How to choose materials for blow molding hollow products?

Injection molding process: the mold is more expensive, the product is very cheap, the market is more transparent, mainly suitable for making products with relatively small sizes, the monthly output is very large, the mold and the product belong to the precision is very high, and the general film is used in various fields. Blow Molding Process:

The mold is moderate, mainly suitable for the production of hollow products, packaging and other special products, the monthly output is only a few thousand sets, and the field of use is limited Rotational molding process: the mold is cheap, the product is expensive, the main sum is the production of hollow products, packaging and super-large products, the size shrinkage ratio is relatively large, the constant is very low and so on.

The materials used for blow molding hollow products should meet the following conditions.

Molded products have sufficient working strength, especially when falling at a certain height, the products should not be damaged or cracked.

Forming containers prevent water vapor, oxygen and carbon dioxide from dissipating inwards or outwards.

The product should have corrosion resistance, chemical resistance, antistatic properties, and should also have certain toughness and extrusion resistance.

The materials used in blow molding hollow products are mainly polyethylene and thermoplastic polyester, in addition to polyvinyl chloride, polypropylene, polystyrene, polycarbonate, polyamide, acetate fiber and polyacetal.

1) Polyethylene.

Polyethylene resins suitable for blow molding hollow products require a melt flow rate (mfr) in the range of ( g 1omin.

Low-density polyethylene (LDPE) products have low strength and good toughness, and are mainly used for thin-walled container packaging such as pesticides and daily cosmetics.

High-density polyethylene (HDPE) products have high strength and good rigidity, and are mainly used for blow molding chemical and pharmaceutical containers.

Ultra-high molecular weight polyethylene (HMWHDPE) products have high impact strength and are more suitable for blow molding large hollow containers.

Linear low-density polyethylene (LLDPE) products have the advantages of good rigidity, good impact resistance and not easy to crack, and are used in more expensive and large-capacity containers.

2) Polyvinyl chloride.

Suspension polymerization and bulk polymerization of PVC resins are most suitable for blow molding hollow products. The k-value is generally 57 65 (the k-value has high mechanical performance). According to the application conditions of the products, resins are divided into general-purpose, high-impact resistance, chemical resistance and food packaging.

3) Polypropylene.

Polypropylene resins suitable for blow molding hollow products require a melt flow rate (mfr) in (, load). MFR resins are mostly used for extrusion or extrusion stretch molding.

4) Thermoplastic polyester.

One of the thermoplastic polyester resins suitable for blow molding hollow products is polyethylene terephthalate (PET). The hollow products of this material blow molded have the advantages of barrier, strong anti-penetration ability, high impact resistance, chemical resistance and pressure resistance, good transparency and gloss, etc., and have been widely used in the field of container packaging.

The earliest application of the blow molding process was during World War II, when it was mainly used to produce small plastic bottles of low-density polyethylene. Later, with the birth of high-density polyethylene and the development of blow molding machine molding process, blow molding technology has been widely used. As early as the last century, the blow molding industry reached the scale of billions of dollars in the United States.

The blow molding process can be divided into the following 4 methods according to the working principle of the blow molding machine:

1: Extrusion blow molding process:

Three-quarters of all blow-molded products are currently manufactured by the extrusion blow molding process. Extrusion blow molding is divided into two types: continuous extrusion blow molding and intermittent extrusion blow molding. Continuous extrusion blow molding is suitable for the rapid production of small products, the use of extruder to continuously manufacture the parison, the parison one by one into the pair of molds on a turntable, and then in turn to close the mold, blow, cool, ejection and other operations.

Batch extrusion blow molding is suitable for the production of large blow molded products, and is often used to produce large containers of high molecular weight polyethylene.

2: Injection blow molding process:

Injection blow molding is obtained by injection molding with a parison, and the advantages of injection blow molding are uniform wall thickness, small weight tolerance, less post-processing, and less waste edges. However, the mold cost of injection blow molding is high, so it is suitable for the production of small refined products with large batches, such as: pharmaceutical bottles, cosmetic bottles, etc.

3: Stretch blow molding process:

Stretch blow molding is divided into two ways: injection pull blowing and extrusion blow blowing, injection pull blowing is to use the injection molding method to prepare the parison for stretch blow molding; Extrusion blow blowing is a stretch blow molding of a parison prepared by extrusion method.

4: Multi-layer blow molding process:

Multi-layer blow molding is to make a multi-layer composite parison with several plastics in an appropriate method, and then mold it with the general blow molding process. Layer blow molding is the introduction of one or several layers of polymer with low permeability to improve the solvent resistance or air permeability of the product.

The above 4 blow molding processes are the most commonly used methods, and the plastic raw materials commonly used in blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc.

Multi-layer blow molding, pressing blow molding, dipping blow molding, foam blow molding, three-dimensional blow molding.

One. The viscoelastic principle of hollow molding is described in mathematical description textual description 1. Mathematical description of polymers due to their long chain structure and the flexibility of molecular chains.

In general, when the polymer is placed at a certain temperature, the morphology of the macromolecule undergoes a series of intermediate states to transition to the equilibrium state adapted to the external force is a relaxation process, and the change of deformation with time is as follows: (1) Above the glass transition temperature of the polymer, the proportion of the general elastic deformation in the total deformation is very small and negligible. So (2) where t* is the relaxation time, which decreases with increasing temperature.

It is viscous and decreases with increasing temperature. Equation (2) can also be written as (3) 2 = high elastic deformation when δe2 is t = viscous deformation when t = 1, that is, viscous deformation per unit time Because the relaxation time of the polymer is very short near the high elastic state or viscous current temperature, if the deformation time t1 in molding is >> t*, then it can be obtained from equation (3), when the external force action time t=t1: after the release of the external force, the high elastic deformation recovers (4) If the temperature is reduced below tg (or crystallization temperature) before the recovery of the high elastic deformation and then the external force is released, then the relaxation time t* then the relaxation time t* becomes that the highly elastic deformation part does not recover, the viscous deformation is irreversible, and the release of the external force does not recover.

Two. The principle description uses the temperature dependence of the polymer to delay the relaxation time of high elastic deformation, and makes the polymer semi-finished products (tubes, hollow profiles, etc.) quickly deform near TF above the glass transition temperature of the polymer, and then maintains the deformation, and cools to below the glass transition temperature or crystallization temperature in a short period of time, so that the deformation of the molded product is frozen, which is the viscoelastic principle of hollow molding. Deduction.

1. Extrusion blow molding.

The extrusion blow molding process is to extrude the parison, when the parison reaches a predetermined length, clamp the parison and position it, and then close it, and shape or size the head of the parison. Compressed air is introduced into the parison for blowing, and the key is made close to the mold cavity to form a product, and the product is opened and removed after the product is cooled and shaped in the mold, and the product is trimmed and finished.

2. Injection blow molding.

The injection blow molding process is more suitable for the production of some large batches, but small containers, cosmetics, daily necessities, medicine and food packaging in the daily production, all of which are produced by the injection blow molding method. The wall thickness of the product is uniform, there is no suture line, waste edges, and less material.

3. Stretch blow molding.

Stretch blow molding is a process method in which parison, stretching and blow molding are completed in one machine, parison is a semi-finished product in the production process, after cooling, the cold parison is reheated for stretching and blow molding. After extrusion, directional stretch blow molding, maintain a certain stretching speed, orient before blow molding, after stretching, transparency, impact strength, hardness and rigidity, surface gloss and barrier properties are all highly improved.

One. Blow molding Blow molding is to blow and deform the hollow plastic parison in a highly elastic or plastic state with the help of compressed air, and then cool and shape it to obtain plastic products. According to the type of blow molding products, this processing method can be divided into hollow blow molding and film blow molding.

1 Hollow blow molding In hollow blow molding, it is necessary to place a hollow plastic parison in a highly elastic state or in a plastic state in a closed blow molding mold cavity, and then introduce compressed air into it to force its surface area to expand (blow) and adhere to the surface wall of the mold cavity, and finally cool and shape to obtain hollow plastic products of a certain shape and size. Suitable for a wide range of plastic containers in bottle and cylindrical shapes. 2 Film blow molding Film blow molding is to use an extruder to form the molten plastic into a thin-walled tube blank, blow compressed air from the center of the machine head to the tube blank, force the tube blank to be blown and deformed at high temperature and transformed into a tubular film (bubble tube), the bubble tube moves to the roller under the action of traction force and is pressed and stacked, and then through the traction roller, the guide roller reaches the take-up roller, and the winding roller becomes a film product through the take-up roller, which is suitable for various packaging plastic bags.

Two. Blister molding Blister molding is also called vacuum molding, which belongs to the scope of plate and sheet molding. It is a processing method that uses a vacuum pump to extract the air in the closed cavity formed by the plastic plate and the mold surface, and then uses the atmospheric pressure to make the plate plastically expand and deform (the surface area increases, the thickness decreases), and the mold surface is transformed into plastic products.

Blister molding is mainly used for the packaging of products, as well as some parts of daily utensils, instrument shells, toys and household appliances.

(1) The forming mold is generally composed of two half-piece molds. The opening and closing mold can be moved horizontally or vertically; It can also be designed to use the hinge as the fulcrum, and the two halves of the die are opened and closed along the arc line with the fulcrum as the axis. This is a mold structure that is more widely used in small and medium-sized products.

For hollow products with concave or space between planes at the bottom of the product, the forming mold should be designed as three or four openings, which is for the smooth demoulding of the product. (2) The end of the forming mold should be equipped with a clamping blank nozzle. From here, the excess material of the product is cut off and the billet tube before blowing is clamped.

45°。(3) Mold.

The hollow blow molding process has the characteristics of simple molding, high production efficiency, low product cost and easy operation. Packaging containers range from eye drop bottles with a capacity of a few milliliters to storage and transport containers with a capacity of several thousand liters. The plastics used for blow molding are:

Polyethylene, polyvinyl chloride, polypropylene, polystyrene, thermoplastic polyester, polyamide, cellulose acetate, etc., among which polyethylene is the most widely used.

The extrusion blow molding mold is generally composed of two halves of the mold with a vertical parting surface, which is installed on the clamping base, and the mold is equipped with an air inlet rod or an air intake needle, and the extruder extrudes the molten parison, and then blows the mold and clamps the mold, clamps the parison, and blows at the same time, and releases the air in the parison after pressure cooling and shaping, and opens the mold to take out the plastic parts. This molding method is characterized by simple equipment and mold structure and low cost.

Vacuum forming is to fix the thermoplastic blank (usually sheet or plate) on the mold, so that it is heated to a softened state, the vacuum pump is used to pump out the air between the blank and the mold, with the help of atmospheric pressure to make the blank close to the mold forming surface and become a plastic part, after the plastic part is fully cooled, and then the plastic part is blown out with compressed air.

The advantage of vacuum forming is that there is no need for a whole set of molds, only a single punch or die is used, and the mold structure is simple and the manufacturing cost is simple.

Low cost, the structure of the plastic parts is clear, bright and detailed, and it can form large, thin-walled, and shallow plastic parts, and the plastic parts can also be observed.

Molding process. However, it is easy to make the wall thickness of the plastic parts uneven, especially the convex and concave parts on the mold, and the wall thickness of the corresponding parts of the plastic parts is the largest.

Thin; When the corners of the plastic part structure are acute angles, wrinkles are easy to occur on the molded plastic parts; Stretching and cold are common on the sides of the part.

but striped; The differential pressure that can be generated by vacuum forming is very small (usually 0.)., so it cannot be formed into thick walls.

and deep-cavity plastic parts.

There are many forms of air intake for hollow blow molding, and there are three types of blowing method: needle blowing, top blowing and bottom blowing. As for which one to use, it can be selected according to the requirements of equipment conditions, finished product size and wall thickness distribution. However, regardless of the form used, the principle should be that the compressed air does not contain oil and water droplets, and its pressure is sufficient to blow the parison to obtain a product with a clear contour and a clear letter pattern.

Using the temperature dependence of the polymer to delay the relaxation time of high elastic deformation, the semi-finished polymer products (tubes, hollow profiles, etc.) are rapidly deformed near the TF above the glass transition temperature of the polymer, and then the deformation is maintained, and the deformation is cooled below the glass transition temperature or crystallization temperature in a short period of time, so that the deformation of the molded product is frozen, which is the viscoelastic principle of hollow molding.

Hollow blow molding has high rigidity and long service life, and the service life of the general blow molding tray can reach 5 10 years, and it can work stably in the environment of -40 40. The dynamic load of its products can reach more than 3t, and the bending strength has reached a high level. It is the first choice for some pallets that require a long life and high-intensity use environment, and its current ** is higher than other plastic pallets, because its service life is particularly long, so its cost performance is the highest among all pallets.

This method will be developed rapidly in recent years with the significant reduction of molding equipment. It will have great advantages for saving limited oil resources and saving logistics costs.

At present, domestic hollow blow molding machine manufacturers have developed a super-large hollow molding machine specializing in high-speed production of high-strength blow molding trays, which is produced by HMWHDPE high-strength plastic, with fast molding speed, high-quality products, and high strength. Hollow blow molding is the most commonly used plastic processing method for the application of thin-shell hollow products, chemicals and packaging.