Plastic part thickness is the most fundamental design requirement for plastic parts and is closely related to part's overall dimensions. Generally speaking, larger products require thicker materials. Plastic part thickness determines product's strength, weight, and assembly capabilities.
Plastic part thickness can be selected based on material and product's overall dimensions. Typical range is 0.6-6.0mm, with a commonly used thickness between 1.5-3.0mm. Recommended thickness values for commonly used plastic parts are: small products have a maximum overall dimension of L < 80.0mm; medium products have a maximum overall dimension of 80.0mm < L < 200.0mm; and large products have a maximum overall dimension of L > 200.0mm.

*Recommended Material Thickness for Common Plastic Parts
Maintain consistent material thickness for plastic parts. Otherwise, uneven filling can lead to defects such as deformation and localized dents. If structural requirements require uneven material thickness, transition should be gradual to avoid excessive thickness variations that could cause stress concentration or localized dents, impacting product's appearance and structural strength.

 

Tips: Material thickness variations should be minimal. Thickness should not exceed 2.0 times thickness from thin to thick, and should not be less than 0.50 times thickness of original plastic.

Angle refers to angle that plastic parts should have in direction of ejection. It is a necessary condition for normal ejection of mold. When designing plastic parts, both appearance and internal structure must have a demolding angle. Demolding angle is related to appearance, material, product size, and product function of product. Design points of the demolding angle are mainly as follows.
(1) For products with high requirements for product appearance, demolding angle should be small.
(2) For products with high requirements for product precision, demolding angle should be small.
(3) For products with large product size, demolding angle should be small.
(4) For plastic materials containing lubricants, demolding angle should be small.
(5) For products with shiny outer surfaces, demolding angle should be appropriately reduced.
(6) For products with rough appearance, demolding angle should be increased.
(7) For products with complex appearance, demolding angle should be increased.
(8) For plastics with poor injection fluidity or reinforced plastics, demolding angle should be increased.
(9) If product material is thick, demoulding angle should be increased appropriately.
(10) Plastics with large shrinkage should use a larger demoulding angle.
(11) Demoulding angle of transparent plastic parts should be increased appropriately.

Tip for demoulding angles of commonly used plastics: Plastic products should be protected from surface damage during demoulding. Regardless of material used, it is recommended that demoulding angle of surface should not be less than 3°.
Tip: When designing structure of plastic products, strictly speaking, all demoulding angles must be made. However, in actual work, demoulding angles of all important mating surfaces must be made. Demoulding angles of non-important surfaces (such as ribs) generally do not need to be made. Mold designer will make draft angle according to company's internal standards.

When designing structure of plastic parts, in order to improve product strength and avoid stress concentration during injection molding and facilitate demolding, transition fillets should be designed between intersections of product surfaces.

 

(1) When there are no special requirements for product structure design, transition fillet is determined by adjacent material thickness. Inner fillet radius (R) is generally in the range of 0.50 to 1.50 times material thickness (t), but minimum fillet radius must not be less than 0.30mm.
(2) When designing fillets at corners of inner and outer surfaces of product, material thickness should be kept uniform, as shown in Figure 4-9, Ra=Rb+t.

 

(3) When designing structure of plastic parts, special attention should be paid to the fact that parting surface of mold should not have fillets, unless there are special requirements for product. If parting surface has fillets, it will increase difficulty of mold production and will also leave clamping marks on outer surface of product, affecting appearance.
(4) There should be no sharp corners or edges where exterior and interior surfaces of product can come into contact. If necessary, fillet processing should be performed. Minimum fillet radius should not be less than 0.30mm to prevent scratches on fingers. Special attention should be paid especially when designing structure of toy products.

 

Sometimes, in order to improve strength and rigidity of plastic parts without changing material thickness, it is necessary to reasonably design reinforcement ribs. Role of reinforcement ribs in plastic products is to improve strength and rigidity of plastic parts, prevent deformation of plastic parts, and also facilitate flow of raw materials during injection molding. Application of reinforcement ribs includes long strip grid reinforcement ribs and circular grid reinforcement ribs.
(1) Design requirements for reinforcement ribs
·Dimension A is thickness of large end of reinforcement rib, ranging from 0.4t to 0.60t, and generally takes value as 50% of material thickness.
·Dimension B is height of reinforcement rib, generally required to be no more than 3t.
·Dimension C is distance between two reinforcement ribs, generally required to be no less than 4t.
·Dimension D is distance between reinforcement rib and surface of part, generally required to be no less than 1.00mm.

 

(2) Reinforcement rib design dimension description for screw column:

 

·Dimension A is flat width of upper end of reinforcement rib, which should be no less than 0.50mm. Dimension B is width of rib at base, ranging from 0.20 to 0.50 times height of screw post.
·Dimension C is distance from rib to the top of screw post and should be no less than 1.00 mm.

Support surface is bottom surface that bears weight of product. For larger products, using the entire surface as support surface will not allow for a smooth bottom. Therefore, it is necessary to design convex edges, bosses, or raised points for support. Height of support surface should be determined based on product's overall dimensions, with a typical range of 0.30-2.00 mm.

 

As shown in the figure, 1-12 are all support surface designs

Hole is often encountered in product structure design. There are two common types of holes: one is circular hole and the other is non-circular hole. When designing hole position, difficulty of mold processing should be minimized without affecting strength of plastic part.
(1) Design dimension description of common holes:

 

·Dimension A is distance between holes. If hole diameter is less than 3.00mm, it is recommended that value of A is not less than D; if hole diameter exceeds 3.00mm, value of A can be 0.70 times hole diameter.
·Dimension B is distance between hole and edge. It is recommended that value of B is not less than D.
(2) Relationship between hole diameter and hole depth dimension description:

 

·Dimension A is depth of blind hole. It is recommended that value of A is not greater than 5D.
·Dimension B is depth of through hole. It is recommended that value of B is not greater than 10D.
Tips: Holes mentioned here do not include inner holes of screw columns.

Characters and patterns on plastic products are divided into two types: convex surface and concave surface. There are generally two processing methods. Small characters and patterns are obtained by mold etching, while slightly larger characters and patterns are obtained directly by mold processing.

 

(1) It is best to use convex surface method for characters and patterns on plastic products. In this way, mold surface is concave, and mold processing is much simpler. If surface does not allow convexity due to structural requirements, area with characters or patterns can be concave to a certain depth, then characters or patterns can be raised in groove. This not only meets structural requirements but also facilitates mold production. Convex character surface is preferably about 0.10mm lower than groove surface.

 

(2) For characters and patterns on plastic products, convex surface height is generally 0.15~0.30mm, concave character and pattern depth is 0.15~0.25mm.

 

Character size description Dimension description:
·Dimension A is depth of blind hole. It is recommended that value of A should not exceed 5D.
Dimension B is depth of through hole. It is recommended that B value should not be greater than 10D.

Threads are used to connect parts and are often used on plastic products. Threads on plastic products are somewhat different from those on hardware products. Threads on plastic products are injection molded by molds, with relatively low precision, and fine threads are difficult to form; while threads on hardware products are made by machining, with high precision, and can process very fine threads.

 

(1) Thread diameter on plastic products cannot be too small. External thread diameter is not less than 3.00mm, internal thread diameter is not less than 2.00mm, and thread pitch is not less than 0.50mm.

 

(2) To ensure good screwing of internal and external threads, fitting length of plastic thread should not be too long. It is recommended that fitting length L is not greater than 2 times thread diameter.
(3) The first circle of plastic thread is prone to cracking or stripping, and in order to facilitate demolding, it is necessary to design a section of unthreaded cylindrical surface at the beginning and end of thread. Height of cylindrical surface is not less than 0.50mm.

 

Inserts on plastic parts refer to parts made of other materials that are implanted into plastic products during mold injection molding and combined with plastic products. The most commonly used inserts are metal parts, small inserts such as screws and nuts; slightly larger inserts such as stainless steel sheets used on the bottom surface under battery compartment in mobile phone products to reduce thickness. Main function of inserts is to improve mechanical strength and wear resistance of plastic parts.
Basic requirements for designing metal inserts are as follows:
(1) Inserts have high dimensional accuracy requirements, such as nuts.
A slight difference between outer dimensions of nut and thread diameter will make it difficult to position it in mold.
(2) Strength of insert must be high enough. Due to high injection pressure of mold, parts with insufficient strength are easily damaged.
(3) Insert must be tightly combined with plastic material and cannot be loose or shaken. Cylindrical inserts need to be treated with a rolling grid pattern on the surface to enhance adhesion.
(4) If insert material is sheet material, such as stainless steel sheet, to prevent it from falling off, more hanging platforms and cutouts should be designed on surrounding side walls to embed it into plastic.
(5) Insert shape is preferably designed to be cylindrical to facilitate placement and positioning in mold.
(6) Insert size should not be too large or too thin to prevent deformation during injection molding.
(7) Design of thickness of plastic covering metal insert.

 

*Thickness of plastic covering metal insert

Self-tapping screws are coarse-thread screws that achieve "tapping", "drilling", "extrusion" and "pressing" through their own threads, thereby fastening two parts together. They are widely used in connections between plastics, softer metals, wood products, etc.
Classification of self-tapping screws:
(1) According to head type, they are divided into round head, countersunk head, round head with washer, hexagonal head, cylindrical head, semi-round head, semi-countersunk head, etc.;
(2) According to groove type, they are divided into cross-shaped, hexagonal, straight-shaped, plum-shaped, chrysanthemum-shaped, triangle, square, etc.

 

(3) According to thread tail type, they are divided into flat tail, pointed tail, flat tail open, pointed tail open, etc.
(4) Examples of naming of self-tapping screws:
① PB2.60mmx4.00mm represents a self-tapping screw with a round head and flat tail of 2.60mm, and a length of 4.00mm.
② PWB (2.60mm x 5.00mm) represents a self-tapping screw with a round head, washer, and flat tail (2.60mm). Length is 5.00mm.
③ PA (2.60mm x 6.00mm) represents a self-tapping screw with a round head and pointed tail (2.60mm). Length is 6.00mm.
④ KB (2.60mm x 5.50mm) represents a self-tapping screw with a countersunk head and flat tail (2.60mm). Length is 5.50mm.
⑤ PAT (2.60mm x 7.00mm) represents a self-tapping screw with a round head and pointed tail (2.60mm). Length is 7.00mm.

 

Plastic products often have limited precision. This precision is affected by many factors, including type of plastic material, shape and size of product, mold design and processing capabilities, and injection molding parameters. Government has established tolerance standards for plastic parts, classifying them into different precision grades.