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It is in your best interest to fill the mold as quickly as possible with liquid plastic and then hold it under pressure to offset thermal shrink.

Bill Tobin

November 17, 2016

7 Min Read
Molding tricks for higher profits, part 4: Filling the mold

When filling the mold, you should keep in mind you are putting a molten liquid into a metal mold that is well below its melt point. The second the plastic touches the mold it solidifies. Fortunately, it cools from the outside in, so there is a period of time that the melt can travel down its center to completely fill the mold. As a general rule, plastic cools at the same rate. For this reason, you should always fill from a thick section into a thin section. If you did the reverse, the thin section would freeze off before you filled and packed the thick section.

Plastic molecules are very long, many of them with branches. Because of this, one end of the molecule can be in a liquid phase while the other isn't. This “stretches” out the molecule and gives us the property of “apparent” viscosity. What this means is that when the plastic is moving quickly through the mold it is a relatively thin liquid. However, plastic molecules prefer to be tangled together. Once the movement slows or stops, the molecules that are still in some form of a liquid state contract back into their preferred tangled geometry. Pressure follows the path of least resistance. Adjust the gate sizes until every cavity gives an identical part weight within 5% of the overall part-weight average. If a cavity's part is too heavy, it's filling first and over-packing—this will be the problem cavity that will stick all the time. If it's too light, you'll over-pack the other cavities to fill this one out, otherwise it will be full of flow lines, sinks and voids.

Because of this phenomenon, it is in your best interest to fill the mold as quickly as possible with liquid plastic and then hold it under pressure to offset the thermal shrink, because it allows the plastic to reform back into its preferred molecular orientation.

Injection molding is a heat-transfer process. We heat the plastic to a molten phase, inject it into the mold and cool it into a preferred shape. You need even heat transfer. Think of the humble box. We tend to put the ejector pins in the corners for ease of ejection. We tend not to put cooling in the corners. Thus, the heat builds up in the corners and edges of the box and the wall warps inward.

Rule: Warp always goes toward the hot side.

For this reason, we try to cheat. We cool the cavity at the recommended temperatures but we lower the core's temperature in an attempt to cheat, using temperature to offset the poor positioning of the core's cooling lines.

Because this is a heat-in, heat-out process, the melt should be at the middle of the recommended temperature range. If the temperature is at the lower end, it will indeed shorten the time it takes to cool, but it will also solidify faster, resulting in short shots. The upper end of the melt point range will allow easier filling but the excess heat will lengthen the cycle time to cool off.

Filling should be done as fast as practically possible. Unless you've put a vacuum pump on the mold (see part three of this series), you have to have the plastic push the air out of the mold through the vents completely. This means the vents have not been squashed with excess or uneven clamp pressure and are clean enough to let the air easily out. More vents are better than fewer vents. As stated in a previous article—vent at the last point of fill, vent the ejector pins and vent the lifters and the parting lines. When the mold is full, the machine continues its injection pressure and compresses the plastic. At some point in time, it can no longer compress and the liquid turns hydraulic. This is where the plastic attempts to open or “blow” the mold. This is why you have a cushion on your load setting. Once the melt approaches becoming hydraulic, the machine should switch over into packing mode until the gate has frozen over and sealed the liquid inside.

The mold will only hold so much volume. Switch to packing mode from the position of the screw.


Plastic is a compressible material. It also is larger in volume when it is liquid and smaller when it is a solid. If the plastic isn't fully pressurized to offset this shrinkage as a liquid, one of two defects will show up. When the material shrinks as it cools, if the outer wall of the part is soft, the shrinkage will pull the wall inward. We call this "sink." If the mold's surface is cold, the outer wall is strong enough to resist this pulling and the material will pull away from itself. This is called a "void." A simple test to determine if you have a void or bubble is to heat the affected area with a flame. If it is a bubble, the internal gas will heat up and expand outward. If it is a void, there is no gas and it will collapse.

Packing time has a geometry/temperature/pressure relationship.

Geometry: Determined by the size of the gate. Large gates take longer to close than smaller gates.

Temperature: Gates will only freeze off when they become solid. The hotter the melt or the mold, the longer it will take to freeze off.

Pressure: The higher the internal pressure of the melt in the mold, the stronger the frozen gate must be to keep it in the mold. High packing pressures will increase the time for the gate to freeze off.

To determine the packing time, first get your fill figured out. Set the packing time too long. Weigh the parts, not the full shot. If they are small parts, weigh many of them and get the average weight. Begin shortening the packing time and continue weighing your sample parts. When the average part weight begins to drop this means the molten plastic in the part is leaking back into the runner system. Increase the time slightly until the part weight stabilizes. Add an additional half second and that's your packing time set point.

Because gate freeze off is a temperature/time relationship, when the overall cycle changes, the gate freeze-off time is also changed. When you speed up the cycle, the mold's temperature increases, and the melt temperature decreases. When you slow down the cycle, the reverse happens. If your water supply isn't at a constant temperature, your gate freeze-off times will vary along with your yield rates.

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, 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/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. If you must give your customer process conditions, do the following:

  • Melt and mold temperature range, per manufacturer range.

  • As stated in a previous article, purchase mold-mounted manifolds and balance your waterlines for even cooling. Remove the manifolds and give the next guy only the hook-up pattern, not the rates of flow.

  • Fill speed—give a number. What you should be doing is filling as fast as possible and switching to packing mold at a predetermined screw position.

  • Shot size and cushions—keep in mind that an eight-ounce shot is a weight. In a machine with a small barrel, there will be a long stroke on the machine. With a larger barrel, the distance will be shorter. Be polite enough not to tell the new molder the size of the barrel. Just give the measurements.

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

"Molding tricks for higher profits, part 3: The mold"

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