Suggestions on the application design of plastic nuts

In many plastic parts, in order to install, connect, etc., it is often necessary to place some metal parts or other material parts in plastic parts, these parts are collectively referred to as the insert in plastic products, of which copper nuts are typical inserts. The figure shows the typical shape of some inserts.

(1) The insert is made of metal, which can increase the strength and stiffness of the plastic part as a whole or a certain part, such as the metal bone rod insert used in the handle and box climbing, as shown in the figure.

a. Can improve the strength of the connection, the strength of the plastic is very low, in order to connect the two plastic parts together, such as screw connection, its strength will be affected, and in the plastic parts in advance placed with Threaded Inserts, can make the connection strength greatly improved, and durable.

b. Plastic is a good insulator, and some metal sheets, wires, plates and other inserts are placed in plastic parts, making them useful in electronics, electrical appliances, and power components.

c. The hardness of plastic is low, the ability to resist external forces is poor, in the need to improve the hardness and wear resistance of the appropriate Metal Insert, will effectively improve the hardness and wear resistance of the insert

(2) Insert material

There are many materials for making inserts, both metal and non-metal materials can be made into inserts, but most of them are based on metal materials. Our company's plastic inserts mainly use C3604 environmentally friendly brass, stainless steel, aluminum alloy as common alternative materials. Copper alloy has high mechanical strength, no rust, easy processing, is a common material for making inserts; However, the thermal expansion coefficient of copper and plastic is relatively large, while the thermal expansion coefficient of aluminum is the largest, the combination with plastic is the most firm, and it is also a commonly used material, but the strength is low.

(3) Design points of inserts

a. Reduce the impact of stress concentration: the plastic part of the metal insert can not be pointed, and should be properly rounded to reduce the impact of the stress concentration generated by the plastic cooling around the insert on the plastic part and improve the strength of the plastic part

b. Ensure the mechanical strength of the plastic insert: if the insert is set on the convex part of the 泎 plastic part, the embedding depth should be greater than the height of the convex part to ensure the mechanical strength of the embedded plastic part.

c. Wall thickness problem: The spacing between the insert and the side wall of the plastic part is generally not less than 0.6mm. If there are inserts on the opposite side of the plastic part, the thickness of the plastic layer separated between the two inserts shall not be less than 3.5mm.

d. Insert and cavity height problem: for internal and external threaded inserts, the height should be slightly lower than the molding height of the cavity about 0.05mm, so as not to crush the insert and cavity, as shown in the figure.

The plastic layer between the bottom surface of the insert and the bottom surface of the plastic part should have a certain thickness, generally not less than 1/6 of the outer diameter of the insert, if the thickness is too small, the plastic part is easy to produce defects such as dents and cracks in this position.

For external threaded inserts, a non-threaded area should be provided to prevent the melt from penetrating into the mold, as shown in the figure.

(4) Fixing of inserts

a. In order to make the insert tightly combined with the plastic part, the surface of the insert should be knurled or processed into a groove to increase the friction. Make the insert not easy to be pulled out in use, do not rotate, etc.

b. In order to facilitate the placement and positioning of the insert in the mold, the part of the insert placed in the mold should be designed to be cylindrical, so that it is easy to place the insert in the relevant positioning round hole on the mold.

c. In order to prevent the metal insert from being pulled out, a ring groove can be turned in the middle part of the insert, as shown in the figure.

d. The height of the insert should be less than 2 times its diameter, and the fit of the fixed part in the mold should be a tight gap fit.

e. If the plate or sheet insert, it can be fixed by the window fixing method, and it should be fixed by the incision or bending method, as shown in the figure.

f. When the insert is set in the convex, in order to ensure the stability of the insert combination and the strength of the plastic matrix, the insert should be extended into the bottom of the convex, but the minimum bottom thickness should be ensured, and the head of the insert should be made of rounded corners, as shown in the figure.

g. For the rod insert, the head needs to be deformed, such as the head flattening, punching, bending, splitting and other forms, this shaped insert and plastic parts can be well combined and fixed, as shown in the figure.

h. The rod insert embedded in the handle, the embedded part should be processed into a square, so as to prevent rotation, as shown in the figure.

i. The shaft inserts are generally inserted into the corresponding holes of the mold and fixed. In order to prevent the molten material from squeezing into the threaded groove, several methods as shown in the figure can be used.

Smooth rod fit, as shown in the figure. It uses the smooth rod part of the insert to fit with the mold.

Shoulder fit, as shown in the picture. The use of a shoulder not only increases the stability of the insert after insertion, but also prevents the flow of molten material into the thread.

Ring fit, as shown in the picture. It has a convex ring, which is pressed on the mold to form a sealing ring during molding to prevent the flow of molten material into the thread.

Shoulder fit, as shown in the picture. This is another form of shoulder fit, but the outer diameter of the knurled part is much different from the outer diameter of the threaded part.

⑸ If there is a threaded insert without a hole in the hole insert, it can be placed as shown in the figure.

a. Blind hole positioning, as shown in the figure. An optical shaft inserted into the mold can be inserted into the blind hole of the insert for positioning.

b. Boss positioning is shown in the figure. A boss is provided on the insert, corresponding to the hole on the die.

c. Concave positioning, as shown in the figure. The insert is concave, and the concave is positioned with the corresponding shaft on the mold.

⑹For the slender insert, if its axis is perpendicular to the direction of the material flow, it is easy to produce bending deformation, as shown in the figure. To this end, some supports can be installed at the location of the elongated insert deformation to increase its rigidity, as shown in the figure, but it should be noted that the additional support holes should not affect the use of plastic parts.