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In today's increasingly competitive market, the appearance of the product has become one of the important factors to attract consumers to buy the product. Due to the plastic material itself and the characteristics of the injection process, plastic parts are easy to produce shrinkage, bubbles, fusion marks, trapped air, jet flow and other appearance defects, which seriously affect the appearance of the quality of the parts. The main reasons for the appearance quality problems of plastic parts are the problems of part design, mold design and incorrect injection process parameters during the injection of parts. For mechanical engineers, the first need to start from the design of the part to solve the appearance of the quality of the part, especially when the part is the product appearance of the parts should be more so. The appearance requirements of the internal parts of the product can be relaxed.
Note: To attribute the selection and layout of gates, mold ventilation and mold ejector structure in mold design to part design is not to say that mechanical engineers need to do the mold design themselves, but because mechanical engineers must understand the impact of mold structure on part appearance quality (or part strength) and check whether the part mold structure has a negative effect on the product appearance (or part strength), and if so require improvements to the mold design, because mold engineers are often unaware of the part appearance (or part strength) requirements. For example, they may design and lay out the gates incorrectly so that fusion marks appear on important cosmetic surfaces of the product, at which point the mechanical engineer needs to point out the error in the mold design and ask it to be corrected.
The selection of plastic material plays an important role in the appearance of the product, and different plastic materials have different appearance quality performance. For example, compared to non-glass fiber-reinforced materials, glass fiber-reinforced materials generally have a lower quality appearance after injection molding, and are prone to warpage.
Part surface shrinkage is one of the most likely to occur in the appearance of plastic parts defects. Shrinkage occurs in the outer surface of the part corresponding to the larger wall thickness of the part, such as strengthening ribs, pillars and wall connections corresponding to the outer surface of the part, as shown in Figure 1.
In the case allowed, the U-shaped groove, the design of the part surface break difference and the surface nibble can be used to cover the surface shrinkage of plastic parts, as shown in Figure 2.
Pillar wall thickness or reinforcement rib wall thickness at the local removal of material, can significantly reduce the possibility of shrinkage in the appearance of parts, as shown in Figure 3. Of course, the "crater" design will reduce the strength of the pillar or reinforcement rib to some extent.
The farther the part is from the gate, the more likely to produce surface shrinkage. For the important surface of the part or the area with high requirement of surface shrinkage, the location of the gate can be reasonably designed so that it is close to the area to reduce the possibility of surface shrinkage of the part. At the same time, the location of the gate should make the plastic melt flow from the wall thickness to the wall thinness, as shown in Figure 4. If the plastic melt flows from the thin wall to the thick wall, the thin wall will be cooled and solidified first, and the thick wall will be easily shrunken on the surface and bubbles will be generated inside.
Part deformation not only causes poor dimensional accuracy of the parts, easy to produce assembly problems and affect the realization of the function of the parts, but also affect the appearance of the parts. Part deformation occurs for many reasons, including four main aspects: parts in the plastic melt flow direction and cross-sectional direction of different shrinkage ratio, parts uneven cooling, parts wall thickness is not uniform, parts geometric characteristics of asymmetry.
Parts in the plastic melt flow direction and cross-sectional direction of the different shrinkage ratio caused by the deformation of the parts, as shown in Figure 5. Non-glass fiber reinforced materials in the plastic melt flow direction shrinkage rate than the cross-sectional direction is large, resulting in plastic in the plastic melt flow direction shrinkage is large, in the cross-sectional shrinkage is small, the part deformation; and glass fiber reinforced materials are just the opposite, in the plastic melt flow direction shrinkage rate than the cross-sectional direction is small, resulting in plastic in the plastic melt flow direction shrinkage is small, in the cross-sectional shrinkage is large, the part deformation. Parts deformation.
Uneven cooling of the part in the direction of wall thickness can cause part deformation. Uneven cooling on the one hand may be due to uneven design of the injection mold runner, on the other hand, the part itself may be the outer heat dissipation area than the inner heat dissipation area, the outer heat dissipation is slower, slower cooling, while the inner heat dissipation is faster, faster cooling (see Figure 6), the direction of the part deformation is always toward the hotter part surface.
Plastic parts shrinkage rate increases with the increase in the wall thickness of the parts, uneven wall thickness caused by the difference in shrinkage is one of the main reasons for the deformation of thermoplastic parts. Specifically, the plastic part profile wall thickness changes usually cause differences in cooling rate and crystallizing differences, resulting in part shrinkage differences and part deformation, as shown in Figure 7.
Asymmetric geometry of the part can lead to uneven cooling and shrinkage differences, resulting in part deformation, as shown in Figure 8.
The previous description of the causes and ways of part deformation, but this is not the focus, the focus of product design is to predict the deformation trend of the part and optimize the part design to reduce or even avoid the occurrence of part deformation, as shown in Figure 9, Figure 10.
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When two appearance plastic parts fit together, because of the existence of parts manufacturing errors and assembly errors, the gap between the two parts and broken difference (refers to the surface of one part is higher than the surface of the other part) will always exist, which will affect the appearance of the product, as shown in Figure 11-a.
The gaps between the two plastic parts can be covered by the design of the aesthetic grooves, thus improving the appearance of the product. There are two common designs of artwork grooves, as shown in Figure 11-b and Figure 11-c. The size of the gutter depends on the size of the product, such as the size of the computer host panel gutter is 0.5mm x 0.5mm. In the two gutter designs, the second method is better than the first. The first design of the artwork groove because of the existence of the gap makes it possible for consumers to see the internal parts of the product, while there is no role of dust control.
When the relationship between two exterior plastic parts is front-to-back or top-to-bottom, such as the top and bottom covers of cell phones, the gap becomes a factor affecting the appearance of the parts. If the back or below the parts higher than the front or above the parts, the product will become very ugly, so in this case the beauty of the ditch design should make the back or below the parts lower than the front or above the parts.
Fusion marks are also common defects on the surface appearance of plastic parts and need to be avoided. Specific methods are as follows.
(1) plastic parts surface nibble can partially cover the fusion marks, but can not completely cover the fusion marks.
(2) Spray paint can cover the fusion marks.
(3) Reasonably set the location and number of gates to avoid melt marks on the important appearance surface of the parts.
(4) Make sure the mold ventilation is smooth.
The intersection of male and female mold, the intersection of core and core, the intersection of core and male and female mold, etc. are easy to have broken differences or burrs. Therefore, mechanical engineers should carefully check the location of the parting surface in the mold structure to avoid the appearance of broken differences or burrs on the important appearance surface of the parts, which will affect the appearance quality of the parts.
In addition, should avoid placing the ejector structure at the important appearance of the part, which will also produce burrs. For transparent plastic parts should pay special attention.
