- Joined
- Nov 18, 2012
- Messages
- 1,352
-
Welcome back Guest! Did you know you can mentor other members here at H-M? If not, please check out our Relaunch of Hobby Machinist Mentoring Program!
Help Troubleshooting DIY Injection Mold for Small Card
Not a stupid question at all! I'm sure I had enough plastic to fill the cavity, probably 3 times at least before refilling.
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.
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.
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?
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.
- Joined
- Mar 15, 2015
- Messages
- 119
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.
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.
- Joined
- Feb 16, 2017
- Messages
- 136
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.
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.
Last edited:
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.
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.
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.
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.
- Joined
- Jan 22, 2012
- Messages
- 655
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.