1. Plastic part thickness and structural design
The plastic part not only has structural and appearance requirements, but also needs to install some parts inside, which requires certain strength and rigidity. At the same time, the requirements of injection molding process must be considered. Therefore, the thickness design of the plastic part is very important.
1.1 Plastic part thickness design
1.1.1 Wall thickness
If the wall thickness of the plastic part is too thin, the flow resistance during molding will be too large, and large plastic parts will be difficult to fill; if the wall thickness of the plastic part is too thick, it is easy to produce defects such as shrinkage pits and bubbles. On the basis of ensuring rigidity and strength, the recommended plastic part wall thickness range is 0.45~6.5mm, and the commonly used range is 1.5~3.0mm. The wall thickness is required to be uniform.
According to the use requirements, when designing the overall wall thickness of the plastic part, the influence of structures such as reinforcing ribs and screw columns on rigidity, strength and appearance must be considered.
1.1.2 Reinforcing ribs
The establishment of reinforcing ribs can improve the strength and rigidity of plastic parts, prevent plastic part deformation and facilitate the flow of plastic melt. The structure and size of the general reinforcing ribs are shown in Figure 2. As can be seen from Figure 2, b = (0.40 ~ 0.75) t, L = (2.5 ~ 5.0) t, α = 0.5° ~ 1.5° . Among them, b is the thickness of the rib, t is the wall thickness, L is the height of the rib, and α is the demoulding angle.
1.1.3 Screw column
Self-tapping screws are usually required to install other components inside the plastic part, so a screw column as shown in Figure 3 can be set up. Screw columns are divided into two types: with ribs and without ribs, where the length of the bottom of the rib c = (0.2 ~ 0.5) × screw column height.
In addition, wall thickness also involves the design of structures such as bosses, corners, through holes and blind holes. However, if a thin-walled structure is used, the structure and size of the above-mentioned ribs and screw columns must be changed.
1.2 Structural design of thin-walled plastic parts
The disposable lunch box of thin-walled plastic parts is shown in Figure 4. Its wall thickness is usually less than 1.2mm. However, the definition of thin-walled plastic parts is not just about the thickness size. The ratio of the melt flow to the wall thickness of the plastic part l/t must also be calculated. When l/t＞150, it is called thin-walled. Since the plastic melt first passes through the main channel, the branch channel, and the gate during the injection molding process, and then is injected into the mold cavity, the actual flow and wall thickness are different at different places (Figure 5). The ratio of the total flow to the wall thickness is equal to the sum of the ratios of each section of the flow to the wall thickness.
The disposable lunch box is relatively soft and the outer surface requirements are not high, but in order to improve the rigidity, most thin-walled plastic parts adopt curved surfaces, ribs, etc. The thin-walled ribs are shown in Figure 6, where the rib thickness b is equal to the wall thickness t, or even less than the wall thickness. The screw column of the thin-walled plastic part for fixing the self-tapping screws is shown in Figure 7.
Due to the thin wall thickness, while the thickness of the reinforcing ribs and bosses (such as screw columns) remains unchanged, according to conventional production methods, plastic parts are prone to defects such as dents, under-injection, and distortion. Therefore, it is also necessary to study the plastic part material, mold structure, and injection molding process.
2. Structural design of thin-walled plastic mold
2.1 Overall mold structure
The plastic part shown in Figure 8 is an item box, made of PP, with a wall thickness of 1.0 mm, which is a typical thin-walled plastic part. Due to the use of thin-wall technology, the material has poor fluidity in the mold, requiring a higher injection pressure, and the rigidity and strength of the mold must be increased accordingly. Therefore, when designing the movable mold plate 12, fixed mold plate 14 and its support plate 8 of the item box mold (Figure 9), its thickness is usually 30% to 50% thicker than that of the traditional mold, and the support column 5 must be added. The mold joint surface fixed mold plate 14 and the movable mold plate 12 should be set with a conical surface positioning (integral conical surface positioning or conical surface positioning block) to ensure precise positioning and good side support to prevent bending and offset. In addition, thin-walled plastic parts require high-speed injection by injection molding machines, which increases the wear of the mold. Therefore, the mold cavity, core, gate and other materials are required to have high hardness, strength, rigidity and wear resistance. Usually, mold steels such as S136, 2344, SKD61, and PMS are used, and pre-hardened or heat-treated to make their surface hardness reach 48-52HRC.
2.2 Casting system
For plastics with excellent fluidity such as PP, point gates can be used; for plastics with medium fluidity (such as ABS, polyoxymethylene, etc.), the gates should be designed as thicker parts of the plastic parts as much as possible, and the injection molding process should transition from thicker to thinner to reduce sinking and warping. Multiple gates can be used (latent gates and thin-sheet gates as shown in Figure 10) to make it easy for the plastic melt to fill the cavity and reduce pressure drop. Hot runner technology can also be used to reduce the viscosity of the plastic melt and achieve the purpose of quickly injecting it into the mold cavity.
2.3 Demolding mechanism
Due to the thin wall thickness of the plastic part, it also has ribs, bosses, etc., which are very easy to be damaged when demolding. The shrinkage of the plastic part along the thickness direction is very small, and the high holding pressure makes it shrink even smaller. The ribs and other parts are easy to bond. In order to avoid penetration and sticking, more and larger ejector rods are required than conventional injection molding. The object box mold shown in Figure 9 adopts a structure in which the center push rod and the push plate are combined.
For some plastic parts with hooks, claws, and screws that need to be installed inside the plastic part, the demolding mechanism is relatively complicated. The screw column adopts a simple push-tube push-out structure =). Because the wall thickness of the plastic part is relatively thin, the screw column in it needs to tighten the ST4.2 self-tapping screw, and its cylindrical thickness is much greater than the wall thickness of the plastic part, and pits are easy to appear on the surface. To avoid this defect, according to the position of the screw column, it is designed as a suspended structure, and the mold demolding is designed as a slanted top structure The screw column is pushed out of the mold, and the distance the push rod moves sideways is e. Although the demoulding of the product sometimes requires manual collection, which greatly increases the complexity, it solves the problem of surface pits on the plastic part.
If there are multiple screw columns, it is necessary to consider the removal of the plastic part after the inclined ejector is ejected, and the inclined ejector direction should be consistent.
