Polymers Used in Plastic Moulding

First, we have to understand that what is Polymer. It is a class of molecules with large numbers of repeating structural units connected through covalent bonds. So how we know about different polymers? It is differed by repetition of many similar, identical or complementary subunits.

Polyethylene is the first choice for plastic moulding process and the reason is its availability, ease of use and also it has many suitable properties. There is 80-90 percent of polymers used in plastic moulding industry are polyethylene compounds (HDPE, LDPE and LLDPE). And also, PVC, nylons and polypropylene compounds are used.

Polyethene is widely used thermoplastic created through polymerization of ethylene.

It is also known as Polyethene, Polythene, PE, LDPE, HDPE, MDPE, LLDPE.

  • LDPE (Low-Density Polyethylene)

LDPE has a density range of 0.910 – 0.940 g/cm3. LDPE has a high degree of short and long chain branching means the chains do not pack into crystal structures. Due to this, it has less intermolecular forces and the instantaneous-dipole induced-dipole attraction is less. Which means it has lower tensile strength and increased ductility. The LDPE is created by free radical polymerization. LDPE gets its to flow properties from the high degree of branches with long chains.

  • HDPE (High-Density Polyethylene)

HDPE has a density of equal or greater than 0.941 g/cm3. It has a low degree of branching gives stronger intermolecular forces and tensile strength. It can be made with chromium/silica catalyst, Ziegler-Natta catalyst or metallocene catalysts.

  • MDPE (Medium Density Polyethylene)

MDPE has a density range of 0.926 – 0.940 g/cm3. The MDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts.

  • LLDPE (Linear-Low Density Polyethylene)

LLDPE has a density range of 0.915 – 0.925 g/cm3. It is a substantially linear polymer and has significant numbers of short branches made by copolymerization of ethylene with a short chain of alpha-olefins like 1-butane, 1-hexene and 1-octene.

Polyvinyl Chloride (PVC)

poly vinyl chloride polymer

Polyvinyl Chloride Polymer

Vinyl Chloride Monomer (VCM

Vinyl Chloride Monomer (VCM) 

Polyvinyl Chloride (PVC) is widely used thermoplastic material for various application. To produced it, the required material is derived from Oil and Salt. The vinyl chloride monomer (VCM) is formed by mixing ethylene which is obtained from oil and Chlorine produced by electrolysis of salt water. The VCM molecules are polymerized to form PVC resin and along with different additives made customized PVC compound.


Nylon is a group of plastics known as polyamides. It is used in the manufacturing of fiber and film and it is also used as a moulding compound.

There are many types of Nylon available like nylon 6, nylon 66, nylon 6/6-6, nylon 6/9, nylon 6/10, nylon 6/10, nylon 6/12, nylon 11, nylon 12. Nylon is used as homopolymer, copolymer or reinforced. Nylon mixes with other engineering plastics to create required material. It is suitable for injection molding, rotational moulding, casting or extrusion into film or fiber.

Chemical Composition

Chemical Composition


The thermoplastic polymer, polypropylene also known as polypropene has properties between LDPE and HDPE and it is more versatile material available.

Chemical Composition


Acrylonitrile Butadiene Styrene

Acrylonitrile Butadiene Styrene (ABS) is an opaque thermoplastic and amorphous polymer. It is a terpolymer consist of Acrylonitrile, Butadiene and Styrene. All these material mixes and creates a product which is flexible and light in weight and it is used in our many daily routine items.

The main benefit of using ABS that it improves impact resistance, toughness and heat resistance. If it is moulded on the high temperature it gives gloss and heat resistance to the product and if it is moulded on the low temperature it gives the highest impact resistance and strength to the product.

ABS is used in drain pipe systems, plastic clarinets, golf clubs heads, automotive parts, kitchen appliances and many more.


Polycarbonate plastics are naturally transparent amorphous thermoplastics. It is used in various products but most in where impact resistance and transparency are required like Bullet-Proof Glass. Opposite of different plastics Polycarbonate can goes to large plastic deformation without any crack or breaks.

It is widely used for greenhouses, DVDs, eyewear lenses, medical devices, automotive components and cellular phones.

High Impact Polystyrene

The high impact polystyrene (HIPS) is very popular and tough plastic in polystyrene family. Polystyrene itself is brittle but can be made more impact resistant if mix with other materials. It is made with modifying crystal styrene with rubber which gives it more impact resistance. HIPS is low cost, good dimensional stability and rigidity.

It is highly flammable but there are also flame retardant and high gloss grades are used for injection moulding.

Plastic Moulding Techniques

The plastic is synthetically produced non-metallic compounds. Plastic can mould into various forms and hardened for commercial use. The plastic products are easily seen. The plastic products we use like jars, protective caps, bottles, toys, kitchen utensils, accessories and many more.

We cannot imagine the world without plastic products. The keyboard and mouse we use is also made with plastic moulding. The plastic part of chairs which you sit is also made with the same process.

The process is just simple. The molten liquid plastic is inserted into a mould like the plastic bottle. After that, it is left for cooling and after that the final product is ready.

The moulds are used to form various plastic products like cabinets, barriers, barricades, traffic signage, garden pots and many more.

If you want to start a plastic moulding process, you should have to understand the different processes first. Choose one of the plastic moulding processes according to your budget, resources and expertise. Below there are various processes for plastic moulding.

Blow Moulding

Blow Molding Process

In the blow moulding process, the air pressure is used to inflate the molten plastic into the mould cavity. This process is used to make one-piece hollow plastic products like bottles and containers. The blow moulded items are widely used in consumer beverages and manufacturers arrange the production for maximum numbers.

The blow molding process has two steps:

 – Starting the process with making the tube of molten plastic called parison.

– Inflate the parison into the final shape.

The Parison is formed from extrusion or injection moulding techniques.

Extrusion Blow Moulding


Manufacturers used extrusion blow moulding for high-production operations for making plastics bottles. The process is automatic and combines with operations like bottle filling and labeling. The blown containers must be rigid. The rigidity depends on various factors like wall thickness.

The extrusion blow moulding process contains below steps.

– Extrude the Parison.

– Cut the Parison from the top and seal at the bottom and the two halves of mould come together.

– After inflating the plastic tube took the mold cavity’s shape.

– Open the mold and remove the final product.

Injection Moulding

injection moulding products

The injection blow molding process contains the injection of mold the prison instead of extrusion. It has low production rate than the extrusion process.

The process has below steps.

– Injection-mould the parison around a blowing rod.

– The parison is transferred to a blow mold.

– Inflate the soft polymer to the mold.

– Open the mould and remove the product.

Stretch Blow Moulding

The manufacturers use the variations of injection blow moulding known as stretch blow moulding. In this technique, the blowing rod is extending downward into the injection moulded parison and this stretch the plastic and create a more stressing of polymer than conventional injection blow moulding. The final product is more rigid, higher transparency and it is more impact-resistant.

The Polyethylene terephthalate (PET) is the most widely used material for plastic injection moulding. The PET is polyester with low permeability and the stretch-blow-molding process gives strength and it is ideal for making containers for carbonated beverages.

The Stretch blow moulding steps:

– Injection moulding

– Stretching

– Blowing

Blow Moulding Materials and Products

The polyethylene (PE) is an ideal for blow moulding process as it has high density and high molecular weight polyethylene.

Materials used for blow moulding:

– Polypropylene

– Polyvinylchloride (PVC)

– Polyethylene terephthalate

Rotational Moulding

Rotational moulding or roto moulding use gravity inside a rotation mold to create a hollow form. This technique is used also as an alternative to blow moulding to make the large, hollow shape. This is used for thermoplastic polymers but now thermoset and elastomer are more common. The roto moulding is better than blow moulding for more complex external geometries, large parts and for lower production quantities.

The roto moulding process has below steps.

– First, load a predetermined amount of polymer powder into a split mould cavity.

– Heating and rotating the mould on two perpendicular axes so the powder can coat the mould’s internal surfaces equally and make sure to make uniform thickness layer.

– Open the mould and unload the ready product.

The speed during the process is slow. The process is done with gravity not with centrifugal force and this coat the mould surfaces.

The Rotomolding Moulds

The rotomoulding process is simple and less expensive than injection moulding and blow moulding. But the production cycle took more time around 10 minutes. To minimize time there is three station moulding machine.

First Station – In this station, unloading of final product and load the powder for the next step in the cavity.

Second Station – the Heating station where hot-air convection heats the mould as rotate and the temperature inside reaches up to 375o C depends on polymer and item molded.

Third Station – The third station is a cooling station that cools the mould with cold air or spray of water to solidify the plastic moulding inside.

The products are made with rotational moulding.

Toys, Boat and canoe hulls, Sandboxes, small swimming pools, Buoys and other floatation items, truck body parts, automotive dashboard, fuel tanks. Furniture, garbage cans, fashion mannequins, containers, storage tanks and many more.

The material used for roto moulding

– Polyethylene, mainly HDPE

– Polypropylene

– Acrylonitrile butadiene styrene (ABS)

– High-impact polystyrene

The plastic moulding process is a technical process which cannot be done by everyone. It requires knowledge about various materials and knows the process in every stage.

So it is advisable to study the business and market of your final product.

The History of Plastic Moulding

History of Plastic Moulding

The injection moulding is a process for manufacturing a product by injecting molten material into a mould. The material of the mould is mostly metal.

In this process the material is injected into a heated barrel and mixed and then injected into a mould. After injected into a mould the material is cool down and gets it strength. The mould is mainly made with metals like aluminum or steel.

injection moulding products

The first injection moulding machine was patented in 1872 by an American inventor John Wesley Hyatt and his brother Isaiah. The design is basic and much simpler than current machines.

The machine uses a hypodermic needle, use plunger to inject plastic into a heated cylinder to the mould. It is used to made products like buttons, combs and collar stays.

The growth of injection moulding products gets hike after American inventor James Watson Hendry invent the screw injection moulding machine. With this technique, the materials can be mixed to get the desired product before an injection. Today all the modern day machine uses screw injection.

How PLASTIC Moulding Works

The moulds are closes. After that, the heated plastic material is injected into mould. When the melted material is entered into mould the air flows through vents in injection pins.

After the mould is filled the mould is left for some time to be cool and the time is fixed for hardening the material. This time depends on the resin is used and the thickness of the product. Every mould has internal cooling lines where water flows continuously to maintain the constant temperature.

In the cooling process, when the resin cools down the screw barrel retracts the material from the hopper. The next step is the mould is open and ejects the ready product. There is a large tub or bin to collect the ready product.

Material Use In Moulding Process

Now which material we can use? The most of materials are polymers generally known as resin and also thermoplastics, some of the thermosets and some elastomers. After 1995 the total numbers of injection moulding materials are increased at 750 per year. There are more than 18000 materials are available.

The materials are generally alloys and are a mixture of materials. It is depending on the product designer that which material they choose. Mostly the materials are choosing on the basis of strength, costs and other properties like when the material bent, heat deflection and water absorption.

The common examples of thermosetting plastics are epoxy and phenolic and the thermoplastic is nylon, polyethylene and polystyrene.

Machine For Moulding

The machine for injection moulding contains a material hopper, a screw-type plunger and a heating unit.

Use of Mould in Moulding Process

There is also used a mould commonly known as die is used to make plastic products.  There are various materials are used to make a mould. The manufacturing cost of a mould is very high and steel moulds are more costly. The materials used for making mould are hardened steel, pre-hardened steel, aluminum and beryllium-copper alloy.

Mold vs Mould – Is There A Difference?

There is a difference between mold and mould but only in spelling, the meaning is the same. The mold is commonly used in American English and meaning is to form the material as mold and the fungi which are developed on fungi. The Mould is British spelling and they don’t use the mold. The mold and mould have the same meaning with a different spelling.

Blow Molding Process

So When using the mold?

The mold can be used as a noun or a verb.

Sometimes mold as a noun meaning fungus that grows on organic matter. Look at this sentence.

I want to eat bread which I made a week ago but it had mold on it.

Mold is also used in a process where liquid turns into solid. Look at these sentences.

The molten plastic poured into mould to get proper shape and size.

The new mold is required for the new design.

Sometimes it is used as a verb as transforming from.

We can mold everything as per your need.

Now, what is the meaning of Mould?

The Mould is mostly used in British English. It is used the same as mold in British English. The difference is only in spelling but the meaning is the same at both places.

In the end, what are mold and mould? Both are the same but use at a different place.

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What is extrusion Moulding?

The extrusion process is used to manufacture those parts, food containers, pipes, garden accessories etc. In this process, the thermoplastic raw material is added from the top hopper into the barrel. The other additives like colorants and UV inhibitors are in liquid or pellet form used and added into a hopper with resin. The manufacturing process is the same as plastic injection molding just this is the continuous process. The injection molding has a similar profile and is in the similar length and the finished product comes from the die and in this process, the fluid is extruded from dying.

Related Article:

Plastic Blow Moulding

The material is fed from the hopper and it reaches to barrel and come in the contact with the screw. The screw is rotating and push the resin ahead into the barrel and heated to set temperature depends on a resin. In the process, a heating profile is set for a barrel to use three or more PID (proportional-integral-derivative) controlled heat zones that increase the temperature time to time of the barrel at the rear where the resin enters to the front. This setup melts plastic resin gradually as the resin flows ahead in the barrel and minimize the risk of overheating which is harmful to product quality. 

At the front of the barrel, the resin leaves and moves ahead through the reinforced screen to remove contaminants. The breaker plate provides extra strength to screen as the pressure is very high around 5000 psi. After a pass from breaker plate, the resin enters into a die. The die gives the shape or profile to the final product, the products have the designed properly as the melted plastic flow through the cylindrical profile to get product’s shape. The flow must be continuous if it is not the product is produced with unwanted stress at some points. This problem cause warping above cooling. We can get any shape if it is a continuous profile.

The cooling process is must required and completed with a water bath. As the plastic is thermal insulated and it is not quickly cooled as steel. The plastic took more time to cool than steel. In the pipe, a sealed water bath is done in a controlled vacuum to keep the shape of the pipe from collapse. There is a set of cooling rollers is used in sheet extrusion process for cooling as it comes from the extruder. The plastic extruders are used for recycled plastic waste or raw material after sorting, cleaning and made it suitable for resin pallet used by the plastic industry.

There are various products are made with extrusion molding. The PVC water pipes are made with extrusion molding. The long cross-section shapes are made with extrusion molding. The insulation on the electrical wire is made with extrusion. The wire is stretch through the die and the insulation is then extruded.

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Choosing the Right Screw and Barrel

The choosing of proper Screw and Barrel combination is a very essential part for the plastic product manufacturing process. However, most people make mistake while choosing a proper screw and barrel. The companies are not serious about choosing screw and barrel. But now the situation is better companies started think about that still the screw and barrel are secondary when purchasing the machine.

The manufacturers are always considering the performance, controls, clamp speed, dry cycle speed and other important features while purchasing the new machine. There is the reason why they focusing on this feature because these features are important for maximum production and performance. The screw and barrel selection comes after these, since a very long time before.

Now companies use more engineered resin and they want more production and minimize the scrap, are the main features that have to be considered before choosing proper screw and barrel. The production growth is increased in the last 10 years as the industry is developed plastic/resin used in products. As the continuous research and development made in plastic and resin, it draws the attention of manufacturers to choose appropriate parts for screw and barrel set up to get maximum production from it.

So how to choose proper screw and barrel?

There are various factors that affect a selection of right screw and barrel. The industrial standards, material use is directly affected. Factors like the size of the machine, heat, cycle time, recovery time, melting temperature and so many other factors direct or indirect effect it.

There is a factor to be considering the ratio of screw and barrel and also metallurgy. The choosing the proper screw and barrel is essential for any machine. The manufacturers forget to upgrade the existing screw and barrel with improved and latest technology. They have to care about the sizing of screw and barrel, coating and metallurgy with material handling against cost and changes of clamp end to improve overall performance. When any plastic manufacturer wants to upgrade existing machinery with the latest or just to improve existing machinery, they have to take care of the screw and barrel for maximum productivity and performance.

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Types of Moulding Processes

There are different types of the moulding processes is used for forming a plastic to make various products of different size and shapes. The plastic turns liquid and becomes soft when heated. It can be easily formed for various products. The melted plastic is inserted into a mold and after it is cooled the mold is removed and the product is ready. The plastic products are made with different processes like casting, injection molding, blow moulding, compression molding and rotational molding.

Let’s start with the casting process. In this process, the plastic molding is completed with casting. The plastic is heated till it turns liquid and then inserted into a mold. There is no additional pressure is applied to this process. In the casting process the liquid turns into solid by various methods like evaporation, cooling, use chemical or applied external heat. The casting process is completed with less equipment and it is not costly. The products have little to no internal stress. The Nylon type 6 and Polyurethanes have widely used a material for casting. It is mainly used for complex designs.

The next is the Injection Molding. The process contains three steps. First is design the product. After designing and testing the second step is mold design. There are various metals used for mold to give strength and durability. In the third step, the thermoplastic material is fed from a hopper to a heating barrel. The plastic is heated and took to the mold by a large screw. After filling the mold the screw applies pressure for cooling. After cooled, the product is removed from a mold and this cycle is repeating more and more time to make lots of products. The dashboard of a car, bottle caps, plastic cup lids etc is made with injection molding process.

Ahead in the list is Blow Molding Process. As the name stated blow molding, in this process, the plastic is melted and made a hollow tube. This tube is inserted into a mold and blows the air and the tube is inflated to mold size and shape. The cold air is blown to cool down and it is ejected automatically. The blow molding process is mainly used for making hollow products like milk containers, water bottles etc.

The Compression molding is more complicated and required more labors. The compression molding is more complicated, it is used for large-scale production not for mass production. In the process, the molten plastic is poured into the mold and the second mold pressed it with high pressure. These molds compress the molten plastic before cooled. After removing the mold we get a product with maximum strength with desired size and shape. The compression molding is used for making car tires.

The rotational molding process the plastic in powder form is added to a mold. After it, the mold is closed and heated while rotating. When the plastic is melted it coated the wall of the mold. Then it took to the cooling area and cooled with air. After the product turns to solid the mold is opened and ejects the product. Products like water storage tank, containers, crates, traffic dividers, furniture and toys are made with the rotational molding process.

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Weight Reduction in Blow Moulding Containers

The more and more companies try to make light-weight containers by reducing the weight of containers. The main challenge is to engineers have to design light-weight container without losing aesthetic of container, impact performance, top load requirement and most important stability. The research took a long time to make perfect light-weight containers. For last few years’ manufacturers of blow moulded containers try to reduce the weight of the traditional design of dairy, water and juice. When reducing the weight of container the main factor to be considered is gram weight.

Continuous white cans making machine

The reducing process is very difficult as the researchers have to starting with an existing design or they have to make a new design. The process is start with analyzing the current weight of the container and other factors like volume capacity, top load, and drop impact with wall mapping of the entire container. The problems has found after the destruct test. If there is a new design of the container there are two steps to follow. One is passing through FEA (Finite Element Analysis) capability. With this test researches find a weak portion that needs to be correct. The other factor is heat transfer which has to be considering as it is cooled down the mould.

After collecting the data the engineers have to research previous data to apply the new attributes which is add to reduction design. These data consists of radius, angles, ribs and the texture of the main part. If the bottle has a handle, the design of the handle is have to be strong, comfortable and have a easy blow to make the whole process easy. The data is collected from a lab where it is tested. There must be the latest equipment which also has a top load tester, drop impact chamber, top wave vision system, vibration stand, and Magna Mike and altitude chamber. These equipments help in the process to get the best design.

The method is how much you saved with new less weight design. First, take the gram weight to save per container and then multiply by a number of containers made in one week. Convert the grams in KGs or pounds, and then multiply by resin cost after that you will get your weekly savings.

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What is blow Moulding?

The blow moulding is worked as its name stated. The molten material is known as parison or preform placing it in the mould and inflate with compressed air until it covers the mould and cools down before the mould is removed. Any hollow plastic product can be made with blow moulding. There are wide ranges of hollow plastic products around us and we use it in our daily routine life. Here are some products that are made with blow moulding.

  • Medical supplies
  • Household items
  • Industry products
  • Toys
  • Plastic bottles
  • Automotive parts

The blow moulding process is similar to glass blowing process. The first production machines and products are developed in 1930 to manufacture blow moulded bottles. After the introducing of low and high-density polyethene which is suitable for blow moulding, the market is hiked for blow moulding products.

The blow moulding process can be done with three different methods according to a product. Which are

  • Extrusion blow moulding
  • Injection blow moulding
  • Injection stretch blow moulding

The main difference between these processes is forming the prison.

Now start with Extrusion Blow Moulding. In this process, the polymer is melted and it is extruded from a die to make a parison or hollow tube. The moulds covered the prison and with the pressurised air blows through the needle and it inflates the shape of a mould. After this, the cool air is blown and moulds are opened and eject the complete product. The extrusion blow moulding there are two methods of extrusion which are continuous and intermittent. In the continuous method, the parison is extruded continuously and mould open and close to complete the process. In the intermittent, the plastic is collected by an extruder and then push it to die to make parison. The products made with this process are bottles, toys, appliances and automotive parts etc.

Let’s discuss Injection Blow Moulding. In this process, the polymer is injected into a core to form a hollow tube known as a preform. Then the preforms are rotated on a core rod to blow mould and are inflated and cooled at blowing station. The injection blow process is suitable for small bottles. The process has three steps namely injection, blow moulding and ejection and all this action done in the single machine. The products like medical supplies, product packages etc.

Next is Injection Stretch Blow Moulding. This process is quite similar to injection blow moulding. In this process, the moulded preform is stretched up to decided length to get better strength and is light in weight. The raw material is used for this process is PET or PP and they are stretched for a better physical characteristic. The wall thickness of the final product is high than extrusion and injection blow moulding. The injection stretch blow moulding is divided in one step and two-step process.

The One Step process the preforms and blowing in the same machine. This process is completed in three or four station machines. At different stations, there is different action performs namely Injection, Conditioning, Blowing and Ejection.  This process is suitable for small to high capacity with different shapes and various size plastic bottles.

In the Two-Step process first, the preform is made with injection moulding. The preforms have necks and threads on the top. These preforms are cooled and stored for re-heat blow moulding. The preforms are heated generally with infrared heaters and then stretched and blow using high-pressure air in the mould.  The Two Step process is suitable for 1-litre bottles and under.

Their materials used for blow moulding are High-Density Polyethylene, Low-Density Polyethylene, PET, PVC, Polycarbonate etc.

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