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As a product structure or mechanical engineer, it is necessary to master basic understanding of injection molds. It can help you better develop products. Details are as follows:

Materials used in mold are mainly as shown in table below

Note: Relative sliding parts use different steel materials and hardness requirements, and hardness differs by 2°.
In United States, mold lines and lifters are generally made of hardened steel, or beryllium copper is used for lifters, unless otherwise specified.

Texture effect to be achieved by mold surface treatment is mainly divided into sparkle pattern and sunburst pattern. Corresponding main standards are VDI and MT pattern plates, which are more commonly used, the others will not be introduced;
Corresponding draft angles and labels are as shown in table below

Note: Reference value, actual situation will be evaluated according to mold factory and product design.

Note: Reference value, actual situation will be evaluated according to mold factory and product design.

Different types of products require customized injection molding and production of different molds to meet different product needs. The most common types of injection molds are two-plate molds, three-plate molds, hot runner molds, etc.

Two-plate mold is also called a single-parting surface injection mold. It is the simplest and most basic type of injection mold. It can be designed into a single-cavity mold or a mold with multiple cavities according to product requirements. It is the most widely used mold.

 

Three-plate mold is also called a double-parting surface injection mold. Difference from single-parting surface injection mold is that three-plate mold adds a stripper plate that can be partially moved in fixed mold part. In this design, runner is ejected between the first and second plates, part will be ejected between second and third plates, thereby separating part and runner. Structure of double parting surface injection mold is complex, manufacturing cost is high, and parts processing is difficult. Generally not suitable for molding of large products.

 

Hot runner mold is similar to traditional cold runner mold, except that it directly injects plastic into mold cavity through nozzle. When hot runner is used in mold, there will be no runners during molding, so there is no waste. Hot runner molds are more expensive to manufacture than cold runner molds, but if molded part is very small, especially if part is smaller than runner, a hot runner mold may be a more economical choice.
Hot runner molds involve more pressure and mechanics, especially multi-cavity molds. This can improve cycle times required to mold parts, especially in high-volume production situations where a few seconds off cycle time can be a big benefit.

 

Plastic parts layout refers to arranging one or more types of plastic parts required according to reasonable injection molding process and mold structure according to customer requirements. Layout of plastic parts complements mold structure and plastic craftsmanship, and directly affects the later injection molding process. Corresponding mold structure must be considered when layout, and layout must be adjusted under conditions of satisfying mold structure.
From perspective of injection molding process, following points need to be considered:

(1) Flow length. Flow length of each rubber material is different. If flow length exceeds process requirements, parts will not be filled.

(2) Runner waste. On the premise that each cavity is filled, length and cross-sectional size of runner should be as small as possible to ensure the least waste in runner.
(3) Gate location. When gate position affects layout of plastic parts, gate position needs to be determined first and then layout. In the case of a multi-cavity piece, gate locations should be uniform.
(4) Glue feeding balance. Glue feed balance means that material fills each cavity at the same time under basically same conditions. In order to meet glue feeding balance, following methods are generally used:
A. According to balanced arrangement (as shown in picture), it is suitable for situation where size of plastic parts is basically same.

 

B. Arrange large plastic parts so that they are close to main runner and small plastic parts are far away from main runner, then adjust size of runner and gate to meet balance of glue feeding.
Note: When weight ratio of large and small plastic parts is greater than 8, adjustments should be made in consultation with product designer. In this case, it is difficult to adjust size of runner and gate to meet balance requirements.
(5) Cavity pressure balance. Cavity pressure is divided into two parts. One is axial pressure parallel to mold opening direction; the other is lateral pressure perpendicular to mold opening direction. Positioning should ensure that axial pressure and lateral pressure are balanced relative to center of mold to prevent glue overflow and batch peaks.
Methods to satisfy pressure balance:

A. Layout is even and symmetrical. Axial balance is shown on the left; lateral balance is shown on the right.

 

B. Utilize mold structure balance as shown in the figure. This is a commonly used method to balance side pressure.

 

Several points need to be considered from perspective of mold structure

Layout should ensure that there is a certain distance between runner and nozzle and edge of front mold cavity to meet sealing requirements. Generally required D1≥5.0mm, D2≥10.0mm, as shown in figure.

 

Distance between slider groove and edge of sealant should be greater than 15.mm.

Space requirements for mold structural parts, such as shovels, sliders, lifters, etc., should be met during alignment. At the same time, following points should be ensured:
A. Mold structural parts have sufficient strength.
B. No interference with other mold base components
C. When there are moving parts, stroke must meet mold ejection requirements. When there are multiple moving parts, there should be no mutual interference.
As picture shows

 

D. Position of cylinder needs to be away from position of top rod hole.

In order to achieve a better cooling effect of mold, attention should be paid to influence of screws and ejector pins on cooling water holes when arranging, and position of cooling water holes should be reserved.

Layout should be as compact as possible to reduce the overall dimensions of mold, and length-to-width ratio should be appropriate. Installation requirements of injection molding machine should also be considered.

Surface where mold is opened to take out plastic parts or pouring system is called parting surface. In addition to being affected by alignment, parting surface is also affected by many factors such as shape, appearance, accuracy, gate position, slider, ejection, and processing of plastic part. A reasonable parting surface is a prerequisite for complete molding of plastic parts. Generally, following aspects should be considered comprehensively:
(1) Comply with basic requirements for demoulding of plastic parts, which means that plastic parts can be taken out of mold. Position of parting surface should be located at the largest projected edge in demoulding direction of plastic parts.
(2) Ensure that plastic part remains on one side of rear mold to facilitate ejection and that traces of ejection pin are not exposed on exterior surface.
(3) Parting line does not affect appearance of plastic parts. Parting surface should try not to damage smooth outer surface of plastic part.
(4) Ensure quality of plastic parts, for example, place parts of plastic parts that require coaxiality on same side of parting surface, etc.
(5) Parting surface should be selected to avoid formation of side holes and side concavities as much as possible. If in-line forming is required, strive to have a simple in-position structure and avoid in-line positioning of front mold.
(6) Reasonably arrange pouring system, especially gate location.
(7) To meet locking requirements of mold, place direction with larger projected area of plastic part in closing direction of front and rear molds, use direction with smaller projected area as lateral parting surface; in addition, when parting surface is a curved surface, bevel locking should be added.
(8) Conducive to mold processing.

All sliders with uphill or downhill slopes must follow this structure when designing slope direction (Figure 1). Structure shown in Figure 2 is strictly prohibited. Only one wear-resistant block at the bottom is allowed in direction of movement, cannot be joined together in the front and back (as shown in Figure 3). Number in width direction can be determined according to actual situation.

 

 

Wear-resistant blocks on back and bottom of slider are 1mm higher than installation body to facilitate mold matching. Matching section between slider and mold core requires a slope of 2 to 3 in movement direction of slider.

 

1. Connection method of slider insert

 

2. Slider limit mechanism

 

 

3. Relationship between core pulling distance of slider

 

Since this set of molds is relatively large, original mold base is used. Considering processing and production of lifter hole, a split lifter is selected, lifter head and lifter base are connected with a round rod. Whether lifter is split or integral, design should be based on principle that lifter can be taken out without removing mold bottom plate.

 

Before designing mold base and accessories, you can first import injection molding machine specifications during production to confirm whether mold base and parts interfere with injection molding machine during design process. If it is found that mold base interferes with injection molding machine, or if changing mold base will involve mold strength issues, you must first ask project manager whether to replace injection molding machine with a larger size for production.
Mold base refers to the overall combination of major mold parts such as fixed and movable mold fixed plates, fixed mold plate, movable mold plate, guide pillars, guide sleeves, support plate pads, push plate and push rod fixed plate. Mold bases are generally standard parts. Domestic mold standard parts manufacturers mainly include: LKM, MINGLEE, Changhui (CFM), etc.; foreign mold standard parts manufacturers mainly include: HASCO (Germany), DME (USA), FUTABA ( Japan).

Guide positioning system is used to ensure precise positioning and relative movement guidance of mold during operation. Including guide posts, guide bushes, tapered positioning blocks, zero-degree positioning blocks and other components. Guide pillars and guide bushes are main components of mold positioning. They achieve accurate positioning of mold through precise cooperation. Tapered positioning blocks and zero-degree positioning blocks are used to realize automatic alignment of mold and improve positioning accuracy.
Guide pillars and guide bushes are guide parts used to accurately position movable mold and fixed mold. If cavity is not accurately positioned when mold is closed, mold cores and other molded parts will be damaged. Functions of guide post and guide bush are: A. Accurate positioning of fixed side and movable side B. Supporting weight of mold C. Protecting mold core.
Design standards for circular guide posts. Diameter of guide posts for general molds is selected according to LKM standard. For diameter of guide posts for non-standard molds, please refer to mold base design reference table. Maximum diameter of guide posts for molds can only be φ80MM.

Ejection system is a system used to eject molded parts from mold, including ejection pin ejection, push plate ejection and other methods. Design and selection of ejection system is determined according to shape, material and size of molded parts to ensure integrity of molded parts and ejection effect.

For flat plastic parts with flanges, if ejection with an ejection plate will adhere to mold, an ejection mechanism should be used. Because push block is an integral part of cavity, it should have higher hardness and lower surface roughness. There are two types of reset: one relies on plastic pressure, and the other uses a reset rod.

Due to structural shape and plastic used, some plastic parts are not suitable to use ejection mechanisms such as ejector pins, ejector tubes, push plates, and push blocks. In this case, molding inserts or concave molds can be used to bring out plastic parts. Push block mentioned earlier belongs to forming insert ejection mechanism.

It refers to combination of several ejection mechanisms mentioned above to achieve purpose of ejection. Commonly used ones include ejector rod plus top plate, ejector tube plus top plate.

Use of air pressure demoulding requires installation of compressed air passages and valves. Processing is relatively simple, is suitable for demoulding light and thin soft plastics.

When plastic part has inner and outer holes or undercuts that are different from mold opening direction, which hinders direct demoulding of plastic part, an inclined slider demoulding mechanism must be used. Parts with side holes or undercuts are made into movable cores.
When plastic part is demoulded, movable core is first pulled out, then plastic part is ejected from mold. Mechanism that completes extraction and reset of movable core is called a core-pulling mechanism.
Purpose of ejection system in plastic molds is to eject products from mold core. Common ejection structures include dome pins, flat tops, cylinders, straight tops, lifters, push plates, push blocks, secondary ejections, etc. Following will introduce to you essentials of ejection system design.

Quality of mold cooling effect will directly affect injection molding cycle and production efficiency, as well as dimensional accuracy, shrinkage effect, product appearance and physical properties of product after molding. If cooling effect is unstable, shrinkage rate will change, and geometric dimensions will fluctuate greatly. Therefore, when designing mold cooling water path, consider being evenly close to product to ensure uniform cooling and prevent defects such as deformation and shrinkage of product.
In order to ensure a good mold waterway, mold workpiece areas that need to be considered include mold core, core, inserts, hot runner hot nozzle (which needs to be cooled separately), etc. For larger or relatively special molds, it is difficult to fully control the heat using only conventional template cooling circuits. In this case, a circulating cooling water structure needs to be installed inside the cavity or core. Application forms of cooling water channels include: conventional mold cooling water channels, cooling water circuit boards, water barriers, water channel jets, using beryllium bronze materials, transporting water outside the core, using cold air cooling, 3D printing conformal water channels, etc.

Exhaust system is used to eliminate gas generated during injection molding process, prevent bubbles and defects from occurring. Exhaust system includes ejector pin exhaust, parting surface exhaust, exhaust needle exhaust and exhaust hole exhaust. Proper design and arrangement of the exhaust system can effectively improve quality and appearance of molded parts.

For molds that form large and medium-sized plastic parts, a large amount of gas needs to be removed, and an exhaust groove should usually be provided, usually on side of die on parting surface. Position of exhaust groove is preferably at the end of melt flow. Size of exhaust groove is based on principle that gas can be discharged smoothly without overflowing. Width of exhaust slot is generally about 3-5mm, depth of front end of exhaust slot is less than 0.05mm, and length is generally 0.7-1.0mm. Depth dimensions of commonly used exhaust slots can be checked in "Plastic Mold Technical Manual".

 

For small molds, parting surface gap can be used for exhaust, but parting surface must be located at the end of melt flow.

For combined concave molds or cavities, gaps between them can be used for exhaust.

Use matching gap between push rod and mold plate or core to exhaust air, or intentionally increase gap between push rod and mold plate.

Powder unsintered alloy is a material made of sintered spherical particle alloy. It has poor strength, but its texture is loose and allows gas to pass through. Placing a piece of such alloy in the area that needs to be vented can meet venting requirements, but diameter of vent hole at the bottom should not be too large to prevent it from being squeezed and deformed by cavity pressure.

A cavity is set outside confluence of plastic melts to allow gas to be discharged into it, and a good exhaust effect can also be obtained.

An exhaust rod is installed in closed gas area. This method has a good exhaust effect but will leave traces of rod on plastic part. Therefore, exhaust rod should be located in a hidden part of plastic part.

Gating system is flow channel for plastic materials in injection mold, which directly affects filling effect and quality of plastic parts. Gating system includes main channels, runner channels, gates, cold wells and other components. Main channel and runner are used to guide molten plastic material to mold filling part. Gate is entrance where plastic material is injected into mold from injection molding machine. Cold slug well is used to collect excess plastic material to prevent defects in molded parts.
For read more, please read Plastic structure design 3 - wall thickness and draft angle.