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Molding Defects


Short shot


 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.

 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

Appearance 写真:乾燥不足による現象 写真:延性破壊 破壊面 写真:延性破壊 破壊面
Location On surfaces Ribs and bosses Corners and tips
Cause Generation of gas Material thickness and gas volume Plastic viscosity

Short Shot Checkpoints


Insufficient plastic fluidity (i.e., viscosity is high) : Checkpoints and countermeasures


Caurses
Countermeasures
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  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

Appearance
Countermeasures
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.
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.
Hesitance
(if flow patterns and gate layouts are defective)
 Modify the gate design (i.e., method, dimensions, locations, and quantity)
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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 UMG ABS Ltd. accepts no responsibility for the quality or safety of any customer products which use our materials or which have made use of any type of data provided by this company.
 Customers are requested to independently determine the suitability of our materials for their products. We also request that sufficient attention also be paid to laws, regulations, and industrial rights.


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