Plastic structure design 7 – other design requirements

To read previous article, please refer to Plastic structure design 6 —part gaps, buckles and stops.
This article mainly introduces requirements for penetration and penetration positions, ultrasonic welding, bottom scraping and surface scraping in design of plastic structures. See below for details;

1. Insertion position and penetration position

Insertion position is often encountered in design of plastic products. Insertion position is a perforation on plastic part formed by contact between steel materials of front and rear molds. Because insertion position is formed by contact of steel materials, design of insertion position will directly affect life of mold. Since injection molding cycle during production is very short, number of openings and closings of mold increases. If design is improper, insertion position in mold will be easily damaged.
At this time, it is necessary to add an appropriate draft angle to insertion position of plastic part diagram. Draft angle of insertion position depends on size of plastic part, generally 3°~5°, but it must not be less than 3°.
SECTION A-A is K/O hole (K/O position is formed by steel material of front and rear molds contacting surface parallel to mold opening direction).
SECTION B-B is S/O hole (S/O position is formed when steel materials of front and rear molds are in contact with surface at a certain angle to mold opening direction, usually an acute angle).

2. Ultrasonic welding design

Ultrasonic plastic welding is a novel plastic processing technology – ultrasonic plastic welding stands out for its advantages such as high efficiency, high quality, beautiful appearance, and energy saving. When welding plastic products, ultrasonic plastic welding machine does not need to add any adhesives, fillers or solvents, nor does it consume a lot of heat. It has advantages of easy operation, fast welding speed, high welding strength and high production efficiency. Therefore, ultrasonic welding technology is becoming more and more widely used. When ultrasonic waves act on thermoplastic plastic contact surface, tens of thousands of high-frequency vibrations per second will be generated. This high-frequency vibration reaching a certain amplitude transmits ultrasonic energy to welding area through upper weldment. Since welding area, that is, interface between the two welds, has large acoustic resistance, local high temperatures will be generated. And due to poor thermal conductivity of plastic, it cannot be dissipated in time and gathers in welding area, causing contact surface of the two plastics to melt rapidly. After a certain pressure is applied, they fuse into one. When ultrasonic waves stop acting, let pressure continue for a few seconds to solidify and form, thus forming a strong molecular chain to achieve purpose of welding, and welding strength can be close to strength of raw material. Quality of ultrasonic plastic welding depends on three factors: amplitude of transducer welding head, applied pressure and welding time. Welding time and welding head pressure can be adjusted, amplitude is determined by transducer and horn. There is an appropriate value for these three quantities to interact with each other. When energy exceeds appropriate value, melting amount of plastic will be large and welded product will be easily deformed; if energy is small, it will not be easy to weld firmly and pressure applied will not be as high. This optimal pressure is product of side length of welded part and optimal pressure per 1mm of edge. Ultrasonic welding is a high-tech technology for welding thermoplastic plastic products. All kinds of thermoplastic rubber parts can be processed by ultrasonic welding without adding solvents, adhesives or other auxiliary products. Its advantages are to increase productivity multiple times, reduce costs, and improve product quality.
Ultrasonic plastic welding principle: Generator generates 20KHZ, (or 15KHZ) high-voltage and high-frequency signals. Through energy conversion system, signal is converted into high-frequency mechanical vibration, which is added to plastic product workpiece, friction between working surface and internal molecules increases temperature transmitted to interface. When temperature reaches melting point of workpiece itself, welding joint of workpiece melts rapidly, then fills gap between interfaces. When vibration stops, workpiece is cooled and shaped under a certain pressure at the same time, and perfect welding is achieved.

Welding surface design
Commonly used welding surfaces include flat welding surfaces, stepped welding surfaces and grooved welding surfaces.

(1) Flat welding surface. This welding method belongs to ordinary two-plane welding. A cone is designed on welding plane that runs through the entire welding plane, as shown in figure. Disadvantage of this welding surface design is that there will be burrs at welding area after welding is completed.

(2) Stepped welding surface. This structure has higher tensile strength than flat welding because molten material easily flows into vertical gap, and it also has high shear strength, as shown in figure. Compared with flat welding surface, melting strength of grooved welding surface is lower than that of flat welding surface due to reduction of welding area. Minimum wall thickness requirement for stepped welding surface design is 2mm.

(3) Grooved welding surface. This type of welding uses pitch displacement welding. Concave and convex surfaces are designed to maintain a certain gap and slope, and are suitable for welding that requires complete sealing. At the same time, grooved welding surface provides self-positioning function and prevents generation of burrs, as shown in the figure. Minimum wall thickness requirement for grooved welding surface design is 2mm.

3. Bottom scraping and surface scraping

For most electronic products used in real life, casing is mainly composed of upper and lower casings. In theory, shapes of upper and lower casings can overlap, but in fact, due to factors such as manufacturing accuracy of mold and injection molding parameters, upper and lower outer dimensions are inconsistent, that is, surface scraping (front shell is larger than bottom shell) or bottom scraping (bottom shell is larger than surface shell). Acceptable surface scraping <0.15mm, acceptable bottom scraping <0.1mm. Therefore, when zero step difference (size difference of bottom shell in cross-sectional view) cannot be guaranteed, try to make product: surface shell > bottom shell.

To read more about Plastic Structure Design, Please refer to Series 1, 2, 3, 4, 5, 6.