Do you know how to design a plastic part embedded in a nut
- Nut embedding method
Hot melt nut
Hot melt embedding is the most common embedding method, generally using hot melt machine and manual electric soldering iron to embed nails.
Injection molding nut
Injection molding embedding generally has strict requirements on the hole diameter of the nut, and the hole diameter is controlled within 0.05mm. Because the product is fixed with Molding Pin and placed in the injection molding mold, the nut hole diameter should be controlled by the size of the PIN needle of the injection molding machine.
Ultrasonic nut
Ultrasonic embedding is a method of increasing the temperature of the interface through ultrasonic vibration, which causes friction between the nut and the surface and internal molecules of the workpiece. When the temperature reaches the softening temperature of the workpiece itself, the nut is embedded in the rubber part. When the vibration stops, the workpiece is cooled and shaped under a certain pressure. (We recommend that you follow the “Mechanical Engineer” official account to get the dry knowledge and industry information at the first time)
- Plastic hole design and nut selection
Figure 1 Basic nut dimensions
Figure 2 Basic plastic dimensions
Note: D: nut outer diameter L: nut length d: nut base C: plastic hole diameter W: plastic hole wall thickness Y: Plastic hole depth - Selection parameters of plastic holes and nuts
- The d size is the nut base, also known as the guide positioning part, which matches the plastic C part before embedding. Therefore, the d part at the bottom of the nut should be smaller than the C size of the inner hole diameter of the plastic BOSS to facilitate positioning; 2. The D size is the outer diameter of the nut, which matches the C size of the inner hole diameter of the plastic BOSS. Generally, the inner hole diameter of the plastic of the mobile phone nut is about 0.25-0.3mm smaller than the outer diameter of the nut; (We recommend you to follow the “Mechanical Engineer” public account to master the dry goods knowledge and industry information at the first time) 3. The L size is the nut length, which matches the Y size of the plastic BOSS hole depth. Generally, the depth of the plastic hole is 0.5-1.0mm greater than the height (length) of the nut for glue storage; 4. W is the wall thickness of the plastic hole. Generally, the meat thickness of the plastic BOSS hole is 0.8-1.0mm or more. The larger the nut size specification, the greater the meat thickness. (See the parameters in the table below)
- The influence of the size of the plastic BOSS hole
If: there is no problem in the selection of plastic and nut, the standard state of the nut after being embedded in the plastic is shown in Figure (I)
If: the nut size is too small, the plastic BOSS hole is too large, and the nut cannot eat the glue after being embedded in the plastic, which will cause insufficient torsional force as shown in Figure (II)
If: the nut size is too large, the BOSS hole is too small, and there will be overflow or bursting as shown in Figure (III)
If there is no problem with the nut size and plastic size, and there are still abnormal phenomena, then we usually consider improving it through some optimization designs;
Example 1: The plastic BOSS hole depth is shallow, which will cause insufficient torsional force
Since the BOSS hole depth is shallow, if a double-twill nut is selected, under the proportion of the nut size, especially the embossing, the embossing step difference is short, such a nut will be embedded in the plastic, and the plastic eating surface of the embossing is too little, which will cause insufficient torsional force. Therefore, it is generally recommended to set the BOSS hole depth to more than 2.5mm, and the nut size length is generally recommended to be more than 2.0mm. (We recommend you to follow the “Mechanical Engineer” WeChat account to get the latest information and industry information)
Improvement plan: In similar cases, if it is inconvenient for customers to change the mold, it is recommended to change the nut flower shape to single-bevel BS1, as shown in the right figure. This will increase the embossing step difference and increase the plastic on the embossed surface, thereby increasing the torsional force.
Example 2: Plastic BOSS hole overflows and bursts
Due to the selection of the nut style shown in the left figure, after the plastic is embedded, A1 and A2 expand, while B shrinks sharply, making it difficult to expel the plastic, and the upper and lower ends of the nut are prone to overflowing, which seriously affects the appearance of the product;
Improvement plan: Use the “C” part as a guide to position the nut so that it can be safely placed in the plastic hole, improving efficiency and yield rate. Due to the addition of the C-end guide part, after A2 expel a certain amount of glue, it also reserves enough glue for the A1 part. At the same time, because the A1 and A2 parts are 90° with an angle of 45°, and can form a high-strength node at an angle, the torsional force on the circumference is resisted, which greatly increases the torsional force.