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In order to ensure the strength and rigidity of the plastic part and not to cause the wall thickness of the plastic part to be too thick, the reinforcing rib can be set at the appropriate part of the plastic part to avoid the deformation of the plastic part. In addition, the stiffener can also act as an internal flow channel, improve the plastic flow in the molding process of plastic parts, and help mold cavity filling.
1. Thickness
Generally, the thickness of the large end of the stiffener A shall not be greater than 1 / 2 of the wall thickness to avoid shrinkage; the thickness of the small end of the stiffener B, PP material shall not be less than 0.9mm, and other ABS / PS materials shall not be less than 1.0mm. The reinforcement section is shown in Figure 1-1.
T-top wall thickness
A-the thickness of the large end of the rib, A≤1/2T
B－Thickness of small end of rib
C-demolding slope
H-rib height
Figure 1-1
The way to increase the strength is to increase the number of bars, not the thickness of the bars. When it is necessary to use deep stiffeners to make the large end of the reinforcement thicker, anti shrinkage structure shall be considered, such as the hanging hook on the inner wall of the disc seat (as shown in Figure 1-2), or the parts easy to form shrinkage marks shall be designed into patterns to cover the shrinkage marks.
Figure 2-2
The following table is the design reference value of wall thickness reinforcement of common plastic products:
Common plastic products wall thickness and rib thickness design reference value | |||||||||||
Product type | Top wall thickness | material | Side wall thickness | Flanging thickness of internal hole | Tendon ≤ | Tendon ≥ | |||||
reference | recommend | reference | recommend | ||||||||
Ordinary appearance products | 3 | ABS/PS | 2.8-2.5 | 2.5 | 2.5-2.0 | 2.0 | 1.4 | 1.0 | |||
PP | 2.8-2.5 | 2.5 | 2.5-1.5 | 1.5 | 1.2 | 0.9 | |||||
2.8 | ABS/PS | 2.5-2.2 | 2.2 | 2.3-1.8 | 1.8 | 1.4 | 1.0 | ||||
PP | 2.5-2.2 | 2.2 | 2.2.1.5 | 1.5 | 1.2 | 0.9 | |||||
2.5 | ABS/PS | 2.5-2.0 | 2.0 | 2.0-1.3 | 1.5 | 1.3 | 1.0 | ||||
PP | 2.5-2.0 | 2.0 | 2.0-1.3 | 1.3 | 1.2 | 0.9 | |||||
Transparent products | 3 | Transparent ABS / Transparent PS / PMMA | 2.8-2.5 | 2.5 | 2.5-2.0 | 2.0 | 2.0 | 1.4 | |||
2.8 | 2.5-2.2 | 2.2 | 2.3-1.8 | 1.8 | 1.8 | 1.2 | |||||
2.5 | 2.5-2.0 | 2.0 | 2.0-1.5 | 1.5 | 1.8 | 1.2 | |||||
Table 2-1
explain:
1. This table is the reference value of wall thickness and reinforcement thickness of common household electrical appliances plastic products, excluding precision products such as mobile phones and remote controls. The size of side wall and stiffener of unconventional products should be discussed separately;
2. The large end value of stiffeners of transparent products given in the table refers to the large end size of transparent products without strength requirements. For transparent products with strength requirements, the large end size of the stiffener can be designed to be equivalent to the basic wall thickness, but the small end of the stiffener cannot be less than the value given in the table above.
2. Height
The height of reinforcement shall not be more than 3 times of the top wall thickness, as shown in Figure 2-1, with dimension h ≤ 3T. In the case of meeting the design requirements, the height of the stiffener shall be as small as possible. Using two or more short stiffeners is better than using a single high stiffener.
To ensure that the plastic parts are basically flat, the end face of the stiffener shall not be flat with the supporting surface of the plastic parts, and shall be no less than 0.5mm lower than the supporting surface, as shown in Figure 2-3:
Figure 2-3
In the design of stiffeners, the position and height of the stiffeners should be taken into account. As shown in Figure 2-4, the design of reinforcement position is unreasonable, and the height from the end face of reinforcement position to each supporting surface shall be as equal as possible. In the improved scheme, the arch opening can be moved to the left or the height of the arch opening can be raised.
Figure 2-4
3. Demoulding inclination
In order to facilitate formwork removal, the angle of demoulding shall be added to the stiffener, generally 0.5 ° ~ 1 ° on one side. If allowed, the angle shall be as large as possible, and the thickness of large and small ends shall be marked for the deeper stiffener.
4. Application of fillet
In order to improve the strength of the plastic part, improve the flow of the plastic part and facilitate the demoulding, the end face of the stiffener should be designed with arc transition. As shown in Figure 2-5, the design of reinforcement position is unreasonable, and the end face of reinforcement position shall be designed as fillet.
Figure 2-5
5. Application of arch and waveform
In the design of stiffener, the application of arch and wave form should be considered properly. The strength and rigidity of the rib can be increased by making the rib surface into arch shape and wave shape, and the phenomenon that the injection molding is not in place due to insufficient materials can be avoided.
6. Strength requirements
In the process of falling, the plastic parts will cause the fracture of special parts, so when designing the stiffener, its function and function should be fully considered, and the strength should meet certain requirements. For example, some stressed structures (screw columns, etc.) are easy to break and affect the function of components. For such structures, the contact area with the bottom surface should be considered. The number of stiffeners should be increased properly. If necessary, fillets should be added to the bottom surface of stiffeners to improve the strength.
According to the different shapes and functions, the stiffeners can be divided into the following categories, and the key points that should be paid attention to are introduced respectively below.
1. Unidirectional bar
Unidirectional reinforcement is generally used in thin and long plastic parts, only in the case of unidirectional rigidity. The unidirectional reinforcement is usually a series of parallel reinforcement combinations. During the design, attention shall be paid to the spacing of the series of reinforcements. Generally, the spacing value shall not be less than 2 times of the wall thickness, as shown in Figure 3-1.
T = top wall thickness
A≤1/2T
H≤3T
L≥2T
C=0.5°～1
Figure 3-1
If there is a strength requirement for the series bars, such as the window of the washing machine, the strength requirement is relatively high here, so the spacing value of the series bars can not meet the above requirements, as shown in the figure.
Figure 3-2
2. Cross bar
The cross bar is the bar where two right angles intersect, and the thick section is easily formed at the intersection, as shown in the figure:
Figure 3-3
Therefore, a core of appropriate size shall be designed at the joint of the intersecting reinforcement to keep the thickness of the reinforcement within 1 / 2 of the wall thickness, so as to meet the design requirements of the wall thickness of the reinforcement, as shown in the figure below.
Figure 3-4
3. Stiffeners on the side walls of plastic parts at both ends
The position and direction of the reinforcing rib should be the same as the filling direction of the rubber melt. The reinforcing rib can be connected with the side wall of the plastic part at both ends, and the length is the same as the length of the plastic part, which can increase the local rigidity of the plastic part. If the reinforcement is not connected to the side wall of the plastic part, the end part shall not be terminated suddenly, and the height shall be gradually reduced until the end, so as to reduce the problems of trapped gas, insufficient filling and scorching marks. These problems often occur in the position of insufficient exhaust or sealing.
Figure 3-5
4. Reinforcement not perpendicular to side wall
When some bars are connected with the side wall, the direction is not perpendicular to the side wall, but at a certain angle. As shown in figure 3-6-1, the thickness of the joint between the bars and the side wall is greater than 1 / 2 of the thickness of the side wall. To avoid shrinkage, the thickness of the joint between the bars and the side wall should be cut to 1 / 2 of the wall thickness, or the joint between the bars and the side wall should be changed to perpendicular intersection, as shown in figure 3-6-2.
Figure 3-6-1
Figure 3-6-2
Such as the bar at the bottom corner of the washing machine
Figure 3-7
In order to reduce the cost of machining and polishing, the following problems should be paid attention to in mold design:
When the thickness of the small end of the rib is less than 1.2 mm, the die should adopt the splicing structure;
When the depth of the reinforcement is more than 10 mm and the small end of the reinforcement is less than 1.5 mm, the die shall be realized by splicing structure;
Other situations can be determined according to specific situations.
1. Under the double barrel washing box
Under the double barrel washing box is the main load-bearing part of the washing machine. The bottom of the reinforcing rib near the installation of the motor cannot be designed too wide, so the stress at the root of the rib is concentrated and easy to break, and the phenomenon of breaking is eliminated after rounding.
Figure 5-1
2. Under the barrel in the washing machine
In order to increase the strength and rigidity of the reinforcement and avoid the uneven stress on each step surface, the end face should be designed with fillet and the arch mouth should be improved.
Figure 5-2
