Plastic is everywhere in our lives. In our daily life, more than 60% of daily necessities are made of plastic products, and "plastic injection molding" is one of the most common production methods of plastic products. In injection molding process, five major elements that affect product quality are "product design", "plastic mold", "plastic material", "molding parameters" and "injection equipment". This article will discuss the first important element-"product design".

To create a successful plastic product, main key comes from good product design. However, in process of transforming "design concept" into mass production, it is necessary to have a basic concept of plastic production process. In this way, designer can skillfully simplify difficulty of mass production in the future during product development process, greatly increase yield rate during mass production. Therefore, before design and development of plastic products, five basic concepts need to be established, namely "plastic material and shrinkage", "thickness design", "surface roughness", "draft angle" and "tolerance setting".

There are many types of plastic materials, mainly divided into two categories: "thermoplastic" and "thermosetting plastic". "Thermoplastics" can be divided into "crystalline plastics" and "non-crystalline plastics". In addition, it can be further divided into "general purpose plastics, engineering plastics, and advanced engineering plastics" according to physical properties and usage requirements of raw materials.
Since there are about 10 to 20 types of common plastic materials, designers in the early stage of product design, after deciding on function and requirements of product, it is recommended to decide which plastic material to use before drawing (3D and 2D drawing). One of main reasons for deciding on plastic material first is that plastic itself has a "shrinkage phenomenon." Different plastic materials have different shrinkage rates. Plastic mold will be made according to shrinkage rate of material. If plastic material is to be replaced in the future, but difference in shrinkage rate is too large, plastic mold may have to be scrapped because it cannot be modified.
For example, there is a considerable difference in shrinkage between crystalline materials and non-crystalline materials. If mold is opened with shrinkage rate of ABS, it is almost impossible to change to PP material later.

When designing product's "thickness design", special attention should be paid to three key points of "avoid competing current effects", "short shots and vacuum bubbles" and "surface shrinkage marks".
• Avoid race flow effect: Main cause of race flow effect is uneven flesh thickness. Therefore, when designing products, it is necessary to achieve “uniform flesh thickness” as much as possible. Because flow resistance is large in thin area, flow resistance is lower in thick area. When thickness of finished product is uneven, resistance in cavity will also be different. When plastic flows in mold cavity, it will be affected by difference in thickness and produce different flow rates, which will lead to other poor injection problems, such as "trapped air encapsulation, hysteresis short shot, confluence cold connection", etc., as shown in Figure 1.

 

Figure 1: Flow lag caused by differences in thickness
• Short shot and vacuum bubble: Too thin can easily lead to two phenomena, such as "excessive injection resistance" or "too fast solidification of flowing core layer, resulting in short shot". If it is too thick, it is easy to cause "vacuum bubble" due to excessive shrinkage of plastic. Generally speaking, easier-to-shape meat thickness is about 1.2~4.0mm. When finished thickness is less than 1.2m and L/T (flow length ratio) is large, it is necessary to consider mold exhaust and injection capacity of machine to evaluate whether it can be formed smoothly. When thickness is greater than 4.0mm, it is necessary to consider shrinkage rate of raw material, design of runner and gate size.
• Surface shrinkage marks: Thickness of product can be divided into "thickness of main structure" and "thickness of ribs". Main cause of shrinkage marks on the surface comes from inconsistency of shrinkage direction of "finished thickness" and "reinforcing rib thickness", which causes the two to pull plastic material each other, causing slight dents on the surface of finished product. If you want to avoid shrinkage marks, thickness of reinforcing ribs should not be too thick, and thickness of ribs should be less than 1/2 of thickness of finished product as much as possible. If it exceeds 1/2, shrinkage tension of reinforcing rib will easily affect appearance of finished product. Especially when surface of finished product is finer and smoother, it is easier to see traces of shrinkage on appearance surface, as shown in Figure 2.

 

Figure 2: Surface dent marks caused by shrinkage of flesh thickness

Roughness of appearance of plastic products comes from transfer of mold surface. The most common processing methods for mold surface are "polishing", "discharge biting", "potion biting", and "laser biting". Any surface that will be directly seen or touched on product needs to define its roughness. For example, if it is a transparent product with a high-gloss surface, you need to specify polishing degree of mold surface as #3000-6000. If product needs to be abrasion-resistant and scratch-resistant, then this surface is suitable for biting surface. The rougher biting surface is, the more difficult it is to see slight scratches.

When demolding, almost all need "draft angle". Because rough surface of mold itself is uneven, when plastic is filled into mold, it will cause subtle demoulding barbs. If product is not designed with a draft angle, when finished product is demolded, it will easily cause appearance scratches or even fail to demold. Therefore, in order to solve barb problem, it is necessary to create an angle to eliminate barb condition. Especially when bite is rougher, its depth will be deeper, and draft angle should be increased appropriately. In practice, a common product design problem is that designer did not set draft angle when drawing 3D drawing, or set draft angle too small, resulting in unsmooth release of finished product, as shown in Figure 3. Another situation is that draft angle of product is determined by manufacturing end, which ultimately results in interference or excessive clearance in assembly of product.

 

Figure 3: The rougher die surface, the greater draft angle required

A good design needs to have basic tolerance concepts when drawing drawing, especially for products with assembly requirements, and special attention is needed. As a simple example, upper cover and lower cover need to be assembled after production. If both dimensions on 3D drawing are 100mm, during actual assembly after mass production, there will be assembly interference problems, as shown in Figure 4. Because size on 3D map is an absolute value, but actual mass-produced size is a range value, so it is unlikely to be exactly same as preset value of 3D map. Therefore, it is necessary to define dimensional tolerance range in advance. According to different sizes, a suitable tolerance range is given. The larger size, the more difficult it is to control size, and a larger allowable tolerance should be given. If allowable tolerance is particularly small, it is recommended to communicate clearly with manufacturing end before opening mold, try to avoid adding dimensional tolerances after mold trial, resulting in failure to accept acceptance or clarification of responsibility.

 

Figure 4: 3D drawing and actual assembly tolerance diagram

Initial product design is the key to determining a plastic product, and responsibility of manufacturing end is to guide customers to commercialize and mass-produce design in the most appropriate and economical way. In addition to above five points, before formal opening of mold, designer and manufacturer should have sufficient communication to jointly create a good plastic product. ■