Thermoplastic Tube Winding Explained

Though laser assisted tape winding has existed for almost 30 years, the process has been industrially available for less than 15 years and isn't generally well known. This article should give you the basic you'll need for a successful configuration. 

Before the configurator can be explained explained, first the terms for a production with this AFP process need to be defined:

  • Mandrel – A rotational body on which the composite is wound. The mandrel is extracted after production is finished.
  • Track – A singular strip of tape wound around the mandrel as a given angle relative to the rotational axis
  • Layer – A collection of tracks that enclose the perimeter of the mandrel without gaps or overlaps.
  • Winding Type / Strategy - The choice of either angle winding or cross finding. Further explanation may be found below.
  • Waste length – The inherent length on either side of the tube where the process is unstable. This is dependent on the winding angle,  tape width and mandrel diameter and dynamically calculated. Waste length is always removed before shipping.
  • Good length / component – The defined length of a part to be delivered.
  • Winding – The cycle of preparing the mandrel, tape and machine for the production of a single part. A winding has minimum one part or one mother tube.
  • Mother tube – A singular winding of multiple components in a long length.
  • Steps - Steps define a collection of layers that have the same winding strategy.
  • Effective Winding Angle - A function of the target winding angle, the diameter of the mandrel and the width of the tape. The effective winding angle is the angle closest to the target angle, which allows a layer to be completely closed without gaps and overlaps. 
  • Component Quality - The quality of a part for a given application. This can be consolidation quality, porosity, fiber-matrix adhesion of dimensions. 
  • Production Quality - The capability and stability of a production process based mainly on the laser control system.
  • Production Certificate -A document which reports the actual winding angle, consolidation pressure, and speed for each layer, as well as the tape material batch information, produciton data and certificate of compliance with our internal process limits for a given polymer. 

The hierarchy of a winding is as follows: 

  1. Track
  2. Layer
  3. Step
  4. Part
  5. Winding
  6. Order


Optimize Price Per Part

Where here is flexibility in the part design and the focus is to optimize the part price, the following two points will contribute the biggest difference.

  1. Tape width
  2. Part length


Tape Width

The dimensions of a UD-thermoplastic prepreg (tape) are typically 0.15-0.25 mm (0.006-0.01 in) thick and 6–50 mm (approx. 0.25-2 in). The standard tapes offered in Alformet's web shop are all in the given range of thickness and have two width ranges: narrow (10-15 mm including 1/2 in) and wide (20-26 mm including 1 in).

A wider tape allows higher placement rates, which means faster productions, which contributes to lower part costs. However, the wider the tape and the smaller the diameter, the worse the prodution quality. Thus only narrow tapes are allowed for smaller mandrel diameters.


Part Length:

The main incurrence of cost for a high mix-low volume production is the secondary time pre and post production. Therefore, the most expensive part prices are singular pieces that can only be done in a singular winding. The maximum winding length is thus dependent on the length of the winding tool. If your single piece doesn’t fit onto available tooling, just ask us and we’ll advise you on options!

To optimize the part price of components where multiple fit into a singular, rule of thumb is that the more pieces that fit on a singular winding, the more that waste and secondary time costs can be spread and thus the lower the part price.

IMPORTANT: There will be scenarios where increasing the number of parts causes a more expensive price per part. These so called price jumps occur when then total number of parts causes multiple windings where the length of a winding mandrel is not optimally used. For example, if 10 components of a given length fit on a specific mandrel, the optimum part price will at 10 components. If the number of components is increased to 11, the price per part will be significantly more than 10 parts because two windings will need to be done to fulfill the order. Increasing the quantity will continue to bring down the part price until the next optimum is reached at 20 parts, which full utilizes the two winding cycles.


Winding Strategy

There are two primary strategies for winding UD tapes on a mandrel. They are called angle and cross winding


Angle winding is the strategy that encloses a singular layer with the respective number of tracks. In this strategy, a track is automatically fed forward, attaches to the substrate with the ‘add-on-the-fly’ function, winds the length of the part, and consolidates the track to the end using the ‘cut-on-the-fly’ function. Finally, there is an empty run back to the beginning of the part where the cycle is restarted.

Cross winding does not have any ‘dead time’ as the material in continuously run back and forth along the length using so called ‘turning zones’, therefore an equivalent plus and minus angle are wound into each other in a type of braided pattern. At the cross-points of the two layers there are fiber undulations and generally a wider variance of wall-thickness.

When choosing between the two strategies, the primary considerations are cost and quality. Cross winding is generally more cost effective because there is no dead time but angle winding has generally higher quality being that each layer is singularly finishes. Another consideration is that cross layers can only be lain in pairs, thus the wall-thickness tolerance is larger with plus or minus two tape thicknesses. Both strategies may be implemented into your component using the step functions.


Tolerances

The inner diameter is determined by the winding tool which is very tightly tolerated with an h7 fitting tolerance. For larger mandrels, the CTE of steel plays a mild role, depending on how the mandrel is pre-heated.

The wall thickness or rather outer diameter tolerance is determined by the discrete application of tape layers, as well as thickness variations of the tape. Parts ordered though Alformet's web shop are produced according to the number of layers defined by the configuration. If a specific outer diameter is important to a component, this may be written in the comment section of the configuration, including the definition of a plus tolerance, minus tolerance, or a ‘close as possible’ tolerance. The tolerance field for wall thickness is defined by one tape thickness (or two tape thickness for cross winding).


For disclaimers, please see our terms and conditions regarding performance of the components. 


Enjoy your tube designing!