[Jim Estes]

Fastturbovet,

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.

Jim