The one-time molding of plastic ornaments refers to the technology of making products or semi-finished products of the desired shape and size from various forms of plastic raw materials, such as powder, granules, fibers, and scraps, as well as resin solutions or pastes. There are many molding methods, and injection, extrusion, compression, and casting methods are currently widely used in production.

(1) Injection Molding

Plastic injection molding is carried out in molds divided into convex and concave molds according to the molding direction; each has its strengths. The former produces products with a strong three-dimensional effect but is more difficult to manufacture; the latter is simpler to produce and has a wider range of applications, but the three-dimensional effect is not very strong. There are four types based on material: metal molds, wooden molds, fiberglass molds, and gypsum molds. Each of these four has its advantages: metal molds are highly flexible, have a wide range of applications, have high precision, and have long service life, but they are also expensive. Wooden molds are flexible, have a wide range of applications, and are cost-effective, but their service life is short. Fiberglass molds combine the advantages of the first two types but are difficult to manufacture. Gypsum molds are low-cost, have high precision, and have a decent service life, but they have limitations on material thickness (generally not exceeding 1 mm). Therefore, it is important to choose the right mold based on the product’s manufacturing process and the materials’ performance.

Injection molding is mainly applied to thermoplastic and highly fluid thermosetting plastics, which can form geometrically complex shapes, precise dimensions, and plastic ornaments with various inserts. Currently, injection products account for about 30% of the total amount of plastic products. In recent years, the development and application of new injection technologies, such as reaction injection, two-color injection, and foam injection, have provided a broader application prospect for injection molding.

(2) Extrusion Molding

Extrusion molding, also known as extrusion forming or extrusion molding, first involves adding granular or powdered plastic into the hopper of an extruder (similar to an injection molding machine). The rotating screw of the extruder then transports the material to the heating zone, where it gradually melts into a viscous flow state. Under the action of the extrusion system, the plastic melt is shaped into a continuous profile with the desired cross-sectional shape through an extrusion die (head) with a specific shape.

The basic molding process includes drying materials, molding, shaping, and cooling products, pulling and curling (or cutting) products, and sometimes post-processing.

The internal structure of extruded plastic parts is uniform and dense, with relatively stable and accurate dimensions. Moreover, their geometric shapes are simple, and the cross-sectional shapes remain unchanged, which makes the mold structure simpler and easier to manufacture and maintain while also allowing for continuous forming, high productivity, and low costs; almost all thermoplastic plastics and a small number of thermosetting plastics can be processed by extrusion molding. Plastic extrusion products include pipes, sheets, rods, films, and various special-shaped materials. Currently, about 50% of all thermoplastic products are produced by extrusion molding. In addition, extrusion molding can also be used for coloring, pelletizing, and blending modification of plastics.

(3) Compression Molding

Compression molding refers to a forming technology that primarily relies on external pressure to achieve the shaping of the molded material in a single forming process. Compression molding is the most traditional plastic processing method widely used for thermosetting plastics. Compression molding can be divided into mold pressing and laminating depending on the material’s characteristics and the processing equipment’s features and technology.

Molding is the process of placing powdered, granular, flake, or fibrous thermosetting plastic materials into a mold, then closing the mold and applying heat and pressure to shape and harden the material in the mold, and finally removing the plastic part from the mold. The equipment used includes hydraulic presses, spinning machines, etc.

Laminated molding involves using sheet materials such as paper, cotton fabric, and glass fabric, which are impregnated in resin, then stacked one by one to the desired thickness, placed in a laminating machine, heated, and pressed. After some time, the resin cures and bonds together to form a shape. The equipment for pressing and molding is simple (the main equipment is a hydraulic press), and the process is mature, making it one of the earliest plastic molding methods. It does not require runners and gates, has minimal material loss, has a wide range of product dimensions, and can press larger products. Still, the molding cycle is long, production efficiency is low, and it isn’t easy to achieve modernized production. Products with complex shapes, dense reinforcing ribs, and many metal inserts are difficult to mold.

(4) Casting and Forming

Casting technology includes static casting, centrifugal casting, cast film casting, and rotational molding.

Static casting is a molding method in which liquid resin monomers or prepolymers are injected into a large mold cavity under normal pressure, and products are formed through polymerization and curing. Static casting can produce various profiles and products, with acrylic being a typical cast product.

Centrifugal casting adds raw materials into a high-speed rotating mold, where the centrifugal force causes the materials to fill the mold, harden, and take shape as products. Centrifugal casting can produce large-diameter pipe products and hollow products.

Cast film casting is the process of dissolving thermoplastic plastic in a solvent to prepare a solution of a certain concentration, which is then spread at a certain speed on a continuously rotating substrate (usually a seamless stainless steel strip). By heating, the solvent evaporates, causing the plastic to harden into a film, which is then peeled off from the substrate to obtain the product.

Rotational molding is adding plastic into a mold, continuously rotating along two perpendicular axes while heated. The plastic inside the mold gradually and evenly coats the cavity surface under gravity and heat, melting and adhering to it, forming the desired shape, and obtaining the product after cooling and solidifying.