Injection molding is a messy process, the whole process cycle is often one to a few minutes, the more critical filling process is usually only a few seconds. In this very short process cycle, plastic has experienced a very messy preheating and rheological history.

The quality of injection parts is closely related to the process conditions (temperature, injection pressure, forming cycle, etc.). In this paper, the selection of injection machine, the preparation before injection (material pretreatment, barrel cleaning, preheating of inserts, application of release agent), and the control of injection process parameters (temperature, injection pressure, forming cycle) are discussed.

1. Selection of Injection Machine

According to some technical requirements of the plastic products to be produced (such as material varieties, trademarks, product components and shape specifications etc.), the function parameters of injection moulding machine in the injection formulation are found. The key data to be controlled in the specification type is the component (or volume) of the product and the share relationship between the shape specification and the parameter value.

That is, the share requirement between the component of plastic products and theoretical injection quantity (or volume) of injection moulding machine, the sum of product length (height) specification and the thickness of moulding mould (thickness of mould on moving template), and the standard requirement condition between the distance of moving template of injection moulding machine.

1.1 Selection of theoretical injection quantity of injection moulding machine according to product component

Components of products refer to the amount of melt required for injection moulding products. The order of accounting methods is as follows.

1) Calculating the total component of fused materials for products

Q = K * (Product Component + Gate System Component)

The K-coefficient in the formula is 1.1-1.3.

2) Calculate the amount of melt that should be injected when injection moulding machine is used for injection moulding of the product.

Qs=Q_1.05 Injection Molding Process

R-product density. RR

The theoretical injection quantity of injection moulding machine for injection moulding of this product should be slightly greater than the Qs value.

1.2 Selection of injection moulding machine according to the clamping force for product moulding

The clamping force of injection moulding machine (also called clamping force) refers to the larger clamping force that is used to form a product cavity with two (or more) die joints in the clamping equipment. When the molten material enters the cavity of the die with relative injection force and flow rate, it has the function of closing force, so that the forming die will not be expanded by the injection force of the molten material. In the standard type specification of injection moulding machine (GB/T12783-1991), the molecular value is the theoretical injection quantity (g or cm3) of injection moulding machine, and the denominator value is the clamping force (t).

The calculation of the clamping force for injection moulding is rather messy. It is related to the injection pressure of the melt, the viscosity of the melt, the plasticizing condition of the material, the shape of the product, the structure of the mould and the cooling setting temperature of the product in the mould. Therefore, it is difficult to calculate a more accurate clamping force. The method of clamping force for injection moulding of accounting products is as follows:

Mould closing force = constant for calculating die closing force * product vertical projection area on the template

2. Preparations before injection moulding

In order to make the injection process go smoothly and ensure product quality, there are a series of preparatory operations before forming, such as raw material pretreatment, insert preheating, barrel cleaning, selection of release agent, etc. Because of the different supply methods (powder or granule) and properties of plastics, the structure (with or without inserts) and application requirements of the parts are different, and the preparatory work of various products before forming is also different, which should depend on the specific situation.

2.1 Pretreatment of Raw Materials

According to the characteristics of various plastics, the raw materials should be pretreated as follows before forming.

1) The inspection of raw materials includes three aspects:

First, whether the raw materials used are correct or not, and whether they can meet the application requirements of products (varieties, specifications, trademarks, etc.);

Second, the appearance inspection (color, particle shape and uniformity, impurities, etc.);

Third, physical function check (including melt activity rate, activity, thermal stability, water content policy and shortening rate, etc.).

2) Granulation and dyeing of raw materials

If the raw material is powder, granulation is needed before forming; if the product requires a certain color, the material should be dyed. The common method of colouring injection moulding parts is to select colour masterbatch, which is to mix raw material particles and colour masterbatch evenly according to the relative share and directly participate in the hopper of injection moulding machine.

This method is simple, useful and uniform in colouring, but the cost is high. It is only suitable for the forming of screw injection moulding machine. If the plunger injection moulding machine is used, the product will be stained or stained because of the uneven plasticization and mixing. As for the injection moulding of powder materials, granulation dyeing is generally used, that is, the powder and color masterbatch are extruded and granulated to obtain uniform color granules.

3) Preheating and monotony of raw materials

Various plastic particles often contain different degrees of water, solvents and other evaporable low molecular substances. Their existence often causes defects such as craze, shortened holes, bubbles and so on in injection moulded products. At high temperatures, they also cause cross-linking and degradation of plastics, and the function and appearance quality of the products are reduced. Therefore, most plastics need to be preheated and monotonously treated before forming.

For example, plastics such as polyamide, polycarbonate, polysulfone, polymethyl methacrylate, polyphenyl ether and so on, which contain hydrophilic groups such as acyl amino, ester group and nitrile group in their molecular structure, have a tendency to absorb moisture, are generally treated monotonously. Polystyrene and ABS plastics, although hydrophilic ability is not strong, but generally also in drying treatment. For polyolefin, polystyrene, polyvinyl chloride, polyformaldehyde and other plastics which are not easy to absorb moisture, it is assumed that the packaging is tight and the storage and transportation are superb, and generally not monotonous.

Monotonous management