chapter 10-part 4
Two-Plate Mold Details
A two-plate mold is used for thermoplastic injection molding consisting of two halves.
Two-Plate Mold Features
Cavity: The inner geometry of the mold is slightly oversized to account for shrinkage leading to a part that is slightly larger than the final product.
Fabrication: The mold halves are precisely machined for better fitting.
Distribution Channel: Facilitates the flow of polymer melt from the nozzle into the mold cavity.
Sprue: Connects the nozzle to the mold.
Runners: Direct the polymer flow from the sprue to the cavities.
Gates: Control the flow of plastic entering the cavity.
More Two-Plate Mold Features
Ejection System: Ejector pins, typically located in the moving half of the mold, eject the part once the molding cycle is complete.
Cooling System: Uses an external pump and channels in the mold to circulate water, cooling and solidifying the molded part.
Air Vents: Allow air to escape from the cavity to ensure smooth filling with the polymer melt.
Three-Plate Mold
Utilizes three plates to separate parts from sprue and runners when the mold opens.
Advantages:
As the mold opens, the runner and parts disconnect, dropping separately into designated containers.
Allows for automated operation of the molding machine.
Hot-Runner Mold
Prevents the solidification of sprue and runner by incorporating heaters along the runner channels.
Keeps the material in runner channels molten while the cavity cools, thus saving material that would otherwise be scrap.
Injection Molding Machines
Machines vary based on injection and clamping units.
Types include:
Reciprocating-screw injection molding machine
Plunger-type injection molding machine
Clamping designs include mechanical toggle and hydraulic configurations.
Shrinkage in Thermoplastics
High Thermal Expansion Coefficients: Plastics experience substantial shrinkage during cooling in molds.
Typical Shrinkage Percentages:
Nylon-6,6: 1.5%
High-density Polyethylene: 4.0%
Low-density Polyethylene: 2.0%
Polystyrene: 0.5%
PVC: 0.5%
Compensation for Shrinkage
Mold cavity dimensions (Dc) must exceed the part dimensions (Dp):
where S is the shrinkage value.
Factors Influencing Shrinkage
Fillers: Can reduce shrinkage in polymers.
Injection Pressure: Increased pressure compacts material, reducing shrinkage.
Compaction Time: Extended time allows more material to fill the cavity, reducing shrinkage.
Molding Temperature: Higher temperatures lower material viscosity, enabling better flow and packing, thus reducing shrinkage.
Thermoplastics Foam Injection Molding
Creates parts with a dense outer skin around a lightweight foam core, enhancing stiffness-to-weight ratio.
Two methods of production: injecting gas into molten plastic or mixing gas-producing additives.
Foam expands to fill the mold, with the cold mold surface forming a dense outer skin.
Injection Molding of Thermosets
Modifications to equipment and processes to avoid premature polymer cross-linking, including using shorter barrel lengths and lower temperatures.
Curing within a heated mold is the slowest part of the process.
Reaction Injection Molding (RIM)
Combines two highly reactive liquids in a mold, leading to solidification through chemical reactions.
Originally developed for polyurethane to manufacture automotive components like bumpers.
Compression Molding
Common for thermosetting plastics and rubber parts.
Can use various forms of molding material, including powders or liquids.
Must control the amount of charge for consistency.
Compression Molding Characteristics
Simpler compared to injection molds, lacking a sprue/runner system.
Limited to simpler geometries due to lower flow capabilities.
Mold must be heated to allow curing of the thermosetting polymer.
Compression Molding Materials
Includes thermosetting polymers like phenolics, melamine, and urethanes.
Typical products: electrical components, household items like pot handles, and dinnerware.
Transfer Molding
A charge is heated and pressed into a mold. There are two methods:
Pot transfer molding: Charge injected from a "pot" into the cavity.
Plunger transfer molding: Plunger injects charge from a heated well.
Compression vs. Transfer Molding
Both generate waste material (scrap) known as cull which cannot be reused.
Transfer molding can create more intricate shapes than compression molding.
Suitable for molding with inserts, integrating materials like metal or ceramics during the process.
Blow Molding
Uses air pressure to inflate soft plastic into a mold, ideal for hollow parts like bottles.
High production rates are typically aimed at consumer beverage markets.
Blow Molding Process
Two-step process involves:
Fabricating a starting tube (parison).
Inflating the tube to the final shape.
Extrusion Blow Molding
Involves:
Extruding the parison.
Sealing it for inflation.
Injection Blow Molding
Involves:
Injecting the parison around the blow rod.
Transferring to mold for inflation and final shape.
Stretch Blow Molding
Variation that involves stretching the soft parison to enhance polymer properties, resulting in strong and transparent materials like PET, ideal for carbonated beverages.
Rotational Molding
Gravity in a rotating mold forms hollow parts, providing an alternative to blow molding for larger, complex shapes.
Thermoforming
Involves heating flat thermoplastic sheets or films until they can be deformed into shape using a mold.
Common in the packaging industry and for items like bathtubs and internal refrigerator parts.
A two-plate mold is a fundamental component used in thermoplastic injection molding, consisting of two halves that create a cavity for shaping plastic. This design is favored for its simplicity and effectiveness in producing high-quality molded parts.
Two-Plate Mold Features
Cavity: The inner geometry of the mold is designed to be slightly oversized to account for the shrinkage that occurs during cooling. This intentional oversizing ensures that the final part dimensions are accurate, leading to a part that is slightly larger than the final product. It is critical to design the mold cavity precisely, taking into account the material's specific shrinkage characteristics.
Fabrication: The mold halves are fabricated with high precision, often using CNC machining to ensure a tight fit between the two plates. This precision is vital to avoid defects in the molded part and to ensure proper sealing during the injection process.
Distribution Channel: This component facilitates the flow of molten polymer from the nozzle into the mold cavity, ensuring even distribution and reducing the possibility of incomplete filling or air entrapment.
Sprue: The sprue acts as the conduit connecting the nozzle to the mold, allowing for the injection of the thermoplastic material into the mold cavity. It must be designed carefully to minimize material waste and to optimize flow.
Runners: Runners are pathways that direct the flow of polymer from the sprue to the individual cavities. Design considerations for runners include size, length, and shape to ensure optimal flow and minimal pressure drop during injection.
Gates: Gates are the entry points for the polymer into the cavity, controlling the amount of material that enters. They play a crucial role in filling the cavity and must be designed to allow for quick filling while minimizing surface defects.
More Two-Plate Mold Features
Ejection System: The ejection system, typically using ejector pins, is located in the moving half of the mold. Once the molding cycle is complete, these pins push the part out of the mold, allowing for efficient cycle times and smooth retrieval of the molded component.
Cooling System: An efficient cooling system typically relies on an external pump and dedicated channels within the mold to circulate cooling water. This rapid cooling helps solidify the molded part, reducing cycle time and increasing production efficiency.
Air Vents: Air vents are strategically placed to allow trapped air to escape during the injection process. Their proper design is critical to ensure that the cavity fills completely and to avoid defects such as short shots or surface flaws caused by air entrapment.
Three-Plate Mold
A three-plate mold incorporates an additional plate that helps separate the molded part from the sprue and runners when the mold opens. This design allows for greater efficiency, as parts and runners can drop into separate containers, streamlining the automation of the molding process.
Hot-Runner Mold
Hot-runner molds prevent the solidification of the sprue and runner by integrating heaters along the runner channels. This technology keeps the material in a molten state, improving material utilization by reducing scrap while ensuring consistent flow into the mold cavity, particularly beneficial for shorter cycle times.
Injection Molding Machines
Injection molding machines come in various designs based on their injection mechanics and clamping configurations. Common types include:
Reciprocating-screw injection molding machine: This machine uses a screw to inject the melt into the mold, offering enhanced control over the injection process.
Plunger-type injection molding machine: This design utilizes a plunger mechanism, generally suited for specific materials and processes.
Clamping units can be hydraulic or mechanical toggle, which influence the force applied to keep the mold closed during injection.
Shrinkage in Thermoplastics
Thermoplastics exhibit significant volume change during cooling due to their high thermal expansion coefficients. Typical shrinkage percentages for common thermoplastics include:
Nylon-6,6: 1.5%
High-density Polyethylene: 4.0%
Low-density Polyethylene: 2.0%
Polystyrene: 0.5%
PVC: 0.5%
Compensation for Shrinkage
To counteract shrinkage, mold cavity dimensions (Dc) must be calculated to exceed part dimensions (Dp). The formula is given as:
where S is the shrinkage value. This calculation is essential to ensure that the final product meets the desired specifications.
Factors Influencing Shrinkage
Several factors can significantly influence the amount of shrinkage in molded parts:
Fillers: The addition of fillers can effectively reduce shrinkage in polymer materials by altering their properties.
Injection Pressure: Higher injection pressure compresses the material more tightly, which can reduce the overall shrinkage.
Compaction Time: Extending the compaction time can lead to better filling of the cavity, further minimizing shrinkage.
Molding Temperature: Higher temperatures lower the viscosity of the material, facilitating better flow and packing of the polymer into the mold, which can also contribute to reduced shrinkage.
Thermoplastics Foam Injection Molding
This process is designed to create parts that feature a dense outer skin surrounding a lightweight foam core, significantly enhancing the stiffness-to-weight ratio. It can be achieved by injecting gas into the molten plastic or by using gas-producing additives during the mixing phase. The foam will expand to fill the mold, with the cold mold surface forming a dense outer skin crucial for the performance characteristics of the final part.
Injection Molding of Thermosets
Injection molding of thermosetting plastics requires modifications to standard equipment and processes to prevent premature polymer cross-linking. This includes shorter barrel lengths and lower processing temperatures. Curing occurs within a heated mold, and it represents the slowest segment of the molding cycle, necessitating careful temperature and time management.
Reaction Injection Molding (RIM)
RIM incorporates two highly reactive liquids that combine and solidify within the mold due to chemical reactions. This method was originally developed for polyurethane components used in automotive applications, such as bumpers, due to its exceptional strength and lightweight characteristics.
Compression Molding
Compression molding is typically employed for thermosetting plastics and rubber materials. This method can utilize a variety of material forms, including powders and liquids, but must control the charge amount to ensure consistency in product quality.
Compression Molding Characteristics
This molding technique is simpler than injection molding as it does not require a sprue or runner system. However, it limits part design to simpler geometries due to inadequate flow capabilities. The mold must also be heated to enable proper curing of the thermosetting polymer.
Compression Molding Materials
Common materials include thermosetting polymers like phenolics, melamine, and urethanes, which are widely used in applications such as electrical components, kitchenware, and consumer items.
Transfer Molding
In transfer molding, a charge is heated and pressed into a mold using one of two methods:
Pot transfer molding: In which the charge is injected from a heated “pot.”
Plunger transfer molding: Where a plunger injects the charge from a heated well.
Compression vs. Transfer Molding
Both methods produce waste material known as cull, which cannot be reused. However, transfer molding allows for the creation of intricate shapes, making it more versatile compared to compression molding. It is particularly suitable for integrating additional materials, such as metal or ceramics, into the molded part.
Blow Molding
Blow molding utilizes air pressure to inflate soft plastic into a mold, primarily designed for producing hollow parts such as bottles. This method achieves high production rates, particularly suited for consumer beverage packaging.
Blow Molding Process
The blow molding process typically involves a two-step procedure: 1. Fabricating a starting tube (parison). 2. Inflating the tube to reach the desired final shape.
Extrusion Blow Molding
This method centers around the following key operations: 1. Extruding the parison and 2. Sealing it for inflation into the mold.
Injection Blow Molding
This technique involves: 1. Injecting the parison around the blow rod, followed by 2. Transferring the setup to the mold for inflation, resulting in the final product shape.
Stretch Blow Molding
Stretch blow molding is a variation that involves stretching the soft parison during the inflation process, enhancing the polymer properties, and yielding strong, transparent materials like PET, which are particularly effective for carbonated beverage containers.
Rotational Molding
Utilizing gravity within a rotating mold, this method forms hollow parts and serves as an effective alternative to blow molding, particularly for larger and more complex shapes.
Thermoforming
Thermoforming encompasses heating flat thermoplastic sheets or films until they become pliable enough to be formed into shape using a mold. This technique is widespread in the packaging industry and for producing items such as bathtubs and