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The holes, slots and pits of plastic parts are formed by the cores on the mold. Core is the raised part of the mold, the size of the core affects the life of the mold and the quality of the parts. In the parts injection process, too high and too long of the core with a high plastic melt impact force, it is easy to cause the position of the core movement, resulting in large error in the size of the hole groove, etc.; or in the long-term impact, the core is prone to fracture and reduce the service life. Therefore, the plastic parts of the hole, groove and pit and other related size design must ensure the appropriate size of the core, so as to ensure the life of the mold and improve the quality of the parts.
The common plastic parts on the hole can be roughly divided into three kinds of holes, such as non-pass hole, through hole and step hole. When the diameter of the hole is less than 5mm, the depth of the hole should not exceed 2 times the diameter of the hole; When the diameter of the hole is greater than 5mm, the depth of the hole does not exceed 3 times the diameter of the hole.
Through-hole is easier to manufacture than non-through-hole, because the core can be distributed on both sides of the male and female mold, the depth of the through-hole can be increased appropriately. When the diameter of the through hole is less than 5mm, the depth of the hole should not exceed 4 times the diameter of the hole; when the diameter of the through hole is greater than 5mm, the depth of the hole should not exceed 6 times the diameter of the hole.
The recommended depth values for non-through and through holes are shown in Figure 1.
If the hole is too deep, it can be formed by the method of stepped hole, as shown in Figure 2.
The thickness of the bottom of the non-through hole should be at least 0.2 times greater than the diameter of the non-through hole, as shown in Figure 3-a. If the bottom is too thin, the strength of the non-through hole is low and the back side is prone to appearance defects. If the bottom is too thin, you can consider using the method shown in Figure 3-b to enhance the strength of the hole.
Between the hole and the hole, the distance between the hole and the edge of the part is at least greater than the hole diameter or the wall thickness of the part 1.5 times, that is, S ≥ 1.5t (or 1.5D), take the maximum value of both, as shown in Figure 4.
The strength of the part is further weakened by the removal of material from the holes, which reduces the strength of the part, as well as the tendency for fusion marks to form around the holes (especially if there are many holes) (see Figure 5). Plastic parts are often weakened by too many holes, so try to avoid placing too many holes in the part under load.
To increase the strength of the hole and prevent deformation of the hole, a flange can be added around the hole (see Fig. 6), and a similar design can be used for long holes or slots that require additional strength.
In order to simplify the mold structure and reduce the cost of the mold, the part design needs to avoid side holes perpendicular to the direction of demolding. The design of the hole should make the mold structure as simple as possible. The side holes that are perpendicular to the part's ejection direction require the use of a lateral core pulling mechanism in the mold, which will increase the complexity of the mold and cause the cost of the mold to rise. In the premise of ensuring the function of the part, the design can be optimized to reduce and avoid the use of lateral core-pulling mechanism. As shown in Figure 7 of the plastic parts, the lower side of the hole needs lateral core-extraction mechanism, the mold structure is complex, and the upper side of the hole due to design optimization can be directly off the mold, do not need lateral core-extraction mechanism, mold structure is simple.
A long hole is a long and narrow hole. The direction of the long hole should be the same with the plastic melt flow direction, avoid perpendicular to the flow direction, so as not to hinder the flow of plastic melt. The design of the long hole is shown in Figure 8.
Due to the need for heat dissipation, it is often necessary to design air holes in products. In general, wind holes for the round hole when the mold core is cylindrical, easy to process, mold cost is low.
Too many air holes will cause the strength of the part to reduce, can be increased by increasing the strengthening ribs or flange, etc. to increase the strength of the part at the air hole.
