Injection Molding: The Complete Guide to Precision Plastic Manufacturing

What is injection molding?

• injecting molten plastic material into a mold cavity,
• allowing it to cool and solidify, and then
• ejecting the finished part from the mold.

Figure 1: Schematic Representation of Injection Molding Machine (Source: Wikipedia)

Polymers processed using injection molding

What are the types of plastic injection molding?

• part design,
• material properties,
• production volume, and
• cost considerations.

List of injection molding types

What are the types of feeding systems in injection molding?

Hot runner injection mold feeding system

• Advantages: It produces less waste. It eliminates any additional processes to remove or recycle the runners. When combined with multi-cavity molds, it helps with large volumes of complex and fine shapes.
• Disadvantages: It is expensive and difficult to ensure that it is clean. It is challenging to work with heat-sensitive materials.

Cold runner injection mold feeding system

• Advantages: It is easier to clean, maintain and work with an extensive range of materials.
• Disadvantages: It produces lots of waste for each cycle.

Insulated runner mold feeding system

• Advantages: It is less expensive. This is because a temperature controller is not required besides easy and faster changing of materials and colors.
• Disadvantages: It is not suitable for demanding engineering-grade plastics.

What is the role of nozzles in injection molding?

• type of material being processed,
• part design,
• molding parameters, and
• specific requirements of the injection molding process.

Different types of nozzles

1. Open nozzle: An open nozzle is the most basic and commonly used type of nozzle. It consists of a simple channel that connects the barrel to the mold cavity. Open nozzles are typically used for general-purpose applications and materials with low viscosity.


2. Shut-off nozzle: It is designed to provide better control over the material flow. It prevents drooling or stringing. They incorporate a valve or shut-off mechanism at the nozzle tip that closes when the injection process is complete. This leads to sealing off the flow of molten plastic. The shut-off nozzle helps to reduce part defects and material waste.


3. Valve gate nozzle: It is also known as hot runner nozzle. It is used in hot runner systems. These systems keep the molten plastic at a consistent temperature throughout the injection molding process. This eliminates the need for the plastic to solidify and re-melt each cycle. They have a valve pin that controls the opening and closing of the nozzle. This precisely controls the flow of plastic into the mold cavity.


4. Multi-tip nozzle: It consists of multiple nozzle tips. These are arranged in a single assembly. They are used for multi-cavity molds where several identical parts are produced simultaneously. Multi-tip nozzles allow the efficient filling of multiple cavities. This is done with a single injection molding machine which reduces cycle time and increases productivity.


5. Mix-head nozzle: It is used for injection molding processes that involve mixing multiple components or colors. These nozzles have separate channels for each component. Here the materials are mixed within the nozzle before being injected into the mold. They are commonly used in applications like multi-color parts or parts with special effects.

What are the types of screws used in injection molding?

• type of plastic material being processed,
• part design requirements, and
• desired melt quality and
• molding parameters (injection speed, cycle time, etc.).

1. Standard general-purpose screw: This is the most common type of screw used in injection molding machines. It has a gradual taper and a constant pitch along its length. They are suitable for processing a wide range of thermoplastics with average melt properties.


2. Barrier screw: This is designed to improve melt homogeneity and mixing. This is particularly for heat-sensitive materials. They feature a section with a barrier flight. This creates a barrier between the feed and melt sections of the screw. Thus, enhancing melt quality and reducing melt temperature.


3. Mixing screw: It is used for materials that need enhanced mixing. For e.g., color concentrates, additives, or materials with different melt viscosities. They incorporate specific flight designs. For e.g., mixing elements or distributive mixing sections. This helps to achieve better dispersion and material blending.


4. Ball check valve screw: It features a valve at the tip of the screw. This prevents the backflow of the molten plastic during the injection phase. This design helps maintain precise control over the injection volume. Thus, eliminates issues like drooling or stringing.


5. High-speed screw: It is designed for fast cycling and high injection rates. They have shorter flight depths and increased feed channels. This helps in quicker melting and higher plasticizing rates.


6. Low compression screw: It is used for processing materials with low melt flow properties or shear-sensitive materials. These screws feature a larger feed channel and a longer transition section. This is to minimize shear and improve melt quality.


7. Venting screw: It is designed to evacuate trapped air or gases from the mold cavity during injection. It features grooves or channels along the screw flights. This allows the air to escape and reduces the chances of air entrapment in the molded part.

What are the types of mold cavities & plates used in injection molding?

Types of mold cavities

• Advantages: It is less expensive and more affordable for low-volume productions. This is because tooling cost for this mold type is lower than other options. It also enables better control of the molding process.
• Disadvantages: It leads to slow production.

• Advantages: It is suitable for large-volume productions. The production speed is faster and the cost for a unit part is lower.
• Disadvantages: Initial cost for the injection mold tooling is usually higher than single cavity molds.

• Advantages: It usually reduces the overall production cost. It can be used to make several parts in one cycle and save a lot of time and operation cost. This is because one family mold can be useful for various components.
• Disadvantages: It is expensive and a simple multi-cavity mold can only produce one iteration in one cycle. It is only suitable for components made from the same material and color.

Types of mold plate systems

1. Two-plate injection mold

2. Three-plate injection mold

3. Stack injection mold

4. Unscrewing injection mold

What is a mold and how is it manufactured?

1. Mold is produced often in collaboration with mold producers and customers. This ensures that the molds meet their specific requirements in terms of:
   • design,
   • fabrication, and
   • production of injection molds.


2. Computer-aided mold design is used as a first step in mold manufacturing to create 2D and 3D designs of molds. This is based on the customer's requirements including:
   • part geometry,
   • material characteristics,
   • cooling requirements,
   • ejection mechanisms, and
   • mold lifespan.


3. Once the mold design is finalized, the mold maker proceeds with fabricating the mold. The mold maker ensures that the mold does not have air traps, warpage, or part defects. Mold also needs to be maintained and repaired throughout its lifespan by regular cleaning, lubrication, and inspection. They are conducted to prevent wear and damage.

Figure 2: Mold for Injection Molding with Different Components (Source: East West)

What are the types of clamping used in injection molding?

• hydraulic clamping and
• toggle clamping.

• size and weight of the mold,
• desired clamping force,
• cycle time, and
• machine specifications.

What are the steps of injection molding process?

Figure 3: A Step-wise Injection Molding Process

1. Filling/melting stage: The plastic pellets are converted into a molten state. This is done by applying appropriate heat and shear. Thus, the molten plastic can be used to mold.


2. Injecting/packing stage: The screw pushes the molten plastic into the mold cavity. This completely packs the molten plastic with adequate holding pressure. Part size and cavitation will determine the injection molding press size. This is required to pack the plastic melt. The injection molding process normally operates at pressures up to 200 MPa. The “high pressure” injection molding machines, operating at 500 MPa, are currently under investigation. Injection molding machines are classified according to the locking force of the platens. It can vary from 1–3000 tons.


3. Cooling stage: Mold is cooled to solidify the molten plastic before being ejected. This is highly important as the cooling phase is the most time-consuming stage of the injection molding cycle. This is due to plastics' insulating properties. When the plastic cools and solidifies, it shrinks (referred to as "mold shrinkage"). Shrinkage is generally between 0.4-2% and must be taken into account by the mold design.


4. Ejection stage: Once molten plastic is cooled in the mold, the mold opens, and the plastic part is ejected by the mold's built-in ejector pins. The mold will then reclose to repeat the process.


5. Post-ejection stage: It is the last stage where the machine operator has to break off the sprue, runner, or gate from the molded part. This is done by twisting or cutting it off manually, depending on the mold design. A hot runner system actually eliminates runners and sprue, which also eliminates waste. Unfortunately, hot runners will make the tool more expensive.

Variables of the injection molding process

• Melt temperature: Increased melt temperature can increase the melt flow but increasing too high melt temperature could cause degradation.
• Moisture content: Polymer drying temperature and time are important key factors for good products in our production.
• Mold temperature: Uniform mold temperature reduces the effects of warpage.
• Residence time: A longer residence time could degrade the polymer.
• Injection speed: Injection speed affects the viscosity of the molten material.
• Injection pressure, pack pressure, and clamp tonnage can also play important factors in successful injection molding of parts.

What is injection blow molding?

• injection molding and
• blow molding.

Figure 4: Injection Blow Molding Machine (Source: ScienceDirect)

Steps of injection blow molding process

• The process involves injecting molten plastic into a mold cavity to form a preform.
• The preform is a hollow tube-shaped object that resembles the final product but is smaller in size.
• The ejected preform is then transferred to a blow molding station. Here it is inflated to the desired shape of the mold cavity. This is done manually or automatically using robotic systems.
• The blow-molded part may require further trimming or secondary operations depending on the specific application.

Applications of injection blow molding

• It ensures high production efficiency.
• It has precise control over wall thickness.
• It offers excellent repeatability.
• It has the ability to produce complex shapes and intricate details.
• It is commonly used in the packaging industry.
• It is used to manufacture bottles, containers, and other hollow plastic objects.
• The process is particularly suitable for:
  
  • small to medium-sized production runs and
  • products that require tight dimensional tolerances and consistent quality.

What are the possible solutions to common injection molding defects?

Automatic or manual injection molding - Which one to choose?

• production volume,
• part complexity,
• required precision,
• desired automation,
• initial investment budget, and
• available skilled labor.

What are the advantages and disadvantages of injection molding process?

References

1. Rosato, D. V.; Rosato, D. V.; Rosato, M. G.; Injection Molding Handbook (3rd Ed.), Kluwer Academic Publishers, 2000
2. Johannaber, F.; Injection Molding Machines – A User’s Guide, (4th Ed.), Hanser Verlag, 2008
3. Bryce, D. M.; Plastic Injection Molding – Manufacturing process fundamentals, Society of Manufacturing Engineers, 1996
4. Osswald, T. A.; Turnig, L.; Gramann, P. J.; Injection Molding Handbook, Hanser Verlag, 2008
5. Potsch, G.; Michaeli, W.; Injection Molding An Introduction, (2nd Ed.), Hanser Verlag, 2008