Page 23 - Plastics News July 2021

FEATURES

number and see if the bubbles are still there. If so, at least Voids
you don’t have to worry about the parameters involved A void occurs during cooling while the part is inside
with second stage. Assuming you still see bubbles, the or outside the mold, usually in thick sections. In thick
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WKH JDV LV DLU WUDSSHG XSRQ ÀOOLQJ WKH SDUW shrinks more, pulling away from itself to form a bubble.
If you run the mold hotter and the bubble goes away,
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volume, do a short-shot study. That is, decrease the shot but you have a sink as a result, that is an indication your
VL]H IURP IXOO WR IXOO E\ LQFUHPHQWV 'R QRW VWDUW bubble was a void. Voids and sinks are signs of internal
short and increase shot size, as you may get a different stress and are warning signs that the part may not perform
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stage is pressure limited, you may not get the consistency packing more material into the cavity is recommended.
needed for accurate results. Make sure you have a consistent cushion and are not
bottoming the screw so that you can properly pack out
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the part. Higher hold pressures or longer hold time may
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is coming around on itself, or if there is a hesitation of help, but many times the gate is freezing before you can
adequately pack out the center of the nominal wall.
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bubbles always in the same area? If so, that means there 7R VROYH YRLGV RU VLQNV WU\ VORZ ÀOO UDWHV XVH RI JDV FRXQWHU
LV D À[HG SODFH IURP ZKLFK WKH EXEEOHV DUH FRPLQJ 1RWH pressure, or increasing backpressure. Ensure the runner or
if there is a race-tracking effect or jetting that can cause gate is not freezing too early and that longer hold times
air to become trapped in the polymer. will allow more packing during the second stage. If the
Check ribs or any projections off the nominal wall. If they gate is freezing off too soon, a slight opening of the gate
are short it means there is air trapped in that area and it may be all that is needed, as a small change in diameter
LV EHLQJ SXVKHG RXW WR IRUP EXEEOHV ZKHQ WKH ULE LV ÀOOHG results in a longer gate-seal time. Also try reducing melt
Sometimes you actually see a trail of bubbles coming from temperature, if it’s possible.
this projection. Do the bubbles only appear after the part Other approache to eliminating voids or sinks is to thin
LV IXOO" ,I VR LW FRXOG EH D YHQWLQJ LVVXH down the nominal wall. Thicker is not always stronger in
plastic parts. Thick nominal walls should be redesigned
Check the vents to be thinner with ribs for strength. This will save plastic
One source of bubbles that is rather odd is the venturi and cycle time as well. Core out the thick section, if it’s
effect. There are several possibilities for a venturi effect to possible.
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of nozzle tip to sprue bushing, nozzle misalignment, and &KDQJLQJ WKH JDWH ORFDWLRQ WR ÀOO WKLFNHU DUHDV LQ WKH PROG
separated plates in a hot runner. These are much harder ÀUVW PD\ DOORZ PRUH SRO\PHU LQWR WKH SDUW EHIRUH WKH JDWH
to detect but as you rule out other sources, the tool must freezes. You may also try raising the mold temperature
be checked. Apply bluing agent near the hot-runner drops and/or ejecting the part sooner, which can prevent voids
and on plate mating surfaces, being careful not to allow by allowing the outside walls to collapse during cooling,
though that may result in sinks. In fact, it’s no exaggeration
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startup, you have found the source of the problem. to say that gate location can make or break a part. The
type and placement of the gate can affect many aspects of
Another common source of bubbles is too much a project including: Structural issues including the overall
decompression, especially in hot-runner molds. Yet strength of the part,Ability to hold your desired roundness
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zone or feed section. General-purpose screws with L/D of pressure is needed,Aesthetics such as the appearance of
18:1 or lower can be the culprit. Try using a lower rear- VLQNV YRLGV DQG ÁRZ OLQHV 1HYHUWKHOHVV JDWH ORFDWLRQ
zone temperature and/or higher backpressure. Another must be high in importance on the list of considerations.
solution may be to pull a vacuum on the mold just before
injection.

23 July 2021 Plastics News