What are the Common Plastic Processing Issues?

Overview

The primary considerations when looking to either prevent moulding issues at the pre-production stage or to find the root cause of a quality concern are:

• Component design
• Production tool design
• The moulding process
• Raw material quality

These can all influence the quality of a moulded component, sometimes in combination.

To get help and advice from PlastikCity partners that specialise in troubleshooting & process optimisation, please visit here.

Below are some common moulding process issues, their most common causes and some possible solutions.

Short Shots

Definition
The tool cavity is not completely filled with plastic. Areas that are furthest from a plastic injection point, or where the material flow is restricted, are the most susceptible to this issue.

Root cause
This is usually a combination of:

• Excessive material flow length
• Speed of filling being insufficient to prevent material from solidifying before it reaches the extremities of the moulding cavity
• Material temperature is too low, or the tool not being heated sufficiently to prevent premature material solidification
• Material is releasing gasses which become trapped within the tool’s cavity and are compressed to a level that can act as a physical barrier.

Possible solutions to short shot issues

1. Modification of the component design:
• Increasing the wall thickness of thin sections, or adding ribbed areas can increase material flow rates
2. Modification of the injection mould tool:
• The addition of vents (slight gaps, or porous surfaces) within areas of mould cavities in which gas is being trapped, can allow these gasses to escape to the atmosphere.
• Adding additional material injection points (gates), or moving/increasing the size of existing gates can reduce flow lengths and increase flow rates. Specialist software allows the effect of such modifications to be simulated. Experts in mould flow analysis provide this service.
3. Modifying the moulding process:
• Increasing barrel temperatures can reduce material viscosity
• If one half of the tool is heated, then raising tool temperature by a few degrees can prevent material from prematurely freezing off.
• After the primary material injection stage, apply a secondary stage of material injection to pack out the mould. This can compensate for material shrinkage during the cooling phase of the moulding cycle.
• Increase primary injection pressure or speed so that the mould fills more rapidly.

Sink Marks & Voids

Definition

Sink marks and voids are caused by the natural shrinkage of plastic materials during the cooling process within the mould tool. Different material types will shrink by differing amounts.

Whereas a sink mark is a visible depressed area on the outer surface of a moulding, a void is hidden within the wall section of the product.

In some respects, a void is more of an issue, as it is not easy to detect and can cause an inherent weakness.

Possible causes

• Not enough molten plastic material has been injected into a cavity during the early stages of the cooling time (packing the mould).
• Sinking is often associated with underlying areas (below the wall section) that are fairly large structures such as supporting ribs of other wall section.

Possible solutions

1. Component design - reduce the thickness of any underlying features as much as possible.
2. Mould tool design - sinks and voids are less likely to occur if there is sufficient material flow to fully fill the mould tool cavity during the main stage of material injection. Attention should be paid to the number and sizes of injection points.
3. Modification of the moulding process
• Reduce the melt temperature (processing temperature of the molten polymer) by a few degrees
• By packing the tool (slowly injecting additional material after the main injection phase has finished) the natural material shrinkage that occurs can be countered.
4. Material conditioning – if a material requires drying before processing, make sure drying is sufficient, as excessive moisture content is a common cause of voiding.

Material Burning

Definition
An area of the moulding is discoloured, or material is degraded by excessive heat or corrosive gasses.

Possible solutions
Burning is usually caused by caustic gases released by certain polymer grades being trapped within a mould tool. Venting of the tool allows this gas to escape.

Material Flashing

Definition

If the material is not kept within the confines of a mould cavity, it will solidify as a thin layer (flash) that may need to be removed. Flashing most commonly occurs where tool faces meet, or around items such as ejector pins that require a small clearance to operate without sticking.

Possible causes

• Clamping force is lower than that generated internally within the tool as the material is injected under high pressure
• There is damage to the mating faces of the tool, or ejector pins have worn through continual use.

Possible solutions

1. Injection mould tool maintenance – worn tool faces can be refinished and worn ejector pins replaced with new ones.
2. Modification of the moulding process:
• Reduce the material injection pressure, if possible
• Either increase the clamping force of the existing injection moulding machine or move the tool to a larger machine if the upper limit has been reached.

Jetting or Silvering on the Moulding Surface

Definition

The surface of the product has lighter coloured lines or swirls that spread out from the material injection point.

Possible causes

• The material has laminated due to excessive stress as it passes through the injection point and into the mould cavity.
• If the plastic material requires pre-process drying, then this may have been insufficient.

Possible solutions

1. Injection mould tool design
• By increasing the surface area of a material injection point, material stressing will be reduced
• Make sure that material injection points do not require any sudden changes in material flow direction, as this can cause turbulence.
2. Modification of the moulding process
• If possible, reducing injection pressure or flow rates will reduce material stressing
• Material stressing may be minimised if it is less viscous. Increasing processing temperature typically reduces viscosity.
3. Material preparation or choice
• Make sure your material dryer can reach the required temperature and has sufficient capacity to allow a long enough drying period
• Select an alternative material with a higher MFI rating. This means it is less viscous at a set processing temperature.

Product Warpage (Distortion)

Definition

Once a product is removed from the mould tool and is no longer supported, it loses its shape.

Possible causes

• Areas of the product cooling and therefore shrinking at different rates. This may be due to variations of material thickness.
• Lack of supporting structures in areas of the product that have large surface areas and relatively thin wall sections.
• Physical distortion caused by the ejection of the component from the mould tool.

Possible solutions

1. Component design
• If possible, make wall thicknesses uniform so that they cool at similar rates
• Add features such as ribs to unsupported areas
2. Tool design
• Analyse where mouldings are sticking to see if there are unnecessary under-cuts or burrs
• Add additional ejector pins to spread the load more evenly, or move pins that are impacting on unsupported areas to thicker and stronger areas.
3. Modification to the moulding process
• If the two halves of the mould tool are kept at widely differing temperatures, this may be the cause of uneven cooling. It may be possible to narrow the temperature gap being used.
• Allow mouldings to cool for longer while supported by the tool. Cooler components will also be more resistant to pressure exerted by the ejector system.

Weld Lines & Flow Marks

Definition

Weld lines are an indication of where two or more material flow paths have converged and combined. Flow marks are a visible indication of where the hot material flow has come into contact with the cooler surface of the mould tool. As well as being cosmetically problematic, both issues can result in structural weaknesses.

Possible causes

In the case of weld lines, the material has cooled to the point that it won’t fully mesh together. This may be because the distance from the material injection point is too long.

Flow marks are typically caused by:

• Sudden changes in the direction of flow that create turbulence
• The tool is too cool and is causing premature material

Solutions

1. Mould tool design
• Reduce flow lengths by moving or adding additional feed gates.
• Reduce flow turbulence by straightening material flow paths (no sharp bends!) or increasing the surface area of the feed system
2. Modification to the moulding process
• In the case of weld lines, increasing material flow rates should ensure the material is at a higher temperature when material flow paths converge.
• Both issues could be improved by raising barrel temperatures slightly so that material is hotter when entering the tool & by increasing the temperature of the mould tool