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Molding Defects: Short Shot or Short Mold
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The term "short shot" or "short mold" is used to refer to a phenomenon
where plastic being injection molded does not reach certain portions
of the inside of the die before solidifying.
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Some of the typical factors affecting this type of problem are
as follows:
- Fluidity and viscosity characteristics of the plastic
- Die design (i.e., gate design, bushing construction for ribs
and bosses, the presence of venting, etc.)
- Direct carving design - Molding conditions
- Molding machine performance
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| Condition |
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| Location |
On surfaces
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Ribs and bosses
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Corners and tips
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| Cause |
Generation of gas
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Material thickness and gas volume
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Plastic viscosity
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 Short
Shot Checkpoints |
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 Insufficient
plastic fluidity (i.e., viscosity is high): Checkpoints and countermeasures
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- Die temperature : Increase within the range for removability
Note that as the molding cycle becomes longer as a result
of this, care must be taken with regard to cylinder residence
time. * Refer to the molding
condition sheet
- Screw failure : Replace the back-flow prevention ring
Note that this is applicable to situations where there is disparity
in the degree of cushioning during injection or back flow.
- Unstable supply
- Lower the temperature at the rear of the cylinder. If plastic
at the hopper opening is bridging
- Increase the cooling water volume at the hopper opening :
If plastic at the hopper opening is bridging
- Correct the back pressure : Insufficient or excess back pressure.
- Injection speed : Increase the injection speed.
If no change in the filling time is noticed after setting
of a high injection rate, there is a possibility that the
performance of the molding machine is insufficient or that
there is excessive pressure loss in the sprues, runners, and
gates; accordingly, separate evaluation and modification of
the molding machine's performance will be needed. *
Refer to the molding condition sheet
- Injection output : a Raise the injection output
If the problem persists even after setting of a high pressure,
it is highly probable that the same factors as described in
the above items are at work.
- Plastic temperature : Raise the temperature within the service
range.
* Refer to the molding condition sheet
- Grade change : Switch to a grade with better fluidity. * Alternative
grade search
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Die design: Checkpoints and countermeasures |
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Uneven thickness (if both thin and thick walled sections are present
within the same cavity)
Standardize the wall thickness in each cavity. Note that
this can result in sink marks on the top of ribs and bosses,
and therefore, special care must be taken in this regard.
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Rib and boss design (if constantly occurring in specific ribs and bosses)
Use shaped-section bushings and ensure smooth extraction
of gas from the extremities of dead-end sections.
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Hesitance (if flow patterns and gate layouts are defective)
a
Modify the gate design (i.e., method, dimensions, locations,
and quantity)
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Defective cooling design (if the distribution of die temperatures
is non-uniform)
Review the adjustment of temperature (i.e., the cooling
design) and the performance of the die's temperature regulator.
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