Introduction to injection moulding defects

Plastics have been widely used in daily life and become a necessary part of the current world. injection moulding is a manufacturing process that involves injecting molten plastic materials into mould cavity under high pressure. The process begins with the selection of a suitable thermoplastic material, which is melted and injected into a mold designed to replicate the desired part’s shape. The molten plastic solidifies within the mold, allowing for the removal of the finished part.

THE CYCLE OF INJECTION MOULDING

when the plastic material enters the injection moulding machine through Hooper, It is turned to melted state after being squeezed by the screw from which a large amount of heat is generated through friction.the melted plastics accumulates in the front of the cylinder and is constantly heated in order to maintain the temperature for injection.This process is called plastification.

Then ,the melted plastic will be pushed forward into closed mould cavity by the screw, a process that is called injection . After the initial injection is completed,when the molecular weight melted has fully filled the mould cavity , more melted plastic is injected under high pressure in order to compensate for the decrease in volume of plastic due to cooling as well as to make sure the mold cavity is perfectly filled until the spree is solidified,a process that is called peaking.

Finally, The movable side moves back until the injection pin reaches the rear plate to eject the molded product,runner system,and waste.This cycle is Known as the molding cycle of injection molding.

Defects of injection molded products

There can certain defects associated with injection molded products. Some common defects include:

short shot: occurred when the injected plastic material does not completely fill the mold cavity, resulting in an incomplete part.therefore,any factor that affect the smooth flowing of the melted plastic is likely to cause short shot defects, such as insufficient amount of plastic injected,too high flow resistance,or insufficient liquidity.

Warp: Is the deformation or twisting of a molded part from its intended shape. It can occur due to non-uniform cooling or inconsistent material flow during the molding process.

Flash: Flash is excess material that escapes from the mold cavity and forms thin, unwanted protrusions along the edges or parting lines of the molded product. It can occur due to:

Mold clamping force is too small:A pushing force is applied to the mold by the melted plastics during the injection molding process, specifically if the central area of cavity is subjected to excessive high presser the mould will separate from the parting plane.

Mold Gap: Is the cause that the moving part(core side) does not fit the cavity in proper way.

Improper molding condition:wrong selection of molding machine,over temperature of melted plastics,and excessive injection pressure are all causes of flash generation.

Improper venting system:flash will be generated if the venting is insufficient or the venting groove is too deep.

Sink mark or void: The formation of sink marks and voids is a phenomenon observed in thick areas where there is not enough plastic supplied during the cooling process.Sink marks are generated as the plastics contacting the mold walls cools and hardens before the inner plastics starts to cool down,and hence the surface is pilled inward by contraction. if the surface strength is sufficient,voids are generated instead of sink marks. there for sink marks and void are often seen at the rib parts or the backside of a convex surface.

Air Trap: Occurred when melt front of the melted plastic traps the air inside the mold cavity, so that the air can not escape from venting holes or the gaps in thee mounting part. and air trap mostly occurs at the area filled at last, where there is no venting hole or the venting are too small.

Burn Mark: The cause of burn marks is very similar to those of air trap.The major cause is that the air trapped inside the mold cavity is overheated by compression and creates dark mark on the plastic surface.

Delamination: The main cause of delamination(layer separation) is due to the mixing of two incompatible material types used in molding process.Delamination occurred when the temperature of melted plastic is too low,the humidity of the material is too high,or the runner and sprue are not smooth.

Flow Mark: Flow marks main occurred if the temperature of melted plastic is not evenly distributed or the melted plastic is of excessive high viscosity.An excessively low temperature cause friction and pushing between the plastics and the mold cavity, and the results in the plastic hardening too quickly leaving the flow mark.

Stress mark: If the thickness of plastic parts varies a lot,the cooling speed at a thinner area differ significantly that of other areas.Then,the uncooled melted plastic will apply stress on the cooled plastics,which generates a stress mark due to inner stress.

Jetting: is usually generated when the melted plastic passes through a narrow sprues or runner into the mold cavity with high speed.

Splays: Splay refers to the appearance of small silver or gray streaks on the surface of the part. It is caused by moisture or contaminants in the molten plastic, which vaporize and leave behind residue during the injection process.

Weld Line: Occurred when there are two or more plastic flow fronts that merge together during the injection process,an incomplete fusion can occur as the melt front is low temperature and hardens first.

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