Anyone know the answers to any of 'em? I'll take partial answers
Thanks guys (and gals)!
I haven't made a mold yet, but I plan to get into it once I get my CNC machine (next 60 days). I have read a lot of stuff online about part design for molds and my background is mechanical engineering. I know about runners, drafts, radiused corners, gussets. etc. etc. There are a few gaps in my understanding of molding though that I hope someone can fill in for me.
First, what is the mold opening supposed to be like? (where the nozzle goes and injects the melt) Is it a taper? What are the specs of the taper? Does it vary by the machine? Is it just a taper or something more involved?
Second, its my understanding that the nozzle would move into the mold opening with each shot - am I wrong? As I understand, the mold closes, pressure is applied, the melt is injected, it holds while it cools, the nozzle retracts, then the mold opens and the part comes out. Or does the nozzle stay in the mold?
Third, is there any info on water cooled molds? Is it as simple as machining water channels in the back side of the mold, putting a plate over them with a gasket and running water lines? How is the mold cooled - would a leftover car radiator with a pump inline and an automotive 12 fan do? I know it's not "professional" but would that be in the ballpark or are we talking $50,000 chillers to accomplish cooling?
How do you determine the thermal aspects of the mold? I mean, if the mass of the mold was big and it radiated a lot of heat wouldn't it potentially not ever get to operating temp because it would cool faster than the shot of plastic heated it up? Or is this not a real concern?
How do you detemine how many tons machine you need to mold a part? Is there a rule of thumb? I want to make car tail light housings (eventually).
About surface finish... if my mold has a mirror finish, do I get a smooth-as-glass finish on the part (think a brake light lens)? If I make the mold in AL and then anodize it, in my experience it reduces the finish, so would this mean I'd need to use a steel mold (I hope not, I'm talking part counts of 500 pieces or 1000 pieces over the life of the mold at most).
I know the layout of a mold machine... 4 guide bars (from what I've seen) that hold the halves of the mold. Do I just mount my mold in these bars? How does the ram attach to the mold? Do I buy a "mold blank" from the molding machine manufacturer or do I incorporate the fixturing for the ram and retraction mechanism onto the back side of my mold?
How do you determine where the shot will enter the mold and where to put a parting line? On a brake light lens, I know whereI would like my parting line which would imply the shot injection point would be at the top. Are there other considerations other than cosmetic about where to part the mold or inject the shot? Such as the further the melt travels the more chance it has to cool and cause problems?
I think thats it for now Sorry to ask so many ???'s but I understand the molding process except these parts. I have been looking at books on Amazon but they are so expensive (like >$300 for some of them). I want to lean what I can on my own before I invest that much. I'm also signing up for a moldmaking course at my local engineering school but it doesn't begin until August and I'd like to play around with some molds before then.
Anyone know the answers to any of 'em? I'll take partial answers
Thanks guys (and gals)!
I don't know a lot of the answers to everything you asked, but the topic is worth a discussion
The molding machines I've read about are rated in "ounces per shot", which is the weight of material it will inject per cycle. So I guess you would need to know quite accurately what the volume of your part is going to be, to match it to a particular size machine.
The molder has its own press platens. One side is fixed, and the other side moves on the 4 guide rods you referred to. All you have to do is mount your mold on the fixed platen, then close the moving platen on it, and fasten the upper half of the mold to the slider. Then, its ready to shoot.
The finish inside your mold is exactly what you'll get on your part. A mirror finish is required for something like a lens cover. You need a good, modern high speed machine to generate a good enough surface directly off the mill, with no hand polishing. I've never done one of these, but I suspect I guy might still need to polish a little bit here and there.
The parting line needs to be in a natural location where the part can be ejected from the cavity without catching or hanging up on something. It is best along an edge, where it visually won't be so noticeable, I suppose.
I believe you'd want to shoot the injection in an out-of-the-way position that will not be noticeable, not right in the middle of the top face. The sprues still have to be cut off.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
The spruee is where the machine injects the plastic into the runners, then the runners carry it past the gate to a cold well, the when the cold well fill, it forces it through the gate, and into the mold cavity / cavities. There will also be some say .5 width x .005 deep slots to let air out of the mold cavities. For a brake light lens you will need to have a ejector pin setup. (Dont want to have to pry out part from the mold, could ruin the polish).
Tonnage differs between people. some say 2 sqin per ton, other say 1.5 or The 2 can be done with certain materials, but most use the later two to have a saftey margin to insure they can clamp the mold properly.It also can make a difference if you are using a toggle locking press or a hydraulic press (about 50 tons max). I would guess a tail light mold would be roughly say 12x8" which would require roughly 144 ton machine for a sinlge lens.
The shot size would be the total part size+ all gate runners, coldwells, spruee. Basically everything that plastic would fill in the mold. My machine has a 3 oz shot which limits it to small parts.
The parting line on tail light molds can get very complex, (and relativley deep). I would double / triple / quadruple check the parting line. Not so much that you will mill and under cut, but you may not get the part you want if not done corectly.
I would say that aluminum would be a poor choice for you mold application, as it will last for the required number of parts, but the parts would most likeley have a optical degrading effect, as the aluminum may loose some of the polish as the cycles run. I have never tried a mirror polished aluminum tool, but all I use is aluminum for my molds, as it is so much easier and faster to mill, and actually pulls heat away great and (in my small pres) negates having to setup my chiller.
I have posted a picture of my molder in my photoalbum if you want to take a look. I will try to get a few better pictures of it.
For finish and het conduction, consider copper plating the aluminum mold.
Caswell plating sells some plating components, which I have bought, but haven't used yet.
Because of its gret conductivity of heat, copper has been used to make molds for casting iron!
Sounds like you are going all out with this moldmaking. If you want to learn about molds and not spend a fortune on books, go and get yourself an account with DME Mold Bases, have them send you a catalog on disc for both DME products and Master Unit Die Products. You could also open an account with Progressive Components, PCS Pins, National Mold Bases, and Misumi of the Americas. DME sells mold bases of all shapes and sizes, but you will see that many of the components are standard features seen throughout all of the molds.
Most "stand alone" type molds have a sprue bushing that is tapered 1/2" per foot, that's just the standard, you could go with a different taper if you like. The small end of the taper will be slightly larger than the orifice in the injection machine's nozzle tip. These tips will either have a 1/2" or 3/4" radius on the end of them. This radius is there so that it is self centering, the sprue bushing will have a corresponding radius on the end of it that the nozzle seats against, this is the mating surface of the machine to the mold (injection wise). Most all molds will not have a sprue bushing that moves, it is part of the mold, but it can be removed for cleaning from time to time. Most "stand alone" molds will have a locating ring that fits a hole in the stationary platen of the molding press. This locating ring will hold the mold in position so that the nozzle can be postiioned against it. The platens of the machine will have threaded holes in a pattern that will allow clamps to hold the mold. There is usually a clamp slot in each half of the mold to allow clamping.
Whenever I build a mold, my water lines usually only constist of drilled holes tapped to accept a quick disconnect type fitting that is sold by DME and various other molding suppliers. The water lines are not generally the first consideration when designing a mold, however you must allow enough space for water lines on all four sides of a part (sometimes with smaller parts, water lines can be straight thru without passing around the part). With very large parts, water lines will usually be placed underneath the part, between ejector pins, and anywhere else that is possible to put them. I have a customer that ran one press for several years with just a 100 gallon poly tank filled with water, and a small tower outside that had a fan to cool the water that circulated thru a large radiator. Your cooling water will be used to cool the barrel of the machine where the plastic pellets enter the screw, as well as the oil cooler for the hydraulics.
When trying to size a machine for a particular part, the part volume it the most important factor. Your machine should have a shot size large enough for the part (or parts) the gates, runners, and the sprue all together, plus enough extra shot for a "cushion" to hold pressure on the plastic while it is cooling. This hold cushion is what prevents "sink" in a part. As for the machine tonnage question, from my experience, as long as the mold will fit between the tie bars of a machine, the machine will hold it shut. Very seldom will you find a mold that a machine can't hold shut, as long as it fits the machine.
If you use 7075 aluminum or QC-7 aluminum, you may not need to anodize your molds, and you could polish the cavities to a mirror finish and get the desired finish you are looking for. I understand how you wouldn't want to machine a steel cavity for only 500 pcs.
As for how the "ram" or injection unit fits the mold. The injection unit is a large barrel with a screw inside of it. This barrel will have heaters wrapped around it and they will be heating the plastic in stages. At the front of this barrel is a cap and the cap will have a small tip with a hole in it, this is the nozzle. The nozzle has the radius that fits the radius in the sprue bushing. The entire injection unit (barrel, screw,heaters, nozzle) are all on guide rails, and can be pushed forward and back by hydraulics. When setting a mold into the press, the inection unit will be pulled back out of the way to allow the locating ring to be positioned. Once the mold is in position, the machine will be closed on the mold, the machine will hold the mold there while you clamp the two halves to the corresponding platen. Once the mold is bolted in place, the screw inside of the injection unit is turned hydraulically to force the plastic pellets forward, as they are melted and pushed forward they squirt out the nozzle tip. This is called "purging" and it is used for various reasons, when changing colors, when changing from one type plastic to another, for cleaing out impurities, or for cleaning our degraded material that has been overheated.
There are many options when choosing what to build your mold out of. You could go with a stand alone mold base, that is clamped into the press by clamps. Or you could opt for a Master Unit Die type mold, where you have a "frame" that bolts into the press, and you have inserts that are interchangable in the frame. The advantage to MUD type molds, is that they are cheaper that a stand alone mold base, and you can do a mold changover in about 1 minute. You could also opt to build you own mold base, which is the lease expensive but most time consuming.
As for where the plastic enters the part, this is called the "gate" and there are several different types of gates.
Sprue gate - where the tapered sprue is actually attached to the part being molded, this allows a larger part to be molded in a smaller mold, the drawback is that this sprue gate must be trimmed from the part and it leaves a mark, however, you could build a fixture and use a wood router to trim it flush and make a nice dot.
Tab Gate - This consists of just a small slot cut betweet the part cavity and the runner of the mold, this tab will need to be cut to free the part from the runner (like on model cars and Beyblades) This trimming is fairly simple and not very noticable.
Fan Gate - just like the tab gate, except that the gate fans out as it goes from the runner to the part.
Sub-gate- this is the gate that I use most, because is allow the part and runner to be seperated when the part is ejected from the mold. This gate is usually tapered (approximately 15 deg) and is cut from the runner to the part on an angle (usually 45 deg) down into the steel. The plastic travels down from the runner into this subterranian tube. Once the plastic cools, and the mold opens, the part is pushed forward by the pins, and usually a pin is placed under this sub-gate to push it out. This seperates the part from the gate and runner. I have used sub-gates that go into the stationary side of the mold, in this case, an undercut "puller" is required to pull the subgate onto the moving side before ejection.
Some other gate types include: ring gate, wedge gate, and cashew gate.
As for where to put the gate, usually whereever the customer will allow it. Most surfaces that can be seen should not have an ugly gate mark on it. With the sub-gates you can't put them on the parting line so they will show either on the inside or outside of the part. Tab gates or wedge gates, or fan gates are placed on the parting line because they will be trimmed. A fan gate can be thinner than a tab gate because it spreads out to allow more flow. A wedge gate can be made to break apart very easily. Most mold builders will determine the gate location by the shape of the part, usually trying to inject as fast as possible without trapping excess gases. If mixing of the plastic during injection is a concern, then shooting the plastic into a thin wall first will cause it to mix better. Bottom line about gates is that every part is different and requires different gating.
I hope I haven't been too long winded here, I love building molds, it's what I do. I am always happy to share what I know.
Send me an email and we can walk you through this way too many ?? at once. I am a tooling engineer - injection molds.
I couldn't tell if you are getting (or have) an injection press. I'll assume that you are.
Jim is right, you're going to need some type of cooling. And a large volume of water is a cost-effective way to start. However, keep in mind that some materials need special processing conditions. For example, when we run ABS, the mold temperature is kept at ~35°C. You won't need a heater (the melted resin heats the mold) , but some sort of regulation is required to keep the mold temperature steady once it gets there. If your parts aren't very critical, watching the mold temperature and adjusting the coolant flow by hand should be enough. Though, this is a CNC forum, so it shouldn't be too difficult to automate.
An aside: A molder friend runs a shop that specializes in high-end engineered resins (i.e $$$). In his travels, he had to inject a resin that requires a mold temperature of ~125°C. He used oil, because water would have turned to steam. They were prototype reeds for a 2 stroke engine and had to withstand high temperatures, flex at 150 Hz, and retain their shape. Really neat material--the part was half a hollow sphere and we couldn't get them to break when we hammered them with a 5 pound sledge.
I'm a plastic part/mold designer by day and parting lines are the single most difficult aspect of my job. On a part like a cup, the parting line is easy. But on ergonomic devices (that everyone likes so much these days), the parting line can run every which way. And trying to hide it for cosmetic purposes only compounds the problem. Good CAD software is really the only tool you have. Whatever software you have, get comfortable with the parting line tools. The learning curve is steep, and nothing is more frustrating to find out that your software can't generate a parting line for the job you were just awarded.
Also, complicated parting lines are a handful from the machining standpoint. The parting line is basically two faces (one on either side of the mold), that mate to create a seal-off that stops resin flow. A planar parting line is easy; both sides of the mold need to be flat and flush--not hard to do with a CNC. But a complicated parting line involves 3D milling. Getting both sides to match up perfectly is difficult, especially when you take into account things like machine stability and setup inaccuracies. And, if they aren't flush, the part will have flashing when it comes out of the mold. However, there is some allowance. Most resins have a minimum gap through which they will not flow. Kmed mentioned a 0.5" x 0.005" air vent. So, the material he's injecting will not flow through that 0.005" gap. On the parting line, it'll be the same thing. But, I wouldn't go to 0.005" though, 0.0005" to 0.001" is probably the maximum.
Injection molded parts tell all. Any tiny nick or gouge in the mold will be magnified on the part (especially when the part is in the hands of the QC guy...). If you need a perfectly smooth surface or the parts are for optical use, you will need to polish the mold to a mirror finish. Just like body work, keep using finer and finer grits. However, no sandpaper will give you a true mirror finish--you'll need diamond paste for that (such as: https://www.travers.com/pdfshow.asp?p=863). Needless to say, it's really tedious work; we usually outsource it.
More often than not, a textured finish will be acceptable (most computer hardware is). The mold will still need to be polished, but not to a mirror polish. It would go for texturing long before the diamond paste is used.
And be sure that your customer really needs a mirror polish. Two years ago, we built 5 molds for a customer. When asked to choose a surface finish (based on our sample cards), he wanted mirror polish. The first shots were delivered to him and he was not happy. The surface finish reflected too much light (making the part look oily), any scratches from handling/installation looked 10 times bigger than they were, and, the parts were installed into an area where every other part near it had a matte finish. Mirror polish sounds cool, but it isn't always required.
However, the runners should be highly polished. Any surface roughness will slow down the melt and increase cycle time and injection pressure.
My last bit of advice: There are a lot of aspects in moldmaking. Taking them on all at once can be daunting. I suggest you assume what you can, and outsource the rest. i.e. Your CNC (3 axis or 5 axis?) will be good for the majority of the work. However, you will eventually come across parts that require a ram EDM to manufacture (such as thin, deep features). Outsource it for now, and buy a machine once it's affordable/required.
You're entering a field that'll never get boring.
I would goto the SPE site ( Sociaty of Plastic Engineers) look for a book on Plastic Mold Design and spend the $60 to $125 bucks. Then you will have a hard copy on how to build a mold. It would be worth the investment.
I have been really busy at work and this thread started off slowly but I am amazed and grateful at the EXCELLENT info provided here! Thanks everyone, especially Chris and Jim. Hey if anyone is near the greater Boston area and wants a free worker for a week, I'd be happy to work for nothing to learn a little more about this
I feel I have a pretty good understanding of the whole process. I have been reading about mold design for a long time.. the thing that always got me was it was described as a "black art" and very specialized, but it didn't seem like there were really THAT many rules to follow, just a few basics and go from there (i.e. drafts, part thicknesses, gate locations, etc). Maybe the intricasies of mold design are being unveiled quicker due to groups like this!
So based on the excellent posts above, I had a few followup questions. I am assuming (could be wrong) that on the two halves of the mold, one will be fixed and the other will move. Generally the fixed piece will contain the sprue bushing so that you align it with the screw nozzle when you insert the mold then you don't need to worry about its alignment from then on (am I right on that?). Now it would seem to me that for most molds, the part will tend to stay in one side or the other but it will generally always stick in the same side and that's where I would want my ejector pins. Now how are ejector pins actuated? Do they get activated by a seperate mechanism, or does the moving platen coming backwards somehow actuate the ejector pins? And if the moving platen actuates them, that would mean the moving half of the mold would be the one where I want my part to "stick" upon mold opening so that the ejectors can remove the part, and given I want my sprue bushing to be on the fixed part, doesn't that limit my options of where to inject the plastic? Or am I way off here?
I was just looking online and I think I am wrong... I read about "ejector mechanisms" that seem to be standalone mechanical mechanisms not related to the opening/closing of the mold. How does an ejector mechanism work - air powered? electric? hydraulic? On ejector systems, when I look online, I see pics like this:
Now for many molds, there may not be a flat surface where you can place the ejector pin to push against... so how does that flat ejector pin fit into the puzzle? Are ejector pins sold as blanks and meant to me machined (or ground?) to the profile you need? I picture the ejector pin being totally flush inside the mold with it's other end sticking out the back of the mold. When you push the pin, a small piece of the mold surface will protrude, knocking the part out. Or do I have this all wrong? Once the pin is actuated to remove the part, what retracts the pin? Is it spring loaded or affixed to a retraction mechanism?
I am looking at specs of machines... I use Pro/E to design parts so it can tell me the size and volume of the part so I can determine what size machine I need. I've heard you want to be between 60-80% of the machines "shot size" per cycle, including runners/gates and the part(s). Does that sound reasonable?
And finally (I know, I know, I ramble on.. sorry) can anyone give some pointers about buying a used molding machine i.e. on eBay? There seems to be some great deals on there. Lets assume I determine the size machine I want, is there any common brands to look for? Or does it not matter? Better to buy a newer machine and see it under power before buying? Any pointers or advice would be very helpful in this area.
You guys have helped out so much and I can't thank y'all enough. Its such a great resource to be able to converse with experts on this subject.
You were correct in most all of your assumptions. The 'ejector mechanism' is usually part of the mold itself. Most molds will have an ejector housing plate that have two other plates that can travel forward and back within this. The ejector housing plate is actually a plate with two shoulder rails that create a space for the ejector plate and it's back-up plate to move forward and back within. Some moldbuilders will spring load their ejector plates, and some will connect the plates to the molding machine so that the hydraulic cylinder of the mold machine will push and pull the ejctor plates.
You were correct with both of your assumptions about how the ejectors are pushed forward. Some old molding machines had a rod that pushed the ejectors forward whenever the platen came back during mold open. Most new machines have a seperate hydraulic cylinder to push the ejectors forward, either during mold open or after the mold is opened, or even push them forward several times, for parts that want to stick to the pins.
I machine 100% of the ejector pins that I buy, they come in standard lengths 5,6,7,8,10,12" etc. etc. and they need to be cut off and ground to length. If there is not a flat surface to put a pin on, you can grind the end of the pin to fit the coutour of the part. There are also other options for ejctors, instead of pins, you could use blades or sleeves. Blades are flat rectangular shaped and work basically the same as pins, however they can be places on thin ribs to help push them out (they are harder to put in because you have to create a rectangular hole that fits them, i use wired EDM for that). Ejector sleeves are used to push parts off of core pins, and they require another plate behind the ejector plates that holds the core pins in place.
As for used machines, my advice is to buy the best machine that you can afford that has all the features that you are looking for. Believe me it won't be as easy as it sounds. You will find yourself comparing many machine features and thinking, "This machine would be perfect if it had _______!"
I'll try and dig up some pictures of the ejctors side of a mold to show you what I am talking about.
The book I recommended is here http://www.4spe.org/training/products/1015.php . I know it is a little expensive but it is worth the investment. It preatty much covers all your questions in detail. As far as machines, keep in mind no matter what machine, if the barrel and screw are bad then your looking at a couple grand and up to fix. There are a few good used machine dealers in the N-East , some of which you can visit in a long day trip. If you can stick with an older relay machine easier to track problem, if you get a cheap enough machine with solid state control that is bad and cost alot to repair, you can switch it over to relay. Good luck with your venture. Richard