Maya Ellison
Injection molding is a manufacturing process that involves injecting molten material into a mold. Once the parts are removed from the mold, they may require additional finishing steps, such as deflashing, to remove excess plastic that has seeped beyond the mold parting line. This excess plastic is called flash.
| Characteristics | Values |
|---|---|
| Term | Flash |
| Description | Excess plastic that seeps beyond the mold parting line |
| Removal | Deflashing, which involves manual or mechanical trimming |
| Finishing techniques | Filing, sanding, milling, tumbling, etc. |
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Finishing techniques
The excess plastic from molding is called a "gate" or "flash." This excess material is removed through a finishing process called "deflashing."
- Degating: The process of removing the gates or excess plastic from the molded part.
- Deflashing: After degating, the deflashing process removes any remaining excess material, such as flash, excess compounds, or resin bleed, which can affect the outcome of subsequent finishing steps.
- Cleaning: A thorough cleaning process ensures that the surface of the plastic part is spotless before moving on to the next finishing step.
- Polishing: Diamond buff polishing is a specialist finishing process that can make plastic parts glossier. Manufacturers use a loose abrasive material on a work wheel, which is then applied to the part's surface with minimal aggression to achieve a glossy finish.
- Buffing: Plastics like acrylic and PC can achieve a very smooth finish through buffing. However, it is challenging to apply this finish to materials with high abrasion resistance, such as TPU.
- Sandpaper Grit Finishing: For parts requiring some glossiness, a B-grade finish using sandpaper grit is ideal. This process is compatible with a wide range of injection molding plastics and produces highly aesthetic parts suitable for consumer products.
- Surface Finishing Options: The Society of Plastic Industry (SPI) has established 12 standard plastic surface finish types, divided into four categories, ranging from smooth to rough finishes. The choice of finish depends on the desired aesthetics, functionality, and cost considerations. For example, a high level of roughness may be preferred for mechanical parts to extend their lifespan or reduce costs.
- Overmolding: This technique involves reinserting previously molded parts to allow a new plastic layer to form around them. It is commonly used in the production of one-piece tires and wheels and small, precise parts.
- In-Mold Decoration: This process involves applying a design or color to the molded plastic part. It can be done through ink deposition or an engraving process using a heated die to transfer pigment.
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Deflashing
During cryogenic deflashing, moulded parts are loaded into a parts basket and exposed to a cryogen, such as liquid nitrogen, to cool them to cryogenic temperatures. Once cooled, the parts are tumbled and may be blasted with media pellets ranging in size from 0.006 to 0.080 inches (0.15 to 2.03 mm). The cryogenic temperatures cause the flash to become stiff or brittle and break away cleanly from the moulded product. This method maintains part integrity and critical tolerances.
Other deflashing methods include manual processes such as filing, sanding, milling, and tumbling. These techniques may be used for a range of materials, including metals, glasses, elastomers, confections, and polymers. The choice of deflashing method depends on various factors, including the shape of the product, the material, and the desired level of precision.
Overall, deflashing is a crucial step in ensuring the quality and precision of moulded products, and the choice of deflashing method depends on various factors, including the product's shape, material, and desired tolerances.
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Rotational molding
The process was first applied to plastics in the 1950s but was initially little used due to its slow speed and restriction to a small number of plastics. Over time, improvements in process control and developments with plastic powders have increased its use. Today, it is commonly used to create products such as road cones, marine buoys, car armrests, and children's plastic slides.
The rotational molding process can be broken down into four main phases: filling, heating, cooling, and removing the product from the molds. First, a custom-designed mold is positioned on a rotating apparatus. The mold is then heated while rotating within a chamber, causing the plastic material to melt and uniformly coat the mold's interior. The correct rotation ratio, usually 4:1 for spherical or cubic molds, ensures a uniform wall thickness.
The heated mold is then moved to a cooling chamber, where it continues to rotate while the plastic solidifies into its final form. The mold rotates at all times during both the heating and cooling phases to avoid sagging or deformation. Finally, the part is removed from the mold and may undergo secondary processes such as trimming, deburring, or removal of excess flash to achieve a precise fit and finish.
The most common material used in rotational molding is polyethylene due to its low cost, thermal stability, and structural integrity after cooling. The molds themselves are typically manufactured from stainless steel or aluminum, with aluminum molds being thicker due to the softness of the metal.
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Extrusion molding
In the plastic extrusion molding process, plastic polymer pellets are fed into the main barrel of the extrusion molding machine from a hopper. The extruder then plasticizes this solid material into a homogeneous melt and pressurizes the viscous fluid through the head and die at a uniform rate. The molten polymer is then passed through a die with a predetermined shape and cooled to form a continuum with a similar cross-sectional shape to the mold. The extrusion molding process includes raw material preparation and pretreatment, extrusion molding, cooling, shaping of the extrudate, traction, coiling (cutting), and post-treatment of the product.
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Flash
The process of removing this excess material is called deflashing, which is typically done through manual or mechanical trimming. A poorly designed, worn, or degraded mould is the most common cause of flash. Excessively high mould temperature or injection pressure can also cause flash, as the material may force its way between the plates when the plate clamping force is inadequate.
To prevent flash, it is important to ensure that the mould is well-designed and in good condition, and to avoid excessively high mould temperatures or injection pressures. Additionally, maintaining an adequate plate clamping force is crucial to prevent the material from forcing its way between the plates.
Finishing techniques, such as filing, sanding, milling, and tumbling, can also be used to remove excess material from plastic mouldings. These techniques help achieve a more refined and precise final product.
Overall, flash is an undesirable occurrence in plastic moulding that can impact the quality of the final product. By understanding the causes of flash and implementing preventive measures, manufacturers can minimise the risk of flash and improve the overall quality of their moulded components.
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Frequently asked questions
Excess plastic that seeps beyond the mold parting line is called "flash". Deflashing refers to the removal of this excess material, which can be done through manual or mechanical trimming, filing, sanding, milling, or tumbling.
The process of removing excess plastic is called "deflashing".
Excess plastic from molding, or "flash", needs to be removed because it is unwanted material that can affect the final appearance of the molded part's surface.
