The main difference between injection molding and extrusion molding is that injection molding is highly restrictive molding by injection mold, and extrusion is semi-restrictive molding by extrusion die.
Source: https://www.pinterest.com/pin/842243567788479328/The Difference Between Extrusion and Injection Molding
Material fluidity
Custom injection molding requires high material fluidity, especially for thin-walled products and large products.
Extrusion has less stringent material flow requirements because plastic extrusion is open-ended and does not have the huge resistance of die reversal.
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Injection moulding is molded in a die, and there are geometrical limitations of the die composition, so the melt strength requirement is not high.
Although there is also the geometrical limitation of the mouth die, plastic extrusion molding is not fully cured after the exit of the die, and in this free state, it cannot be well-formed due to low melt strength, such as spinning.
In addition, the extrusion method is often accompanied by subsequent free state processing molding, such as thermoforming, hollow Blow Molding, in these processes, there are often different degrees of melt stretching, if the melt strength is low, it is easy to pull through.
Therefore, injection moulding generally does not require high melt strength, while the extrusion method is generally required.
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Injection molding generally does not have strict requirements for exit expansion.
The exit expansion of extrusion moulding is difficult to handle. Therefore, the control of viscoelasticity of injection moulding and extrusion materials will also be different.
There are many ways to mold plastics, such as injection moulding, blow molding, extrusion, and blister molding, but in principle, all of these plastic molding processes go through three basic stages: melting (plasticize stage), flow (molding stage), and curing (cooling stage).
These methods have their purposes and advantages, and both are valuable to plastic parts manufacturers. Here we will mainly explain the difference between extrusion moulding and injection moulding.
Source: https://www.pinterest.compin604186106262440921What is the difference between these two processes?
The molten die-casting method is the basis of the injection moulding process. The injection-molding unit consists of two elements: the clamping unit, and the injection unit. Unlike extrusion, injection molding forms three-dimensional shapes.
Extrusion and Injection Moulding Basics Simply put, the extrusion process is required to create continuous linear, two-dimensional shapes. injection moulding produces three-dimensional shapes that do not remain constant in a parallel line.
Source: https://www.pinterest.compin669699407097767307A. Different tools
1. Injection moulding: The main tool is the injection moulding machine.
2. Extrusion moulding: The main tool is a non-rubber extruder.
B. Process is different
1. Injection moulding: Plastic is plasticized in the heated barrel of an injection moulding machine and then injected by a plunger or reciprocating screw into the cavity of a closed mold to form a product.
2. Extrusion moulding: Extrusion moulding is a plastic processing method in which molten plastic is passed through a mold to create a profile.
Source: https://www.pinterest.com/pin/1005147210554615365/What is injection molding?
Injection molding is a process in which the plastic resin is put into the hopper, which then releases the plastic pellets from the feed section into the compression section, where the frictional heat is created and molten form plastic from an injection machine is thrust into a mold by a plunger or screw and cooled to obtain a product.
The plastic is plasticized in the heated barrel of the injection molding machine and then injected by the plunger or reciprocating screw into the mold cavity of the closed mold to form the product.
This method is capable of processing products with the desired shape, precise dimensions, or inserts, and is highly productive. Most thermoplastics and some thermoset plastics (such as phenolic plastics) can be processed by this method.
The material used for injection molding typically must have a good flow to fill the mold cavity to obtain the product.
Source: https://www.pinterest.compin362469470021836254What is the injection molding process?
The injection molding process is based on the principle of melting solid plastic at a certain melting point, injecting it into the mold structure at a certain speed through the pressure of the injection machine, and then cooling the plastic in the waterway to obtain a product with the same cavity as the designed mold.
It is mainly used for the plastic molding of thermoplastics, but can also be used for thermosetting plastic mold manufacturing. The complete injection molding process includes preparation before molding; injection process; post-processing of plastic products.
Source: https://www.pinterest.com/pin/853995148094370658/1. Preparation before molding
To make plastic injection molding manufacturing go smoothly and ensure the quality of products, a series of preparations such as raw material pretreatment, barrel cleaning, preheating insert, and selecting a release agent is needed before the continuous plastic injection production process.
2. Injection process
The injection process generally consists of: adding material – plasticizing – injecting – cooling – demolding.
3. Charging
Since plastic injection molding service is an intermittent process, it is necessary to add material in a quantitative (constant volume) manner to ensure stable operation, uniform plasticization, and finally, obtain high-quality plastic parts manufacturing.
4. Plasticizing
The molding material is heated, compacted, and mixed in the barrel of the injection molding machine to transform from a loose powder or granular solid into a continuous homogenized melt.
5. Injection
The plunger or screw, starting from the metering position in the barrel, applies high pressure through the injection cylinder and piston and passes the plasticized plastic melt through the nozzle at the front of the barrel and the nozzle in the mold.
The plastic injection molding manufacturing process of injecting the plasticized melt through the nozzle at the front of the barrel and the pouring system in the mold into the closed mold cavity quickly. The injection unit can be subdivided into three stages: flow filling, pressure retaining and shrinkage, and backflow.
6. Cooling
When the plastic in the pouring system as well as frozen, continued pressure-holding is no longer necessary, so the plunger or screw can be returned to unload the plastic in the barrel
Therefore, the plunger or screw can be returned, the pressure of the plastic melt in the barrel can be removed, and new material can be added. At the same time, cooling media such as cooling water, oil, or air can be introduced into the mold to further cool the mold. The cooling process starts when the plastic melt is injected into the cavity, which includes the period from mold filling and pressure-holding to the time before demolding.
7. Demolding
The plastic part is cooled to a certain temperature to open the mold, and the plastic part is pushed out of the mold under the action of the pushing mechanism.
8. Post-processing of products
A. Annealing: to eliminate the residual stress;
B. Humidification: to stabilize the color, performance, and size of the molded part.
What is extrusion molding?
Extrusion molding is also known as extrusion in plastic processing, in the non-rubber extruder processing using hydraulic press pressure in the die itself extrusion is called press out.
It is a processing method in which the molten material is thermoplasticized by the action between the extruder barrel and the screw while being pushed forward by the screw, and is continuously pushed through the head to create complex cross-sections and various cross-sectional products or semi-products.
Extrusion molding is a molding method in which a polymer material is forced through a die with a constant complex cross-section by the extrusion of a screw or a plunger under pressure.
The Process The machine motor, an extrusion machine, turns a screw that feeds melted plastic through a heater. The extrusion process mainly includes molten plastic material addition, melting and plasticizing, extrusion, shaping, and cooling.
Source: https://www.pinterest.com/pin/315111305176123015/The extrusion process can be divided into two stages
The first stage is to plasticize the solid plastic and pass it through a specially shaped die under pressure to become a continuous body with a cross-section similar to the desired shape of the die;
The second stage is to make the extruded continuum lose its plasticity and become solid by a suitable method, that is, to obtain the desired product.
Source: https://www.pinterest.com/pin/598626975452772688/Extrusion molding classification
The extrusion process can be divided into dry and wet methods according to the way of plasticizing plastics.
The plasticization of the dry process is done by heating the plastic into a melt, plasticization and pressurization can be done in the same equipment, and the shaping process is only simple cooling.
The plasticization of the wet process is done by softening the plastic with solvent, so plasticization and pressurization have to be divided into two separate processes, and the shaping process has to be done with more troublesome solvent removal and solvent recovery.
Although wet extrusion has advantages in terms of homogeneous plasticization and avoiding overheating of the plastic, it is limited to the extrusion of nitrocellulose and a few cellulose acetate plastics due to the above disadvantages.
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