1. Mold opening direction and parting line
At the beginning of the design of each injection molding product, its mold opening direction and parting line must be determined first to ensure that the core-pulling slider mechanism is minimized and the influence of the parting line on the appearance is eliminated.
1. After the mold opening direction is determined, the product's ribs, buckles, protrusions and other structures are designed to be consistent with the mold opening direction as much as possible, so as to avoid core pulling and reduce stitching lines, and prolong the life of the mold.
2. After the mold opening direction is determined, an appropriate parting line can be selected to avoid undercuts in the mold opening direction to improve appearance and performance.
Second, the draft slope
1. Appropriate demoulding slope can avoid product pulling (garbage). The demoulding slope of the smooth surface should be ≥ 0.5 degrees, the surface of the fine grain (sand surface) should be greater than 1 degree, and the surface of the rough grain should be greater than 1.5 degrees.
2. Proper demoulding slope can avoid product top damage, such as top whitening, top deformation, and top breaking.
3. When designing products with deep cavity structure, the slope of the outer surface should be greater than the slope of the inner surface as much as possible, so as to ensure that the mold core does not deviate during injection molding, obtain a uniform product wall thickness, and ensure the material strength of the product opening.
3. Product wall thickness
1. All kinds of plastics have a certain range of wall thickness, generally 0.5 ~ 4mm, when the wall thickness exceeds 4mm, it will cause too long cooling time, shrinkage printing and other problems, you should consider changing the product structure.
2. Uneven wall thickness will cause surface shrinkage.
3. Uneven wall thickness will cause pores and weld marks.
4. Ribs
1. Reasonable application of ribs can increase product rigidity and reduce deformation.
2. The thickness of the reinforcing rib must be ≤ (0.5-0.7) T product wall thickness, otherwise the surface will shrink.
3. The single-side slope of the rib should be greater than 1.5° to avoid top damage.
5. Fillet
1. If the fillet is too small, it may cause stress concentration of the product, resulting in cracking of the product.
2. If the fillet is too small, it may cause stress concentration in the mold cavity, resulting in cracking of the cavity.
3. Setting reasonable rounded corners can also improve the processing technology of the mold. For example, the cavity can be directly milled with R cutter to avoid low-efficiency electric machining.
4. Different rounded corners may cause the movement of the parting line, and different rounded corners or clear corners should be selected according to the actual situation.
Six, hole
1. The shape of the hole should be as simple as possible, generally round.
2. The axial direction of the hole is consistent with the mold opening direction, which can avoid core pulling.
3. When the length-to-diameter ratio of the hole is greater than 2, the demoulding slope should be set. At this time, the diameter of the hole should be calculated according to the minor diameter size (maximum physical size).
4. The length-to-diameter ratio of blind holes generally does not exceed 4. Anti-hole needle punching
5. The distance between the hole and the edge of the product is generally larger than the hole size.
7. Core pulling, slider mechanism and avoidance of injection mold
1. When the plastic part cannot be demoulded smoothly according to the mold opening direction, a core-pulling slider mechanism should be designed. The slider of the core-pulling mechanism can form a complex product structure, but it is easy to cause defects such as stitching lines and shrinkage of the product, and increase the cost of the mold and shorten the life of the mold.
2. When designing injection molding products, if there is no special requirement, try to avoid the core-pulling structure. For example, the direction of the axial direction of the hole and the direction of the ribs is changed to the direction of the mold opening, and methods such as punching through the cavity core are used.
8. Integrated hinge
1. Using the toughness of PP material, the hinge can be designed to be integrated with the product.
2. The size of the film used as a hinge should be less than 0.5mm and kept uniform,
3. When injecting an integrated hinge, the gate can only be designed on one side of the hinge.
9. Insert
1. Inserting inserts in injection molding products can increase local strength, hardness, dimensional accuracy and set small threaded holes (shafts) to meet various special needs. At the same time, it will increase the product cost.
2. The insert is generally copper, but it can also be other metal or plastic parts.
3. The part of the insert embedded in the plastic should be designed with anti-rotation and pull-out prevention structures. Such as: knurling, hole, bending, flattening, shoulder, etc.
4. The plastic around the insert should be properly thickened to prevent stress cracking of the plastic part.
5. When designing an insert, its positioning method in the mold (hole, pin, magnetism) should be fully considered
10. Identification
The product logo is generally set on the relatively flat part of the inner surface of the product, and adopts a raised form, and the logo is set on the surface where the normal direction is likely to be consistent with the mold opening direction ruler, which can avoid strain.
11. Precision of injection molded parts
Due to the unevenness and uncertainty of the shrinkage rate during injection molding, the precision of injection molded parts is obviously lower than that of metal parts, and the dimensional tolerances of mechanical parts cannot be simply applied. Appropriate tolerance requirements should be selected according to standards. my country also issued GB/ T14486-93 "Dimensional Tolerance of Plastic Parts Molded by Engineering Plastics", the designer can determine the dimensional tolerance of the part according to the requirements of the plastic raw materials used and the use requirements of the part according to the regulations in the standard. At the same time, it is necessary to determine the appropriate design tolerance accuracy based on the comprehensive strength of the factory and the design accuracy of peer products.
12. Deformation of injection molded parts
Improve the rigidity of the injection molded product structure and reduce deformation. Avoid the flat structure as much as possible, and set the flanging and concave-convex structure reasonably. Set reasonable stiffeners.
Thirteen, deduction
1. The buckle device is designed to be shared by multiple buckles at the same time, so that the overall device will not fail to operate due to damage to individual buckles, thereby increasing its service life, and adding rounded corners to increase strength.
2. The tolerance requirements for the relevant dimensions of the buckle are very strict. If there are too many undercut positions, it will easily cause damage to the buckle; on the contrary, if there are too few undercut positions, it will be difficult to control the assembly position or the combination part will be too loose. The solution is to reserve a way to modify the mold and add glue easily.
14. Welding (hot plate welding, ultrasonic welding, vibration welding)
1. Welding can improve the connection strength.
2. Using welding can simplify product design.
15. Reasonably consider the contradiction between process and product performance
1 When designing injection molding products, the contradiction between product appearance, performance and process must be considered comprehensively. Sometimes a good appearance or performance can be obtained by sacrificing part of the manufacturability.
2. When the structural design cannot avoid injection defects, try to make the defects occur in the hidden parts of the product.
Sixteen, the relationship between the hole diameter of the screw column and the diameter of the self-tapping screw
Self-tapping screw hole diameter
M2 1.7mm
M2.3 2.0mm
M2.6 2.2mm
M3 2.5mm
17. Design principles of BOSS:
1. The pillar should not be used alone as much as possible. It should be connected to the outer wall or used together with the reinforcing rib. The purpose is to strengthen the strength of the pillar and make the rubber flow more smoothly.
2. The height of the pillar is generally not more than two and a half times the diameter of the pillar. Excessively high pillars will cause air trapping during molding of plastic parts (too long will cause air holes, burnt, insufficient filling, etc.).
3. If the height of the pillar exceeds two and a half times the diameter of the pillar, especially the pillar far away from the outer wall, the method to strengthen the strength of the pillar is to use reinforcing ribs
4. The shape of BOSS is mainly round, and other shapes are not easy to process
5. The position of the BOSS should not be too close to the corner or the outer wall, and a certain distance should be kept from the outer wall of the product
6) Part of the thickness of the flesh can be removed around the BOSS (that is, the crater) to prevent shrinkage and subsidence
7), BOSS's dial angle: usually 0.5° outside, 0.5° inside or 1
The purpose of mold parting is to make the product can be taken out of the mold smoothly, let's take a picture as an example to illustrate.
In the picture above, apart from separating the mold cavity from the A-A line, the product cannot be taken out smoothly at any part of the split mold, because there is an undercut buckle (the shaded part in the picture).
In the picture above, if the mold is parted from the line B-B, we can see that the shaded part in the picture is the blocked part of the product, that is, the glue plane between A and B is an undercut.
Therefore, we can draw a conclusion that in order to ensure that the product can be demolded smoothly, the parting line of the product should be selected at the maximum projection side of the mold opening direction.
Projection, the mathematics teacher in junior high school talked about it, and some students said, I forgot it a long time ago, okay, let’s make it up a little bit.
picture
As shown in the figure, parallel light shines from the direction of product opening, forming a shadow (yellow line) on the vertical plane, and the largest side of the shadow is the largest projection side of the product.
There are many principles for parting line selection, and this is the most basic one. With this one, the simple flat mold, parting mold is completely enough
There is also an important knowledge point here, that is, the mold opening direction of the product. Everything is based on this expansion.
The mold opening direction of most products is relatively easy to choose, that is, the vertical direction of the largest projection surface of the product is the mold opening direction of the product.
According to the principle of three views, we can find out the three projection surfaces of any product, and the sizes of the projection areas are different, just find the largest projection surface.
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In view of the fact that in the following parting courses, there will be many products with undercuts, here, we will explain all the principles of parting surface selection:
1: To ensure that the product can be taken out of the mold, the position of the parting surface should be the position of the largest projection side shadow in the direction of product release.
2: For appearance parts, the mold parting cannot affect the appearance, and the appearance quality and precision must be guaranteed.
3: Ensure that the product can stay in the rear mold after the mold is opened, which is conducive to demoulding and lateral core pulling.
4: Guarantee the quality of the product and the strength and rigidity of the parts during molding.
5: Consider the clamping force of the mold, and place the direction with the largest projected area of the product on the clamping direction of the front and rear molds.
The theoretical knowledge ends here, next, explain a few mold words that appear in this lesson
1: Undercut refers to the part of the mold that blocks the release of the product, so that the product cannot be taken out smoothly.
2: The parting line is the line formed on the product at the joint of several parts that make up the cavity.
3: The parting surface constitutes the mutual contact surface of several parts of the mold cavity.
4: The glue surface is on the mold, and the contact surface between the product and the mold is collectively called the glue surface
5: The action that the demoulding product is smoothly taken out of the mold
6: The demoulding direction is the direction in which the product can be taken out of the mold smoothly.
7: Appearance surface The outer surface of the product, the surface that can be seen directly after assembly.
8: Clamping force The force exerted by the injection molding machine on the mold against the injection pressure
