GOAL:
The purpose of this project was to gain a basic understanding of engineering design through the process of reverse engineering a household object, in this case a hot glue gun, as well as become more familiar with engineering report writing.

COMPONENT LAYOUT:

Sketch of hot glue gun assembly. Functional “chunks” of hot glue gun are labeled and shown in their approximate relative positions.

MOCK BILL OF MATERIALS:

Part	Quantity	Material	Function	Dimensions (mm±0.2mm)	Specifications
External casing	1	Plastic, metal	Support internal function of hot glue gun	Max. barrel length: 97.4 Handle length: 71.3	Must accept glue stick of 8±0.2mm diameter
Trigger Mechanism	1	Plastic, metal	Push glue stick into heated tubing	Spring length: 6.0 Internal diameter: 8.1	Must accept glue stick of 8±0.2mm diameter
Heating element	1	Aluminum	Melt glue stick	Barrel length: 52.3 Barrel diameter: 10.0	Must reach melt glue stick without melting external casing
Power cord	1	Metal, plastic, ceramic	Conduct electricity	100cm (tape measure)	Standard 2 prong plug

An electronic digital caliper was used (Accuracy ±0.2mm). The external casing is held together by 5 identical screws. The heating element is held together by a two metal clips and a rubber adapter collar. The trigger mechanism is wedged between the two halves of the external casing. The power cord is clipped to the heating element on one end, while the other runs through the bottom of the external casing to an external power source.

FUNCTIONAL ANALYSIS:

HAND DRAWINGS:

HEATING ELEMENT
Drawing of the heating element (hot glue gun, “functional chunk”). The heating element was removed from the external casing, and a square of ceramic and wires from the power cord (source) were unclipped from the slot shown in the bottom view of the drawing. Units are in mm±0.2mm.
TRIGGER MECHANISM
Exploded view sketch of the trigger mechanism. Exact dimensions and spring stiffness depend on the size of the external casing. Parts 1 and 2 are held together with a metal bar (not shown). The protruding arms of part 5 are threaded through parts 4 and 2, and through part 3.

DESIGN EVALUATION:
The basic functions of a hot glue gun are to pass glue into the heating element, melt the glue, and eject the liquid glue. The first basic function is to melt the glue sticks. When the hot glue gun is plugged into a standard 120 volt home wall outlet, a circuit is completed. Electricity flows through one wire of the power cord through the ceramic plate, where some amount of electrical energy is released as heat, and back out through the second wire. The aluminum body transfers some of that heat to the glue stick, which then melts. The interior of the hot glue gun does not overheat because the ceramic plate is a PTC heater, a “positive temperature coefficient” material, which means that its resistance to current flow increases as temperature increases. The use of PTC ceramic allows the hot glue gun to “self-regulate” temperature. The manufacturer of the hot glue gun seems to have chosen to use PTC ceramic and an aluminum body for the heating element to avoid material and manufacturing costs associated with a heat sensor/warning light system and separate temperature controls.

The second function, linear movement of a glue stick through the barrel of a hot glue gun, is achieved by a trigger mechanism. Though possible to push a glue stick through the glue gun barrel manually, it is not the most convenient method as it requires either greater single-handed dexterity or the use of two hands. The inconvenience of Push-fed glue guns outweighs the benefit of requiring fewer parts (simpler manufacture), which is likely why an Amazon.com search of ‘hot glue guns,’ and ‘craft glue guns,’ will pull up only products with trigger mechanisms. The majority of glue guns with triggers are manual, though “automatic,” motor-driven glue guns do exist. The mechanical trigger was likely selected for the hot glue gun model used for this reverse engineering project because it strikes a balance between low manufacturing costs and ease of use for the consumer.