What principle is the injection molding in plastic mold processing?


I believe that many friends who are engaged in the mold industry do not know what is going on in the plastic mold injection molding, now to enrich the knowledge of your peers in the mold!

Mold injection: Thermoplastic injection molding is the process of melting plastic material and then injecting it into a membrane cavity.Once the molten plastic is in the mold, it is cooled and moulded into a shape according to the mold cavity.The resulting shape is often the final product and no additional processing is required before installation or use as a final product.Many details, such as protrusions, ribs, and threads, can be molded in a single injection molding operation.

One, the injection mold machine has two basic components:

     Injecting and closing devices for melting and feeding plastics into molds.

     The function of the die closing device is: 1. Make the die close under the condition of bearing the injection pressure; 2.2. Take out the product The injection device melts the plastic before it is injected into the mold, and then controls the pressure and speed to inject the melt into the mold.

     There are two designs available for injection devices: screw preplasticizers or two-stage devices, and reciprocating screws.

     The screw preplasticizer uses a preplasticizer screw (stage 1) to feed the molten plastic into a feed rod (stage 2).The advantages of screw preplasticizers are constant melt mass, high pressure and speed, and precise injection volume control (using mechanical thrust devices at both ends of the piston stroke).These advantages are required for transparent, thin-walled products and high production rates.Disadvantages include uneven residence time (resulting in material degradation), high equipment costs and maintenance costs.

     Reciprocating screw injection devices, the most commonly used, do not require a plunger to melt and inject the plastic.

Two, extrusion blow molding

     Extrusion blow molding is a method of making hollow thermoplastic parts.The widely used blow moulding objects are bottles, barrels, cans, boxes, and all containers for food, beverages, cosmetics, medicines, and daily necessities.Large blow - moulded containers are commonly used for packaging of chemicals, lubricants and bulk materials.Other blow moulds are balls, bellows, and toys.For the automotive industry, fuel tanks, car shock absorbers, seat backs, center brackets, and armrests and headrest claddings are blow moulded.For machinery and furniture manufacturing, blow moulded parts may be casings, door frames, frames, pottery POTS, or boxes with an open side.


     The most common blow moulding material is high-density polyethylene, the polymer from which most milk is made.Other polyolefins are also often processed by blow molding.Styrene polymers, polyvinyl chloride, polyesters, polyurethane, polycarbonate and other thermoplastics can also be used for blow molding, depending on their use.

     Engineered plastics have recently gained widespread acceptance in the automotive industry.Material selection is based on mechanical strength, weather resistance, electrical, optical and other properties.


     At present, plastic mold injection molding processing 3/4 of the blow molding products are manufactured by extrusion blow molding method.The extrusion process is to force the material through a hole or die to make a product.

     Extrusion and blow molding process consists of four steps:

1. Plastic embryo (extrusion of hollow plastic tube);

2. Close the flap closing mold on the mold, clamp the mold and cut off the mold;

3. Blow the mold to the cold wall of the mold cavity, adjust the opening and maintain a certain pressure during cooling, open the mold, and write down the blown parts;

4. Trim the flaps to get the finished product.

Five, polymer mixing equipment

     Polymer mixing is defined as the process of improving the grade of a polymer or polymer system by mixing with melt.

     Mixing processes range from the addition of a single additive to the treatment of multiple additives, polymer alloys and reactive mixtures.It is estimated that one-third of all polymer production in the United States goes through blending.Blended ingredients can be customized according to the performance requirements of the final application.Blend products have hybrid properties, such as high gloss and excellent impact strength, or precision molding and good stiffness.

     The blended polymer is usually granulated for further processing.However, there is growing interest in the industry in combining blending with next steps, such as extrusion of profiles, to avoid reheating the polymer.

Six, mixed

     Various types of melt mixing equipment are used, from roll mills and batch mixers to single - and twin-screw extruders.Continuous compounding is the most commonly used device because it provides consistent quality and reduces operating costs.

There are two types of mixing:

     1. The distributed mixture can be evenly distributed without high shear stress in the remixing ingredients.Such mixtures are referred to as extended or laminar mixtures.

     2. Dispersed mixing, also known as strong mixing, in which high shear stresses are applied to break up cohesive solids.For example, when additive pellets are broken, the actual particle size decreases.Mixing operations often require two types of mixing in the same process.

Seven, rotomolding

     Rolling molding, also known as rotary molding, is a process used to manufacture hollow seamless products of various sizes and shapes.Traditionally, it is mainly used in thermoplastic materials, in recent years, such as crosslinked polyethylene thermosetting materials rolling plastic also developed rapidly.Because roll molding does not require higher injection pressures, higher shear rates or precise compound gauges.Therefore, the mold and the machine are cheaper, and the service life is longer.Its main advantages are: the performance of the machine/price is relatively high;Forming complex parts does not require post-assembly;A variety of products and a variety of colors can be formed at the same time;The cost of the mold is lower;Easy to change colors and materials;Less loss of scrap material.

     The basic process is simple. A powdered or liquid polymer is placed in a mold, heated and rotated simultaneously around two vertical axes, and then cooled.At the beginning of the heating phase, a porous layer is formed on the surface of the mold if the powder material is used, and then gradually melts as the cycle goes on to form a homogeneous layer of uniform thickness. If the liquid material is used, the flow is first and coated on the surface of the mold, and the flow stops completely when the gel point is reached.The molds are then transferred to the cooling area, cooled by forced ventilation or water spray, and placed in the working area, where the molds are opened, the finished parts removed, and the next cycle begins.