Injection Molding defects And How To Control Them – SMLease Design

During Injection Molding process some plastic defects come-up that affects product performance, function & aesthetics. We need to follow plastic design guidelines to reduce injection moulding defects. Injection molding defects can be because of design, material or process.

In this article we are going to discuss in detail about some of common injection molding defects.

Short Shot / Short Moulding

In Short Shot / Short Mould defect, injection molded plastic, does not reach at certain portions of the injection mold before solidifying. 

Short shot may occurs because, in between the flow of the molten plastic, the front end of the flow gets cooled and solidifies another reason can be during molten plastic flow air traps are generated.

Countermeasures

Part Design Modifications :

  • Increase wall thickness or provide rib in the area where molten plastic is solidifying.
  • Select a less viscous plastic with higher flowability.
  • By Changing Injection Moulding Conditions.
  • Increase Injection Plastic Temperature
  • Increase injection pressure.
  • Reduce Mold clamping Force.
  • Increase core and cavity Temperature

By Changing Injection Mould Design

  • Increase Gate and Runner Size.
  • Change gate position.
  • Provide effective Air Vents where air is trapping.

Flow Marks / Lines

Flow mark is a phenomenon where a striped pattern(commonly off-toned in colour) is formed around the gates when plastic has flowed through the Injection Mold.

Low melting and low mould temperature can cause the cold material to run inside cavity and partially solidified material takes the foam of flow lines.

It occurs when the plastic flows through sections with varying wall thickness and when the injection speed is too low causing the plastic to solidify at different speeds.

Countermeasures

Modification in Part Design

  • Maintain wherever possible uniform wall thickness in part design.
  • Avoid any sudden Change in material wall thickness to avoid any sudden change in direction and flow of material.

By Changing Injection Moulding Conditions

  • Increase Injection Plastic Temperature
  • Increase injection speeds and pressure to the optimal level, which will ensure the cavities are filled properly.

By Changing Injection Mould Design

  • Locate the gate at a spot in the tool cavity with thin walls.

Sink Marks

Sink marks are small craters or depressions that develop in thicker areas of the injection molded parts when shrinkage occurs in the inner portions of the finished product.

 

Sink marks are treated as a problem depends on the required quality of appearance.

Causes of Sink Marks

Sink Marks occurs during the cooling of molten plastic into solid state. Molten plastic inside mould begins to cool and solidify from the mould surface. Continues cooling and hardening of plastic from the outside result in changes in volume of the plastic (i.e., volumetric shrinkage). Which result in the plastic at the surface of the mold drawn towards the inside of the molding when volumetric shrinkage occurs in the molten plastic, and this results in the cosmetic defect referred to as sink marks.

When the outer layer of the molded component has sufficient strength to resist the pull of volumetric shrinkage, voids will be generated at the interior, and in some cases, this will not be manifested as an appearance-related problem. 

Sink marks are often caused when the cooling time or the cooling mechanism is insufficient for the plastic to fully cool and cure while in the mold.

Countermeasures

By Modification in Part Design:

  • Reduce the thickness of the thickest wall sections will also ensure faster cooling and help reduce the likelihood of sink marks.

By Changing Injection Moulding Conditions

  • Increase Dwelling Time & Dwelling Pressure
  • Mold temperatures should be lowered, holding pressure increased, and holding time prolonged to allow for more adequate cooling and curing.
  • Reduce Nozzle Temp.

By Changing Injection Mould Design

  • Changing gate position to thick wall side
  • Increasing Cooling efficiency
  • Provide effective Air Vents where air is trappping

Warpage

Warpage is the deformation that occurs when there is uneven shrinkage in the different areas of the molded component. The result is a twisted, uneven, or bent shape where one was not intended.

Causes Of Warpage

Warping is usually caused by non-uniform cooling of the mold material. Different cooling rates in different parts of the mold cause the plastic to cool differently and thus create internal stresses. These stresses, when released, lead to warping.

Countermeasures

Modification in Part Design:

  • Design the mold with uniform wall thickness and so that the plastic flows in a single direction.

By Changing Injection Moulding Conditions

  • Ensure that the cooling time is sufficiently long and that it is slow enough to avoid the development of residual stresses being locked into the part.

Voids

Voids are pockets of air trapped inside or close to the surface of an injection molded component.

Causes Of Voids

Vacuum voids are often caused by uneven solidification between the outer surface and the inner sections of the component. This can be aggravated when the holding pressure is insufficient to condense the molten plastic in the mold (and thereby force out air that would otherwise get trapped).

Countermeasures

By Modification in Part Design:

  • Switch to a less viscous plastic. This will ensure that less gas is trapped as air is able to escape more rapidly.
  • By Changing Injection Moulding Conditions
  • Increase holding pressure as well as holding time

By Changing Injection Mould Design

  • Ensure that mould parts are perfectly aligned.
  • Locate the gate at the thickest part of the moulding.

Flash

Flash is a molding defect that occurs when some molten plastic escapes from the mold cavity. Generally this happens through the parting line or ejector pin locations. This extrusion cools and remains attached to the finished product.

Causes of Flashes

Flash can occur when the mold is not clamped together with enough force (a force strong enough to withstand the opposing forces generated by the molten plastic flowing through the mold), which allows the plastic to seep through. The use of molds that have exceeded their lifespan will be worn out and contribute to the possibility of flash. Additionally, excessive injection pressure may force the plastic out through the route of least resistance.

Countermeasures

By Changing Injection Moulding Conditions

  • Increase the clamp pressure to ensure that the mold parts remain shut during shots.
  • Ensure that the mold is properly maintained and cleaned (or replaced when it has reached the end of its useful lifespan).
  • Adopt optimal molding conditions like injection speed, injection pressure, mold temperature, and proper gas venting.

Knit / Weld Lines:

Weld / Knit lines are actually more like a plane than a line that appears in a part where molten plastics meet each other as they flow from two different parts of the Injection Mold.

Causes Of Knit / Weld Lines

Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic.

Countermeasures

By Modification in Part Design:

  • Switch to a less viscous plastic or one with a lower melting temperature

By Changing Injection Moulding Conditions

  • Raise the temperature of the mold or molten plastic.
  • Increase the injection speed.

By Changing Injection Mould Design

  • Adjust the design for the flow pattern to be a single source flow. 

Surface Delamination

Thin surface layers appear on the part due to a contaminant material. These layers appear like coatings and can usually be peeled off.

Causes of Surface Delamination

Foreign materials that find their way into the molten plastic separate from the finished product because the contaminant and the plastic cannot bond. The fact that they cannot bond not only has an affect on the appearance of the component, but also on its strength. The contaminant acts as a localized fault trapped within the plastic.

When the mould and plastic temperatures are extremely low, the difference in temperatures between the outer walls and the fluid layer results in the development of a thin hard coating which then peels.

Countermeasures

By Modification in Part Design:

  • Smooth out the corners and sharp turns in the mold design to avoid sudden changes in melt flow.
  • By Changing Injection Moulding Conditions
  • Pre-dry the plastic properly before molding
  • Increase the mold temperature.

By Changing Injection Mould Design

  • Focus more on the ejection mechanism in the mold design to reduce or eliminate the dependence on mold release agents

Burn Marks

Burn marks are discolorations, usually rust coloured that appear on the surface of the injection molded prototypes.

Causes of Burn Marks

Burn marks are caused either by the degradation of the plastic material due to excessive heating or by injection speeds that are too fast. Burn marks can also be caused by the overheating of trapped air, which etches the surface of the molded part.

Countermeasures

By Modification in Part Design:

  • Switch to a less viscous plastic. This will ensure that less gas is trapped as air is able to escape more rapidly.

By Changing Injection Moulding Conditions

  • Reduce injection speeds
  • Reduce mold and melt temperatures.

By Changing Injection Mould Design

  • Optimize gas venting and degassing.

Jetting

Jetting refers to a situation where molten plastic fails to stick to the mold surface due to the speed of injection. Being fluid, the molten plastic solidifies in a state that shows the wavy folds of the jet stream on the surface of the injection molded part.

Causes Of Jetting

Jetting occurs mostly when the melt temperature is too low and the viscosity of the molten plastic becomes too high, thereby increasing the resistance of its flow through the mold. When the plastic comes in contact with the mold walls, it is rapidly cooled and the viscosity is increased. The material that flows through behind that viscous plastic pushes the viscous plastic further, leaving scrape marks on the surface of the finished product.

Countermeasures

By Changing Injection Moulding Conditions

  • Increase mold and melt temperatures.

By Changing Injection Mould Design

  • Increase the size of the gate so that the injection speed becomes slower.
  • Locate the gate at the thickest part of the moulding.
  • Optimize gate design to ensure adequate contact between the molten plastic and the mold. 
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