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A variety of commercial methods are used to produce thermoplastic products by injection molding. Each has its specific design requirements, as well as limitations.
A variety of commercial methods are used to produce thermoplastic plastic products. Each has its specific design requirements, as well as limitations. Usually part design, size, and shape clearly determine the best process. Occasionally, the part concept lends itself to more than one process. Because product development differs depending upon the process, your design team must decide which process to pursue early in product development. This section briefly explains the common processes used for thermoplastics from Bayer Corporation.Injection Molding, today lots of company buy molding parts from injection molding China companies, if you need injection molded parts for your business, you need really think about this.
The most common processing method for Bayer thermoplastics, injection molding, involves forcing molten plastic into molds at high pressure. The plastic then forms to the shape of the mold as it cools and solidifies. Usually a quick-cycle process, injection molding can produce large quantities of parts, accommodate a wide variety of part sizes, offer excellent part-to-part repeatability, and make parts with relatively tight tolerances. Moulds can produce intricate features and textures, as well as structural and assembly elements such as ribs and bosses. Undercuts and threads usually require mold mechanisms that add to mold cost.
The injection molding process generally requires large order quantities to offset high mold costs. For example, a $50,000 mold producing only 1,000 parts would contribute $50 to the cost of each part. The same mold producing 500,000 parts would contribute only $0.10 to part cost. Additionally, mold modifications for product design changes can be very expensive. Very large parts, such as automotive bumpers and fenders, require large and expensive molds and presses.
In extrusion forming, molten material continuously passes through a die that forms a profile which is sized, cooled, and solidified. It produces continuous, straight profiles, which are cut to length. Most commonly used for sheet, film, and pipe production, extrusion also produces profiles used in applications such as road markers, automotive trim, store-shelf price holders, and window frames. Production rates, measured in linear units, such as feet/minute, ordinarily are reasonably high. Typically inexpensive for simple profiles, extrusion dies usually contribute little to the product cost. Part features such as holes or notches require secondary operations that add to the final cost.
Thermoforming creates shapes from a thermoplastic sheet that has been heated to its softening point. Applied vacuum or pressure draws or pushes the softened sheet over an open mold or form where it is then cooled to the conforming shape. The process of stretching the sheet over the form or mold causes thinning of the wall, especially along the sides of deep-drawn features. Mold or form costs for this low-pressure process are much lower than for injection molds of comparable size.
Thermoforming can produce large parts on relatively inexpensive molds and equipment. Because the plastic is purchased as sheet stock, materials tend to be costly. Material selection is limited to extrusion grades. Secondary operations can play a large role in part cost. Thermoformed parts usually need to be trimmed to remove excess sheet at the part periphery. This process cannot produce features that project from the part surface such as ribs and bosses. Cutouts and holes require secondary machining operations.
Blow molding efficiently produces hollow items such as bottles, containers, and light globes. Design permitting, the process may also produce hollow shapes such as automotive air ducts and gas tanks. Wall thickness can vary throughout the part and may change with processing. Blow molding cannot produce features that project from the surface such as ribs and bosses. Part geometry determines mold and equipment costs, which can range as high as those for injection molding.
The two most common types of blow molding are extrusion and injection. In extrusion blow molding, mold halves pinch the end of a hanging extruded tube — called a parison — until it seals. Air pressure applied into the tube expands the tube and forces it against the walls of the hollow mold. The blown shape then cools as a thin-walled hollow shape. A secondary step removes the vestige at the pinch-off area.
Injection blow molding substitutes a molded shape in place of the extruded parison. Air pressure applied from inside the still-soft molded shape expands the shape into the form of the hollow mold. This process eliminates pinch-off vestige and facilitates molded features on the open end such as screw threads for lids.
In rotomolding, a measured quantity of thermoplastic resin, usually powdered, is placed inside a mold, which is then externally heated. As the mold rotates on two perpendicular axes, the resin coats the heated mold surface. This continues until all the plastic melts to form the walls of the hollow, molded shape. While still rotating, the mold is cooled to solidify the shape.
This process is used for hollow shapes with large open volumes that promote uniform material distribution, including decorative streetlight globes or hollow yard toys. Mold and equipment costs are typically low, and the process is suited to low-production quantities and large parts. Cycle times run very long. Large production runs may require multiple sets of mold,
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