Design of plastic parts buckle—DFM key points

The buckle is the simplest, fastest, lowest cost and most environmentally-friendly method of assembly of plastic parts. There is no need to use screwdrivers during the buckle assembly. The assembly process is simple, and only one simple insertion action can complete two or Assembly of multiple parts.

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Classification of buckles

There are many ways to classify buckles. According to the shape of the buckle, the commonly used buckles can be divided into straight-arm buckles, L-shaped buckles, U-shaped buckles, and circumferential buckles. Compared with the straight arm type buckle, due to the increase of the effective length, the shape and U-shaped buckle have greater elasticity.

According to whether the buckle can be removed, the buckle is divided into a detachable buckle and a non-removable buckle.

According to whether the cross-section of the buckle changes, the buckle is divided into a straight arm buckle and tapered buckle.

The tapered buckle starts to become thinner from the root of the buckle. Compared with the straight arm buckle, the tapered buckle can effectively reduce the stress of the buckle and allow larger buckles when the thickness of the root of the buckle is the same. The amount of deformation. Another application of the tapered buckle is that when the strength of the straight arm buckle is insufficient, a tapered buckle can be formed by adding material at its root to increase the strength.

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Precautions for buckle design

Before designing the buckle, you need to understand the following important factors: >The mechanical properties of the plastic materials used. >Number of times to be assembled and disassembled. >The stress and strain that the buckle can withstand during the assembly process. >The mechanical pressure acting on the buckle after assembly.

2.1 Dimensions of the buckle

The size of the buckle needs to ensure that the buckle has sufficient strength and elasticity so that the buckle will not break and fail during assembly or disassembly. Therefore, a reasonable buckle size design is very important.


Buckle thickness t = 0.5~0.6T buckle root fillet Rmin = 0.5t buckle height H = 5~10t buckle assembly lead angle α= 25°~ 35° buckle disassembly angle β: β≈ 35° is used for detachable assembly that does not require external force; β≈45° is used for detachable assembly that requires small external force; β≈80°~90° is used for non-detachable assembly that requires large external force; buckle The top thickness Y≤t

The thickness and height of the buckle are the main factors that determine the strength and elasticity of the buckle. If the thickness of the buckle is too thin, the strength is weak, and the buckle cannot withstand greater assembly force; if the thickness of the buckle is too thick, the buckle has no elasticity, and it will break due to insufficient offset during the assembly process. At the same time, the buckle corresponds to The plastic wall is prone to shrinkage defects.

Different plastic materials have different buckle sizes because of their different elastic modulus and other parameters. The required buckle size can be calculated through related formulas. Of course, the best way is to verify whether the size design of the buckle meets the force requirements through finite element analysis.

2.1 Increase the fillet at the root of the buckle to avoid stress concentration

The most common failure mode of the buckle is due to the sharp connection between the root of the buckle and the wall of the part, which leads to stress concentration at the root of the buckle, so that it breaks during assembly or disassembly. Therefore, sharp corners should be avoided at the root of the buckle, and at least half the thickness of the buckle should be rounded.


2.3 The buckles are evenly distributed

If the two parts are fitted by snaps, the snaps need to be evenly arranged around the parts to evenly bear the load. If the part is prone to deformation, consider placing the buckle close to the part where the part is prone to deformation, such as the corner of the part.

The spacing of the buckles needs to be reasonably considered based on the actual situation of the buckles.

2.4 Use the positioning column to assist the assembly of parts and ensure the dimensional accuracy of the assembly

 If the Plastic seat belt buckle is completely fitted by buckles, due to the low dimensional accuracy of the buckles, the accuracy requirements of the assembly between the parts are guaranteed. This is the disadvantage of the buckle assembly. At this time, by adding positioning pillars and positioning holes to ensure the assembly size between the parts and improve the assembly accuracy.

There are two other benefits of using positioning posts and positioning holes. First, in the assembly process of two parts, the positioning column and the positioning hole of the appropriate height contact between the assembly features of the buckle (that is, the height of the positioning column on the plastic part is higher than the height of the buckle), which can be The assembly process of the parts provides guidance and improves assembly efficiency. At this time, the role of the positioning column plays a guiding role; second, the use of the positioning column can effectively avoid the snapping damage caused by the rough assembly action.


2.5 The buckle design avoids increasing the complexity of the mold

Unreasonable buckle design can easily increase the complexity of the injection mold, and the parts require a lateral core-pulling mechanism, which increases the cost of the mold. Proper buckle design optimization can simplify the mold structure. Opening a hole at the root of the buckle can avoid reverse buckling, and the injection mold does not require a lateral core-pulling mechanism, which simplifies the mold structure.


2.6 The buckle design needs to consider the convenience of mold modification

The buckle design generally requires multiple design modifications (including modifying the length, thickness, offset, etc. of the buckle) to meet the assembly requirements of the parts. Therefore, the design size of the buckle can be slightly smaller instead of one-time Make the size of the buckle sufficient to provide convenience for subsequent mold modification.