Coming to the plastic extrusion dies there are standard dies or custom dies. Standard dies are those used to produce shapes like rods or T-shapes. They can be both simple and complex. Complex dies are those made according to the requirement of the customer typically for shorter production runs. An image is shown here of a modular complex die for producing window profiles that has a very tight tolerance level for the finished product.
A typical schematic is shown here of a melt flow of plastic through the extrusion die.


Types of Plastic Extrusion Dies
Various types of dies are used in the plastic extrusion process. Some of them are described here:

• Sheeting Dies
  Coat hanger or Sheeting dies are very popular for extruding plastic profiles. One of the primary advantage of this type of die head is that they evenly distribute melt to the approach or land region. They are available in single or muti channel versions.
  
  
  
• In Line Dies
  In line dies offer the most popular choice for processing of pipes. In line dies have single or multiple hollow sections. The shapes formed by in line dies are generally complex. For pipes, the in-line die system comprises an outer section that forms the outside of the pipe and a mandrel/torpedo that helps to form the inner surface. The mandrel gets mounted on an generally on an appropriate carrier like spiders within the body of a die.
  
  
  
• Coating Dies
  These dies are used for coating applications. For example the die shown below can coat a pipe up to 21 inches in diameter.
  
  
  
• Slot Coating Dies
  Slot die coating gives an efficient and controllable alternative. It can pre-meter coating fluid at an uniform rate. Another positive feature is that it offers a fully enclosed system which effectively addresses the problems of airborne emissions. Slot coating die is able to maintain fluids at the proper temperature for application.
  
  
  
• Profile Dies
  Profile die with virtually almost unlimited complex shapes are designed today. This gives a near uniform exit flow all over the entire profile.
  
  
  
• Pipes and Tubing dies
  Pipe and Tubing dies these days are designed using the latest computer simulated methods. These dies are further CNC machined for getting tight tolerances. A good design of the tubing dies would mean even distribution of the polymer melt, to achieve tight tolerance along with uniform pipe thickness distribution.
  

Design Criteria for Plastic Extrusion Dies
The following points play a decisive role in making die designs to work successfully for the plastic extrusion dies.

• Making Dies simple
  A question that needs to be answered is whether the complicated cross section is at all needed.
  
  
• Eliminating unnecessary designs
  There is need to eliminate designs where there is chance of melt hang up.
  
  
• Uniform outside wall thickness.
  
  
• Interior supports/wall
  Interior supports/wall should be uniform.
  
  
• Symmetrical Profile
  The target should always be for a symmetrical profile.