manufacturing Manufacturing is the creation or production of goods with the help of equipment, labor, machines, tools, and chemical or biological processing or formulation. It is the essence of secondary sector of the economy. The term may refer to a r ...

, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "

knit Knitting is a method by which yarn is manipulated to create a textile, or fabric. It is used to create many types of garments. Knitting may be done by hand or by machine. Knitting creates stitches: loops of yarn in a row, either flat or i ...

" together, or " weld", during the molding process. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part.KNIT LINES, Texas Plastic Technologies
/ref> Knit lines are considered molding defects, and occur when the mold or/and material

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

s are set too low: thus the materials will be cold when they meet, so that they do not bond perfectly. This can cause a weak area in the part which can cause breakage when the part is under stress. Weld lines therefore occur during machine start-up, when equilibrium conditions have not been met. Mouldings made in this setting-up period must be rejected. There are many Computer Aided Engineering tools that are available that can predict where these areas could occur, but a skilled designer will be able to predict where such defects can be found by examining the tool or product. Weld lines are not found in other manufacturing methods such as

Rotational moulding Rotational molding (BrE: moulding) involves a heated mold which is filled with a charge or shot weight of material. It is then slowly rotated (usually around two perpendicular axes), causing the softened material to disperse and stick to the ...

, but can exist in

extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...

, especially where there are internal metal supports for a die. The defects are then known as

spider lines Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and rank ...

Causes

Weld lines can be caused by several different problems: * Low temperature of

injection moulding machine An injection molding machine (also spelled as injection moulding machine in BrE), also known as an injection press, is a machine for manufacturing plastic products by the injection molding process. It consists of two main parts, an ''injectio ...

barrel * Inadequate back pressure * Injection pressure or injection speed is too low * Low mold temperature * Small injection gates and/or runners * Improper location of injection gate * Too many gates * Excessive gate land length * Improper

flow rate Flow rate may refer to: * Flow measurement, a quantification of bulk fluid movement * Mass flow rate, the mass of a substance which passes per unit of time * Volumetric flow rate, the volume of fluid which passes per unit time ** Discharge (hydrol ...

of injected materials * Inconsistent process cycle * poor

product design Product design as a verb is to create a new product to be sold by a business to its customers. A very broad coefficient and effective generation and development of ideas through a process that leads to new products. Thus, it is a major aspect of n ...

References

Injection molding {{Manufacturing-stub