Elements of a Mold — From Components to Final Product

Mold tooling is a critical aspect of all plastic injection molding projects, determining the final shape and quality of designed parts or products. However, an injection mold is not a single item that takes the molten plastic flows and solidifies them. Instead, various components of an injection mold perform distinct functions in a compact design structure throughout the process.

This article will discuss the various systems and components and how they impact the overall structure and functionality of mold tooling. Additionally, it will briefly describe possible defects and materials for mold manufacturing to help readers make better decisions.

Let&#;s go through it!

Types of Injection Molds

Before molds and their types, what is injection molding? It is a process of shaping thermoplastic parts by injecting and solidifying the liquid material inside the mold. Cavities in molds mimic the negative geometry of the intended part geometry.

Next, the injection mold has several variations, each with unique production capabilities and assembly structures for included components and systems. The following are some popular types of injection molds;

Family Molds

First, let&#;s understand the single and multi-cavity molds. A single cavity produces a single item in a production cycle, while a multi-cavity produces multiple identical items. Furthermore, the family molds involve multiple cavities with distinct geometries. This means manufacturing multiple designs on a single mold setup, for instance, molding housing, buttons, and internal brackets of a medical diagnostic with the same material.

The complexity of mold design and operation poses the risk of certain defects, such as uneven solidification and dimensional inconsistencies.

Two-Plate Molds

It is a simple form of mold that only includes a moving half and a fixed mold half, both meeting at a parting line. The main characteristic of two plate molds is that the single parting line facilitates the straightforward opening of the injection molding cavity and core to eject the cured part.

If the mold has multiple cavities, the runner and gates remain near this parting line. Manufacturers use these molds for small parts without any intricate features at a low cost. However, high pressures can cause flash, and the simple structure limits the molding design flexibility.

Three-Plate Molds

Three plate molds contain two or more parting lines; the final parts cannot be achieved by simply opening the cavity and core. Instead, the extra runner plate separates the runner and gate so you can separately remove the molded object from the runner.

The addition section or plate does not need the position of the runner and gate near the parting line of the cavity and core, so you can place gates separate from runners. These types of molds are suitable for complex shapes and multi-point gate requirements. However, both injection mold tooling and production costs are relatively expensive.

Stack Molds

The multiple molds combine in precise alignment with a single face to form a stack mold. So, the cavities are double or higher than those of a standard injection mold. While one mold ejects the parts, the other mold is injected, and this cycle works simultaneously; that&#;s how a single cycle doubles the number of parts. Additionally, the shape or size of cavities does not need to be the same on all sides. This is very beneficial when different injection mold components are required for assembly.

The stack molds boost production efficiency and make the high-volume molding more seamless. The advanced machines can automate the injection processes and maintain tight precision.

Unscrewing Molds

The unscrewing molds are mainly popular for making screw surfaces like bottle taps. It involves a threaded core that pushes out the solidified parts through unscrewing cycles. Meanwhile, a rack and pinion mechanism supports the unscrewing of the internal core.  

If you need high volumes of precise threaded parts with similar features on their surface, unscrewing molds is the best option.

Insert Molds

These specially designed molds incorporate metallic inserts inside the injection molding parts. The injected material flows around these inserts and encapsulates upon solidification. Insert molding is mainly popular for inserting threading components and electronic metal connectors in plastic parts.

Manual or automatic mechanisms place and hold the insert inside the mold. In manual holding, it is placed inside the mold with a hand. Pins, slots, or magnetic holders provide the alignment and correct positioning.  On the other hand, robotic systems or feeders execute the insertion automatically before each molding cycle.

Multi-Shot Molds

This tooling is used to produce multi-colored and multi-material parts. The mold involves multiple injectors that can simultaneously inject the molten material into the cavity. Once the first shot takes shape, successive shots are built over it. To facilitate the multi-shot injection, the mold is attached to a rotating, shifting, or core-back mechanism.

The multi-shot molds are suitable for combining thermoset and thermoplastic material in a single part and parts with multiple thermoplastic materials into a single item. For example, they can be used to add grips on thermoset tool handles, tooth bulrushes, seals, gaskets, O-rings, etc.

The Core Components of Injection Molds

Two fundamental sections of any injection mold are Cavity Side A (stationary) and cavity side B (moving). The stationary section defines the outside profiles of the part and forms the cavity to fill the material, while side B moves to the parting line.

Cavity A Side (Stationary Side)

Cavity side A is attached to the stationary plate of the molding machine and does not move during the molding process. It houses the runner system and maintains the precise alignment with the moving side B using guide pins and bushings. Consequently, this side also incorporates the cooling channels to flow the coolants during solidification. 

Cavity B Side (Moving Side)

The B side of the cavity plays a crucial role in opening and closing the mold. Often, it contains an ejector system and an insert holding mechanism. Furthermore, the molding machine&#;s moving platen connects this side and facilitates opening and closing the mold. The movement and alignment of this cavity are essential for precise dimensions and smooth release of final parts.

Components by Function

After the core components, here are the components of an injection mold according to their functionality. Some of them transfer the raw material, guide the opening and closing, and provide cooling. This means a specific set of parts performs some specific functions to achieve the desired results.

Runner System

Let&#;s understand this way: The barrel injects the molten liquid through a nozzle, and some channels are required to transfer the flow from the barrel nozzle to the injection gate, from where the material will enter a cavity. Here, the runner system facilitates this material transformation to the gate. Furthermore, the runner system can have a network of channels for distribution in case of multi-cavity molds.  

A typical runner system parts of an injection mold are;   

• Sprue Bushing:  It is typically a conical or cylindrical channel that transmits molten plastic from the nozzle tip to the intake point of the runner. In a single cavity mold, the sprue directly extends to the gate position.
• Runner Network: It sprue divides the intake material into different cavity gates with a sub-runner&#;s network.
• Gate: The runner network delivers the flow to the gate, a small opening to the mold cavity. The gate can be an edge, pin, fan, or other type.

You might be thinking about pressure and temperature while discussing these runner components of an injection mold. The nozzle itself maintains the high injection pressure. So, the material flows uniformly within the desired viscous level.

Furthermore, runners can be of two types: cold runners and hot runners. The hot runner contains a high-temperature runner with additional heating arrangements, and it maintains the flow temperature to avoid premature solidification. On the other hand, the cold runner just supplies the intake flow without further heating.

Cooling System

The cooling stage takes 50 to 80% of the injection molding process time, so you can imagine how important it is to produce defect-free plastic parts. Essentially, the cooling system is a network of water lines near injection mold components, mainly surrounding the main cavity that shapes the molten feed. Although water is most familiar as a coolant, ethylene glycol or other oils are circulated in high-temperature molding.

A cooling system provides more control over operations as it can regulate and adjust the temperature and flow rate. As a result, proper cooling prevents wrapping, increases production efficiency, and slows down mold wear.

The water circulation is conformal instead of straight lines for the complex and large molds (e.g., core with size 50 mm or more). The following are the parts of an injection mold that fall under the cooling system;

• Baffles: They redirect the coolant to the sub-channels, typically blade-shaped metal strips.
• Bubblers:  These hollow tubes connect the channels placed inside the drilled holes.
• Thermal Pins: They are fluid-filled cylinders that absorb and dissipate heat through continuous circulation.
• External Pump: It provides sufficient pressure for a determined flow rate and maintains the cooling cycles.

Molding Components System 

These are the central components of an injection mold responsible for final geometry, dimensions, alignment, and precision. As the name suggested, they mold(shape) the parts, giving them the details of the cavities&#; surface and internal features. The molding components include a core, cavity, molding rod, lifter, etc.

You can quickly identify these components. Every component that is in contact with feed materials once it enters the cavity from the gate.

Here are the commons of the molding component system;

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• Mold Cavity: It remains stationary with the machine and withstands the injection pressure of the plunger.
• Core: Another movable half interlocks with the cavity during the process, forming the complete internal features.
• Molding Rod: A core pin that creates narrow and elongated features like shafts or holes within the part.
• Lifters: They maintain fixed draft angles for various features to ease the mold closing and opening.

Venting System

The molten flow can bring the air inside the cavity, and the solidification processes produce molding gases. These entrapments can cause voids, bubbles, weak spots, burn marks, and incomplete filling. Therefore, a venting system in injection molds and dies is essential to remove the trapped air and tackle these issues. Additionally, vents help to limit the excessive injection pressure.

In the small and standard molding process, vents are placed in the plating line along with the vent pins on the body of the central cavity. However, the system parts of an injection mold become more complex with the mold complexity.

Some other typical venting systems are;

• Grooves and Channels: Narrow channels or grooves along with pins and venting points at the parting line:
• Air Vacuuming: Removing the air by an external vacuum pump before injection.  
• Venting Valves: The micro-valves in both the internal and external sides of the cavity body.
• Vents Around the Components: Often, vents are placed all over the components associated with the heated flow, such as sprue, runner, and gate.

Guiding System

The guiding system parts ensure the alignment of two mold halves and other components during opening and closing. Thus, their role is crucial in ensuring precision and consistency across each cycle. Subsequently, the clamping forces in repetitive cycles can deviate the position. Hence, the guiding system components like guide pins, bushes, and plates work to avoid this.

Guiding Pins and Bushes: These two components act together to guide the movement of mold halves. Guiding pins are cylindrical extensions attached to one halve that interlock with the counterpart bushes (sleeves) on another halve and maintain alignment.

Ejector System

Once the cooling period is over, the mold opens, and the ejector system facilitates the safe and smooth removal of the parts and runners. Typically, ejector pins are used for this purpose. These thin cylindrical pins are fixed in an ejector plate attached to the moving side. The contact points of the pins are flat plate surfaces, so the force is evenly distributed and does not damage the part.

Other components include;

• Return Pins: These parts provide positioning and stability to mold halves while opening them. They restrict the pushing force of ejector pins at the stationary side.
• Ejector Sleeves: Sleeves are used for removal from cylindrical cavities. A thin sleeve covers the mold surface, and the returning force ejects the part from the mold.

Components by Structure

The structure categorization of injection mold components includes the mold base, core, and various auxiliary parts and systems.

Mold Base

It is the foundation on which all other components of an injection mold are built or set. The mold base is typically made with solid, rigid materials like hardened steel. However, the injection molding term &#;base&#; does not refer to a single piece. Instead, different types of plates are combined into a single plate with various assembly features like drilled holes.

Different plates are sandwiched between the rare and top clamp plates. The rare clamp plate connects the mold with the injection molding machine: mold plate, ejector plate, ejector retention plate, etc., depending on the particular mold characteristic.

Mold Core

A mold core forms the cavities for hollow and internal geometries while combining with the cavity. It provides the structure and withstands some portion of clamping pressure. The shape of the core typically involves round corners and edges with suitable draft angles. When you interlock the core and cavity with correct alignment, they form a void or cavity to intake the molten plastic feed.

After the molding, the core is pulled back, and the ejection system removes the part from the stationary cavity section. Common core pulling mechanisms are mechanical, hydraulic, and pneumatic pull.

Auxiliary Parts

The auxiliary parts refer to supporting items not installed under the mold structure. They are temporarily assembled to ease the function of enclosed injection mold parts. Although auxiliary parts do not have a role in shape and geometry, they are crucial for maintaining tight tolerances, structural integrity, and overall quality of injection molded plastic parts.

• Locating Ring: A circular ring on the movable side that guides the mold while securing it on the machine. It provides the correct positioning for the nozzle tip, sprue busing, and similar parts concerning the mold position.
• Sprue Bushing: A small intermediate channel between the nozzle tip and the runner&#;s intake.
• Ejector Pins: They provide the safe ejection of the final part.
• Material Grabber: A mechanism that holds and guides the plastic pellets into the machine&#;s barrel.
• Support Pillers: The vertical solid structures between the rare clamp and top plate of mold. They provide structural support and distribute the pressure.
• Ejector Plate: A plate in the base that fixes the ejector pins.
• Guide Pins and Bushes: The extended guide pins on one half and bushes on the other interlock to ensure the proper alignment.
• Ejector Retainer Pin: They hold the ejector assembly while ejector pins remove the part.

Auxiliary Systems

Like auxiliary parts, auxiliary systems are the supporting systems for the injection molding process. Typical examples are runner, ejection, and cooling systems, which we discussed earlier in this article.

Auxiliary Setups

Two main auxiliary setups in injection mold are lifting eye bolt holes and KO holes. These setups offer the mechanism for transferring or relocating the mold and assist the ejection procedure.

• Lifting Eye Bolt Holes: These threaded holes secure the eye bolts. Meanwhile, the bolts transfer the large mold using a crane or hoist system.
• KO Hole: The KO hole position is in the rare clamping plate; it incorporates the ejector rods and provides a push-back space for pushing the ejector plate and parts.

Dead Corner Handling Structures

First, dead corners refer to areas or corners that are hard to reach for processing (filling, cooling, etc.). Examples include undercuts, sharp corners, deep channels, etc. Here, structures like angle ejectors, hydraulic cylinders, and slides counter this complexity.

• Slide:  A slide remains on the side where undercuts are present. A slide insert and bolt mechanism support the undercut during solidification and help remove the undercut side without physical damage.
• Hydraulic Cylinder: A cylinder that provides the necessary force to move the slides.
• Angled Ejector: An ejector pin moves at a specific angle to push parts out of the mold from tricky or hard-to-reach areas.

Common Defects and Adjustment Methods for Injection Molds

The complex structure and assembly of mold parts also risk some defects in the final parts. These defects are mainly associated with incorrect alignment, setup, and operation of different components of an injection mold. However, considering the possible defects during designing and processing allows you to make counter-adjustments.

The table below shows the common defects, possible causes, and counter-adjustment methods;

Defect 1: Mold opening, closing, ejection, and reset actions are not smooth

Cause:

• The guide pin and guide bushing in the mold base are not sliding smoothly or are too tight. 
• The slider or ejector pin is not sliding smoothly. 
• The reset spring lacks sufficient force or pre-load.

Solution:

1. Repair or replace the guide pin and guide bushing. 
2. Inspect and repair the fit of the slider and ejector pin. 
3. Increase or replace the spring.

Defect 2: Mold and injection machine mismatch

Cause:

• The position of the locating ring is incorrect, or its size is too large or too small. 
• The width of the mold is too large; the height of the mold is too small. 
• The position or size of the ejection hole is incorrect; the position or size of the forced reset hole is incorrect.

Solution:

1. Replace the locating ring; adjust the size and position of the locating ring. 
2. Use a higher tonnage injection machine; increase the thickness of the mold. 
3. Adjust the position and size of the ejection hole; adjust the position and size of the reset hole.

Defect 3: Difficulty in filling and removing parts

Cause:

• The gating system is obstructed, the cross-sectional size of the runner is too small, the gate layout is unreasonable, and the gate size is small. 
• The mold&#;s limiting stroke is insufficient, the mold&#;s core-pulling stroke is insufficient, and the mold&#;s ejection stroke is insufficient.

Solution:

1. Inspect all segments of the gating system and gates, and repair the relevant parts. 
2. Check whether the limiting, core-pulling, and ejection strokes meet the design requirements, and adjust the strokes that do not meet the requirements.

Defect 4: Mold water channels blocked or leaking

Cause:

• Adjust the clearance appropriately and grind the parting surface of the working parts. 
• Add material locally and improve ventilation Increase the size of the ejector pins and distribute them evenly. 
• Repair burrs, increase the draft angle, and perform nitriding. 
• Adjust the gate, ensure even pressure, and strengthen the product. 
• Rework the machining. 
• Improve the gate and increase the mold temperature.

Solution:

1. Inspect the connection method of the cooling system&#;s water inlet and outlet pipe joints and all segments of the water channel, and repair the relevant parts. 
2. Inspect the sealing ring and water pipe joint, and repair or replace the relevant parts.

Defect 5: Poor part quality (Flash, short shot, ejector marks, drag marks, significant warping, excessive tolerances, visible weld lines)

Cause:

• Excessive clearance in fitting.
• Poor material flow, trapped air.
• Ejector pins are too small, leading to uneven ejection.
• Insufficient draft angle, burrs, insufficient hardness.
• Uneven injection pressure, and insufficient product strength.
• Machining errors.
• The distance from the gate is too far, low mold temperature.

Solution:

1. Adjust the clearance appropriately and grind the parting surface of the working parts. 
2. Add material locally and improve ventilation. 
3. Increase the size of the ejector pins and distribute them evenly. 
4. Repair burrs, increase the draft angle, and perform nitriding. 
5. Adjust the gate, ensure even pressure, and strengthen the product. 
6. Rework the machining. 
7. Improve the gate and increase the mold temperature.

Materials for Producing Injection Molds

Carbon steel, stainless steel, aluminum, titanium, beryllium copper, and other various metals and alloys are key options of materials for producing injection molds. However, ceramic molds are also prevalent for raw materials with high melting points.

The mold material for any specific project or plastic part depends on the desired production volume, type of injecting material, complexity, machinability, and tolerances. For example, stainless steel can withstand up to a million cycles, and aluminum is suitable for a few thousand cycles. Saying all that, the minimum requirement for mold material is that it should have a higher melting point than the injected plastic.

Here is a brief elaboration of common injection mold materials;

Steel

Steel is an evergreen material for the mold manufacturing process with excellent durability. It can withstand up to 5,000 cycles and accommodate ABS, Nylon, PP, PC, Acrylic, and numerous other plastics. Steel A-2, D-2, and M-2 can make the core, cavity, and other components of an injection mold.

Stainless Steel

The composition of additional chromium and carbon enhances the corrosion, wear, and abrasion, resistance. So, stainless steel grades 420, 316-L, 174-PH, etc., make more complex and durable molds. However, the cycle time can be longer due to a low heat dissociation rate.

Tool Steel

Tool steels are cast iron alloys with carbon and other alloying elements. The variation of tool steel alloys and grades allows for machine mold with custom properties. Examples are H-10, H-13, T-15, A6, and M2 tool steels.

Aluminum

Aluminum cannot withstand several batches, but it is famous as a rapid tooling material. This means aluminum injection molds can be prepared at a low cost and short lead time due to the material cost and excellent machinability. Consequently, the high thermal conductivity of and also significantly reduces the cycle time.

Beryllium Copper

This copper alloy is known for exceptional thermal conductivity and corrosion resistance, which makes it a beneficial mold material for high-precision plastic parts. Manufacturers use this metal for hot runners, mold inserts, cores, and other parts.

Conclusion

In addition to the core and cavity, several other systems and components act together to shape the molten material that passes by the nozzle tip of the heated barrel. Runner components transfer the flow to the gate and mold cavity, the cooling system controls the solidification, guiding component signs the mod halves, ejection pins remove the parts from the cavity, and several other in-bilt and auxiliary components execute specialized functions.

The proper material selection, precise manufacturing, cavity finishing, and accurate alignments are essential to make a mold that can fulfill all of the intended specifications. Besides that, the expertise of engineers and operators also influences the final quality.

Our comprehensive Injection Molding Services encompass everything, from mold machining with CNC, EDM, and other methods to diverse surface fishing options for injection molding products. Our expert engineers also help you optimize various parts and systems of your injection mold design. So, upload your drawing file to our online platform today!

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FAQs

What are the four fundamental steps in the injection molding process?

The four fundamental injection molding steps are claiming the mold in the machine, injecting the pallet into a heated barrel and further into the mold cavity, controlled cooling, and ejection. All of these steps have critical roles in the overall success of plastic molding.

How many cycles can an injection mold typically endure?

The production cycle capability of an injection mold depends on factors like mold material, raw plastic type, and processing conditions. For example, a rapid aluminum mold can last for a few thousand cycles, whereas a heat-treated steel alloy mold can withstand up to a million cycles.

What is the standard temperature for an injection mold?

During the injection molding, the melting temperature of plastic pallets ranges between 204°C to 249°C (400 to 480 °F), whereas the mold temperature ranges from 80°C to 90°C (176 to 194 °F).

From which direction should plastic be injected in the molding process?

The plastic should be injected from a direction that allows the material to flow evenly throughout the mold, often through the thickest section first. This ensures proper filling, minimizes air traps, and reduces the risk of defects.

What is the maximum thickness possible for an injection mold?

The maximum thickness for an injection-molded part typically ranges from 4 mm to 6 mm (0.16 to 0.24&#;). However, depending on the material type and part design, it can go up to 10 mm.

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