1.1 Overview of injection molding machines and selection of processing materials

Injection molding machines (injection molding machines) are the main molding equipment for making thermoplastic or thermosetting additives into various plastic parts. Injection molding has the characteristics of complex shape, high dimensional accuracy and metal inserts only once. The variety of plastics produced by injection is very extensive, except for most thermoplastic resins such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene and copolymer resins (PS, AB, ABS). In addition to general plastics and engineering plastics such as polyamide (PA), polymethyl (POM), polycarbonate (PC), polysulfone (PSF), polyphenylene oxide (PPO), and polymethyl acrylate (PMMA), There are other copolymerization, blending, reinforcement, filling materials and the like.

According to the requirements of use, various products with different physical, mechanical, dielectric and thermal properties can be produced by injection method, which can be divided into four categories according to the application.

1.2 Selection of injection molding machine

When the customer wants to choose an injection molding machine, the first step is to determine the structural form of the injection molding machine. The structural forms of the major manufacturers of the rubber injection mechanism are not much different, and the structural differences are mainly in the form of clamping.

The advantages and disadvantages of these two types of mode-locked structures are listed in Table 2-2.

Comparison of Hydraulic, Machine Hinge and Molding Devices After selecting the structural form of the machine, the specifications and parameters of the injection molding machine are determined.

a) Determination of the amount of injection The size of the injection volume to be determined first. The injection volume refers to the maximum amount of injection that the injection device can reach when the injection screw is used for the maximum injection stroke under the condition of air injection. This parameter reflects the processing capability of the injection molding machine to some extent.

First calculate the weight of the product: W = product weight + gate system weight type: W required for injection. In the company's product samples, all actual injection volume parameters are calculated using polystyrene as an example, so there are two cases to consider in the actual situation.

When the injection molded product is made of polystyrene (PS), the injection molding machine should have a injection volume of WpsWps = (1.3 ~ 1.1) × (product weight + total weight of the gate system), when the quality requirements of the product are high The coefficient in the above formula should take a large value, and vice versa.

When the product is other plastic (named X plastic) to follow the method, first calculate the theoretical injection quantity of the plastic should be WxWx = (1.3 ~ 1.1) × (product weight + total weight of the gate system) and then The density of this plastic (X) is converted into the actual weight Wps of the PS material. The conversion formula is: Wps=Wx ×1.05/Vx where: Vx?D?D?D-The specific gravity of a certain plastic is compared with the product sample according to the calculation result. Choose the right injection molding machine.

The specific gravity (Vx) of commonly used plastics is now listed in Table 2-3.

Common plastic density (g/cm3)

for example:

An injection molded product is made of polyethylene (PE). It has been calculated that the product itself weighs 185 grams. The estimated sprue system weighs 20 grams. It is calculated by the above formula: Wx=1.2X(185+20)=246g from the manual. Or the proportion of PE material found in the above table is Vx=0.92, so Wps=246*1.05/0.92=280.8 g comparison parameter table, should buy 312 g (11 蛊) injection molding machine of our factory as well.

Note: 1 蛊 = = 28.35 g 1 gram = 0.035 蛊 b b) Clamping force to determine the clamping force (also known as the clamping force) of the city's injection molding machine important parameters, that is, the clamping force applied by the injection molding machine to the mold.

The clamping force, like the amount of injection, reflects to some extent the ability of the machined product to be used as a primary parameter for representing the size of the machine.

According to the vertical projection area of the injection molded product on the template (head plate or two plates), calculate the clamping force P: clamping force = clamping force constant X The projected area of the product is P=kp.S P-clamping force (t) Vertical projection area (cm2) of the S-product in the template Kp-clamping force constant (t/cm2) Kp values are listed in the following table (Table 2-4)

for example:

It is assumed that the projected area of a certain product in the vertical direction of the head plate or the second plate is 410 cm 2 , and the product material is PE, and the required clamping force is calculated.

Calculate the following formula from the above formula: P=Kp.S=0.32X410=131.2 (ton) Compared with the model of the company, we should use our company's 160-ton injection molding machine, ie K160V, to pay attention to: insufficient clamping force, product production The flashing edge may not be formed, and if the clamping force is greater, the system resources are wasted, and the hydraulic system components may work for a long time under high pressure, which may prematurely age and the mechanical structure wears too fast.

Determination of other parameters When purchasing an injection molding machine, in addition to the main consideration of the injection volume and clamping force, the main technical parameters to be considered are: the distance between the connecting column and the connecting column (also called the guiding column) determines the mold. Dimensions, such as the right icon, the inner diameter of the guide post can accommodate the mold is also small. However, when the mold is large in one direction and the other direction is larger than the inner distance of the connecting column, it can also be accommodated, that is, the special structure of the widened door as described later.

For the hydraulic machine, the modulus is the maximum distance between the moving and fixed templates; and for the maximum and the minimum modulus of the machine-type injection molding machine, when the hinge is straightened, when the mold is adjusted to the maximum and minimum, , the spacing between the templates, as shown on the right.

This parameter determines the size of the mold opening space and the depth of the product. If the modulus is large, the deeper the depth of the product, and vice versa, the shallower the depth of the product. The ejection force and the ejection voyage, this parameter makes sense for the removal of the product.

The injection pressure screw applies pressure on the molten material in order to overcome the flow resistance of the melt flowing through the nozzle, the runner, the cavity, and the like. For a certain type of screw of a certain type, the maximum injection pressure is constant.

If the injection pressure setting is too high, the product may have burrs, mold release is difficult, affecting the smoothness of the product, and the product generates a large internal stress. If the injection pressure is too low, the material is likely to be completely dissatisfied with the cavity or even formed.

Generally, low-density, low-density plastics such as polyethylene and polyamide, the injection pressure can be less than or equal to 70-80Mpa: processing medium-viscosity plastics such as modified polystyrene, polycarbonate, etc., shape Generally, products with certain accuracy requirements, injection pressure selection of 100-140Mpa; processing of high-viscosity engineering plastics such as polysulfone, polyphenylene ether, etc., thin-walled long process, uneven thickness and precision requirements of the product, injection pressure About 140-170Mpa; when processing high-quality precision micro-products, the injection pressure can be used above 230-250Mpa.

Table 2-5 lists the range of common plastic injection pressures.

Injection Rate The injection rate is the amount of solution that is ejected from the nozzle per unit time.

The calculated value is the product of the cleansing of the barrel and the speed.

The injection rate is low, the dissolution time of the dissolution material is long, and the product is prone to residual cold joints, uneven density, and high stress. When injecting the product and simultaneously injecting at a high speed, a low-temperature mold can be used to shorten the molding cycle, and high-speed injection can also reduce the required clamping force without forming overfilling.

However, when the injection speed is too high, when the molten material passes through the gate or the like, irregular flow, material scorching, and inhalation of gas and exhaust gas are easily formed, and high-speed injection cannot ensure the stable replacement of the injection and the holding pressure. Overfilling is formed to cause the article to overflow.

The ratio of the screw diameter and the length diameter to the screw length to the screw diameter is the screw length to diameter ratio.

When the diameter of the screw is constant, the large aspect ratio can increase the length of each section of the screw, improve the plasticizing quality, the length to diameter ratio is too large, the length of the screw is long, the screw weight is increased, and the front part of the screw is drooped to cause the screw and the screw. The unevenness of the barrel gap, even when the screw rotates, is scraped with the barrel to reduce its service life.

Increasing the screw speed and increasing the screw speed can greatly improve the production efficiency, but it will also increase the shear heat of the molten material, which may cause the solution to overheat.

At the same time, the increase of the screw rotation should be matched with the hydraulic motor with large displacement hydraulic quantity, and the hydraulic system must also have high requirements, which greatly increases the overall cost.

The hydraulic system pressure is the maximum working pressure that can be generated when the hydraulic pump is not overloaded when the cylinder is working.

When the pressure of the hydraulic system is large, the working pressure of each part of the injection molding machine will produce more force when the external dimensions are constant, but the system pressure is too large, and the requirements for hydraulic valves, pipelines and mail seals are correspondingly improved. It is difficult to manufacture and maintain.

The total energy consumed by the total power injection molding machine is mainly the motor power, the total power of each heating coil and the power consumed by some auxiliary equipment. Dimensions and Machine Weight This parameter has reference significance for the loading and installation of injection molding machines.

The control system's deterministic control system includes computer and hydraulic system. At present, the company mainly has: US Barber-Colman MACO4000 high-end closed-loop computer, Japan CPC2 ordinary computer, Japan MPC-2 computer and Taiwan Hongxun SA60 computer are available.