Community
Fusion Manufacture
Talk shop with the Fusion (formerly Fusion 360) Manufacture Community. Share tool strategies, tips, get advice and solve problems together with the best minds in the industry.
cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 

Advice on Injection Mold Runners and Gates

19 REPLIES 19
Reply
Message 1 of 20
johnXH8PE
3556 Views, 19 Replies

Advice on Injection Mold Runners and Gates

Hi,

 

I've been making DIY injection molds for some time, and usually guess at where to put the gates, and what size to make the gates and runners. I did this with a recent mold I made for running on a desktop injection molding machine. This machine has limited clamping force, and I'm right at the edge of the available clamping force for my machine as it is now:

 

Mold Runners and Gates.PNG

I have some ideas on how to reduce the pressure required in order to fill this mold, and I'd like to get input to see if I'm going in the right direction. First some details. The mold is 6 inches wide and the part is 0.083 thick for most of the cross section. I made the gates (which are only on one side of the mold due to geometry) are 0.05" deep and about 0.08" wide.

 

So here are my thoughts. First, I can increase the size of the runners. Second, I could add two more gates near the left and right side, while also increasing the sprue and runners to accommodate the new gates.

 

Suggestions?

 

Here is a video of my first attempt to make this part: https://youtu.be/kIXllkno8wo

 

Since that video I was able to reliably make parts, but just barely.

 

Thanks,

  -- John

 

 

 

19 REPLIES 19
Message 2 of 20
CGPM
in reply to: johnXH8PE

Do you have enough injection pressure and speed to "reduce viscosity with shear", ie can you fill the cavity in 1 second?  If so then gate design is everything.  What material are you running?  What about mold temperature?

 

Just looking at your runners and gates I would say runners are small, you need some room for a cold slug, and the gates look a little big.  I would only try more than one gate when I could not get one gate to work. TBH I have never molded a part with multiple gates.

 

Just watched your video, yeah your machine is a little different from the 200 ton toggle press I worked with.  First thought is mold temp, you should probably be around 200f to start.  For your gates how wide are your lands?  I think .02" would be my recommendation.  I would also say no more gates and they are already too big.  If anything make your runners bigger.  By the way how big are your runners?

Message 3 of 20
HughesTooling
in reply to: johnXH8PE

I used to work for a business that made small machines like yours for schools and colleges. One trick when the part was too big is to bolt the tool together as well. It slows the production down but you have to work with what you have.  You need to use 10 or 12mm bolts, not sure your plates are thick enough to fit the bolts in. Like @CGPM suggests I'd use one gate, probably pointing at the boss on the right so you get less problems with jetting. 

 

Mark

Mark Hughes
Owner, Hughes Tooling
Did you find this post helpful? Feel free to Like this post.
Did your question get successfully answered? Then click on the ACCEPT SOLUTION button.

EESignature


Message 4 of 20
johnXH8PE
in reply to: CGPM

My machine has about 7,000 PSI maximum of injection pressure and about 4.5 tons of clamping force (http://www.travin.co.uk/tp1.html). So very different from the "real" machines like you work with. I'm using polystyrene and did not pre-heat the molds, so they were at room temperature. I'll have to look closer to see if this mold fills in 1 second--I think it does.

 

The sprue is currently 0.2" in diameter, and the runners are 0.142" in diameter. I made the cross section of each runner 1/2 the cross section of the main sprue.

 

Regarding the runners, you mention that I need room for a cold slug. I believe that's what I have that at the end of the sprue, before it braches into the two runners. Are you saying I should make this larger?

 

What do you mean by the landing for the gates? Do you mean the length of the gates from the runner to the cavity? If so, it's 0.32" at the top and 0.075" at the bottom of the gate.

 

For this mold, since it's a test mold, I can easily make the sprue and runner larger to see what happens.

 

As to using a single gate, I didn't think I'd be able to fill the entire cavity from a single gate because of the limits I have on injection and clamping pressure. Perhaps I'm wrong, as I'm completely self taught, and all of my gates and runners in previous molds (I've made 20-30) were based on guesses. Most of my molds are a lot smaller (well less than 1 square inch). So this is trying to push what I can do with my machine.

 

Thanks. I really appreciate your help.

Message 5 of 20
johnXH8PE
in reply to: HughesTooling

Thanks Mark. I've used bolts in the past for parts that were low volume (a few hundred). I don't want to go that route with these parts as the volume will be higher (in the thousands). I was able to reliably fill this mold after making a few changes to my setup, and I'm currently editing a video that will show that. Basically, I added some 1.5" thick by 6" blocks of aluminum on either side of the mold so I could get better clamping force and distribution. That allowed me to fill the molds and make several test parts with only a couple of sink marks where I have the thick bosses.

 

Given that I'm limited to 7,000 PSI injection pressure, and 4.5 tons of clamping force (and the part has a cross section of 4.6 square inches, do you think I would be able to fill the entire part from a single gate without exceeding the limits of my machine?

 

I've been reading more about gates, and it's really hard to find much good information. And almost all of the information is for machines with much higher injection and clamping forces, so I'm not sure how to apply that advice (or how much of it is applicable) to my machine. But from what I've read, it sounds like smaller gates can actually reduce viscosity of the plastic as it enters the cavity. Do you happen to know how this applies to such a small machine as the one I have?

Message 6 of 20
johnXH8PE
in reply to: johnXH8PE

By the way, here are the most recent results:

 

Success.PNG

Message 7 of 20
CGPM
in reply to: johnXH8PE

Those parts look pretty good all things considered.  You really want only one gate towards one end of the part with a feature in front of it so the plastic doesn't go jetting across the cavity.  You want to fill the cavity from one end with a consistent, smooth melt front.  Once the cavity is filled you should "hold" some pressure, say 5000 psi, while the plastic in the cavity cools and shrinks, this means both your runners and gate need to not freeze off prematurely.

 

To reduce the melt viscosity you either increase the temperature or you can do it through shear with proper gate design and injection speed.  THIS WILL GREATLY REDUCE INJECTION PRESSURE, but you may still need the higher pressure to get it started, I don't know.  With the 200 ton press I used the pressure would drop off after the first .1 second or so when injecting the shot but that was while lowering the barrel temps down 10% below the recommended melt temp because of the gate design.  With a properly designed runner with cold slug wells and that part I would start with a .125" runner with a .03" round gate.  The land, or straight section, should be around 50% of the gate diameter, perhaps less may help with your setup.  I like round gates because they have a constant land size and the sharpest edge to shear the plastic as it enters the cavity.  If you need to go square then it should be square, not rectangular.  A square gate size should be thought of as the size a round pin you could fit though it.  The land at the parting line will be tiny.  When doing test shots you're molds NEED to be the proper temperature for that plastic.  Assuming you're mold temperature is correct then if you have problems filling your mold I would first increase the runner size before increasing the gate size.  If the runner is too big you waste a little plastic, if the gate is too big your, oh let's just say you need to start over.  You want the gate to freeze off before the runner.  You may need to figure a way to pump hot water through your molds and insulate them from the clamp platens, even a few sheets of paper may be enough.  When I tried doing test shots without a way to control the mold temperature we would only get 3-5 shots before the mold got too hot.  We also scrapped a few shots just trying to get the mold warm enough.  You really need to control the mold temperature.

 

You want space for the cold slug to go when changing direction in the runner or for the gate.  I have been told the best way to do this is with a J to feed the gate, with the gate at the bottom of the J.  Keep the J runner around 3/16" from the cavity and feed the gate with a stub runner.  You want the runner to pass the change in direction or gate by around 2 or 3 diameters long.  I never tried a J runner because all of the molds I ever made were with manual machines so that J was too hard to do.

 

Since your mill is right beside your molding machine and your molds are easy to disassemble to do some machining play with runners and gates when test shooting.  Start small and see how it works, make a modification and see what that does.  When I did test shots the mill was 50 feet from the molding machine so I could "adjust" things while the molder sat waiting.  It was always a race to get the mold back in the machine in time to use the shot in the barrel.

 

Any way here are some ideas to think of when trying to get your setup to work.  It is all about making a change and observing the effect.

Message 8 of 20
johnXH8PE
in reply to: CGPM

Thanks @CGPM. I took your advice and increased the size of the sprue and runners. Woooa, did that every make a difference. Increasing the diameter of the sprue from 0.2" to 0.25" meant that I could use about 65 psi for the same results that previously took 105 psi.

 

I've been pretty busy at work this week (had to work unexpectedly on Monday), and I hope this weekend will be better. In any case, my plan is to mill a simplified mold to try out a single small gate and work may way up to see if I can get the shearing that you described. I guess with the small molds I've made in the past, I got lucky.

Message 9 of 20
CGPM
in reply to: johnXH8PE

Glad to hear you have your mold working better.  I forgot to mention in my last post that when running the molding machine with gates that reduce the viscosity through shear we would open the pressure valve wide open and control the fill time with the speed valve, this machine was not computer controlled.  We set the boost pressure to cut out with injection unit location instead of time.  You want to use boot pressure to fill the cavity as much as possible before dropping to your hold pressure.  Set hold pressure time with a very sensitive scale.  Too little hold and the parts get lighter, too much hold pressure and they get heavier.  You know you have it right when you can make small changes and the weight does not change.  We just used a triple beam scale, the exact weight didn't matter, just tiny changes in weight.  IMO you can not really set up a molding machine without a scale, you just don't get the information needed about what is going on with the shot.

 

I love aluminum for molds, they generally run around 50% faster than steel, are much easier to machine, and for all of the molds I made lasted longer than the part designs.  The one downside is the extra thermal conductivity is bad for the runners so you have to enlarge them to compensate.  The faster you can fill the cavity the smaller the runners need to be, as long as they don't freeze off before you are done packing the part.  I contemplated inserting the runners with stainless steel to keep them warmer but never got the chance.

 

When you do your test mold make the cavity somewhat difficult to fill.  Maybe make an L shaped part with the far end thinner, or at least some thinner areas.  You may also want to do some test tabs off the runner near the gate.  A stepped extension of the runner with a flat mill ending in a vent.  Maybe .015" to .002" in .0025" increments with each step about .1" long.  These let you know how much pressure is getting to the gate.  You may need to do two test molds that are identical.  The first will have oversize runners so you are only testing the gate.  Once you get the gate size figured out then the second is to test runner sizes.  As a reference on gate size for a 1 lb part made with 10% glass filled polycarbonate the gate was .08" round, and may have been a little big. 

 

I have been out of injection molding for 20 years but back then the resin manufacturers used to have part and mold design and processing information available that was extremely informative, I think Dupont did the best job with providing this information.

 

Any way have fun and don't get too hung up thinking you know anything when figuring this all out, keep an open mind and keep questioning everything.  You also NEED TO CONTROL THE MOLD TEMPERATURE!  And run it on the high side since you are using aluminum for your molds.  Until you do this you don't know what a limitation it is not to.  Your runners and gates will be much smaller and your part quality and consistency will be far better.

Message 10 of 20
johnXH8PE
in reply to: CGPM

Thanks David. It's going to take me a little while to digest and try everything you suggested. For example, I don't have an easy way to heat the mold while it's in the machine (but I'm working on getting another used machine that has a heater plate). Is this what you're suggesting for the single gate?

 

 

Message 11 of 20
johnXH8PE
in reply to: CGPM

Thanks David. It's going to take me a little while to digest and try everything you suggested. For example, I don't have an easy way to heat the mold while it's in the machine (but I'm working on getting another used machine that has a heater plate). Is this what you're suggesting for the single gate?

 

Single Gate.PNG

 

Message 12 of 20
johnXH8PE
in reply to: CGPM

Thanks David. It's going to take me a little while to digest and try everything you suggested. For example, I don't have an easy way to heat the mold while it's in the machine (but I'm working on getting another used machine that has a heater plate). Is this what you're suggesting for the single gate?

 

Single Gate.PNG

I put it about as far to the right as I can get it, with a single air vent now on the left side. I did some reading about gates, and I've switched to a symmetric gate (so it extends into both the core and cavity. Right now I have it set to 0.08" wide and 0.020" tall (0.010" in each mold half). And the gate is about 0.03" from the inside wall of the cavity to the runner (is this what you call the land?). Of course, I could also switch to a circular cross section for the gate, as you suggested. Just wanted to make sure I have about the right starting point, with a smaller gate. That way I can re-mill the gate, as you suggested, to try different sizes.

Message 13 of 20
CGPM
in reply to: johnXH8PE

You should put the gate on the other side of the part so the resin flows onto the boss/standup.  Having the plastic impinge on a boss is better than a cavity wall.  You also don't have a cold slug well before your gate.  If nothing else make the runner nearly parallel with the cavity extending 2 diameters past where it Ts off to feed the gate.  You DO NOT want a long runner ending at the gate and don't worry about sharp direction changes with runners, they help the cold slug flow into the well.  Also, runners and gates should be ROUND.  If you have to put both in one side then they should be as close to square as possible, except runners should have round bottoms, not sharp corners.  When gauging the size of either that size is what diameter of round you can fit in it.  A 1/4" square runner will flow the same amount of plastic as a 1/4" round runner even though it has more area.  This also applies to gates.  I think round works best for shear since you get the hardest edge leading into the gate when it is round being fed with a round runner, although I don't think the difference between square and round is noticeable.  The gate design you propose will not work to shear the plastic, I know this from experience.  My employer who taught me mold making loved fan gates so all of the molds had them when we learned about reducing viscosity with shear.  If you don't want to trust me on this then take the time to prove it to yourself.  I believe that the golden rule of cold runner gates is they are round or square.  If you need to vary something it should be the size of the gate and the size of the land, not the shape.

Message 14 of 20
johnXH8PE
in reply to: CGPM

OK, I misunderstood. I've moved the runner over to the other side, as you can see here:

Single Gate 2.PNG

I'm still not sure what to do about the gate, as there is no cavity on the other mold half where I have the gate. So I've draw a half circle cross section for the gate. Is that what you're suggesting? Or should I switch to a square gate? I've set the diameter of the runner to be .25" to make sure it's not the limiting factor. And I'll start with a gate "diameter" of 0.03.

Also, can you explain why it's better to have the gate so the plastic hits the boss instead of going into other parts of the cavity?

Thanks again for your input. I've I'm not doing what you suggested, it's most likely because I'm not following every detail. I like to understand the reasons, but I haven't been able to find a good reference to explain gate design.

-- John

Message 15 of 20
johnXH8PE
in reply to: johnXH8PE

I didn't check carefully enough before saying I could only use a half circle for the gate. When I turned on the part, I saw this:

 

Single Gate 3.PNG

 

 

So that means it's right on the dividing line between the wall and the section that will hit the boss.

Message 16 of 20
CGPM
in reply to: johnXH8PE

If you can't have your gate in both sides of the P/L then use a square gate, I have had to do this many times and it doesn't make much of a difference, if at all.  The only real problem is the land.  Make it as small as you dare, .015"-.02", at the P/L and just live with the bottom being long.  The new gate location is perfect!  I think the reason you want the flow to not hit the parting line is two fold.  First, if it hits a boss it has more places to go and second is you want to protect the cavity parting lines as much as possible.  With your first gate location, I think it would increase the possible pressure and wear locally on the parting line.  The one problem I see is you have a radiused entry to the gate runner.  You need to make is a straight T and increase the main runner by 2 diameters past the T for the cold slug.  Think of it as a hard cold front to the plastic as it flows through the runners to get to the gate.  You do not want that cold slug getting in the way of the gate when you are trying to fill the mold.  You want nice, hot plastic to get to the gate first with as much velocity and pressure as possible.  The only reason you may want to slow down filling the cavity is if you can't vent it fast enough.  This is why the J runner sounds like it really would be the best style since the gate is right off the side of the runner.  I would think the cold slug would go right past it.  If I were doing the runner I would have it extrude along a spline so I could create a freeform path having the runner curve down to the side of the cavity to your gate location then curve away from the cavity for 2 diameters for the cold slug.  You want to vent the ends of the cold slug wells when possible by the way.  

 

When you do your test shots on the mold once you get the mold filling correctly then slowly lower the melt temp.  As you lower the melt temp increase the injection pressure, the velocity valve should be wide open.  Once you find the lowest temp you can run then bump it up 10 F and you are good to go.  The reason you increase the temp is this was what gave us the biggest molding window.  This is where molding was most stable with changes in shop temperature or other variables. How much control do you have of the injection cycle?  Do you have boost and hold pressures and if so how do you control where/when boost transitions to hold?

 

The reason I keep harping about controlling your mold temperature is because it needs to be at a certain temperature for best results.  Stray 20 or 30 degrees f and the difference can be very noticeable to the point of not getting good shots.  To low and it gets harder to fill the cavity and the part surfaces don't look as nice, it will also draw the glass fill to the part surfaces on filled resins.  Too hot and it takes forever for the resin to set making it very hard to get a solid packed part.  Until you control this variable the quality of your test shots is significantly hampered, at best.

 

Did you get any processing information with your plastic?  It should tell you not only the melt temp but also the mold temp.  Have you talked with any application or technical engineers at the plastic manufacturer about mold and part design?  They may have manuals or technical bulletins that would be very informative.

 

I don't mean to sound too brusque but I want to be as clear as possible and if you don't understand what I am trying to explain please say so. How I learned about gate design using shear to reduce the viscosity of the plastic was from a seminar a local plastic supplier did.  I didn't get to go but I did get the information and later talked with some application engineers that would occasionally come out with the salesman, which was a wonderful bonus.    I then applied this information to the molds I was making, and repairing, and learned the rest through trial and error.

Message 17 of 20
CGPM
in reply to: CGPM

I found some design info on Dupont's web site. This is the type of design information some plastic manufacturers have available that I asked earlier if you had ever look for.  Some of the documents, like the Delrin Molding Guide, had very specific information on mold and part design. 

Message 18 of 20
johnXH8PE
in reply to: CGPM

I was busy the past few nights, so I just now got time to work on the design changes you suggested. I think I finally may have gotten what you meant by a J runner:

 

Single Gate 4.PNG

 

The land is currently 0.015", so it doesn't show up that well in this picture. I've managed to keep it circular since there is a cavity on both sides of the mold in this location. I've extended the runner and added a vent at the end. And used a T for the short runner to the gate.

 

I haven't ignored your suggestions about the mold temperature, and I've been mulling over an idea. I'm thinking about creating a heater block that I can attach to the back of one half (or both halves) that would have a cartridge heater in it attached to a PID controller. I would use the same 24-volt cartridge heaters that I use on my 3D printer. What I don't know is if I'll also need cooling, or if I can get away with a fan for cooling the blocks. I'd prefer to avoid water heating/cooling if I can. Since I'm using polystyrene at the moment, the suggestion I've found is 120 F for the mold temperature.

 

As to my plastic supplier, it's not a direct source. Because my machine is a plunger style machine, I purchased pre-colored pellets from a model railroad company that will sell small batches (I think I bought 10 pounds at a time).

 

Thanks again for all of your suggestions and information. I wish I'd had access to this when I first started making molds about 10 years ago.

Message 19 of 20
CGPM
in reply to: johnXH8PE

Mold cooling will be more important than warming if you go anything other than a few shots at a time.  The one time we tried doing very rudimentary test shots without being able to control the mold temperature was a total waste of time, other than teaching us that no matter what we had to have water channels in the molds.  We had trouble warming the mold up by running it as we were haveing problems filling the mold, then we got about 3 good shots, then the mold got too hot.  We tried fans to cool it but the residence time in the barrel was getting too long.  When molding you need to get into a smooth consistent rhythm and the only way to do this is to control all of the variables.

 

The runner and gate look good but it looks like the vent is pretty big.  I would suggest .001" deep x the width of the runner, with a 3/16"-1/4" land.  A post I read while looking for plastic information may be interesting.  The person said that you only needed cold slug wells for the sprue.  The melt front does not push forward leaving a cold slug in front of it.  It moved forward like lava with the melt front sticking to the walls of the runner exposing fresh resin from the center.  This goes contrary from what I have always been told but it is worth thinking about.

 

Do you dry the resin before running it?

Message 20 of 20
johnXH8PE
in reply to: CGPM

Hi David,

 

I made a new set of test molds to try out the ideas. I also heated the mold as well as the 1.5" thick spacer blocks, all aluminum. So far, I've only been able to fill about 1/2 of the mold without the mold halves starting to separate. I started with a 0.030" diameter gate and have been working my way up in size by 0.005" increments of diameter. My most recent test was a 0.045" diameter gate. What's interesting is that I'm not seeing much difference between the 0.030" diameter and 0.045" diameter gate. So my guess is that my pressure requirements are all about filling the cavity, and my machine just doesn't have enough clamping force to be able to fill it from only one side. Even with the molds heated, and with the injection temperature higher. My maximum clamping force is 4 1/2 tons under the best of conditions.

 

The molds and blanks actually cool off pretty quickly. So after about 10 shots I can easily handle them with my hands instead of gloves. That, I'm guessing, means the plastic I'm injecting doesn't have enough thermal mass to be able to keep the molds heated.

 

I can try increasing the size of the gate even more. But based on what you described, that probably won't make any difference. So, it seems like my options are either to feed in two places like I was before. Or use a machine with more clamping force. And in any event if I want the molds heated, I'll need to add a heater to keep the up to temperature. So the bottom line, unless I'm missing something, seems to be that these desktop machines have very different mold requirements than the big machines. Do you have any other thoughts?

 

Thanks,

  -- John

Can't find what you're looking for? Ask the community or share your knowledge.

Post to forums  

Autodesk Design & Make Report