Plastics are classified according to their heating properties. They are divided into thermosets and thermoplastics. Rule to distinguish between two types of plastics is generally whether there is a chemical reaction and hardening after heating at a certain temperature for a period of time or adding a hardener. Thermosetting plastics that harden by a chemical reaction are called thermosetting plastics. On the contrary, it is called thermoplastic.
According to basic performance classification, plastics are mainly divided into general plastics, engineering plastics, and functional plastics. General-purpose plastics mainly include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and ABS. They are characterized by large output, low price, and average performance. Engineering plastics mainly include polyamide (PA), polycarbonate (PC), polyoxymethylene, polyphenylene oxide, etc., which are characterized by excellent mechanical properties, electrical properties, chemical properties, heat resistance, wear resistance and dimensional stability. Output of engineering plastic is relatively small and price is more expensive. Functional plastics mainly include medical plastics, photosensitive plastics, etc., plastics with special functions.

Generally speaking, plastic molding methods are as follows: injection molding, extrusion molding, die-casting molding, foaming, blow molding, vacuum molding, hollow molding, machining, etc.
Due to influence of types and properties of plastics, use occasions, molding processes and other conditions, structural design of plastic parts will naturally produce some special requirements and methods, which involve a lot of content. My main contact is structural design of injection molded thermoplastic parts, and my sharing is mainly for this module.
1. When using injection molding process to produce products, due to uneven cooling and uneven shrinkage of plastic in mold cavity, unreasonable product structure design, it is easy to cause various defects in product: shrinkage, weld marks, pores, deformation, napping, top damage, flashing.
2. In order to obtain high-quality injection molded products, we must fully consider its structural manufacturability when designing product. Following combined with main structural characteristics of injection molded product to analyze methods to avoid injection defects.

At the beginning of design of each injection product, mold opening direction and parting line must be determined first to ensure that core pulling mechanism is minimized and effect of parting line on the appearance is eliminated.
2.1.1 After mold opening direction is determined, product's ribs, buckles, protrusions and other structures should be designed to be consistent with mold opening direction as much as possible to avoid core pulling, reduce seam lines, extend life of mold.
2.1.2 For example: mold opening direction of bumper is generally car body coordinate x direction. If mold opening direction is designed to be inconsistent with x axis, included angle must be indicated in product drawing.
2.1.3 After mold opening direction is determined, an appropriate parting line can be selected to improve appearance and performance.

2.2.1 Proper demolding slope can avoid product fuzzing. Demolding slope of smooth surface should be greater than 0.5 degrees, fine grained surface should be greater than 1 degree, and coarse grained surface should be greater than 1.5 degrees.
2.2.2 Proper demolding slope can avoid product top damage.
2.2.3 When designing deep cavity structure products, outer surface slope must be smaller than inner surface slope to ensure that mold core is not deviated during injection molding, to obtain uniform product wall thickness, and to ensure material density strength of product opening.

2.3.1 All kinds of plastics have a certain range of wall thickness, generally 0.5～4mm. When wall thickness exceeds 4mm, it will cause excessive cooling time and shrink printing, so we should consider changing product structure.
2.3.2 Uneven wall thickness will cause surface shrinkage.
2.3.3 Uneven wall thickness will cause pores and weld marks.

2.4.1 Reasonable application of stiffeners can increase product rigidity and reduce deformation.
2.4.2 Thickness of ribs must be less than 1/3 of product wall thickness, otherwise it will cause surface shrinkage.
2.4.3 Single-sided slope of stiffener should be greater than 1.5° to avoid top damage.

2.5.1 Too small rounded corners may cause stress concentration in the product, leading to product cracking.
2.5.2 Too small rounded corners may cause stress concentration in mold cavity, resulting in cavity cracking.
2.5.3 Setting a reasonable fillet can also improve processing technology of mold. For example, cavity can be directly milled with an R cutter to avoid inefficient electrical processing.
2.5.4 Different rounded corners may cause parting line to move, so you should choose different rounded corners or clear corners based on actual situation.

2.6.1 Shape of hole should be as simple as possible, generally round.
2.6.2 Axial direction of hole is consistent with opening direction, which can avoid core pulling.
2.6.3 When length to diameter ratio of hole is greater than 2, demolding slope should be set. At this time, diameter of hole should be calculated according to minor diameter (the largest physical size).
2.6.4 Aspect ratio of blind holes generally does not exceed 4.
2.6.5 Distance between hole and edge of product is generally greater than hole size.

2.7.1 When plastic part cannot be demolded smoothly in opening direction, core pulling mechanism should be designed. Core pulling mechanism can form a complex product structure, but it is easy to cause defects such as product stitching, shrinkage, etc., increase mold cost and shorten mold life.
2.7.2. When designing injection products, if there is no special requirement, try to avoid core pulling structure. For example, direction of hole axis and ribs is changed to mold opening direction, using methods such as cavity core collision.

2.8.1 Utilizing toughness of PP material, hinge can be designed to be integrated with product.
2.8.2 Size of film used as a hinge should be less than 0.5mm and kept uniform.
2.8.3 When injecting an integral hinge, gate can only be designed on one side of hinge.

2.9.1 Inserting inserts in injection molded products can increase local strength, hardness, dimensional accuracy and set small threaded holes (shafts) to meet various special needs. At the same time it will increase product costs.
2.9.2 Inserts are generally copper, but also other metal or plastic parts.
2.9.3 Part of insert embedded in plastic should be designed with anti-rotation and anti-pull-out structure. Such as: knurling, hole, bending, flattening, shaft shoulder, etc.
2.9.4 Plastic around insert should be appropriately thickened to prevent stress cracking of plastic.
2.9.5 When designing inserts, fully consider its positioning method in mold (holes, pins, magnetism)

Product identification is generally set on inner surface of product on a relatively flat surface, and adopts a convex form. Select surface where normal direction and mold opening direction ruler may be consistent to set mark to avoid strain.

Due to inhomogeneity and uncertainty of shrinkage rate during injection molding, accuracy of injection molded parts is significantly lower than that of metal parts, appropriate tolerance requirements should be selected according to standard (OSJ1372-1978).

Try to avoid flat structure, reasonably set flanging, concave-convex structure, and set reasonable reinforcing ribs.

2.13.1 Gas-assisted injection molding can improve product rigidity and reduce deformation.
2.13.2 Gas-assisted injection molding can avoid shrinkage.
2.13.3 Gas-assisted injection molding can save raw materials and shorten cooling time.

2.14.1 Use of welding can improve strength of connection.
2.14.2 Use of welding can simplify product design.
3. Reasonably consider contradiction between process and product performance
3.1 Contradiction between product appearance, performance and process must be considered comprehensively when designing injection products. Sometimes a part of manufacturability is sacrificed to get a good appearance or performance.
3.2 When structural design cannot avoid injection molding defects, try to make defects occur in hidden parts of product as much as possible.

1. Mold opening direction and parting line
2. Demoulding angle
3. Part wall thickness
4. Reinforcing ribs
5. Fillet and hole
6. Core-pulling mechanism and avoidance
7. Deformation of plastic parts
8. Integrated hinge
9. Insert
10. Gas-assisted injection molding
11. Comprehensive consideration of manufacturability and part performance

Place granular or powdered plastic in barrel of injection machine, heat it to melt, apply pressure with a push rod or a rotating screw to inject rubber cluster nozzle in barrel and casting system of mold into mold cavity for cooling and molding. See Figure 1 and Figure 2 below for details.

 

Figure 1 Schematic diagram of injection molding

 

Figure 2 Overall structure of injection molding machine