Help Troubleshooting DIY Injection Mold for Small Card

Greetings,

First, I'd like to say thank you to the members of this forum. I began machining just under 10 months ago, and the advice people have given me here has really propelled me forward as I try to climb each mountain.

I am trying to successfully inject a mold I made for a card, but I keep getting short shots. Here is a picture of the mold and the short shot:







I am concerned that I'm not designing the mold properly, as I'm completely self-taught. I have successfully made several other molds; however, this was the largest and thinnest I have attempted. I did some research online and found a card mold with a fan gate, so I based it off of that.

I am using an LNS 150A (a benchtop, hand operated injection machine), and the material is ABS with a melt mass-flow rate nominal value of 21g/10 min .

Part Size: 3" x 2" x .03"
Vent Depth: .002"
Fan Gate Depth: 0.0225"
Runner Width: .25"

I realize this part is difficult to inject largely due to it being very wide and thin, along with material's viscosity. I'm thinking perhaps the land on the fan gate is simply too much, and my guess is I should mill the runner longer so it leaves a gate with a land of about .02" . Again, this is just a guess.

I've heated the mold to 130 F with a barrel temperature of 460 F, and still cannot get much better. Any advice would be appreciated!

- Michael

Heat the mold if not already done? Or maybe a lack of pressure and venting.

I'm not sure what kind of pressure you can get from your equipment, but perhaps increasing the mold temp 20 degrees might help compensate for my assumed lower injection pressures than you might expect from a hydraulically operated unit.

I agree that increasing the gate size can help the material inject quicker, but you would lose some pressure at the gate. Not really being familiar with injection molding, I dont know how important the amount of pressure at the gate you have is.

Either way, I wish you good luck!

Sent from my Nexus 7 using Tapatalk

Wreck™Wreck said:

Heat the mold if not already done? Or maybe a lack of pressure and venting.

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emccarthy25 said:

I'm not sure what kind of pressure you can get from your equipment, but perhaps increasing the mold temp 20 degrees might help compensate for my assumed lower injection pressures than you might expect from a hydraulically operated unit.

I agree that increasing the gate size can help the material inject quicker, but you would lose some pressure at the gate. Not really being familiar with injection molding, I dont know how important the amount of pressure at the gate you have is.

Click to expand...

I just did another test with the max recommended mold heat for the material (159F), and it didn't get any better. As for mold venting, I followed the information found on this page for my vents: http://www.plastictroubleshooter.com/ThePlasticTroubleshooter/mold_venting.htm

As for pressure, it's as much as I can do with my arm strength. Thank you for trying to help! I think you're right emccarthy25, about the pressure being lost with a large gate. I'm going to look into that more.

Pressure loss means flow velocity loss which means temperature loss which means viscosity increase which requires more pressure. Lack of adequate pressure may be the root of the problem, and you will have to make adjustments in other parameters to succeed. I've built a fair number of molds, but it's not my strong suit. So take my ideas for what they cost you, that's what they may be worth. Most of my mold work has been in PE, which seems to me to be one of the easier to work with polymers.

Most of the molds I have built were high volume, high speed molds with liquid cooling and electric heating....so the rules are going to be a bit different.

I would heat the mold. Given the thinness of the card and large associated surface area, all the heat is being sucked out of the plastic before it can travel the length of the mold.

I would heat the mold to just slightly above the glass transition temperature of the ABS you are using. You may need to bump the temp up or down from there until you get a complete shot. I've often used a digital hot plate to zero in on the temp to heat small molds. It will slow your cycle time down. But given, you would be getting parts, cycle time might not matter?

You could also try multiple gates to get polymer to all parts of the mold more quickly, though you could end up with knit lines and weak points if the polymer cools too much before knitting.

Chris

Tony Wells said:

Pressure loss means flow velocity loss which means temperature loss which means viscosity increase which requires more pressure. Lack of adequate pressure may be the root of the problem, and you will have to make adjustments in other parameters to succeed.

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Thank you for the advice, Tony. Yes, the pressure I achieve with this little hand-operated machine is nothing compared to what most pros work with. You mention making adjustments to other parameters to make up for lack of pressure - could you name a few? Currently, I'm thinking of making the gates smaller, but it's all guesswork.

DAT510 said:

I would heat the mold to just slightly above the glass transition temperature of the ABS you are using. You may need to bump the temp up or down from there until you get a complete shot.

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Thanks, Chris - I first tried 230F, then I maxed the mold temp controller at 240F, and still get short shots. You're correct, the cycle time is not as critical for this level of work.

I'll save adding more gates for a later solution, as it already barely gets past the fan gate. I would need to add a lot to get it to fill completely. But perhaps my gate is the problem.

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

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. If you get closer to a full shot, it tells you the mold is cooling the shot too quickly, and you can adjust the mold temp from there.

I love working with this kind of problem so here goes a bunch of random thoughts that may or may not help. Given the parameters you're working with, here's some things I'd try to get a full shot.

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. Ramping the gate will (in my experience) dramatically decrease cooling in this critical area and keep your material flowing. Considering you're using a hand cranked press, this is probably what I'd try first.

Aside from that, I would play with both material and tool temperatures. With the hand-cranked press, increasing injection velocity may not be so much of an option. I wouldn't think slowing it down would help at all, and you may already be injecting near maximum speed.

It'll be interesting watching this play out!