• DuPont Plastics

• Design Tips
  • Comparison of materials
  • Material selection
  • Wall thickness
  • Ribbing
  • Gate positioning
  • Cost-saving designs
  • General assembly technology
  • Welding technology
  • Tolerances
  • Check list
• Moulding Problems
  • Moisture in the Granules
  • Feed System Too Small
  • Wrong Gate Position
  • Hold Time Too Short
  • Wrong Melt Temperature
  • Wrong Tool Temperature
  • Poor Surface Finish
  • Problems with Hot Runners
  • Warpage
  • Mould Deposit
• What Can Plastics Do?
  • Chapter  1
  • Chapter  2
  • Chapter  3
  • Chapter  4
  • Chapter  5
  • Chapter  6
  • Chapter  7
  • Chapter  8
  • Chapter  9
  • Chapter 10
  • Chapter 11
  • Chapter 12

Partially crystalline engineering thermoplastics such as POM (acetal), PA (nylon), PBT and PET (polyesters) are used primarily because of their outstanding mechanical, thermal and electrical properties. Further advantages over amorphous materials include their excellent chemical resistance and low tendency towards stress cracking. In many kinds of applications a high quality surface finish is an additional requirement. This article is intended to help eliminate possible surface defects.

Localisation and definition of surface defects

To solve the problem of surface defects one must first examine the precise location of the defect, and when it actually became evident. Here it is advisable to observe the surface during the actual injection moulding process. The points that need clarification are listed below.

• Does the defect occur with every shot or irregularly?
• Does the defect always occur in the same cavity?
• Does the defect always occur at the same place in the moulding?
• Can the defect be predicted already during a mould filling study?
• Is the defect already evident on the sprue?
• How does the defect react when a new batch of moulding compound is used?
• Does the defect occur with only one machine or with others as well?

Analysis of possible causes of surface defects

Surface defects may be caused by many different factors such as:

• Compounding: drying, compound quality, presence of contaminants (foreign bodies)
• Injection moulding conditions: melt temperature, injection speed and change-over point
• Condition of the injection unit, e.g. wear and dead spots
• Design of the hot runner system (runners, material stoppage etc.)
• Mould design, position of gate and gate cross-section, cold slug interceptor, venting etc.
• Additives such as pigments
• The polymer contained in the moulding compound.

Conclusions to be drawn from surface defects

1. Regular local defects

If surface defects regularly occur in the same place this indicates that there is a problem in the injection nozzle or the hot runner nozzle. The shape and design of the runner, gate or the moulded part itself may be responsible, e.g. sharp edges, sudden changes in wall thickness etc. Another cause may be moulding conditions such as the injection profile or the change-over point.

2. Irregular local defects

Where surface defects occur irregularly in different places, one should look at compounding (compound quality, presence of dust). Factors such as low melt temperature, back pressure, screw speed and screw retraction can also play an important part.

3. Surface defects covering large areas

This kind of defect usually extends over the entire moulding and is often visible already on the sprue. Here one should check whether melt decomposition has occurred. This is done by forcing a shot of melt into the open and observing whether, for example, it contains bubbles. In the case of hot runner systems, this method can be used with only limited success. Melt decomposition may be due to polymer degradation or decomposition of additives, caused by overheating or excessively long residence times. In the case of hygroscopic polymers, an important part is also played by hydrolytic degradation if the moulding compound has not been dried sufficiently.

General recommendations

Parts made from partially crystalline engineering polymers should preferably not be made by hot runner injection moulding if a perfect surface finish is essential. It is advisable to make use of a subsidiary runner, which thermally isolates the nozzle from the moulded part, thereby reducing the risk of surface defects.

The cold slug coming from the injection or hot runner nozzle should be intercepted by a special device opposite the sprue so that it cannot get into the moulded part.

The following table lists various surface defects and ways and means of eliminating them. In practice however, different surface defects appear simultaneously, which makes any investigation as to their origin and their elimination much more difficult.