Plastic structure design 6 —part gaps, buckles and stops

Time:2024-04-25 15:56:18 / Popularity: / Source:

To read previous article, please refer to Plastic structural design5 —hole, thread, and insert design requirements.

This article mainly introduces requirements for parts clearance, buckle positions and stops in design of plastic structures. See below for details;

1. Part clearance

In production and design of plastic products, it is very important to have a certain gap between parts (commonly known as virtual space). With size of product, shape and function of parts, value given to virtual position should also change accordingly. When designing plastic parts, try to achieve first-time success. For certain areas that are difficult to guarantee, considering difficulty of adding materials to mold and ease of removing materials during mold repair, a certain gap can be reserved for plastic parts in advance.
Gap value between plastic product parts (commonly known as virtual position) *for reference only*
1 Virtual position between general parts (non-movable parts) Single side 0.1mm~0.2mm
2 Virtual position between parts (movable parts) Single side 0.3mm~0.5mm
3 Empty position around battery door and shell body. Single side 0.2mm
4 Insertion hole between battery door and case body. Single side 0.5mm
5 Requires insertion holes that match other parts. Single side 0.3mm~0.5mm
6 Inner wall of gears, pulleys and gearbox housing 1.0mm
7 Virtual position between gears (large gear radius + small gear radius + virtual position) Virtual bit = modulus × 0.1
8 Bone position of battery sheet and electrical box and empty position of battery door. 0.2mm
9 Virtual position between battery and bottom plane of battery box. 0.25mm
10 Parts of skipped part line match virtual position of parts. Single side 0.1mm~0.2mm

2. Buckle design

(1) Buckling function and introduction

Buckle provides a convenient, fast and economical product assembly method, because combined parts of button are formed at the same time when producing finished product, there is no need to use other locking accessories such as screws, mesons (commonly known as washers or washers) during assembly, as long as buckles on both sides of combination need to be fastened together.
Although design of buckle can have a variety of geometric shapes, operating principle is roughly same: when two parts are fastened, hook-shaped protruding part of one part is pushed away by flange part of connecting part until flange part is completed; then, with elasticity of plastic, hook-shaped protruding part is reset in real time, and groove behind it is embedded by flange part of connecting part. This undercut position immediately forms a mutually interlocking state.
If distinguished by function, buckle design can be divided into two types: permanent type and removable type. Design of permanent buckle is easy to install but not easy to remove, while design of detachable buckle is very convenient to install and remove. Principle is that hook-shaped extension of detachable buckle is equipped with appropriate lead-in and lead-out angles to facilitate buckling and separation. Size of lead-in and lead-out angles directly affects strength required for buckling and detaching. Permanent buckle has only an inlet angle and no outlet angle design, so once it is buckled, connecting parts form a self-locking state and are not easy to remove.

(2) Basic principles of buckle design

a. Buckle must be strong enough to avoid assembly damage.
b. Amount of buckling (amount of buckling position) must be sufficient, otherwise effect will not be obvious.
c. Buckle position must have deformation space for disassembly and assembly.
d. Buckle positions of the whole machine should be evenly distributed.
e. Where strength of rubber shell is weak, adding buckles will strengthen it.
f. Normal button cloth method: female button is clothed on the shell of male stop. Similarly, male button is clothed on the shell of female stop.
g. Reverse buckle: Side where female button is clothed is called reverse button. When making a reverse buckle, be careful to cut off at least 6mm of male openings on both sides of male buckle. Otherwise, buckle will not be deformed, will lose its function and become a dead buckle.

(3) Buckle size

Size of buckle needs to ensure that buckle has sufficient strength and elasticity so that buckle will not break and fail during assembly or disassembly. Therefore, reasonable buckle size design is crucial. A typical straight arm buckle size design is shown in figure.
Plastic structure design 
Buckle thickness t=(0.5~0.6)T.
Root fillet radius of buckle Rmin=0.5.
Height of buckle H=(5~10)t.
Assembly lead-in angle of buckle is a=25°~35°.
Buckle disassembly angle B:
B=35° is used for detachable assembly that does not require external force. B=45° is used for detachable assembly that requires small external force. B=80°~90° is used for non-detachable assembly that requires large external force.
Top thickness of buckle Y≤t.
Plastic structure design 
1) a is buckling amount of buckle, which is related to whether buckle is functional and whether it is difficult to remove after being buckled. It is generally between 0.3-0.6. Here, 0.5 is taken (0.5 is also commonly used by individuals);
2) b is thickness of male buckle, which is also one of factors that ensures success of buckle position design. It generally takes 0.8-1.2, and here takes 1;
3) c is thickness of female buckle, which is as important as male buckle. It usually takes 0.8-1.2, here it takes 1;
4) d is width of female buckle, which ensures same strength as bc. It usually takes 0.8-12, here it takes 1;
5) e is gap between buckles. It cannot be too large or too small. Generally, 0.05 is sufficient.
6) f is chamfer of male buckle, which is designed to facilitate assembly. Generally, chamfer is 0.3-0.5, here we take 0.4:
7) g is thickness of the thinnest part of male buckle. In order to ensure elastic strength of male buckle, 0.8-1.25 is generally taken here, and 0.88 is taken here. H is also avoidance gap of buckle, which is generally not less than 0.2, and 0.2 is taken here.
8) i is avoidance clearance of buckle position, which is generally not less than 0.2, here it is taken as 0.2.
9) j is also avoidance clearance of buckle position, which is generally not less than 0.2. Here it is taken as 0.2.
10)k is convenient for mold design, usually 0.05-0.2, here it is 0.05 (personally, I usually take 1)
Thickness of the thinnest part of female buckle can actually be calculated. According to above content, d is 1, fastening amount a is 0.5, and buckle clearance is 02. Extra margin is the thinnest part of female buckle. Thickness of female button is 0.3; chamfer of female button, without further explanation, is same as that of male button, 0.4.

(4) Precautions for buckle design

1) Buckle segmented design
a. Middle partition form of female buckle forms segments, which increases strength of female buckle, makes it less likely to be damaged during disassembly and assembly.
b. With segmented style of female buckle, male buckle also needs to be partitioned in the middle, so load-bearing strength is correspondingly weakened, male buckle is easily damaged during disassembly and assembly.
2) Buckle-out glue design
a. Main function of buckling glue is to prevent glue from being too thick and shrinking, which affects appearance.
b. In order to avoid affecting appearance of male buckle, glue can be washed appropriately. After glue is washed, the thinnest part should not be less than 0.7mm.
c. Rear part of female buckle can be sealed or partially glued. It is not recommended to fully penetrate female buckle due to impact on strength.
3) Distribution requirements of buckle bits
a. Buckles must be roughly symmetrical and evenly distributed so that forces are balanced everywhere.
b. Distance between the two buckles is set to about 30mm. If distance is too large or too small, it must be adjusted.
4) Buckle design needs to consider convenience of mold modification. Buckle design generally requires multiple design modifications (including modifying length, thickness, offset, etc. of buckle) to meet assembly requirements of parts. Therefore, design size of buckle can be slightly smaller, instead of making buckle full size at one time, to facilitate subsequent mold modifications.

3. Stop design

(1) Definition

1) Stop can be understood as stop structure at junction of upper and lower shells, also called lip, which is divided into a male stop and a female stop.
2) Male joint is usually formed by adding glue on inner edge of plastic part.
3) Female seam is usually formed on inner edge of plastic part by reducing glue.

(2) Design principles

1) Male stopper is generally found on relatively thin plastic parts.
2) Female seams are generally made on relatively thick plastic parts, which can reduce or avoid thick and thin marks.

(3) Design method

1) Really stop talking
Usage: For comparison purposes during production and assembly, and for applying glue. Structural design dimensions are shown in figure below.
Plastic structure design 
2) False stop
Purpose: It can be used to cover up ugliness in appearance, usually called aesthetic line. It is often used for assembly guidance and positioning of bottom and surface shells (or front and rear shells) of plastic products. Structural design dimensions are shown in figure below.
Plastic structure design 
3) Double stop
Usage: Mostly used on some finished products that need to be waterproof. Moreover, ultrasonic welding will be used for assembly to enhance effectiveness. Structure is shown in figure below.
Plastic structure design 
1) Wishbone (commonly known as mouth-stopping bone)
Wishbone is often used to fix stop of plastic products, so it is commonly known as stop pipe bone to prevent bottom and surface shells (or front and rear shells) from separating from shell to cause bottom scraping or surface scraping, which affects product quality. Wishbones should be evenly spaced around plastic parts to play a positioning role. Because plastic has greater elasticity, distance between them should not be too large.
2) Straight-shaped wishbone
Straight-shaped wishbones are often used in plastic products with smaller dimensions or smaller wishbone heights. Structural design dimensions are shown in figure below.
Plastic structure design 
3) Symmetrical L-shaped wishbone
Symmetrical L-shaped wishbones are often used in plastic parts with larger overall dimensions or larger wishbone height dimensions. General structural design dimensions are as shown in figure below, and distance between two adjacent bone positions must be reasonable.
Plastic structure design 

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