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Quality Mold Shop, Inc.
Plastic Injection MoldingPlastic Injection Molding
There are two types of injection molding machines- Manual molding machine (like an orange juice machine)- Automatic machine (like a mincing machine)The injection molding process requires the use of an - Injection molding machine- Raw plastic material (Granular/Powder form) - Die
The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidi�es into the �nal part. Injection molding is used to produce thin-walled plastic parts for a wide variety of applications, one of the most common being plastic housings. These housings are used in a variety of products including household appliances, consumer electronics, power tools, and as automotive dashboards.
Injection molding is also used to produce several everyday items such as toothbrushes or small plastic toys. Many medical devices, including valves and syringes, are manufactured using injection molding as well.
Process:- Clamping- Injection- Cooling- Ejection
Clamping:Prior to the injection of the material into the die, the two halves of the die must �rst be securely closedby the clamping unit. One half of the die is attached to the injection molding machine and one half is allowed to slide.
The hydraulically powered clampingunit pushes the die halves together and exerts su�cient force to keep the mold securely closed while the material is injected. The time required to close and clamp the mold is dependent upon the machine - larger machines will require more time.
Clamp force:The force that is applied to a mold by the molding machine in order to keep it securely closed while the material is injected.
The clamp force is typically some factor of safety greater than the separating force, which is the outward force exerted on the mold halves by the injected material.
Injection:The raw plastic material, usually in the form of granules, is fed into the injection molding machine, and advanced towards the mold by the injection unit. During this process, the material is melted by heat and pressure.
The molten plastic is then injected into the mold very quickly and the buildup of pressure packs and holds the material. The amount of material that is injected is referred to as the shot.
The injection time is di�cult to calculate accurately due to the complex and changing �ow of the molten plastic into the mold. However, the injection time can be estimated by the shot volume, injection pressure, and injection power.
Shot:The amount of material that is injected or poured into a mold. The shot volume includes the volume of all part cavities, as well as the feed system which delivers the material.
Cooling:The molten plastic that is inside the mold begins to cool as soon as it makes contact with the interior mold surfaces. As the plastic cools, it will solidify into the shape of the desired part. However, during cooling some shrinkage of the part may occur.
The packing of material in the injection stage allows additional material to �ow into the mold and reduce the amount of visible shrinkage. The mold cannot be opened until the required cooling time has elapsed.
The cooling time can be estimated from - Thermodynamic properties of the plastic and - The maximum wall thickness of the part
Ejection:After su�cient time has passed, the cooled part may be ejected from the mold by the ejection system, which is attached to the rear half of the mold. In order to facilitate the ejection of the part, a mold release agent can be sprayed onto the surfaces of the mold cavity prior to injection of the material.
Once the part is ejected, the mold can be clamped shut for the next shot to be injected. During cooling, the material in the channels of the mold will solidify attached to the part.
Defects Flash:
The occurrence of molten material seeping out of the mold cavity and solidifying.
Once the part is ejected, a thin layer of material will have formed attached to the part along the parting line.
Causes:- Injection pressure is too high.- Clamp force is too low.
Warping:
The permanent bending of a part that occurs when certain section of the part shrink faster than others, as result of a non-uniform cooling rate.
Causes:- Non-uniform cooling rate
Bubbles:
Balloon shaped cavities.
Causes:- Injection temperature too high- Too much moisture in material - Non-uniform cooling rate
Un�lled sections:
Incomplete parts
Causes:- Insu�cient shot volume- Low �ow rate of material
Defects
A part defect in the form of small indentations that are made where the ejection system pushed the part out of the mold.
Causes:- Cooling time too short- Ejection force too high
Sink marks:
When molten material isinjected into a mold, voids can occur if certain sections solidify �rst. The remaining material will �ll these voids as it continues to cool and shrink. This shrinkage causes marks on the part where the material sunk into the void.
Causes:- Injection pressure too low- Non-uniform cooling rate/Non-uniform wall thickness
Ejector marks:
Defects
