I certainly can - my reasoning for the 90 degree turn was to be rid of the initial plastic by having the cold slug like you see on many other molds. Do you think I should try a short runner straight into the part?

 

[dennys502]

I was thinking something more along these lines. A short sprue so its easier to push around the corner and not cool as quick. a little deeper for the cold slug.

 

[ghostdncr]

Thank you for all the advice ghostdncr. I currently have 1/4" wide vents taking up a little more than 30% of the parting line. I'll be modifying the mold today to have a tiny gate to see if that gives me the pressure I need. While I'm at it, I'll go ahead and add vents to the corners.

You mention increasing the depth to .003 - would that not cause flashing in the vents if the recommended vent depth is .002"? I'm quite new to all this, and vent tolerance is something I've been thinking about in the recent weeks.

I think you're spot on concerning the fan gate being too thin for a hand operated injection. If the small pin gate doesn't get me there, I'll likely go back to using the fan gate with a ramp as you've recommended.

You are most welcome, Mike. I really enjoy troubleshooting molds, even though I haven't actively done it for a few years now.

My thoughts on the vent depth are based on your hand-cranked press. I've done no research on this, but was thinking our ABS vents were always cut at .002" for use in Cincinnati Milacron presses. I swear, our mold techs could crank those machines up and flash parting line on any tool they put in them! With the lower injection pressures you are surely running with your press, I suspect you can get away with a somewhat deeper vent than what's normally recommended. That is, IF my memory is correct on the production specs.

I particularly like Denny's suggestion (post 13 and 15) of moving the sprue closer to the part and gating directly off the sprue. If this is an option with your setup, that may prove to be a very good solution.

 

[dennys502]

I agree with ghostdncer - you are trying to design your mold based on criteria for large presses.
I looked at the video they have for your press and they are injecting straight into the mold but its hard to say what the material is.
It's been almost 50 years now since I was around injection molding. I worked at West Bend Thermo - Serve as a press operator.
It will take some experimenting for you but that's part of the fun.

 

[rzbill]

This is 100% due to injection pressure limitation. Pressure loss is proportinal to 1/t^3 with t being the thickness dimension of the fan or the card depending on where you are calculating the loss. Since you are getting through the gate, I suspect the main problem is the thickness of the card itself but opening the fan and gate thickness will help

ABS is an amorphous polymer which means the viscocity is less affected by injection rate but more affected by resin temp and mold temperature. Earlier, it was recomended that mold temp be raised. That was a very good recomendation. If you are only making a few, you could raise the mold temp up to the resin temp, inject it and turn everyting off (keeping pressure on the handle until the gate freezes) and let it cool for ejection.

You could try a semi-crystaline polymer whos viscocity is affected by injection rate and whos viscocity above melt temperature is usually quite a bit lower than an amorphous polymer. I'd go for the common ones first such as Polypropylene or Polyethylene. Run the mold at the warmer end of your machine settings (will help warp).

Lastly, if you must have ABS, you may need to go to a center gate in order to reduce flow length (this is simply to allow your machine to shoot it). Try the other stuff first. Center gating will introduce goofy flow patterns in the corners that promote warp.

 

[Mike28303]

View attachment 232550 I was thinking something more along these lines.

Ah, that makes sense. Thanks dennys502, the CAD model really helps me understand. That design would certainly keep the plastic warmer for longer. It would make clamping a little difficult though. I tend to think the most convenient option for my setup would be dealing with the mold temperature.

With the lower injection pressures you are surely running with your press, I suspect you can get away with a somewhat deeper vent than what's normally recommended. That is, IF my memory is correct on the production specs.

In my last two tests I made the vents .003" deep, but I'm afraid the plastic is just cooling too fast. The plastic that reached a vent ended up going back and flashing a little at them, but that's simply because the front of the flow was frozen off and it had nowhere else to go. At least we can rule the vents out as the cause of the shorts shots now! I appreciate the information.

I agree with ghostdncer - you are trying to design your mold based on criteria for large presses.
I looked at the video they have for your press and they are injecting straight into the mold but its hard to say what the material is.

Yes, I think the pressure isn't enough to get the plastic through before it cools. As DAT510 has said, sticking the mold in the oven to 400F and injecting would likely give the results I want.

ABS doesn't appear to be the typical resin used with the small machine. I think it came with Polypropylene. When I bought my ABS, I ensured that it had a compatible flow rate. Up to this point, all my mold parts have been very small and not this thin, so filling the cavity was never an issue.

This is 100% due to injection pressure limitation. Pressure loss is proportinal to 1/t^3 with t being the thickness dimension of the fan or the card depending on where you are calculating the loss. Since you are getting through the gate, I suspect the main problem is the thickness of the card itself but opening the fan and gate thickness will help

ABS is an amorphous polymer which means the viscocity is less affected by injection rate but more affected by resin temp and mold temperature. Earlier, it was recomended that mold temp be raised. That was a very good recomendation.

I'm quite convinced at this point. Just as you and others have said, the pressure capabilities of the hand operated machine is just not cutting it, and heating it up might be the only reasonable option.


Here's a few short shots from the last several days of troubleshooting different designs:



Thank you everyone! This forum really is incredible, as I've never had access to veteran machinists and fellow DIYers like you all until I came here.

 

[sanddan]

A question on the part design. What is the thickness? Thin sections are very hard to fill, on parts I designed (using nylon 6) we used .050" as the absolute thinnest section that would fill without issues. .080" to .090" would be even better.