Causes and Countermeasures for Deformation of Injection Molded Parts - 5

Time:2024-03-06 19:02:29 / Popularity: / Source:

Serial No. 5 (Friends who are interested can follow Gud Mould and check out serials 1, 2, 3, and 4 in historical news.)

2. Product structure

As mentioned before, if product structure is rigid enough to resist internal stress caused by uneven shrinkage of plastic molecules, product may not deform. This is just one aspect. Product structure has a deeper and broader impact on product deformation. Generally speaking, there are several aspects.

1) Whether rigidity of product can overcome influence of internal stress

We can understand rigidity of product structure as stability of product structure and its resistance to external forces (as mentioned earlier, effect of internal stress on product is similar to external forces). Therefore, there is no doubt that a product with good rigidity and stable structure has a small chance of deformation. On the contrary, a product with good rigidity and stable structure has a high chance of deformation. On the other hand, products with good rigidity have higher requirements on product deformation, because once deformation occurs, it is difficult to overcome it. Forcing to rely on assembly to overcome deformation will produce great assembly stress; and if rigid product structure is unreasonable (such as thickness difference is too large), structural deformation will be difficult to overcome through technology. On the contrary, products with poor rigidity have low requirements for deformation due to their weak strength. Even if deformation occurs, it can be overcome by assembly without causing large assembly stress.
Therefore, products with poorer structural rigidity have a larger space for process adjustment; products that are more difficult to deform have a smaller space for process adjustment. Just like a person with a weak personality is easy to be influenced by others and change his mind, and it is also easy to change his mind; but a person with a stubborn personality is difficult to be influenced by others and change his mind.
Internal stress undoubtedly has a negative impact on product quality. So-called internal stress refers to stress that remains inside object after external load is removed. It is generated by uneven volume changes in macro or micro structure of material. This is a standard academic explanation. It's very abstract, so I might as well use an intuitive metaphor to explain it. It may be biased, but it may help better understanding. Function of internal stress is like two people of similar strength wrestling with each other, and they are in a stalemate stage. At this time, it seems that the two hands are in a static state (product does not deform or deforms not much, but internal force is very strong), but in fact mutual force between the two hands is very large. Assuming that both of them have exerted 10% of their force at this time, then this stalemate stage cannot be sustained (this is similar to when internal stress of product is very large, because product structure is very rigid and does not show great deformation, but it may crack when exposed to external force; or deformation may occur after being left for a long time); if two people each use only 10% of force to maintain this balance, they can maintain it for a long time (this is similar to product that can withstand a larger load when internal stress of product is small), there is no doubt about it. Therefore, regardless of whether product shows deformation, we must find ways to reduce internal stress of product as much as possible.
Although sometimes products have a very rigid structure, which can help resist large internal stresses without causing deformation, this does not mean that internal stresses have been overcome. Internal stresses still exist inside product. When ambient temperature rises during storage or use, plastic molecules move due to heat, and product may deform, releasing internal stress. This process can be regarded as process of internal stress release.
Product structure design considers stability or functional requirements of the overall product structure and designs a large number of ribs. Many designed ribs are connected throughout the entire product. Designers believe that this design is beneficial to stability and strength of entire product structure. However, this design causes the entire product to have a very high internal stress after injection molding. Reason is that due to limitation of appearance shrinkage marks, thickness of ribs is very different from main wall thickness of product (usually thickness of ribs is 0.4-0.7 of main wall thickness, depending on plastic raw material and distance from gate), which naturally produces uneven shrinkage. In addition, ribs are usually wrapped with mold steel from both sides, which makes heat transfer easy. However, due to heat insulation effect of ribs, heat conduction of main wall thickness is relatively difficult, which further aggravates uneven shrinkage. Huge uneven shrinkage causes product deformation. If product rigidity is very good, degree of residual stress inside product will be very high. Adopting reinforcement breaking method to deal with this situation will greatly improve deformation. However, impact on the overall strength of product needs to be tested and verified. Although reinforcement breaking method reduces the overall strength of product, because its internal residual stress is greatly reduced, ability to withstand load will be enhanced, which needs to be considered comprehensively. Rib breaking method is shown in Figure 4.6.
Mold design 
Mold design 
Figure 4.6 Ribs of designed product in picture above are through ribs. If product does not deform after injection molding, its internal stress will be very large.
Picture below shows improved product design. All through ribs are cut into U-shaped notches (broken rib method). In this way, optimized design will have very low internal stress in product. Additional effect is improved filling and improved cooling of rear mold. The only shortcoming is that strength of product may be reduced, but in the face of this problem, two aspects need to be considered: first, whether improved strength can meet product function; second, stress of the original design will reduce product's load capacity to a certain extent.

2) Whether thickness distribution of product meets uniform shrinkage of product

As mentioned before, fundamental cause of product deformation is effect of internal stress. Internal stress is generated from uneven shrinkage. Therefore, uniform shrinkage of product determines degree of deformation of product, uniformity of product's thickness determines uniform shrinkage of product to a large extent (note not all), so uniformity of product's thickness is very important to product quality.
It should be pointed out that uniform thickness is not goal, but a means to achieve uniform shrinkage. This relationship must be clarified, otherwise if uniform thickness is regarded as goal, big mistakes will be made in practice. Due to complexity of product structure and mold structure, as well as position of glue inlet point, heat conduction speed and pressure distribution of melted glue in mold cavity cannot be uniform. Therefore, in order to compensate for this unevenness, thickness of mold is intentionally uneven to achieve uniform shrinkage. That is, the areas where heat conduction is naturally slow are reduced accordingly to make product thinner to reduce accumulation of heat, thereby reducing shrinkage; the areas subject to higher pressure are appropriately thicker and extend cooling time to increase shrinkage. In this way, wall thickness of product is artificially made uneven to compensate for uneven shrinkage caused by cooling and gate position restrictions. It is very effective in practice and can greatly widen molding window.
Therefore, we must look at issue of product thickness uniformity from a dynamic perspective, rather than rigidly believing that uniform meat thickness is necessary to achieve uniform shrinkage.

3) Whether structure of product limits design of mold waterway, so product cannot achieve uniform cooling during molding process.

Complexity of product structure limits design of waterway. In order to achieve uniform cooling of product to the greatest extent, waterway design of mold must be carefully considered. Depth of product, narrow grooves and other places that cannot be reached by waterways greatly affect uniform shrinkage of product, leading to product deformation and seriously affecting production cycle.
Mold design needs to focus on cooling design in these areas: reducing heat by reducing meat thickness, and enhancing cooling to improve heat transfer efficiency.

4) Whether appearance requirements of product restrict design of gate, resulting in need for more stringent process requirements?

Shape, size and location of gate have a great impact on appearance of product. Generally speaking, small gates such as pinpoint gates have greater flow restrictions, initial flow rate is difficult to control, and it is not conducive to pressure maintaining, so requirements for molding process are relatively strict; from another perspective, larger gates such as side gates have relatively small flow restrictions and are conducive to pressure transmission, which is more beneficial to appearance of product. However, residual traces of gate have a fatal impact on appearance of product. These need to be considered comprehensively when designing mold.
Excessive internal stress is likely to remain in gate area due to relationship between filling and pressure holding. Excessive residual stress will affect strength and appearance of product, so try to stay away from these areas when defining gate location. Otherwise, more stringent process control will be required, and results will not be very good.

5) Product dimensional tolerance and geometric tolerance requirements

For product design engineers, defining overly strict and unrealistic tolerances is a form of self-protection and is also a sign of insufficient technology. Do this, because if there is a problem later, product design can completely shift responsibility: it is not that my design is not good, but that you did not meet my requirements. However, such unnecessary strict tolerances bring great difficulties to production and mold making, and increase many unnecessary costs. Ultimately it is business that suffers. Plastic materials have their own unique characteristics. For amorphous materials, they have never truly solidified; for semi-crystalline plastics, only their crystalline areas are truly solidified. From this perspective, although plastic materials have extremely strong plasticity, their stability is relatively poor because of this. Therefore, referring to industry plastic product tolerance definition standards, it is recommended that customers relax tolerances appropriately; or use appropriate design measures to make up for strict tolerances that are necessary (hard to achieve in practice) in some situations. This is very important for mold factories and injection molding factories.
Obtaining high-quality and low-cost injection molded products has never been just responsibility of mold factory and injection molding factory. It is a systematic project. It requires systematic collaboration from all aspects of materials, product design, mold design and injection molding factory to achieve this.
Strict dimensional tolerance and geometric tolerance requirements place higher requirements on product deformation. This type of mold design requires more careful consideration. Effect of deformation on size and shape of product goes without saying. Product design needs to consider actual conditions of molding process and define reasonable tolerances. Generally speaking, the larger size of product, the more difficult it is to control size; structural rigidity of product is poor and it is easier to deform; the greater shrinkage rate of material, the more difficult it is to control size. Therefore, it is very necessary for product designers to find a compromise between desired design and achievable design.
Very strict appearance (such as decorative requirements for high mirror surfaces, especially black mirror requirements), very complex structures (such as bone position) and very strict size requirements, it is best not to assign them to same product, this is what product design engineers need to consider of. Because of complexity of product structure:
Firstly, existence of uneven shrinkage may lead to high residual internal stress, and appearance defects such as stress marks will appear on the surface of high-gloss products;
Second, sometimes size of the entire product may be too large in order to overcome a slight shrinkage; in order to reduce internal stress of product and control holding pressure, size may be too small;

6) Whether assembly of product has ability to overcome deformation, and how much it can overcome.

Assembly of product can overcome deformation of product to a certain extent. However, as mentioned before, if deformation is too large or rigidity of product is too large, it cannot be overcome by assembly. Even if it can be overcome, great assembly stress will occur. If assembly stress is too high, it will have a negative impact on service life of product. Deformation is overcome by assembly, and assembly stress points of product will bear assembly stress. The greater deformation of product, the greater assembly stress.

7) Product functions and stress conditions

It is necessary to consider function of product and stress condition: long-term load or cyclic load. As mentioned before, effect of internal stress on product is similar to that of external load. Therefore, if residual internal stress of product is too large, ability to withstand external loads will be adversely affected.
Rigidity of product structure is beneficial to overcome product deformation, but on the other hand, if a product with good rigidity has a large potential deformation tendency, it will be difficult to overcome it once deformed.
Regardless of whether product is deformed or not, doing everything possible to reduce internal stress of product is very beneficial to product quality, especially ability to withstand use load.
For design of connected ribs, use of "broken rib method" is very beneficial to reduce tendency of product deformation, reduce degree of internal stress, improve filling, and shorten cycle.
Uniform wall thickness is not goal, uniform shrinkage of product is. Relationship between ends and means must be clarified. Don't use means for the sake of means.
Overly strict and unrealistic tolerance definitions significantly increase mold development and injection molding production costs. A deep understanding of characteristics of plastics and defining reasonable tolerances according to plastic product tolerance definition standards are very effective in reducing unnecessary costs.
Plastic product design engineers must make a compromise between their desired design and a productive design. Different products have different requirements. Decorative plastic parts that require extremely high appearance should not be given too many other functions.
(To be continued: Series 5. A paragraph will be published every day. If you are interested, you can follow Gud Mould.)

Go To Top