Below molding may be fit for your products

Posted on : November25 , 2023 , By  GREFEE

In addition to injection molding, many plastic products can also be processed by blow molding. For certain products, blow molding has natural advantages of structure and cost.

1. Introduction of below molding

Below modling is a manufacturing technique to make hollow plastic components by inflating the heated platic tubes until the whole mold is filled out and the required shapes has been formed. During this process, the raw material is small or granular thermoplastic, which is first melted and formed into a hollow tube, called a parison. Then clamp the blank between the two halves of the mold and inflate it with pressurized air until it matches the internal shape of the mold cavity. There are various methods for forming billets, as described as below. The typical pressure is 25 to 150 pounds per square inch, much lower than injection molding. Finally, after the parts have been cooled down, separating the two halves of the mold and ejecting he parts.

The parts is produced by below molding is plasitc, hollow and thin wall, like bottles and containers with various shapes and sizes. Small size products may include bottles of water, liquid soap, shampoo, engine oil, and milk, while large containers include plastic buckets, basins, and storage tanks. Blow molded components can be formed from different thermoplastic materials.

2. Common process of below molding

Extrusion below molding

The extrusion machine uses rotating screw to force the melted plastic to pass through the mold head, forming a blank around the below needle. The blank is  crimped vertically in between the two opening molds so they can be closed on  blank and below needles. Inflating the parison with the pressurized air flows through the blowing needle.  This is the most common type of below molding which is used for producing parts that are large volume and simple.

Injection blow molding

The melted plastic is molded around the core in the parison mold to form hollow parison. When the parison mold opens, the parison and core will be moved to below mold and clamped tightly. Then, the core is opened, allowing the pressured air to inflate the parison, which is not often used due to low production rate. However, it can produce more complicated parts and the accuracy is higher. Thus, injection below molding is often the first choice for small and complicated bottles, like the medical care used bottles.

Stretch below molding

The formation of parison is similar to injection below molding. However, once it is moved to below mold, it will be heated and stretched down by the core before inflating. This stretch increases the intensity of plastic. It is often sed to produce parts that for bearing internal pressure and parts with durance, like beverage bottles.

3. Advantages of below molding

A. The Low unit price and mold processing cost of blow molding products: The cost of blow molding machinery is lower, and the product cost of blow molding products is also lower.

B. Below molding can produce products with thin wall and this type of products cannot be produced with injection molding.

C. As below molding grinding tools are only composed of male molds so the thickness of products can be simply adjusted by adjusting the engine mold gap or extrusion standard, which is effective for products that cannot accurate calculate the required wall thickness in advance.

D.The relative molecular weight of below mold plastic (like PE) is much higher than injection molding plastic. Thus, the below molding products have higher  fracture toughness and high resistance to natural environmental stress cracking are suitable for the production, manufacturing, packaging, or transportation of cleaning agents and blow molded small toys, as well as blow molded industrial products.

E. During below molding process, the parison is formed according to engine under low working pressure and is inflated under bottom pressure. Thus, the below molding products has low inner stress and high strain resistance, impact resistance bending and natural environmental characteristics. It has good performance indicators.

4. materials  fit for blow molding

Low density polyethylene (LDPE)

High density polyethylene (HDPE)

Polyethylene terephthalate (PET)

Polypropylene (PP)

Polyvinyl chloride (PVC)

5. Industries and products involved

A.Food industry

In food industry, below molding is very useful as it can produce transparent, durable plastic containers, like beverage bottles, cans, fresh boxes, juice bottles, food bags, etc. In addition, it also can produce some small tools, like spoon, fox, knife, etc. These products are suitable for packaging and shipping food.

B. Medical care

In medical care, below molding can produce various medical equipments and tools, like medical syringes, oxygen inhalation equipment, blood bags, infusion sets, drug bottles, etc. The uniqueness of blow molded products lies in their accuracy and repeatability, which makes them very safe and reliable for in-depth medical operations and treatments.


Below molding can produce different chemical substance containers like packaging bags and industrial pipelines, etc. Especially for highly corrosive chemicals, blow molding can produce high-density plastic containers, making them very suitable for storing and transporting these highly hazardous materials.

D. Entertainment industry

In entertainment industry, below molding can produce all kinds of entertainment equipments like swimming rings, slides, home amusement equipment, park supplies, etc. The price is cheap and more cost effective comparing to traditional material manufacturing that is more heavy and more expensive. It also creates more demands as it is being more and more acceptable for entertainment industries.

E. Automobile

The manufacturing of automobile also needs below molding, such as the interior dashboard, door inner panel, front grille, etc. The main feature of blow molding applications is their strong control over cost, weight, and design freedom during the production process.

6. Difference of injection molding products and below molding products


Below molding can only be used for simple structure products, like seats that cannot be adjusted in any part, and detachable seats. The injection molding process can be used for the production of safety seats with complex structures, such as adjustable headrests, adjustable wings on both sides, adjustable sitting and lying down, and foldable functions.


Blow molded products are hollow and mostly black in color, as black can mask defects such as burrs and surface unevenness caused by the blow molding process. Due to the hollow mouth of the blow molding process, the weight is relatively light. Injection molded products have a smooth surface, better texture than blow molded products, and better flexibility and compression resistance compared to blow molded products. Thus, the cost is high. The final selling price is higher than blow molded products

7. Why choose below molding

A. Low cost

The production is more simple and less machines. There is no need of expensive material and chemicals during production. The cost of below molding is lower than that of injection molding and extrusion molding.

B. High quality

The simple process makes the quality better. In the meantime, the products has low internal strain, so the less likely the parts distorted. This indicates below molding can produce more liable and stable products. 

C. High durability

The addition of low-density polyethylene, polypropylene, polyethylene glycol copolymer and other raw materials during the production process of blow molded products can improve the strength characteristics and wear resistance of the products, which is why compared to other methods such as injection molding and extrusion, blow molding have better durability.

D. environmental protection

Blow molded products are composed of melting plastic particles. Thus, blow molded products are clean, free from any chemical or pollutant emissions. In addition,  the low energy consumption is also more environmentally friendly.  Meanwhile, compared to other production processes, blow molded products are easy to recycle, which can effectively reduce the pollution of plastic waste to the environment.


Below molding makes container with all kinds of forms and designs, like bottles, cups, boxes, etc. At the same time, it also has different colors and textures, which makes the application range of blow molded products more extensive.

In conclusion, blow molded products are cheap, and also have stable quality, good durability, and superior environmental performance. It also produce containers in various designs and shapes. After years of development, it is estimated  to become the most popular production process in the plastic manufacturing industry.


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