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After discussing the injection molding learning process and the underpinning philosophy in the first two parts of this series, Bill Tobin gets to the heart of the matter—the mold—and shares a few tricks of the trade.

Bill Tobin

November 4, 2016

7 Min Read
Molding tricks for higher profits, part 3: The mold

While it might sound dumb, the mold will determine the cycle and the quality of the parts. The mold should accomplish two things: First, it's a hole we fill with plastic. This determines the shape of the parts. Second, it's a big heat exchanger. But these two concepts must work together.

Parting lines and vents

We first need to determine how the mold will open and close—this is the parting line. Many times, it's contoured, so the fit must be perfect. It also needs to stay closed because momentarily there will be a lot of hydraulic pressure pushing the mold open while the machine's clamp is trying to push it closed. If you do it properly, the parting line has a few thousandths of an inch of preload. This is when the two mold halves have touched; when full clamp is applied, the steel compresses like a spring to ensure a complete seal.

Trick #1: Determine your preload (let's pretend it's 0.002 in.) but instead of closing up one entire mold half, grind a 2-x-2-in. square around the leader pins, 0.002 in. lower. Then grind away any steel that isn't the shut off or the leader pins. Since pressure is pounds/square inch, you've reduced a substantial amount of square inches. This means you can achieve the required pressure to keep the mold closed with a lot less pressure from the press, lowering energy costs and reducing machine wear and tear.

Since the mold is full of air when it closes, the plastic has to displace it to fill the mold. This offers us two options: Push the air out through the vents or don't have any air in the first place.

Pushing the air out or not is the difference between a good part or burns and flow lines. Vents have three components: The vent depth, the land, and the vent to atmosphere. The liquid viscosity of the melt determines the vent depth. Materials like nylon have a thin and watery melt and require a very shallow vent depth. ABS has a relatively thick molten melt and can allow a deeper vent. To help the molders the material manufacturers usually specify the vent depth. The land is ground at the vent depth long enough so that if any material squeezes through the vent it will immediately cool to a solid. But we have to do something with the air and the “fumes” that precede the melt. This is easily done with a channel to the outside air. If you used trick #1, you need only get past the shut-off area. The advantage to this is that the gunk that condenses from the resin must now fill up the area you've ground away before it will inhibit the fill.

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Vents need to be in front of the material flow so that the material doesn't trap it. Here's where your flow simulations can assist you. More vents are better than fewer vents.

Trick #2: Grind a flat down as many ejector pins as possible equal to the vent depth. Each pin is an extra vent. Because the pins move, they are also self-cleaning. This is also an excellent solution to the weld lines/burning around through holes in the part because the plastic will trap air as it goes around the pins.

Trick #3: The easiest way to deal with venting problems is to remove the air from the cavity before you fill it. Here's how:

  1. Block off all the vents.

  2. Install a normally open (a few thousandths of an inch) poppet valve in the mold and connect it to a vacuum line.

  3. The vacuum line hooks into a solenoid that is attached to a small tank.

  4. The tank has a separate line that connects to a vacuum pump.

When the mold closes, the solenoid opens the line to the tank that immediately sucks out all the air. The vacuum is usually large enough to offset the leaks from the ejector pins, slides and occasionally damaged parting line. The machine injects a quarter second after the solenoid opens. As the material is injected, it flows over the poppet valve and seals it closed. When the machine goes into packing mode, the solenoid closes and the pump again empties the tank. No air, no venting issues . . . problem solved.

Mold cooling

There are only a few concepts to keep in mind when it comes to cooling the mold.

  1. Heat travels from hot to cold—never the reverse—depending on the heat conductivity of the material. Metal conducts heat relatively fast compared to the heat transfer properties of plastic.

  2. Thick sections contain more heat than thin sections per square inch of cooling.

  3. Heat transfer between two surfaces is inefficient. Air is an excellent insulator.

  4. Water transfers heat best with turbulent flow. Water will always follow the path of least resistance. The volume of water through a line controls cooling better than the temperature.

Even cooling is the key. A normal waterline will control a mass of metal three diameters around it. Place your waterlines accordingly. Direct cooling into the core and cavity is always preferred to cooling the plate and hoping the part will cool.

Try not to loop water circuits. Looping causes two problems. The first is simple line resistance—the longer the circuit, the harder it is to get water through it. Each time you loop a circuit you run two risks: You can kink the hose and restrict the circuit or you can hook the loop back into itself, causing a blind circuit where no water flows at all.

Balance the flow: When everything is hooked up, use a flow meter on each circuit and restrict the lines with high Reynolds numbers (extremely turbulent). This will force pressure to the lines with restricted flow and improve their turbulent flow. This is accomplished using the valves on the manifold. But this has to be done each time you start up the mold. Of course, the techs don't do it, and the cycle slows down as a function of the least efficient circuit.

Trick #4: Buy and install waterline manifolds on each half of the mold. Use a fire hose connector on the machine's manifold. This way you hook up the mold and balance the waterlines once. Remove the handles on the manifolds so no one can readjust them. Faster setups, balanced circuits . . . problem solved.

Insert molding

Inserts are sometimes a necessity, but hand loading them usually means inconsistent cycle times and low yields. If you address this when the mold is built, you can vastly improve your profits. Start out by realizing that insert loading and part removal can be done automatically without very spendy robots. You would be amazed what you can do with the lowly sprue picker and some innovative end-of-arm tooling. The picker can come in to unload the molded parts, move laterally and load a new set of inserts and then get out of the way, unloading the molded part and picking up new inserts.

Even if you have an operator hand loading the inserts in a jig during the molding cycle for the picker to pick up, your cycle time will be consistent and your yields will be higher, with more parts per hour produced than by hand loading and unloading the inserts and parts.

Tricks when the customer pulls the job. A customer is really buying your expertise, because all your competition has are machines, molds and resin. For this reason, don't give it away. Tell your customers that no one is allowed on your production floor for insurance reasons. This keeps your tricks private. When the job goes, keep in touch with the buyer. After a few weeks or months, the buyer will want to give you the job back because nobody can do it as well as you do. Raise the price, re-install your tricks and enjoy the profits.

  1. Purchase your own manifolds for your cooling (ROI is weeks in reduced productivity and shorter setup times). Pull the manifolds off the mold and keep the waterline hoses. The next guy will have to figure out your cooling pattern.

  2. If you used “vacuum venting,” simply remove the equipment and close the poppet valve, and let the next guy deal with burns, shorts and flow lines.

  3. Keep your robot, end-of-arm tooling and the program you wrote to control the robot. Let the next guy hand load the inserts.

Bill Tobin is a consultant who teaches seminars and helps clients improve their productivity. He can be contacted at www.wjtassociates.com or [email protected].

Read the previous articles in this series:

"Molding tricks for higher profits: The expert syndrome"

Molding tricks for higher profits, part 2: The philosophy

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