# Help Troubleshooting DIY Injection Mold for Small Card



## Mike28303 (Apr 28, 2017)

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


----------



## Wreck™Wreck (Apr 28, 2017)

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


----------



## emccarthy25 (Apr 28, 2017)

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


----------



## Mike28303 (Apr 28, 2017)

Wreck™Wreck said:


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





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.



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.


----------



## Tony Wells (Apr 28, 2017)

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.


----------



## DAT510 (Apr 28, 2017)

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


----------



## Mike28303 (Apr 28, 2017)

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.



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.



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.


----------



## vtcnc (Apr 28, 2017)

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


----------



## DAT510 (Apr 28, 2017)

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.


----------



## ghostdncr (Apr 29, 2017)

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!


----------



## David S (Apr 29, 2017)

vtcnc said:


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



I am wondering the same thing.

David


----------



## Mike28303 (Apr 29, 2017)

vtcnc said:


> 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.



DAT510 said:


> 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. 



ghostdncr said:


> 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 (Apr 29, 2017)

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


----------



## Mike28303 (Apr 29, 2017)

dennys502 said:


> 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 (Apr 29, 2017)

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 (Apr 30, 2017)

Mike28303 said:


> 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 (Apr 30, 2017)

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 (Apr 30, 2017)

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 (May 1, 2017)

dennys502 said:


> 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.



ghostdncr said:


> 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.



dennys502 said:


> 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. 



rzbill said:


> 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 (May 2, 2017)

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.


----------



## Mike28303 (May 3, 2017)

sanddan said:


> 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.



The thickness is .03", across the whole length of the 2" x 3" card. I'm about to do a test heating the mold to the melting temperature of the plastic, as others here have suggested. I don't think the hand-operated injection machine was meant for a part like this, but I really want to make it work.


----------



## Plas62 (May 5, 2017)

.030" is pretty thin. If you can increase your part thickness as Sanddan suggested you will have a better chance at success.

If you goggle "recommended wall thickness of abs parts" you will get some hits. I copied and pasted some info below that I found.


*Material*

*Recommended Wall Thickness*
ABS 0.045 in. - 0.140 in.
Acetal 0.030 in. - 0.120 in.
Acrylic 0.025 in. - 0.500 in.
Liquid crystal polymer 0.030 in. - 0.120 in.
Long-fiber reinforced plastics 0.075 in. - 1.000 in.
Nylon 0.030 in. - 0.115 in.
Polycarbonate 0.040 in. - 0.150 in.
Polyester 0.025 in. - 0.125 in.
Polyethylene 0.030 in. - 0.200 in.
Polyphenylene sulfide 0.020 in. - 0.180 in.
Polypropylene 0.025 in. - 0.150 in.
Polystyrene 0.035 in. - 0.150 in.
Polyurethane 0.080 in. - 0.750 in.


----------



## petertha (May 5, 2017)

Hope you get your issues resolved. This is fascinating. I'd like to see some pics of your setup one day. My hack amateur work is pouring vacuum de-gassed 2-part urethane resin into molds, but that's certainly not a production operation & limited to the properties of the resin which are probably different to the plastics you are using.


----------



## sanddan (May 5, 2017)

Plas62 said:


> .030" is pretty thin. If you can increase your part thickness as Sanddan suggested you will have a better chance at success.
> 
> If you goggle "recommended wall thickness of abs parts" you will get some hits. I copied and pasted some info below that I found.
> 
> ...



Those numbers make sense, my parts were 30% glass filled nylon so my min of .080 closely matches the chart's .075.


----------

