For previous read, please refer to Plastic structural design 4 —reinforcement ribs and columns. 
This article mainly introduces requirements for design of holes, formed threads and metal inserts in plastic structure design. See below for details:

It is a common method to open holes in plastic parts to connect them with other parts or to increase functional combination of product. Size and location of holes should try not to affect strength of product or increase complexity of production. Following are several factors that need to be considered when designing holes.
Distance between connected holes or distance between a hole and straight edge of an adjacent product shall not be less than diameter of hole, such as distance between hole and edge of inner wall as shown in key diagram. At the same time, wall thickness of hole should be as large as possible, otherwise breakage will easily occur at perforation location. If there are threads in hole, design requirements become complicated because location of threads can easily create stress concentrations.

 

Hole design principles
(1) Distance between holes is more than 2 times hole diameter.
(2) Distance between hole and edge of finished product is more than 3 times hole diameter.
(3) Distance between hole and side wall is more than 3/4 times hole diameter.

 

(4) Thickness of flesh around hole should be strengthened (especially for holes with assembly properties and stress), and thickness around cut hole should also be strengthened.

 

(5) For blind holes perpendicular to direction of material flow, when hole diameter is less than 1.5 mm, hole depth shall not exceed 2 times hole diameter, and thickness of bottom of closed hole shall be at least greater than 0.2 times diameter of closed hole. Bottom is too thin, strength is low without through holes, and back surface is prone to appearance defects. If bottom is too thin, you can consider using method shown in Figure 3-21 b to enhance strength of blind hole.

 

 

(6) Through holes with different apertures should not be designed with two sides facing each other, as this will cause eccentricity. Hole diameter on either side can be increased, or holes can be designed without facing each other.

 

(7) Avoid side holes that are perpendicular to demoulding direction of part
In order to simplify mold structure and reduce mold cost, part design needs to avoid side holes perpendicular to demoulding direction. Hole design should try to make mold structure as simple as possible.
Side holes perpendicular to demoulding direction of part require use of a lateral core-pulling mechanism on mold, which will increase complexity of mold and increase cost of mold. On the premise of ensuring function of parts, use of lateral core pulling mechanisms can be reduced and avoided through design optimization. As shown in figure for plastic part, hole on lower side requires a lateral core-pulling mechanism, and mold structure is complex; while upper hole can be directly demoulded due to optimized design, without need for a lateral core-pulling mechanism, and mold structure is simple.

 

(8) Long hole design avoids impeding flow of plastic melt.
An elongated hole refers to a long and narrow hole. Direction of elongated hole should be consistent with flow direction of plastic melt, should not be perpendicular to flow direction to avoid impeding flow of plastic melt. Design of long hole is shown in figure.

 

(9) Design of air holes
Due to need for heat dissipation, air holes are often designed in products. In general, when air holes are round holes, mold core is cylindrical, which is easy to process and mold cost is low.
Too many air holes will reduce strength of parts. You can increase strength of parts at air holes by adding reinforcing ribs or flanges.

Things to note when designing plastic forming threads:
① Avoid using threads below 32 threads/inch (pitch 0.75mm). Maximum thread pitch can be 5mm.
② Long threads will distort pitch due to shrinkage and should be avoided. If structure requires it, self-tapping screws can be used for locking.
③ Avoid using it when thread tolerance is smaller than shrinkage of forming material.
④ External thread must not be extended to the end of finished product. Sharp part produced will cause end face of mold and thread to crack, reduce service life. Therefore, at least a 0.8mm flat part must be left (as shown in figure).

 

①Due to fluidity, plastic parts will produce weld marks around metal inserts; due to different shrinkage rates of plastics and metals, cracks are prone to occur after forming.
②When using metal insert molding, cycle time will be extended.
③Metal insert is slightly higher than molded plastic part to prevent it from being pulled and loosened during assembly.

 

Installation method of embedded nuts. When plastic parts need to be disassembled multiple times, using self-tapping screws is not the best choice. In this case, you can choose to embed nuts in plastic parts and then use screws to fix them. Of course, cost of embedded nuts is higher. According to installation method of embedded nuts, embedded nuts can be divided into ultrasonic nuts/hot melt nuts, press-in nuts and in-mold inlaid nuts, as shown in figure.

 

Types of embedded nuts
Ultrasonic/hot melt nut b) Press-in nut c) In-mold insert nut

Nut Support Design Guide. Generally speaking, thickness of plastic material around nut is 1/2 - 1 times outer diameter of nut. If nut is buried in post, diameter of post should be at least 2 times diameter of nut, as shown in picture. Smaller wall thickness and smaller post diameter can easily affect performance of nut. When mechanical engineer is unsure about size design of pillar, he can consult nut manufacturer. Many manufacturers will provide design guidelines for pillar.

 

For in-mold nuts, since thermal expansion coefficient of plastic materials is much greater than that of metal, plastic will generate large internal stress when cooling, which can easily lead to rupture of pillars. Therefore, when selecting in-mold nuts, product design should pay attention to reducing generation of internal stress and increasing strength of pillars to avoid rupture of pillars.
1) Nuts should be preheated before use.
2) Add reinforcing ribs around pillars to increase strength.
3) Nuts should avoid sharp corners. Features such as knurling can easily cause pillar to break, especially for notch-sensitive materials such as PC.
For later article, please refer to Plastic structure design 6 —part gaps, buckles and stops.