Basic knowledge of injection molding and plastics

1 Basic knowledge of injection molding.

1.1 Characteristics and composition of injection molding.

Injection molding consists in pouring molten casting material into a closed mold under high pressure. The pressure in injection molding cavity is about 400 kgf/cm2, which is about 400 atmospheres. Products are manufactured under such high pressure. its characteristic, which is its advantage and its disadvantage. That is, mold must be strong enough, so price of mold is also quite high, so it must be produced in large quantities to compensate for high cost of mold. For example, production volume of each batch In other words, there must be more than 10,000 ., injection molding work must be carried out in large quantities.

A few steps in molding process:

1.1. 1 close door

The gate of safety is just beginning to take shape.

1.1.2 Blocking

Move moving plate on moving side to close mold, and fix mold securely after closing it.

1.1.3 Injection (including holding pressure)

The screw advances rapidly to inject molten mold material into cavity to fill and mold. After filling, pressure must be maintained. This action is specifically called "holding pressure". The pressure that a mold can withstand when it is freshly filled is commonly referred to as injection pressure or "primary pressure".

1.1.4 Cooling (and next step plastification project)

The process of waiting for mold material to cool and solidify in mold cavity is called "cooling". At this time, injection device is also ready for next time. This process is called "plasticizing process". The molding material placed in hopper enters heated material tube for heating, and raw material is turned into a molten state according to rotation of screw. The screw is similar to screw pulling principle, and it turns and retreats at same time. The front end of screw will store melt Molding material. Screw In rotation, pressure that resists backing of screw is called "back pressure" of screw.

1.1.5 Open form

Move movable plate to movable side and mold will open accordingly.

1.1.6 Open protective door

The protective door is open, molding machine is in standby mode.

1.1.7 Pickup

Take out finished product, and then make sure that there are no items left in mold before closing door. The entire shaping operation described above is called CYCLIC shaping.

The finished product is formed according to shape of mold. The mold consists of a female mold and a male module. There is a gap between male and female molds, into which material flows and contracts to form a product. Passes before feeding forming maMaterials in male and female forms include main channel (GATE), gutter (RUNNER), gate (GATE), etc.

1.2 Injection Molding Machine

Injection molding machines are characterized by larger elements, which can be divided into two elements: a clamping device and an injection molding device.

1.2.2 Clamping device

The mold closes and does not open, but mold opens after mold material has cooled and solidified in mold cavity, and then finished product is taken out.

1.2.3 The equipment that injects molding material and fills mold cavity with it is called injection device. These two devices are combined into an injection molding machine.

Continue to explain capability of injection molding machine The capability of injection molding machine is mainly determined by following three rules.

Clamping force

During injection, maximum clamping force without opening mold is expressed in tons.

Shot Volume B

The weight of one serving is usually expressed in grams.

C Plasticity

How much resin can be melted in a certain period of time, usually expressed in grams.

The most important is clamping force. The projection area of ​​the molded product refers to projection area of ​​the shadow perpendicular to opening and closing direction of mold (in fact, it can also be called area of ​​the mold). The average pressure in mold added to projected area is called clamping force. If clamping force is greater than "projection area × average pressure" of mold, male and female molds will be pushed back.

Clamping force = projection area × average mold pressure

Generally speaking, pressure that a mold can withstand is 400 kgf/cm2, so clamping force is usually calculated using this value. However, clamping force often depends on shape of mold material and molded article. Large parameters such as PE, PP, PS, ABS raw materials, these materials are used to make boxes with shallow depth, parameter is 300kgf/cm2 if used for deep box, parameter is 400kgf/cm2 if used for small precision is high When design area finished product is about 10cm2 or less, its parameter is 600kg/cm2. PVC, PC, POM, AS and other raw materials are used to make boxes with shallower depth. , parameter is 400kg/cm2. The deep box has parameter 500kg/cm2, and if it is a small and high-precision finished product, when projection area is about 10cm2 or less, parameter is 800kg/cm2. (in form tables)

Average cavity pressure (unit: kgf/cm2)

Finished product


Shallow box

Deep box

Small and complex finished product















Generally speaking, current injection molding machine has a relatively large injection capacity. If clamping force is sufficient, there will be no problems using it. Includes total weight of main rail (SPRUE) and rail (RUNNER). .

Generally speaking, there should be no problem with plasticizing ability, but when weight of molded product is large enough and molding speed is high, if molding time of each cycle is set too short, plasticizing ability will be affected, there will be insufficient phenomena to pay special attention to attention.

The so-called big column is core of injection molding machine to support base plate and fixed plate. The mold can only be installed inside TIE-BAR.

The so-called mold opening stroke refers to distance between movable plate and fixed plate when movable plate of molding machine moves back down. In general, clamping device can be divided into two types: direct pressure type and crank type.< /p >

1. 3 Equipment included

1.3.1 Mold Temperature Controller

The mold temperature controller is used to cool mold and solidify molded product. Basically, mold temperature is temperature of male mold, and it is ideal to control female mold separately. If you want to reduce mold temperature controller water to a very low level. When temperature is eg 5°C, mold temperature controller should be replaced with a freezer.

In addition, ordinary water cannot be used normally when it is above 95 ° C. At this time, mold temperature controller must use special heat-resistant oil, and sometimes mold temperature controller is also used together with electric heating. kernel.

1.3.2 Raw material dryer and material feeder

Usually, injection molding machine is equipped with a drying hopper, and a windmill is installed on it. The drying hopper is used to dry molding material to remove moisture.

For ordinary polystyrene, polypropylene and other molding materials, it is enough to dry them with a drying hopper. However, when it is necessary to mold engineering plastics requiring special drying and high performance plastics, they must be dried with a desiccant. The hot air drying method is not suitable for drying materials.

1.3.3 Grinder (for sprue)

The sprue and chute crusher is mainly used to crush mold sprue and chute so that crushed material can be reused. The gate after crushing is returned to hopper for use. Pay attention not to put too much recycled material, usually within 20%.

1.3.4 Hot key channel controllerher

It can effectively adjust temperature of runner to control finished product, saving raw materials, stable quality, short molding cycle and high efficiency, but expensive and easily damaged.

2. Basic knowledge of plastic

2. 1 Types and applications of plastic

Plastic is made ofIt is extracted from oil and artificially compounded, has high molecular organic matter and has plasticity.

The advantages of plastic: finished product can be formed in one time, high productivity, ability to make any shape, beautiful appearance and light weight of product. Due to above characteristics, plastic products are of good quality, can be mass-produced and are cheap. Plastic has its drawbacks: poor heat resistance, low rigidity and easy brittleness, not oil resistant, easily flammable, especially heat resistant. Compared to metal, plastic has very poor heat resistance.

If we classify plastics into larger items, they can be divided into two categories:

One of them is thermoplastics, which become stronger when heated and become stronger when cooled, which can be recycled and reused.

One of these is thermoset plastic: it changes when heated, becomes solid when cooled, and stays same when reheated, so it cannot be recycled.

Thermoplastic: its molecular structure is shown in table: it is long and linear, moves and softens when heated, and forms a solid object that does not move when cooled.

Thermosetting Plastics: At beginning, short filaments will move, but when heated, they will fuse with each other to form a mesh, becoming an immovable solid object, and cannot return to their original state after cooling.

Currently widely used plastics include thermoplastics and thermosetting plastics, which can be classified in following table.

Classification of plastics


Thermoset plastic








Thermoplastics can be divided into two categories: crystalline plastics and non-crystalline plastics

Crystallinity and non-crystallinity

Crystal plastic

Amorphous plastic

Molecule with high regularity




Regular low-quality molecules




Crystallic plastics have higher molecular arrangement rules.

Amorphous plastics are molecules that form an amorphous structure called non-crystalline plastics.

When forming crystalline plastics and non-crystalline plastics, their shrinkage rate is quite high. Among crystalline plastics, shrinkage ratePE is 2.5%, PP is 1.6% and POM is 1.7%. It can be known that amorphous plastics injection molding products can achieve higher precision than crystalline plastics.

Example of mold shrinkage (%)

Crystal plastic

Amorphous plastic

Shrinkage of polyethylene 2.5%

Polypropylene shrinkage 1.6%

Shrinkage POM 1.7%

Polystyrene shrinkage 0.4%

PVC shrinkage ratio 0.3%

PC Shrinkage: 0.6%

When choosing plastic materials, in addition to accuracy, strength, heat resistance, chemical resistance and weather resistance of finished product, factors such as complexity of molding, total cost, price, etc. are taken into account. Everything must be confirmed and evaluated.

Reference table for plastic selection

1. What is approximate temperature range

2. Will oil (medicine) be attached to finished product?

3. Used outside home?

First, discuss temperature range of use. As a rule, molded products are used at temperatures from -20°C to +65°C. If they are used outside this range, special materials must be selected.

Let's discuss problem of oil and chemicals sticking. As a rule, plastic products are used on condition that they will not stick to oil and chemicals. The raw material cannot be used. In this case, PP, POM, PA and other raw materials should be used.

Let's discuss situation with outdoor use. As a rule, plastic products are mainly used indoors. If they are to be used outdoors, materials with high weather resistance must be used.

Table 9 Typical plastics and their properties





Heat resistance

Oil resistant

Weather resistance


It can be shaped even if it is thin and thick


High rigidity and abrasion resistance


Easy to shape and transparent


Good strength and fluidity


Good strength and transparency

Indicates "Excellent" Indicates "Typical" Indicates "Bad"

Description of above table

PP is cheap, and it can be molded even if meat is thin. The heat resistance is medium, oil resistance is very good, accuracy is low, weather resistance is poor.

POM is characterized by high rigidity, small coefficient of friction, small heat resistance and good oil resistance, poor weather resistance and high price, and accuracy cannot be very accurate.

PS is cheap, high precision, easy to mold, and can meet requirements of transparency, but on other hand, it has poor heat resistance, oil resistance and weather resistance.

ABS plastic ensures high precision. It is characterized by good strength and good physical properties.and, but has an average heat resistance.

PC is very durable, transparent and heat resistant.

Application of various plastic materials

List of representative plastic applications


Plastic buckets, tanks of washing machines, deflectors for automatic machines (auto)


VCR Transmission Oil Shaft Seat


TV front cover fluorescent lamp cover


Interior telephone panel


Camera housing CD case Mobile phone housing

Polypropylene is used in plastic buckets, washing machine sinks and car dividers because of its good chemical resistance and flexibility.

POM is used in VTR gears and oil-soaked shaft seats due to its high strength and high friction.

PS is used as a front cover for TVs and car interiors because of its good looks and durability.

The PC is used as a camera and mobile phone case because of its high precision and durability, and as a CD case because of its good transparency.