Date Approved


Graduate Degree Type


Degree Name

Engineering (M.S.E.)

Degree Program

School of Engineering

First Advisor

Dr. Sung-Hwan Joo

Second Advisor

Dr. Wael Mokhtar

Third Advisor

Dr. Huihui Qi

Academic Year



In producing a plastic part in an injection molding machine many physical factors related to the machine, such as injection pressure, holding pressure, heater temperature, etc. play their roles in the overall quality of the produced part. There are many aspects of the quality of a plastic part, such as part strength, part dimensions, and part surface finish. If any of those aspects gets compromised, the part is considered to have some defects. Of all defects found in injection-molded plastic parts, one most common defect is the presence of weld lines.Weld lines are formed when the flow of the molten plastic inside the mold cavity gets divided into more than one path and then rejoins again at the weld line location.

Weld lines in plastic parts can be reduced or eliminated by introducing different kinds of heating processes. Research works have been done on reducing weld lines by applying different methods of induction heating of the mold. However, no research has been found on applying a spot-heating technique in a mold in order to reduce weld lines. In this thesis, spot-heating technique is applied at the weld line region of a mold, and the effectiveness of spot-heating in reducing weld line is investigated.

A plastic part that normally contains a weld line when produced without any additional processing or heating was simulated in Moldex3D mold flow simulation software. The generation of the weld line was observed in the simulation. Further simulation with applying a heater unit to spot-heat the weld line region of themold indicated some reduction in the weld line. An experiment has been carried out by producing the plastic parts, first with no spot heating and then adding a heater unit in the actual mold. A close investigation of the surface near the weld line region and a tensile strength test of the molded plastic parts show some noticeable reduction in weld line, and accordingly, relatively greater mechanical strength in the parts produced with spot-heating.