Complexity of shape and structure of product obviously increases complexity of mold structure and difficulty of mold manufacturing, which will eventually affect instability of product performance and economic cost. From a process perspective, the simpler shape and structure design, the easier it is to fill mold with melt and the more guaranteed quality.
Ideal simplified product design should be: ① conducive to molding processing; ② conducive to reducing costs and saving raw materials; ③ conducive to reflecting aesthetic value of simplicity and beauty; ④ in line with principles of green design.

 

Following are some suggestions and tips for simplifying design.
(1) Simple structure, symmetrical shape, avoid irregular geometric figures;
(2) Avoid concave and convex shape design of side holes and inner surface of side wall of part. Concave and convex shape of side wall holes and inner surface of side wall of part is difficult for some molding processes and requires secondary processing after product is molded.

 

Figure 5-15 Typical products with concave and convex shapes on inner and outer surfaces of side wall

 

Figure 5-14 Example of cost saving by improving design
For example, for injection molded parts, a relatively complex demolding mechanism must be used in mold structure to demold parts. Usually, side holes are realized by side parting and core pulling mechanisms, which will undoubtedly make mold structure complicated. In order to avoid increasing complexity of mold structure design, design of such products can be improved. Figure 5-16 shows a design that avoids side core pulling.
(3) Possibility of molding should be considered in dimensional design. Different molding processes will have certain restrictions on dimensional design of parts, including size and dimensional changes.

When determining wall thickness, uniform wall thickness is an important principle. This principle is mainly proposed from perspective of process and quality issues caused by process. Uniform wall thickness can make melt flow and cooling of parts balanced during molding process. Difference in cooling shrinkage of thin wall parts will produce certain shrinkage stress. Internal stress will cause parts to warp and deform in a short period of time or after a long period of time. Figure 5-17 is an example of warping of a workpiece caused by uneven wall thickness. Figure 5-18 is a circular hole set in uneven wall thickness area. Due to uneven shrinkage, it is difficult to call it a perfect circle.

 

Figure 5-16 Design to avoid lateral core pulling
Following are three common treatment methods for uneven wall thickness:
(1) Smooth transition at junction of thick and thin. When thickness of product is inevitably designed to be inconsistent, transition should be gradual at junction of thick and thin to avoid sudden changes. Thickness ratio changes within a suitable range (generally not more than 3:1). Some molding processes may be exceptions, such as structural foam injection molding and gas-assisted injection molding.

 

Wall thickness transition form is shown in Figure 5-19. (a) in figure is a step-type transition, which should be avoided as much as possible; (b) is a conical transition, which is better; (c) is an arc transition, which should be the best.
(2) Changing sharp corner to a rounded corner, two walls with same wall thickness are connected at right angles, which destroys principle of uniform wall thickness. As shown in Figure 5-20, maximum thickness at the corner is 1.4 times wall thickness. If inner corner is rounded and outer corner is still a right angle, maximum thickness (W) at the corner can be increased to 1.6-1.7 times wall thickness. Correct design should be to round both inner and outer corners to ensure uniform wall thickness. Rounding can also avoid stress concentration, improve fluidity and formability of melt during plastic molding.

 

Figure 5-20 Rounded corner design
(3) Thinning thick-walled parts to make them uniform. For parts with large differences in wall thickness, thick-walled parts can be thinned to make them uniform by adding process holes, slotting or setting reinforcing ribs. Figure 5-21 shows several examples of making thickness of plastic parts uniform through design improvements.

 

Figure 5-21 Making the thickness of plastic parts uniform through design improvements

When a certain force is applied to geometric discontinuities such as holes, cuts, and corners on parts, a stress much greater than given apparent stress will be generated on cross section of this part. This phenomenon is called angular stress concentration. Ratio of large local stress to apparent stress is called stress concentration coefficient. Plastics are materials that are sensitive to notches and sharp corners. Under stress, these parts will gradually produce fine cracks, which will then gradually expand into large cracks. Continuous extension of cracks will eventually lead to damage to parts. Therefore, avoiding stress concentration should be a basic principle in product design.
The most direct and effective way to avoid stress concentration is to use arc transition at corners, edges, grooves, and transitions between thick and thin parts. Since logarithmic wall is similar to classic cantilever beam structure, stress concentration factor can be calculated for different wall thicknesses and fillet radii. Calculated structure is shown in Figure 5-22. Curve in figure shows that ratio of radius R to wall thickness T, that is, R/T, becomes flat after 0.6. It can be seen that radius of inner fillet should be at least half of wall thickness, preferably 0.6-0.75 of wall thickness.

 

Figure 5-22 Relationship curve of stress concentration factor of cantilever beam to R/T

For products that may cause deformation, warping, creep due to external loads and deadweight, enhancing rigidity of product must be considered. For products with rigidity requirements, we must first select materials. After materials are confirmed, we can enhance rigidity of product through appearance and structural design of product.
Following methods can usually be considered.
(1) Changes in geometric shape. For thin-shell flat parts, surface is designed to be corrugated, corrugated, arched, spherical, or parabolic. Its rigidity is much higher than that of a flat plate of same weight. Figure 5-25 shows several commonly used design schemes.

 

Figure 5-25 Surface design to improve rigidity
Above structural theory is often cleverly used in reinforcement design of bottom of container, as shown in Figure 5-26.

 

Figure 5-26 Reinforcement design of bottom of container
Figure 5-27 is design of bottom of plastic bottle, which is a common design method for strengthening bottom stiffness. Among them, (b) is a spherical bottle bottom with a bottle holder attached, which is design used in the past for cola bottles. It is rarely used now and is replaced by (c).

 

Figure 5-27 Reinforcement design of bottom of plastic bottle
(2) Design and application of reinforcing ribs. Design of edge of container shown in Figure 5-30 plays the role of edge reinforcement. In fact, this sudden edge can be regarded as a variation of reinforcing ribs.

 

Figure 5-30 Design of container edge reinforcement
Reinforcing ribs used to support vertical wall are also called corner braces. Corner brace design on the side surface of sleeve (Figure 5-32) is an effective way to improve torsional stiffness and bending stiffness of sleeve.

 

Figure 5-32 Design of sleeve rigidity enhancement
1-sleeve: 2-corner brace; 3-rounded corner
(3) Reinforcing effect of inserts. Metal inserts are set in parts to improve local or overall strength of plastic parts. Typical examples in this regard include car steering wheels, movable handles, plastic door and window frames, plastic gears with metal inserts, etc.
(4) Structural design. In product design, there are several structures with a relatively high rigidity/mass ratio.
① Honeycomb sandwich structure, as shown in Figure 5-36, this structure is usually behind car horn cover. This structure has a good rigidity design effect, but its disadvantage is that process is relatively complicated, cost and price are high.

 

Figure 5-36 Honeycomb sandwich structure
② Structural foam parts. Parts formed by structural foam molding process have a dense skin layer and a core with a microporous structure. This structure has a high specific strength and can be used in load-bearing structures.
③ U-shaped structure, T-shaped structure and I-beam structure. Compared with solid structure of rectangular cross-section, this structure can save materials without reducing rigidity.
④ Conical structure: Compared with cylindrical structure, this structure can withstand large compression load and has good bending stability.
⑤ Double-wall structure: There are many processes that can form parts with double-wall structure. Parts with this structure have higher rigidity, impact toughness and bending resistance. A double-wall structure part formed by blow molding is shown in Figure 5-37.

 

Figure 5-37 Flat and wide double-wall part

There are two situations that can cause deformation of parts and require targeted preventive design: one is warping deformation caused by internal stress of parts, and the other is thermal deformation caused by thermal effect.
(I) Part deformation caused by internal stress. This deformation is caused by internal stress in parts. Usually, uneven distribution of internal stress is main cause of warping deformation, and uneven distribution of internal stress may be result of combined effect of processing conditions (such as uneven distribution of temperature and pressure, anisotropy of shrinkage rate, etc.), material composition (tendency of material of product type to deform over a long period of time is greater), mold structure (especially gate design) and product shape.
Aforementioned measures to avoid stress concentration and rigid design are also helpful to prevent or reduce deformation of parts.
① Side wall of a rectangular thin-walled container is prone to concave deformation. For this reason, side wall can be designed to be slightly convex, as shown in (c) in Figure 5-38.

 

Figure 5-38 Preventing side wall of a rectangular thin-walled container from being concave
For shallow box products, to avoid warping and deformation, bottom edge can be designed to be chamfered, as shown in Figure 5-39(b).

 

Figure 5-39 Designing bottom edge to be chamfered to prevent deformation
② When there is a groove as shown in Figure 5-40 in injection molded part, top of groove will be arched due to different solidification speeds of thick and thin wall parts. To avoid this, correct design should be as shown in Figure 5-41.

 

Figure 5-41 Correct design can avoid deformation
When x≤5, A≤1/3B; when x>5, A≤1/4B
③ For component shown in Figure 5-42, due to different wall thicknesses, after plastic at thick wall is completely solidified, it will exert tension on thin-walled part that solidified first, causing deformation of part. Two measures shown in Figure 5-43 can avoid this situation, among which (a) adopts method of uniform wall thickness; (b) adopts method of increasing height of rib.

 

④ Frame structure is easy to deform. Design shown in Figure 5-44 is a design measure that uses reinforcing ribs to prevent deformation.

 

Figure 5-44 Design using reinforcing ribs to prevent deformation
⑤ Due to uneven heat diffusion during melt flow, U-shaped injection molded part shrinks in the right angle direction, resulting in warping deformation as shown in Figure 5-51 (a). In addition to setting reinforcing ribs, solution to this phenomenon can also be to open a small groove at the right angle as shown in (b).

 

Figure 5-51 A small groove is opened at the corner of a U-shaped injection molded part to prevent deformation
2. Dimensional tolerance of molded plastic parts
For dimensional tolerance of injection molded parts, my country still uses SJ1372-78 standard of the former Fourth Ministry of Machine Building and WJ1266-81 standard of the former Fifth Ministry of Machine Building. Two standards have same content, and dimensional accuracy of plastic parts is divided into eight levels.
Dimensional tolerance of engineering plastic molded plastic parts GB/T14486-93 has been implemented. This standard specifies dimensional tolerance of thermosetting and thermoplastic engineering plastic molded plastic parts. It is applicable to plastic parts molded by engineering plastics for injection molding, compression molding, transfer molding and casting, but not for extrusion molding, blow molding, sintering and foam products. Code for molding dimensional tolerance is MT. Tolerance grades are divided into seven levels. Numerical table of tolerances for each level is listed in Table 5.4. Selection of tolerance grades for molded parts of common materials is shown in Table 5.5
This standard only specifies tolerance, upper and lower deviations of basic size can be allocated according to actual needs of project. See Table 5.5 for dimension grades without tolerance.
Table 5.4 Molded Parts Dimension Tolerance Table

 

 

Note: A is tolerance value of dimension not affected by movable part of mold: B is tolerance value of dimension affected by movable part of mold.
Table 5.5 Selection of tolerance grade of molded parts made of common materials

This standard specifies molding shrinkage VS, difference between corresponding dimensions of molded part and mold used at room temperature, expressed as a percentage of corresponding dimensions of mold.
Where LF... size of plastic part after being placed in a standard environment for 24 hours after molding, mm;

 

Lw... corresponding size of mold, mm;
This standard divides molding dimensions into two categories
① Dimension a not affected by movable part of mold, as shown in Figure 5.6, it refers to dimension molded in parts of same movable mold or fixed mold.
② Dimension b affected by movable part of mold,

 

Figure 5.6 Dimension a not affected by movable part of mold
As shown in Figure 5.7, it refers to dimension formed by joint action of movable mold parts. For example, wall thickness and bottom thickness dimensions; dimensions affected by movable mold parts, fixed mold parts and sliders.

 

Figure 5.7 Dimension b affected by movable part of mold
This standard also stipulates that demoulding slope is not included in tolerance range. If there are special requirements, location of basic tolerance dimension should be indicated on drawing. Size of demoulding angle must be marked on drawing.
Table 5.3 Molding shrinkage range of commonly used plastics Unit: mm/mm

②Mold. For small-sized plastic parts, manufacturing error of mold accounts for 1/3 of plastic tolerance. Precision of plastic part size related to moving parts on mold is low. Improper design of pouring system and cooling system on mold will cause uneven shrinkage of molded plastic parts. Improper force of demoulding system will cause ejected plastic parts to deform. All of these will affect precision of plastic parts.
③ Plastic part structure. Wall thickness of plastic part is uniform and shape is symmetrical, which can make shrinkage of plastic part balanced. Improving rigidity of plastic part, such as reasonable setting of reinforcing ribs or use of metal inserts, can reduce warping deformation of plastic part, which is conducive to improving precision of plastic part.
④Process. Temperature, pressure and time at each stage of injection cycle will affect shrinkage, orientation and residual stress of plastic part. There is an optimal process for precision requirements of plastic part. Stability of process parameters is more important to ensure precision of injection molded parts. Error caused by fluctuation of molding conditions accounts for 1/3 of tolerance of plastic part.
⑤Use. Sensitivity of plastic materials to time, temperature, humidity and environmental conditions will be manifested after injection molded products are used for a long time. Stability of size and shape accuracy of injection molded parts is poor.