For previous article, Please read Plastic Structural Design 2 - Mold (Introduction). 
This article mainly introduces requirements for wall thickness and draft angle in plastic structure design. See below for details;

Thickness of wall depends on external force that product needs to withstand, whether it serves as a support for other parts, number of supporting columns, number of protruding parts, and selected plastic material. Wall thickness design of general thermoplastics should be 4mm as limit. Shell is usually 1.5, 1.8, 2.0, 2.2, 2.5, 2.8, 3.0mm, etc. (precision structures are not included in this list). Empirical value is maximum size of the whole machine x1/100mm about. From an economic point of view, excessively thick product design not only increases material costs, prolongs production cycle (cooling time), and increases production costs. From a product design perspective, products that are too thick increase possibility of causing cavities (bubbles), which greatly weakens rigidity and strength of product. Since internal structural plastic parts do not have appearance problems, their thickness and size are much more flexible.
The most ideal wall thickness distribution is undoubtedly that thickness of cut surface is uniform everywhere, but it is always inevitable to change wall thickness to meet functional requirements. In this case, transition from areas of thick rubber to areas of thin rubber should be as smooth as possible. Too sudden wall thickness transitions can cause dimensional instability and surface problems due to differential cooling rates and turbulence.
Table shows minimum wall thickness of commonly used materials and recommended values of common wall thicknesses.

For general thermoplastics, when shrinkage rate is lower than 0.01mm/mm, product can allow thickness to change by up to . However, when shrinkage rate is higher than 0.01mm/mm, product wall thickness should not change by more than. For general thermosetting plastics, too thin product thickness often causes product to overheat during operation, resulting in waste parts. In addition, fiber-filled thermosetting plastics often form insufficient fillers when they are too thin. However, some easy-flowing thermosetting plastics such as epoxy resin, etc., if thickness is uniform, minimum thickness can reach 0.25mm, even some small products and precision products can have a stable wall thickness of 0.1-0.2mm; actual situation needs to be communicated with mold factory and material factory.

1) Whether structural strength of parts is sufficient. Generally speaking, the thicker wall, the stronger part. However, when wall thickness of part exceeds a certain range, due to quality problems such as shrinkage and pores, increasing wall thickness of part will actually reduce strength of part.
2) Whether part can resist demoulding force during molding. Parts that are too thin are easily deformed by ejection.
3) Whether it can resist tightening force during assembly.
4) When there are metal embedded parts, is strength around embedded parts sufficient? Generally, shrinkage of embedded metal parts and surrounding plastic materials is uneven, which is prone to stress concentration and low strength.
5) Whether parts can evenly distribute impact force they bear.
6) Is strength of hole sufficient? Strength of hole is easily reduced due to influence of welding marks.
7) On premise that above requirements are met and injection molding does not cause quality problems, wall thickness of plastic parts should be kept to a minimum, because thicker wall thicknesses not only increase material cost and weight of parts, but also prolongs molding cycle of parts, thereby increasing production costs.
In order to ensure and improve strength of parts, mechanical engineers often tend to choose larger wall thicknesses. In fact, it is not the best way to ensure and improve strength of parts by choosing a larger wall thickness. Strength of parts can be improved by adding reinforcing ribs, designing curves or wavy part sections, etc. This can not only reduce material waste of parts, but also shorten injection molding cycle of parts.

The key to uniform wall thickness is also required at corners to avoid inconsistent cooling times. Where cooling times are long, shrinkage occurs, causing part deformation and deflection. In addition, sharp rounded corners often lead to component defects and stress concentrations, and sharp corners often cause undesirable material accumulation after plating process. Areas of concentrated stress can crack under load or impact. Larger fillets provide a solution to this shortcoming, not only reducing stress concentration factors, but also making flowing plastic flow smoother and finished product easier to demold. Picture below is for reference.

 

In most hot melt process operations, including extrusion and cure molding, uniform wall thickness is very important. Area with thick glue cools slower than area with thin glue next to it, and shrinkage marks appear on the surface of connecting areas after gate solidifies. What's more, it will cause shrinkage marks, thermal internal stress, distortion of flexure part, different colors or different transparency. If it is unavoidable that thick glue will gradually turn into thin glue, try to design a gradual change and do not exceed wall thickness ratio of 3:1. Picture below is for reference.

 

In fact, most thick plastic designs can be eliminated by using reinforcing ribs and changing cross-sectional shape. In addition to saving materials and thus reducing production costs, design after cancellation can also retain same rigidity, strength and functionality as original design. If metal gear in picture below is changed to use plastic material, changed design should be as shown in picture. Compared with original metal design, this plastic gear design not only saves material, but also eliminates increase in internal stress caused by uneven thickness and deformation of the overall gear caused by shrinkage of crown part.

 

Plastic parts must have sufficient draft to avoid whitening, scratching and dragging. Draft of mold is related to properties of rubber material, shape of rubber parts, and surface requirements.
Recommended values for minimum draft angle of commonly used rubber materials. For parts of 3D file of plastic parts that do not have draft requirements, refer to general draft requirements in technical description. Appearance surface of plastic parts requires smooth or textured surface, and its demoulding slope is also different. Slope value is as follows:
1) Demoulding angle of small plastic parts with smooth outer surface is 1˚, and demoulding angle of large plastic parts is 3˚;
2) Outer surface etching surface Ra<6.3 draft angle/3˚, Ra6.3 draft angle/4˚;
3) Spark pattern surface of outer surface Ra<3.2 draft angle/3˚, Ra3.2 draft angle/4˚.

 

Schematic diagram of demoulding slope
Design principles for draft angle of parts are as follows:
1) If there are no special requirements for parts, draft angle is generally 1°~2°.
2) For plastic parts with a large shrinkage rate, a larger demoulding slope should be selected.
3) A smaller draft angle should be used for parts with high dimensional accuracy requirements.
4) Draft angle of male mold side is generally smaller than draft angle of female mold side to facilitate part demoulding.
5) When wall thickness of plastic parts is larger, molding shrinkage increases, so demoulding slope should be larger.
6) Draft angle of embossed surface and complex surface should be larger. Size of embossed surface determines draft angle. You can refer to previous article Plastic Structural Design - Mold (Introduction)
7) For glass fiber reinforced plastics, draft angle should be larger.
8) Size and direction of demoulding slope cannot affect function realization of product.
9) Some planes of part do not need to have a draft angle due to functional requirements, but mold needs to be designed with a side core-pulling structure. Mold structure is complex and cost is high.
10) When function and appearance of part allow, draft of part should be as large as possible. A smaller draft angle increases possibility of surface scratches and damage to parts during ejection process; at the same time, a smaller draft angle requires mold surface polishing or a complex mold ejection mechanism, which increases mold costs.
For read more, to Plastic structural design 4 —reinforcement ribs and columns .