Plastics engineers all know principle that excellent products require excellent molds, and excellent molds require excellent mold design. In the entire injection molding process, quality of mold design and processing is crucial to success of product.
Generally, after plastic materials and product design are determined, injection mold design begins. Before designing mold, it's essential to understand production volume, whether plastic part requires secondary processing, and product's appearance, physical properties, dimensional accuracy, and other usage requirements. This determines mold's lifespan, machinability, machining accuracy level, and cost. At the same time, attention must be paid to injection molding machine model and other relevant matters.
As a mold engineer, in addition to designing molds based on practical experience, it is essential to refer to mechanical and mold design manuals. Mold design is a highly technical and experience-intensive branch; therefore, this chapter will not discuss mold design in detail, but will focus on knowledge relevant to injection molding process.
As a field plastic molding engineer, to better develop products, accept molds, and repair molds, it is also necessary to master general mold basics, specifically understanding following five fundamental principles of injection mold design.

First, based on information regarding development of plastic product, determine mold material. Then, calculate mold strength based on material, thereby determining mold's external dimensions.
In injection mold design, strength and rigidity are not necessarily simultaneously achieved. In actual mold production practice, large cavities are often calculated and designed primarily for rigidity, while small cavities are calculated and designed primarily for strength. Practical experience tells us that during injection molding, regardless of cavity size, if sidewalls and bottom walls of mold cavity are not thick enough, this part of mold's strength is most easily compromised. Therefore, this area is a key consideration in design. Many failures have occurred in practice due to insufficient mold strength. However, larger mold sizes are not always better. Excessive size not only increases mold processing costs but also leads to injection molding machine being oversized for job. It is worth noting that for molds requiring surface finishing such as electroplating or orange peel texture, mold material selection must consider its abrasion resistance and chemical corrosiveness during electroplating.
Molding plastics such as PVC, POM, and FEP, which produce corrosive gases like chlorine, or low-foaming, flame-retardant, and acetate-based plastics, also generates significant amounts of corrosive gases during molding. Therefore, when molding these types of plastics, corrosion resistance of mold material must be considered.
For molding high-temperature plastics such as PC, PETP, FEP, and PPO, high processing temperature, coupled with the resulting increase in mold temperature, can easily lead to wear and tear on top plates and sliders. Therefore, when designing sliding components, a mold material with a low coefficient of thermal expansion should be selected.

Ejection mechanism of a plastic part consists of three main parts: guiding, ejection, and resetting. Basic design principles are:
Part should remain on moving mold side as much as possible; Ejection marks should not affect appearance of part; ejected parts should not deform or damage quality of plastic part; Resetting should be smooth, stable, and reliable when mold closes.
Components of mold's ejection mechanism were among the first to be standardized. Therefore, in mold design, regardless of part, standard parts that can be matched with mold should be considered and prioritized.
China's plastic mold industry chain has now formed an independent system and scale, with increasingly complete professional supporting facilities. Therefore, in mold design, standard parts should be used as much as possible for nozzles, ejector plates, ejector pins, guide pillars, guide pillar sleeves, sprues, bolts, etc., to facilitate future mold maintenance and reduce mold processing costs.

When receiving design drawings of a plastic product, the first consideration is mold parting surface, while also considering gate design. Once mold parting line is determined, design of gate and runner becomes crucial, and it's also the area most prone to problems in injection molding. Therefore, in mold design and initial machining, while ensuring design dimensions, a certain amount of correction dimensions must be reserved to allow for fine-tuning of gate and runner during on-site trial molding of new mold. Because on-site repairs are limited to simple adjustments to gate, runner, venting, water channels, and locating rings, this section will focus on explaining design relationship between gate and runner, understanding technical knowledge of gates and runners. This will have an unexpectedly positive effect on on-site mold correction and solving various problems that arise in plastic molding.

Gate, also known as feed gate, is channel for molten plastic connecting runner and cavity. Location, number, shape, and size of gate in a plastic mold directly affect appearance, physical properties, efficiency, and dimensional accuracy of finished product. Gate design has relatively standard formulas, which can be found in any plastic mold design book. Here, we'll begin by understanding some general knowledge and experience regarding gate and runner design, which is essential for on-site mold trials and adjustments.
Gate Classification and Design Principles: Restricted gates are various gate shapes determined by restriction of their cross-sectional area. These gates allow molten resin to rapidly solidify as it fills gate after passing through runner.
Restricted gates offer advantages such as shorter molding cycles, reduced warpage and cracking, lower residual stress near gate, and improved product appearance. Examples include pin-point gates, ear-type gates, and submarine gates.
Unrestricted gates are the largest cross-sectional area in the entire gating system. They are primarily used for medium to large cylindrical and shell-shaped plastic parts. Unrestricted gates simplify mold and reduce pressure loss, but they have longer curing times, leading to longer molding cycles and a higher risk of residual stress and overfilling. Examples include direct gates.
Single parting line gates can employ various gate types, including direct gates, center gates, side gates, ring gates, spoke gates, claw gates, and submarine gates. It's worth noting that ear-type gates are suitable for transparent products, especially for plastics like PMMA and SAN. Ear-type gates are also suitable for plastics with poor thermal stability and high melt viscosity, such as PVC and PC.
Gate design principles and objectives: Ultimate goal of gate design is to quickly and evenly fill cavity, prevent backflow of melt, achieve a clean separation between molded part and gate.

Regardless of type of gate used, its location significantly impacts not only mold structure but also moldability and quality of molded part. When selecting gate location, factors such as part's structure, product requirements, and molding state must be considered comprehensively. General principles for gate location selection are as follows:
① Minimize flow distance. Selection of gate location should ensure rapid and uniform filling of mold cavity by melt, minimizing melt flow distance and changes in material flow direction. This is especially important for large plastic parts.
② Gate should be located at the thickest part of plastic part. When wall thickness of plastic part varies significantly, if gate is located at thinner wall, flow resistance of plastic melt will increase after entering cavity, and it will also cool more easily, affecting flow distance and making it difficult to fill the entire cavity. To ensure smooth filling of cavity by plastic melt, effective transmission of injection pressure, sufficient compensation during melt solidification and shrinkage, gate should generally be located at the thickest part of plastic part.
③ Consider molecular orientation. Shrinkage rate of product differs depending on whether it flows vertically or horizontally, resulting in differences in strength and stress cracking tendency. Therefore, this should be carefully considered when selecting gate location.
④ Improve weld strength. Due to gate location, filling of mold cavity by plastic melt may cause merging of two or more melt streams. At the point where material flows converge, front end is gaseous and at its lowest temperature, resulting in a weld line on plastic part, as shown in Figure 3-2.

 

Figure 3-2 Weld mark on plastic part
There are two types of weld lines. One occurs when a core or insert is present in mold cavity. As melt passes through this obstacle, flow splits into two streams, as shown on left side of Figure 3-2. These two streams then merge again, forming a weld line. Because contact time between melt and obstacle is short, melt temperature drops only slightly, hence it is called a hot weld line.
The other type is due to design of plastic part. To reduce melt flow and mold filling time, two or more gates are often used. In this case, melt enters mold cavity from each gate separately, as shown on right side of Figure 3-2. A weld line is also formed where front ends of melts meet. Because flow through mold cavity is long, melt temperature drops, hence it is called a cold weld line.
Table 3-2 Relationship between Common Plastics and Gates/Runners

Note: ◎ indicates applicability, ○ indicates can be used, blank indicates not suitable for use
Regardless of whether it is a cold or hot weld line, and whether butt joint is made first or later, strength of weld line area on plastic part will be significantly reduced. If process is not properly adjusted at this stage, it will affect the overall appearance quality of plastic part. This phenomenon is particularly serious when molding glass fiber reinforced plastic parts. For some plastic parts, gate design must also consider facilitating end venting and avoiding high-pressure impact of melt, which can cause deformation of core and inserts.
If design structure of plastic part and mold cannot be changed, then injection molding process must be considered. To ensure strength of weld line, based on above analysis of root causes of weld lines, we need to adjust mold temperature, injection rate, injection pressure, and other process conditions accordingly.

Table 3-2 lists the gate types suitable for various plastics, for reference only when designing and inspecting molds. On-site considerations also include size, shape, wall thickness, appearance, and whether it is a modified material, etc., to make a final decision.

Standard gates are commonly used in molds, so they will be discussed in detail. Here's an example using a rectangle:
Depth of rectangular gate is represented by h (mm), then: h(mm) = nt, where t is product wall thickness (mm), and n is an empirical reference coefficient for plastics. Generally, empirical values for n are: PS and PE 0.6, POM, PC, and PP 0.7, PVAC, PMMA, PA 0.8, and PVC 0.9.
Gate width W = (n√A)/30, where A is outer surface area of product (unit: mm²). This allows for a rough judgment of whether rectangular gate size meets molding requirements of product, or for minor adjustments to gate. If gate width W is larger than runner diameter, a fan-shaped gate is recommended. Generally, when choosing dimensions of W and h, dimension of h should be given priority; a ratio of W to h of 3:1 is generally considered good.
Based on established experience, general gate thickness is 0.5–1.5 mm, width is 1.5–5 mm, and length is 1.5–2.5 mm. For large products and complex-shaped plastic parts, a wider range is used: gate thickness 2.0–2.5 mm (generally 70%–80% of part's wall thickness), width 7–10 mm, and length 2.0–3.0 mm.
Runner is also an area that requires frequent adjustments during mold trials. Table 3-3 recommends runner diameters for various plastics for reference during on-site runner repairs.
Table 3-3 Reference Flow Channel Diameters for Various Plastics

Plastic products in use inevitably incorporate side grooves and side holes. If forced demolding is not feasible, especially for side hole structures in non-elastic plastics, mold design must consider structures such as sliders, side core pulls, springs, or inclined guide pillars.
However, such structures frequently experience grinding failures during production. Furthermore, mold processing requires high precision and specific material selection criteria. Therefore, product design should be as simple as possible.
When designing mold, in addition to considering parting surface and its position, it's also necessary to consider type of ejector plate or ejector pin mechanism, ensuring ejection position is balanced and effective. This effectively avoids potential mold grinding during subsequent injection molding.
In mold design, sometimes to improve production efficiency and reduce product costs, multi-cavity structures are often considered. In this case, parting surface of product must be fully considered, OC configuration scientifically implemented, ejection positions and methods rationally designed.

Mold temperature plays a crucial role in product shrinkage, production efficiency, and product appearance, especially for crystalline plastics and specialty engineering plastics.
It is particularly important to note that for box-shaped and cylindrical products, cooling of mold top and sealing of cooling system, especially preventing water leakage, are critical design and manufacturing processes. Selection of sealing materials, fastening mechanisms, quality of accessory selection and manufacturing directly affect subsequent plastic molding and mold maintenance.
Cooling water channels are a vital aspect of mold design. Based on theory and experience, principle for cooling water channel design is: uniform and rapid heat dissipation from mold. Design location and method of cooling water channels must be considered according to shape of product.
Figure 3-3 shows two different cooling water channel designs for same plastic part, resulting in drastically different mold heat transfer effects. Mold on the left in Figure 3-3 uses a large-hole, few-channel water channel design, but this design results in uneven cooling and poor cooling effect. Mold on the right in Figure 3-3 features a multi-channel cooling design with small holes. This design ensures very uniform cooling and excellent heat dissipation, resulting in no problems with finished product.

 

Figure 3-3 Mold Cooling
Table 3-4 provides empirical values for mold cooling design based on above information, for reference only during mold design and maintenance. Referring to graph in Figure 3-3, symbols in Table 3-4 represent: product wall thickness (W), water hole diameter (D), distance between water holes (C), and distance from center of water hole to product surface (H). Therefore, when designing mold cooling, data given in Table 3-4 should be followed, distribution and size of cooling holes should be considered in conjunction with specific plastic part.
Table 3-4 Empirical Values for Uniform Mold Cooling

Some plastics require mold heating during injection molding. If mold temperature requirement is not high, a mold temperature controller is usually used. However, when higher mold temperatures are needed, it is necessary to consider installing heat sources such as electric heating rods on mold. Mold heating system should also adhere to principle of uniform heating, while also considering mold insulation.
When using electric heating rods to heat mold, factors such as efficiency of heating systems like thermocouples and heat conduction must be considered. Assembly gap between electric heating element and mold is particularly important, as it directly affects mold's heat transfer efficiency and heating uniformity. To ensure uniform mold temperature, thermometers must be installed on both moving and stationary molds to monitor temperature changes in real time.a