Thermoset Injection Mold Design Tips

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Magneto
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Thermoset Injection Mold Design Tips

Post by Magneto » Sun Nov 22, 2009 10:30 pm

When designing a mold for an injection molded part, it is important to keep in mind that the goal
is to produce parts with the best quality, in as short a cycle as possible, with a minimum of scrap.
To achieve this goal, you will need a mold that has a uniform mold temperature, balanced fill,
and is properly vented.

MOLD HEATING

A uniform mold temperature means that the temperature of each half of the mold is the same
(within ±3°C (5°F)) for all locations when the mold is heated by oil or steam. Molds that are
heated with electric cartridge heaters can vary by as much as 6°C (10°F). A mold with a uniform
temperature will fill easier and produce parts with less warpage, improved dimensional stability
and a uniform surface appearance. Achieving a uniform mold temperature is dependent on your
method of mold heating.
A mold that is heated by steam or oil will have a uniform mold temperature because the heat
source maintains a constant temperature. However, oil, as a heat source, is only about half as
efficient as steam. Therefore, when using oil to heat a mold, it is necessary to set the oil
temperature higher than the desired mold temperature.
Electrically heated molds are more difficult to maintain at a uniform temperature because the
cartridge heaters are constantly cycling on and off. When they are on, they generate a great deal
of heat at the source, but this heat must be distributed throughout the mold in a way that
produces a uniform mold temperature.
To determine the amount of wattage needed to heat a mold, the use of the following formula
might be helpful: 1¼ kilowatts for every 45 kg (100 pounds) of mold steel. Note: This
formula normally will allow the mold to be heated to molding temperatures in 1 to 2 hours.
Locating a heater on the centerline of the mold is not recommended, because the center of the
mold is normally hot enough without adding any additional heat. Typically, the cartridge
heaters are located in the support plates with a distance of 64 mm (2 ½") between heaters.
NOTE: Deep draw molds may need to also have heaters in the retainer plate. There should be a
minimum of one thermocouple to control each half of the mold. In larger molds, it is
recommended to have more than one thermocouple in each mold half. This will result in better
control and more uniform mold temperatures. The thermocouples should be located in the “A”
and “B” plates, between two heaters if possible and at a distance of 32 mm - 38 mm (1¼" - 1½")
from the closest cartridge heater. This distance is to be measured from the

edge of thermocouple hole to the edge of the cartridge heater hole. The distance from the
thermocouple to the heater is important because a heater that is too close will cause the
thermocouple to turn off the heat before the mold is at temperature. A heater that is too far away
from the thermocouple will result in a mold that overheats and then gets too cool. Likewise, it is
not a good practice to position a thermocouple so it senses the external surface temperature of the
mold. If possible, it should be located 38 mm - 51 mm (1½" - 2") inside the mold, since the
temperature taken there, is less susceptible to outside influences and therefore more stable.

BALANCE MOLD FILL

When injection molding with multiple cavity molds, it is important that all the cavities are filled
simultaneously. The most common means to achieve a balanced fill is to make the distance the
material travels from the sprue to each cavity the same. This approach will work as long as the
material flows directly from the sprue to the gate of the part. However, if the runner is divided
two or three times in going from the sprue to the gate, it is unlikely that the fill will be balanced.
An effective way of balancing the fill is to have one main runner that extends from the last

cavity on one end of the mold to the last cavity on the opposite end, with sub-runners feeding the
individual cavities. To balance the fill of the cavities, flow resistor pins are placed in the
subrunners. These pins are adjusted to inhibit the flow of material to the individual cavities so
all the cavities are filled at the same time.


VENTING

When molding thermosets, the polymerization process that takes place produces volatiles, which
along with the air already within the cavity chamber, can become trapped and superheat to 375°C -
425°C (700°F - 800°F). If the gases are not allowed to escape through vents, they may oxidize the
lubricants leaving burn marks on the part. The vents allow the volatiles to escape to atmosphere.
In addition to visual problems, improper venting will result in parts that cannot be filled, have
dimensional problems or have less than the expected physical and/or electrical strengths.
The first question that has to be addressed is vent location. It is important that all vents must lead
to the atmosphere otherwise the vent will be useless. Unless the part geometry shows some obvious
locations for vents, a brief molding trial should be conducted to observe where the gas voids occur.
Whenever possible, vents should be located in the movable half of the mold, wherever a gas void or
knitline is seen on a part.

Vents for phenolic parts should be 6 mm (¼") wide and 0.08 mm - 0.09 mm (0.003" - 0.0035")
deep and vents for polyester parts should be 6 mm (¼") wide and 0.05 mm - 0.06 mm (0.002" -
0.0025") deep. The width is not as critical as the depth. A vent that is 0.025 mm (0.001") or less, is
too shallow and may seal when the mold is closed. A vent that is 0.13 mm (0.005") is normally too
deep and may not seal. As a result, internal cavity pressure will be low and the shrinkage, the
physical and the electrical properties may not match data sheet values.
Of equal importance to the location and depth of the vents is vent length, which is the distance from
the part that the vent maintains its 0.08 mm (0.003") depth. The vent should be approximately 25
mm (1") long to allow pressure to build in the cavity after the material in the vent cures. After this
point, the vent can be relieved to a depth of 0.25 mm - 0.50 mm (0.01" - 0.02"). To help the vent
stay with the part, the corner of the vent at the part edge can be radiused or chamfered.
It is sometimes necessary to vent “dead” areas of the mold with vented ejector pins. Before adding
the vents, an ejector pin should fit the hole in which it will operate within 0.025 mm (0.001"). A flat
is then ground on the diameter no deeper than 0.13 mm (0.005") for a distance that will take the vent
3 mm (") below the fit length of the pin. Normally, the fit length should be 13 mm - 16 mm (½" -
") . (See sketch below) In addition, the stroke of the ejectors should be long enough for the entire
vent plus 3 mm (") to come up above the bottom of the cavity. This is so the vent can be selfcleaning
or so an operator can blow the flash off the pins.
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