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Molding Defects

The following pages will describe the various types of problem that we have experienced in the molding of ABS.
Product design, material selection, and molding conditions
The following problems either occur individually or in combination. Click on one of the following items for more details about the corresponding problem.

Silver Streaks

Gaseous components in the plastic appear at the molding surface and collapse.

Appearance ??:????????? ??:???? ??? ??:???? ???
Characteristics Thin,radial lines
  • Relatively few occurrences over a wide area
  • Random occurrence at points removed from the gate
  • Thin,long sparkler-type pattern
  • Thermal decomposition and deterioration
Cause /

Insufficient drying

Inclusion of air



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.

Appearance ??:????????? ??:???? ??? ??:???? ???
Location On surfaces Ribs and bosses Corners and tips
Cause Generation of gas Material thickness and gas volume Plastic viscosity




The term "jetting" is used to describe the phenomenon where plastic passing through a gate does not adhere and where flow patterns are formed on the surface of the molded product.

Appearance Plastic at a relatively low temperature is injected from the nozzle during the initial stage of molding, upon coming into contact with the mold wall, this plastic becomes highly viscous and swirling takes place; furthermore,as hotter plastic is continually injected into the die, the original material is pushed deeper into the die and leaves flow marks.
  • When the plastic temperature is low, the viscosity of the molten material is high, and this become higher in the case of plastic which has been injected into the die; consequently, the resistance to flow is large and jetting occurs.
  • When the die temperature is low, the material injected into it will be rapidly cooled, and the corresponding increase in viscosity leads to the occurrence of this problem.
  • When gates are small, the speed of plastic injected into the cavity will be relatively fast, and this leads to the occurrence of jetting in many cases.
    (Gate sectional area x flow speed = Fixed injection amount )
Countermeasures Molding conditions
  • - Increase the temperature of the plastic to lower its viscosity.
  • - In the case of amorphous plastics, the ideal temperature of the die is between 20deg.C and 30deg.C lower than the plastic's thermal deformation temperature.
  • - It is also beneficial to reduce the injection speed.
  • Dies
  • - It is advantageous to increase the sectional area of the gates so that the speed of the material passing through the gates becomes slower.


Flow Marks

The term "flow mark" is used to describe the phenomenon where a striped pattern is formed around the gates when plastic has flowed through the die.

Appearance ??:????????? ??:??????????? ??:?????????
Characteristics Uneven thickness type Record type Staggered type
Cause /


Color Streaks

The phenomenon of color streaking is characterized by localized changes in the color of a molded component or by streaking.

Appearance ??:?????? ??:??
Characteristics Variation from the original color

A streaking pattern in a molded product

Cause This type of problem is principally caused by poor thermal stability of the coloring agent. Furthermore, it occurs more easily in locations such as welds, ribs, and the like which are likely to induce shear flow.
  • Poor distribution of coloring agent.
  • Cleaning effect of plastic deposited within the molding machine (or cylinder).
  • Switch to a coloring agent with good thermal stability.
  • In terms of molding conditions, it is desirable for the plastic and die temperature to be high, and for the speed of flow to be low (i.e., hot-slow molding).
  • Improve the quality of distribution during compounding.
  • Fully clean the inside of the cylinders.(We recommend our own UMG Clean™ for this purpose.)
  • Check the screw head, back-flow prevention rings, and other similar items for defects.
  • Change the molding machine.


Weld Lines

The term "weld line" is used to describe the narrow V-shaped line that occurs at the point where two different flow fronts meet.



The terms "flash", "fins", and "spew" are used to refer to excess molding material that penetrates into mold gaps (i.e., between parting faces, slide push-out faces, and inserts, etc.) in a molten state.

Appearance picture:normal picture:flash
Characteristics Normal Flash
Cause /



The term "delamination" is used to refer to a condition where a thin micaceous layer develops on the molded product and then becomes separated.

Cause(1) Inclusion of a plastic which has poor compatibility with ABS (i.e., PP or PS, etc.)
Countermeasures Carry out cleaning.
- Inside the cylinder
- Inside the hopper
- Inside the air feed lines
- Inside the dryer
Cause (2) When the die and plastic temperatures are extremely low, the difference in temperatures between the outer walls and the fluid layer results in the development of a thin hard coating which then peels.
Countermeasures Standardize the resin temperatures.
- Raise the temperature of the resin
- Raise the temperature of the die



The term "stringiness" is used to refer to a phenomenon where string-line sections of plastic which are formed upon opening of the die adhere to the inside of the die and are transferred to the product during the next shot, resulting in string-line unevenness on the surface of the molded component.

Appearance ??:????????? ??:???????????

Thin plastic string formed at the tip of a sprue

Plastic string after transfer to the molded component
Cause High nozzle temperatures
  • Lower the nozzle temperature; alternatively,make combined use of pull-back and cylinder repetition.
  • Strings can be forced to break by increasing the speed of die opening, however, care must be taken as this can result in reduced die service lives for certain die designs (i.e., slides, etc.).
  • Use a string-prevent ring (commercially available).


Sink Marks

 Sink marks or shrink marks are hollows or indentations that occur on the outer surfaces of molded components.


Warping or Twisting

"Warping" is a term used to describe the deformation which occurs when there are differences in the degree of shrinkage at different locations within the molded component.



 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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