Plastic injection molding manufacturing method is considered both science and art. The injection molding process can be described as heating plastic pallets to molten state, injecting molten plastic tin the cavities then cooling the plastic to solidify the plastic. The injection molding process does have some drawback in the form of defects. The defects can be observed on the components. The defects in plastic injection molding can be attributed to material selection, part design, tool/mold design and process settings or any combination of material, part, tool and process. The defects can be first observed during the tool/mold verification/validation trials. The defects can also be observed during production. The defects can be resolved by modifications to part, tool, process settings or any combination of material, part, tool and process. Resolving defects by changes in process settings is very cost effective.
Process settings in the plastic injection molding process involves very good understanding of material properties, tool design and capabilities of injection molding machine/press. Most of the defects are caused due to thermal imbalances in the molten material flow path. This article discusses causes and possible solutions to some of the plastic injection molding defects that can be observed on a part.
1. Flow Lines
Flow lines are visual defects, as wavy and slightly discolored than the component color. Flow lines can be seen on or near thinner cross section of the component including the gate location. As flow lines considered visual defect, do not pose any structural integrity concerns.
- Thin cross sections on the component – gradual change in wall thickness
- Long flow length - add flow channel ribs to assist flow
- Long flow length- add gates to reduce flow length
- Significant pressure loss at gate- adjust gate size
- Sharp edges near gate- round off sharp corners/edges
- Flow lines away from gate- increase fill speed, melt temp, mold temp
- Flow lines near gate- reduce injection pressure, speed
2. Burn marks
Burn marks are usually caused due to trapped hot air in the flow path. Molten plastic is forced through the flow path and into the cavity. The molten plastic forces out the air from the flow path and the cavity. Air can get trapped due to 4 major factors:
- Sharp corners/bends – avoid sharp corners, add radius
- Sharp corners/bends in the flow path - smooth any sharp corners
- Inadequate venting- add/increase venting
- Obstacles in the flow path- clean sprue/runner/gate
- High injection speed - reduce injection speed
- High injection pressure- reduce injection pressure
- High melt temp – reduce melt temperature
- High back pressure – reduce back pressure
Injection molding machine/ press
- Faulty seal at the press
Warping is usually caused due to uneven cooling of the part. Portion/side of the part cooler than the other when the part is ejected from the tool/mold. The uneven cooling leads to twisted, uneven or bent shape.
- Non-uniform wall thickness- transition thin to thick wall, uniform wall thickness
- Ineffective cooling circuit – adjust temperature in the circuits to maintain mold surface temp even
- Long fill time – increase gate size
- Wrong gate location – move gate location to thicker sections of the part
- Long fill time – increase injection speed
- Not enough cooling – increase cooling time
- High melt temp – lower cylinder melt temperature
- High mold temp – lower cooling water temperature
Voids are the vacuum holes inside molded parts. During the cooling process the molten plastic stays aligned with cavity wall and leaves a vacuum in the middle. This occurs mostly in the thick sections or at rib intersections of the part. Visually the void is undetectable if material is opaque, can be seen in translucent or clear materials like a bubble. Appearance of sink mark is reduced with void is present.
- Uneven wall thickness – uniform wall thickness
- Thick sections at rib joints – avoid multiple ribs joining at same location
- Thick sections at bosses – deeper boss holes, thin boss walls with ribs
- Gate at thin wall section – move gate to thick cross section
- Long flow length – add multiple gates to reduce
- Uneven mold surface temperature – cooling circuit design for uniform mold temp
- High melt temp – reduce melt temp
- Low back pressure – increase back pressure
- Moist material- material should be dry before molding
- Insufficient injection pressure- increase injection pressure
5. Sink marks
During the cooling phase of injection molding process when the molten material shrinks away from mold cavity walls, shrink marks are visible. Some portions of the surface of the molded part are visibly depressed or sunk. The sink marks are usually effect of thick cross sections shrinking more than the nominal wall thickness. Commonly sink marks can be observed near the ribs and bosses due to higher cross sectional thickness than nominal wall thickness
- Excessive rib thickness – should follow material recommendation for rib thickness to nominal wall thickness ratio
- Non uniform wall thickness – avoid wall thickness variations, use techniques like core out, ribs or other methods to reduce wall thickness variation
- Gate location – gate should be located near the high wall thickness
- Water lines – higher than nominal wall thickness regions should be accommodated by the cooling circuit
- Low density- increase back pressure
- Cold mold surface- reduce cooling time.
- High melt temp- reduce melt temp
- High fill time- increase injection speed
- Fast gate freeze- increase packing pressure and speed
6. Weld lines
Weld lines are formed when 2 flow fronts merge but are not joined completely. Temperature of flow front drops with flow length and time. The weld lines are structurally weaker than the continuous material. The thicker and longer weld lines make the part structurally weaker. The weld lines are also visible defects.
- Sudden change in wall thickness- gradual change in wall thickness
- Flow split at thin sections- avoid flow splits at thin sections.
- Flow line at long flow length- add gate near the flow lines
- Moving flow lines- optimize cooling circuit
- Flow lines in the structural region- change gate size to move the weld lines
- Long flow lines- increase mold temp
- Thick flow lines- increase melt temp, speed and pressure
- Thick weld lines- switch material with higher MFR, lower melt temp
The squirt marks on the walls of the component are called jetting. The jetting as layers of plastics are formed within the cavity. Jetting is mostly caused by large pressure drop across the gate, also high melt flow velocity at the gate.
- High pressure drop at gate- increase the gate size, change the gate angle
- No resistance to flow at the gate- change gate angle
- Lower injection speed
- Lower injection pressure
- Change material to lower MFR
- Lower melt temp
8. Short shot
The cavities in the tool/mold are not filled completely during injection phase are short shots. The short shot result in incomplete component after cooling. Short shots are mostly seen in multiple cavity tool. Short shot are major defects as parts produced are incomplete. Short shots are observed when the molten plastic turns solid before reaching the end of the flow length.
- Smallest cross section in the flow path should be the gate
- Increase melt temp
- Increase injection speed
- Increase injection pressure
- Increase mold temperature
A thin layer of plastic that flows outside the cavity of the injection tool/mold is called flash. The flash is caused due to excess molten plastic forced in the cavity. Also when the two halves of the injection molding tool/mold leaves gaps at parting surface. The flash can be a major defect when the component is used in an assembly. Also a secondary process of trimming can be used to remove flash from the components.
- Conduct a die test to check gaps in the parting surface
- Check mold/tool halves alignment at parting lines
- Reduce venting
- Check and adjust sprue position
- Check and adjust back pressure and speed
- Reduce melt temp
- Reduce injection speed and pressure
- Check and adjust clamping pressure