Plastic parts are structurally designed on the basis of industrial molding. First, check if there are similar products for reference, then conduct detailed functional decomposition of products and parts, determine main process issues such as part folding, wall thickness, demoulding slope, transition treatment between parts, connection treatment, and strength treatment of parts.
Similar reference
Before designing, first look for similar products of company and peers, what problems have occurred with original products, what deficiencies, refer to existing mature structures, and avoid problematic structural forms.
Determine part division, transition, connection and clearance treatment between parts
Understand styling style from styling diagram and rendering, cooperate with functional decomposition of product, determine number of part divisions (different surface states are either divided into different parts, or there must be transition treatment between different surfaces), determine transition treatment between part surfaces, determine connection method between parts, and matching clearance between parts.
Determination of part strength and connection strength
Determine wall thickness of part body according to size of product. Strength of part itself is determined by wall thickness of plastic part, structural form (flat-shaped plastic part has the worst strength), reinforcement ribs and reinforcement bones. While determining individual strength of part, connection strength between parts must be determined. Methods to change connection strength include adding screw columns, adding stoppers, adding buckles, and adding reinforcement bones to support bottom.
Determination of demoulding slope
Demoulding slope should be determined based on material (PP, PE silicone, rubber can be demoulded forcibly), surface state (slope of decorative pattern should be larger than smooth surface, slope of etched surface should be as large as possible than sample requirement by 0.5 degrees to ensure that etched surface is not damaged and improve product yield), transparency or not determines demoulding slope of part (transparent slope should be larger) and other factors.
Types of materials recommended by company's different product series

Surface treatment of plastic parts

Wall thickness selection of plastic parts

Plastic parts have requirements for wall thickness uniformity. Workpieces with uneven wall thickness will have shrinkage marks. Ratio of rib to main wall thickness is preferably less than 0.4, and maximum ratio does not exceed 0.6.
Demolding slope of plastic parts
In construction of stereogram, slope needs to be drawn in places that affect appearance and assembly, and ribs are generally not drawn with slopes.
Demoulding angle of plastic parts is determined by material, surface decoration state, and whether part is transparent or not. Hard plastics have a larger demoulding angle than soft plastics. The higher part and the deeper hole, the smaller angle.

Recommended demoulding angles for different materials

Dimensional accuracy of plastic parts
Plastic parts are generally not very precise. In actual use, we mainly check assembly dimensions and mainly mark the overall dimensions, assembly dimensions, and other dimensions that need to be controlled on plan.
In practice, we mainly consider consistency of dimensions, such as need to align edges of upper and lower covers,

 

Economic accuracy used by different materials

Surface roughness of plastics
1) Roughness that cannot be marked on etched surfaces. In places where plastic surface finish is particularly high, circle this range and mark surface state as mirror.
2) Surface of plastic parts is generally smooth and bright, and surface roughness is generally Ra2.5~0.2um..
3) Surface roughness of plastic mainly depends on roughness of mold cavity surface. Roughness of mold surface is required to be one to two levels higher than surface roughness of plastic parts. Mold surface can reach Ra0.05 by ultrasonic and electrolytic polishing.
Fillet
Injection molding fillet value is determined by adjacent wall thickness, generally 0.5~1.5 times wall thickness, but not less than 0.5mm.
Position of parting surface should be carefully selected for fillet. There is a fillet on parting surface. Fillet part needs to be on the other side of mold. It is difficult to make, and there are subtle traces at fillet. But fillets are needed to prevent cutting hands.
Problems with reinforcing ribs
Injection molding process is similar to casting process. Unevenness of wall thickness will cause shrinkage defects. Generally, wall thickness of rib is 0.4 times thickness of main body, and maximum does not exceed 0.6 times. Spacing between ribs is greater than 4t, and height of ribs is less than 3t, as shown in Figure 6-1. In method of improving strength of parts, reinforcement is generally added without increasing the wall thickness, as shown in Figure 6-1.
Ribs of screw column are at least 1.0mm lower than end face of column, and ribs need to be at least 1.0mm lower than surface of part, or parting surface.
When multiple ribs intersect, pay attention to problem of uneven wall thickness caused by intersection.

Support surface
Plastics are easy to deform, and should be classified as positioning of blank in terms of positioning. Positioning area should be small. For example, support of plane should be changed to a small convex point or a convex ring.
Problems with lifter and slide position
Lifter and slide position move in parting direction and perpendicular to parting direction. There should be no glue blocking movement of lifter and sliding position in direction perpendicular to mold splitting direction, and there should be enough space for movement, as shown in figure below.

 

Methods for handling limit process problems of plastics
1) Special treatment method for wall thickness
For particularly large workpieces, such as toy car shells, multi-point glue injection method can make wall thickness relatively thinner. Local glue position of column is thick, and method shown in figure below is used to handle it.

 

2) Handling of small slopes and vertical surfaces
Mold surface has high dimensional accuracy, high surface finish, low demoulding resistance, and demoulding slope can be small. To achieve this goal, small slope of workpiece is separately inlaid, inlay is processed by wire cutting and grinding, as shown in figure.

 

 
To ensure verticality of side wall, a running position or a bevel is required. There is an interface line when running position is in place. To avoid obvious interfaces, wiring is generally placed at intersection of fillet and large surface.

 

To ensure verticality of the side wall, a running position or a bevel is required. There is an interface line when running position is in place. To avoid obvious interfaces, wiring is generally placed at intersection of fillet and large surface.

1) Transition processing problems
Precision of plastic parts is generally not high, transition processing is required between adjacent parts and different surfaces of same part.
Small grooves are generally used to transition between different surfaces of same part, small grooves can be used between different parts to handle high and low staggered surfaces, as shown in figure

 

2) Clearance value of plastic parts
When parts are directly assembled and there is no movement between them, 0.1mm is generally taken; Stop is generally taken as 0.15mm; Minimum gap between parts without contact is 0.3mm, generally 0.5mm.
3) Common forms of stop and gap selection of plastic parts are shown in the figure

 

Plastic part hole diameter corresponding to self-tapping screws

1. Replace sheet metal chassis cover with plastic chassis cover;
2. Replace electroformed label with two-color injection molding label;
3. Company is replacing metal antistatic panel with antistatic plastic panel;
4. Company is replacing metal shielding panel with plastic shielding panel