To design an advanced plastic mold, we first need to have a high-level design idea, and we must also study product processability, characteristics and uses of plastic materials, selection of mold steel, processing methods, mold structure design, molding schemes and injection machine models.
Among them, it is very necessary to study processability of mold design from perspective of mold design and injection molding. Purpose is to reduce trouble caused by poor mold processability to mold manufacturing and molding.

Plastic refers to a material with polymer synthetic resin as main component, which has plasticity and fluidity at a certain temperature and pressure, can be molded into a certain shape, maintains same shape under certain conditions. Commonly used plastics are divided into two categories: thermosetting plastics and thermoplastics. Characteristic of thermosetting plastics is that they can solidify into insoluble materials under heat or other conditions. Characteristic of thermoplastics is that they can be repeatedly heated to soften or cooled to solidify within a specific temperature range.
In terms of performance, plastics have advantages of light weight, good strength, corrosion resistance, good insulation, easy coloring, products can be processed into any shape, high production efficiency, and low price.

Wide application of plastic products is inseparable from its own properties and characteristics. Following introduces composition characteristics of plastics.
(1) Molecular structure of plastics
Main component of plastics is resin, which is divided into natural resin and synthetic resin.
(2) Composition of plastics
Resin: Main function is to bond other components of plastics and determine main properties of plastics, such as mechanical, physical, electrical, and chemical properties. Proportion of resin in plastics is generally 40% to 65%.
Filler: Also known as additives, correct selection of fillers can improve performance of plastics and expand its scope of use.
Plasticizer: Some resins have very low plasticity and poor softness. In order to reduce melt viscosity and melting temperature of resin, improve its molding and processing performance, improve flexibility, elasticity and other necessary properties of plastic, high-boiling organic compounds that are compatible with resin and not volatile are usually added. This type of substance is called a plasticizer.
Colorant: Also known as colorant, it mainly plays a role in beauty and decoration, including coating part.
Stabilizer: Any substance that can delay deterioration of plastics is called a stabilizer, which is divided into light stabilizers, heat stabilizers and antioxidants.
Lubricant: Improve fluidity of plastic melt, reduce or avoid friction and adhesion to equipment or molds, and improve surface finish of plastic parts.

Table 1-1 Thermoplastic abbreviations and Chinese comparison

Plastics of different compositions reflect their different use values. Table 1-2 shows performance and uses of commonly used thermoplastics.
Table 1-2 Characteristics and uses of commonly used thermoplastics

Table 1-3 Characteristics and identification of commonly used plastics

In order to facilitate removal of plastic parts from closed mold cavity, and to facilitate placement of inserts or removal of pouring system, mold must be divided into two or more parts. Surface that can be separated from mold to remove plastic part is usually called parting surface. At the same time, with parting surface as boundary, mold can be divided into two major parts, namely movable mold and fixed mold part. The other surfaces are called separation surfaces or parting surfaces. Injection mold has only one parting surface.
Selection of parting surface is a relatively complex issue because it is affected by geometry of plastic part, wall thickness, dimensional accuracy, surface roughness, insert position, demolding method, molding position of plastic part in mold, ejection method, design of pouring system, and mold exhaust method.
Forms of parting surfaces are generally as follows: (a) in Figure 1-1 shows a horizontal parting surface; (b) shows an oblique parting surface; (c) shows a stepped parting surface; (d) shows a curved parting surface. Therefore, for mold designers, correct selection of parting surfaces has a vital impact on mold manufacturing and operation.

 

When selecting location of parting surface, parting surface is generally not located on decorative outer surface or at a corner with a circular arc. Parting surface must be set at the largest cross-section of product, and it is convenient for plastic part to remain on the side of movable mold after mold is opened to ensure that ejection mechanism can be demolded smoothly. Following principles should be followed when selecting parting surface.

In order to facilitate demolding of plastic parts, plastic parts should be retained in lower mold as much as possible when mold is opened. Since ejection mechanism of plastic part is usually set in lower mold, especially mold used for automated production, it is more important to correctly select mold retention method for plastic part.
Whether mold retention method is selected correctly will directly affect product quality and production efficiency.
As shown in Figure 1-2 (a), since core is located in fixed mold, plastic part will shrink and wrap core tightly after mold is opened, leaving plastic part on fixed mold side. Therefore, it is more difficult to demold and mold structure is complicated. If structure shown in Figure 1-2 (b) is used instead, it will be more reasonable.

 

Parting surface should be selected as far as possible in a position that does not affect appearance of plastic part, and flash generated at parting surface should be easy to trim. As shown in Figure 1-3, if plastic part without a circular arc spherical surface on one side adopts structure of Figure (a), surface quality of plastic part will be damaged, while structure of Figure (b) is more reasonable.

 

Figure 1-4 shows a pair of gear molds. Gear rim and outer circle of step part have coaxiality requirements. If parts with coaxiality requirements are molded in movable mold and fixed mold respectively, as shown in Figure 1-4 (a), it will be difficult to ensure their coaxiality requirements due to inaccurate mold clamping; if structure shown in Figure 1-4 (b) is used instead, so that parts with coaxiality requirements are all molded in movable mold, coaxiality requirements can be met.

When selecting parting surface, location of flash on plastic part should be considered according to use requirements of plastic part and plastic used. If plastic part is not allowed to have horizontal flash, structure shown in Figure 1-5 (a) can be used, which is conducive to demoulding. Especially for nylon with good fluidity, this structure can also reduce generation of flash. However, structure shown in Figure 1-5 (b) is not appropriate.

 

 

Gap between parting surface after mold closing is generally not more than 0.01~0.03mm, otherwise flash will be formed, so parting surface must be ground by a "surface grinder".

When selecting parting surface, consider reducing size difference between large and small ends of plastic part caused by demoulding slope. For plastic part shown in Figure 1-6 (a), if cavity is set on one side of mold, size difference of plastic part will be large due to demoulding slope. When plastic part is not allowed to have a large demoulding slope, use of this structure will inevitably make demoulding difficult. If plastic part has no strict requirements on appearance, parting surface can be selected in the middle of plastic part, as shown in Figure 1-6 (b). It can adopt a smaller demoulding slope, which is conducive to demoulding.

 

Venting function of parting surface can discharge part of high-temperature gas inside cavity out of cavity, ensuring that there are no pores on the surface of product, which is conducive to improving appearance quality of product.
Generally, a venting groove with a depth of 0.025-0.1mm and a width of 1.5-6mm is opened on one side of parting surface concave mold. Ejector, cavity, and core insert can also be used for venting.

Parting surface should divide mold into parts that are easy to process to reduce difficulty of mechanical processing.

Casting system refers to flow channel of plastic melt in mold from nozzle to cavity. Function is to smoothly fill various parts of cavity with plastic melt, transmit injection pressure to various parts of cavity during filling and solidification process.
Pouring system consists of main channel, branch channel, cold material hole and gate, as shown in Figure 1-7.

 

Design of pouring system is an important part of injection mold design, which has a direct impact on efficiency of injection molding and quality of plastic parts;
Therefore, following principles must be paid attention to when designing pouring system:
Understand molding characteristics of plastics, that is, temperature and shear rate of plastics.
Prevent deformation of cores and plastic parts, avoid material flow from rushing out of small diameter cores or fragile metal inserts, and prevent deformation of plastic parts due to excessive shrinkage stress at gate.
Good exhaust to ensure smooth, fast and non-turbulent flow of fluid.
Reduce process and plastic consumption to shorten molding cycle, improve molding effect, and reduce amount of plastic.
Position and shape of feed port should be determined in combination with shape and technical requirements of plastic part.
When one mold has multiple cavities, prevent parts with different sizes from being placed in one cavity.
Convenient trimming to ensure appearance quality of plastic parts.

Runner is part between main runner and gate, which plays the role of diversion. Requirement is that heat and pressure loss of plastic melt during flow is minimized, and amount of plastic in runner is minimized. Shape of runner is shown in Figure 1-8. Shape and size of runner must be determined based on factors such as molding volume of plastic part, wall thickness and shape of plastic part, process characteristics of plastic, injection molding speed, and length of runner.

 

Key points of runner design:
Surface of runner does not need to be very smooth, and surface accuracy is generally 1.25. Because surface of runner is not too smooth, it can fix cooling skin of molten plastic, which is conducive to heat preservation.
When runner is long, a cold material hole should be opened at the end of runner to accommodate cold material generated at the beginning of injection molding to ensure quality of plastic part.

Gate refers to small channel between end of runner and cavity. Main function is to increase flow rate and temperature of plastic and prevent side flow of plastic into cavity.

Pin-point gate is also called water point gate. It can be automatically demolded by using a three-plate mold frame. Mold structure is relatively complex. Gate is automatically pulled off during production process. It is suitable for automated injection production. It can be used in a one-cavity mold or a multi-cavity mold. It can inject both small and large products, especially plastic parts with patterns. It does not affect appearance. Figure 1-9 shows pin-point gate structure.

Submerged gate is also called submerged gate. Feeding position is selected in a more hidden place of product to avoid affecting appearance of product. Runner and plastic part are automatically separated during ejection, so a larger ejection force is required. Submerged gate is not suitable for overly tough plastics. Figure 1-10 shows submerged gate structure.

 

Side gate is also called edge gate or side water gate. It is generally opened on parting surface and feeds from edge of plastic part. Its shape is rectangular or nearly rectangular. Processing is convenient and simple, and application is flexible. It can be fed from outside or inside of product. Figure 1-11 shows side gate structure.

Direct gate is also called large water gate or center gate. There is no branch channel. Plastic enters cavity directly through main channel. Therefore, it has advantages of short plastic flow, small flow resistance, fast feeding, small kinetic energy loss, and good pressure transmission. However, it is difficult to cool gate, and plastic parts have obvious gate marks. Because heat is concentrated near gate, condensation is late there, internal stress is large, and it is easy to produce defects such as bubbles and shrinkage holes there. Figure 1-12 shows direct gate structure.

 

When designing casting system, gate position should be selected first. Whether gate position is correct or not will directly affect molding quality of product and whether injection process can proceed smoothly.
Following principles should be followed when selecting gate position:
Gate position should be selected on parting surface as much as possible to facilitate mold processing and gate cleaning.
Distance between gate position and each part of cavity should be as consistent as possible, and its flow should be as short as possible.
Gate position should ensure that when plastic flows into cavity, it faces wide and thick-walled part of cavity to facilitate flow of plastic.
Avoid plastic from directly hitting cavity wall, core or insert when flowing into cavity. Plastic should flow into each part of cavity as quickly as possible to avoid deformation of core or insert.
Try to avoid welding marks on product or melting marks on important parts of product.
When plastic flows into cavity, it should flow evenly in direction parallel to cavity, and it is conducive to discharge of gas in cavity.
Gate should be set at part on product that is easiest to remove, and at the same time, it should not affect appearance of product as much as possible.

Mechanism that removes plastic part from mold is called an ejector mechanism or demolding mechanism. Direction of action of ejector mechanism is consistent with direction of mold opening. A good ejection mechanism requires that plastic part is not deformed or damaged during demolding, and ejection mechanism should be located in an inconspicuous part of part.
Ejection mechanism can be summarized into three categories: mechanical ejection, hydraulic ejection and pneumatic ejection. Following principles should be followed when designing ejection system:
In order to prevent product from being deformed due to ejection, thrust point should be as close as possible to core or part that is difficult to demold, such as slender hollow cylinder on product, which is often ejected by a push tube (standard parts are usually a sleeve). Arrangement of thrust points should be as uniform as possible.
Thrust point should act on part of product that bears the greatest force, that is, part with good rigidity, such as ribs, flanges and wall edges of shell-shaped products.
Try to avoid thrust point acting on thin surface of product to prevent product from breaking and perforating. For example, shell-shaped products and cylindrical products are mostly ejected by push plates.
In order to avoid the traces of ejection affecting appearance of product, ejection device should be located on hidden surface or non-decorative surface of product. For transparent products, special attention should be paid to selection of ejection position and ejection form.
For further reading, please refer to Injection molding materials and mold design professional knowledge sharing (Part 2).