Injection Moulding machine parameter,
MACHINE
EXCESSIVE INJECTION FILL SPEED
Explanation: The speed and pressure of the melt as it enters the mold
determine both density and consistency of melt in packing the mold. If
the fill is too fast, the material tends to ``slip'' over the surface
and will ``skin'' over before the rest of the material solidifies. The
slipped skin area does not faithfully reproduce the mold steel surface,
as does the material in other areas, because it has not been packed
tightly against the steel.
Solution: One solution is to adjust the fill speed rate until the
optimum has been achieved. This will help eliminate blushing.
MELT TEMPERATURE TOO HIGH OR TOO LOW
Explanation: Although this may sound contradictory, either condition
might cause blushing. If the injection barrel heat is too high, the
material will flow too quickly, resulting in slippage of the surface
skin, as mentioned above. If the barrel heat is too low, the material
may solidify before full packing occurs and the plastic will not be
pushed against the mold steel, especially in the gate area because that
is the last area to pack.
Solution: Melt temperature must be adjusted to the optimum for a
specific material and specific product design.
LOW INJECTION PRESSURE
Explanation: The plastic material must be injected into the mold in such
a way as to cause proper filling and packing while maintaining
consistent solidification of the melt. Injection pressure is one of the
main control variables of the machine and must be high enough to pack
the plastic molecules against the steel of the mold while the plastic
cools. Low pressure will not achieve this packing and the material will
appear dull in local areas that do not have enough pressure.
Solution: Increasing the injection pressure forces the material against
the mold surface, producing a truer finish that replicates the steel
finish.
NOZZLE DIAMETER TOO SMALL
Explanation: The nozzle diameter controls the time during which the
material fills the mold. If the diameter is too small, the material may
begin to solidify before the mold is filled. Then, packing cannot occur
because the material is already rigid. Blush will occur because the
plastic has not been forced against the steel surface.
Solution: Enlarging the nozzle diameter will minimize the condition. The
nozzle tip is interchangeable and a tip with the opening the same as, or
slightly smaller than, the sprue bushing opening is recommended.
LOW NOZZLE TEMPERATURE
Explanation: A nozzle that has a low temperature will cause the material
going through it to cool off too soon and not be allowed to pack out the
mold. The non-packed molecules will form blush because they cannot
replicate the steel finish.
Solution: Increasing the mold temperature will allow the material to
flow easier, and for a longer time, thereby packing the mold and
replicating the steel finish. Normally, the nozzle temperature should be
set the same as, or 10 degrees hotter than, the front zone of the
barrel.
MOLD
LOW MOLD TEMPERATURE
Explanation: A low mold temperature may cause the molten material to
slow down and solidify before the mold is packed out. This will cause
dull areas where the plastic was not forced against the steel finish.
Solution: Increasing the mold temperature allows the material to flow
farther and pack properly. The material temperature could also be raised
to accomplish the same effect.
IMPROPER VENTING
Explanation: Trapped air can cause blushing if the air is trapped in an
area that does not compress the air enough to ignite it. The air takes
up space where the plastic should be, so the plastic is not forced
against the steel finish.
Solution: Vent the mold by grinding thin (0.0005''-0.002'') pathways on
the shutoff area of the cavity blocks. Vents should take up
approximately 30% of the perimeter of the molded part. Add vents in
local areas that show blush. Vent the runner, too. Any air that is
trapped in the runner will be pushed into the part.
SMALL SPRUE BUSHING DIAMETER
Explanation: A small sprue-bushing diameter will keep the material from
packing because the small opening reduces the ability of the plastic to
flow far enough to fill the mold. An unpacked mold will cause blushing
where the material is not forced against the mold steel.
Solution: Size the sprue bushing major diameter so its cross-sectional
area is equal to (or greater than) the sum of the cross-sectional area
of all the runners leading from it. Then, taper the sprue diameter to
match the nozzle. That will ensure proper pressure drop adjustments to
pack the mold.
IMPROPER GATE LOCATION
Explanation: If a mold is gated such that the thinnest areas fill first,
those areas will begin to solidify before the thicker areas are packed.
Blush will form in the thicker areas because there is no pressure left
to pack the plastic against the steel surface.
Solution: Make sure the gate is located so the thicker sections fill
first. The material should flow from thick section to thin section. That
ensures equal packing of all areas.
SHARP CORNERS
Explanation: If the product design contains sharp corners, the material
tends to slip by those corners without fill them in. The corners are not
packed with material and blush occurs due to that non-packing.
Solution: Radius all sharp corners, especially in the gate area, as that
is the last place to pack. Sharp corners should not be allowed on any
molded part.
MATERIAL
EXCESSIVE MOISTURE
Explanation: In some cases, excessive moisture in a melt will accumulate
at the gate area. The reason for this is that the gate area is the last
place the pressure builds up. Moisture trapped in other areas may be
forced into the gate area due to this pressure buildup. The gate area
will appear dull due to the moisture that gets screened out. Usually,
this type of blush is accompanied by splay.
Solution: Dry the material to the supplier's recommendations and make
sure it is used within two hours of that drying activity.
OPERATOR
INCONSISTENT PROCESS CYCLE
Explanation: It is possible that the machine operator is the cause of
delayed or inconsistent cycles. This will result in excessive residence
time of the material in the injection barrel. If such a condition
exists, materials may fill at a faster speed and cause slippage as
explained earlier. Slippage causes blush.
Solution: If possible, run the machine on automatic cycle, using the
operator only to interrupt the cycle if an emergency occurs. Use a robot
if an ``operator'' is really necessary. And, instruct all employees on
the importance of maintaining consistent cycles.