Vacuum Pultrusion: A Process for Manufacture of Large Cross-section Profiles
December 30th, 2019
PPS 17, Montreal, Canada, 21-24 May 200l
J M Methven, S F Bush and A J Hulme, Polymer Engineering Research Lab, UMIST Centre for Manufacture, Manchester M60 1QD
The conventional pultrusion process allows continuous manufacture of a fibre-reinforced composite profile of constant cross-section. Important refinements to the basic process have been made within the UMIST Centre for Manufacture over the past few years (1,2) and this work describes a further development.
For pultrusion of a solid profile, a heated steel die that has the required section shape is used to form the package of resin and fibre that makes up the profile composition. For manufacture of a hollow profile, the steel die must be complemented by a heated steel mandrel that is located within it and supported as a cantilevered beam at its upstream end. In this configuration the outer surface of the mandrel forms the inner surface of the profile and the inner surface of the die forms the outer surface of the profile. As well as an increase in the cost of tooling, the mandrel surface imposes an additional friction loss over that of the die surface and this results in either a lower pulling speed or in the need to use a puller that can exert a greater pulling force. Moreover, the provision of a mandrel inevitably results in an increase in length of the die assembly while the design of its form and supports that ensure it lies coaxial with the die are not trivial tasks.
In many applications, the fact that a profile is hollow reflects simply the need for it to be light. Under these circumstances it is possible to replace the hollow channel in the profile with a lightweight composition and achieve approximately the same goal. The most obvious light-weight material for this purpose is a polymeric foam such as a polyurethane or a phenolic. However, synchronising the blowing and gelation of a foam composition within a pultrusion die with the gelation of the pultruded profile as it passes through the die is difficult to achieve continuously in a factory environment.
A viable alternative is to create continuously within the pultrusion die a foam in the form of resin-impregnated mineral particles such as expanded clay or glass (3). In essence this performs as a mandrel that becomes incorporated into the profile. It is this approach which forms the substance of the present work.
The product selected as a vehicle for this project is a 30mm diameter cylindrical rod to replace a solid hardwood dowel currently used as a decorative curtain pole. The pultruded replacement must incorporate a decorative surface veil or film that simulates the surface appearance of light mahogany or other hardwoods. The preferred density of the profile is around 750kg/m3. An additional requirement is that the pultruded profile should exhibit a centre-line bow of no greater than 4mm over a 1.5m length. A schematic of the profile composition is shown in Figure 1.
Expanded clay is a relatively new product made by English China Clays (ECC). It is available in the form of cylindrical prills of density around 400kg/m3. Alternatively, expanded glass “marbles” (Poreva) may be used. They are used widely in the horticulture market for drainage and for decoration, and are readily available.
The vacuum die assembly is shown in Figure 2. Upstream elements (to the right of the symbol ‘A’ in Figure 2) include the assembly of the veils, mats and roving, and the delivery of the (dry) expanded core. These elements will be described in the full paper.
In Figure 2, the formed core and inner veil enter at point A. At the entrance to the vacuum chamber the dry reinforcement that forms the skin of the profile is introduced and the total package is pulled through a perforated tube that is located along the axis of a vacuum chamber. The profile passes through the vacuum chamber into the resin injection element and from here to the heated die wherein the resin crosslinks prior to the haul-off.
The resin injection step of Figure 2 is not uncommon in pultrusion. By contrast it is the combination of the vacuum chamber and the upstream process elements that make the vacuum pultrusion process unique. For example, upstream of the resin injection element the integrity (quality) of the profile is governed by matching the supply of dry core (by means of the applied vacuum and the upstream screw feed) to the line speed – that is to say the linear speed of the mat, roving and veil. If the supply of core is too great then the inner veil will expand and will either rupture or will block the vacuum tube. Too low a supply of core will result in poor consolidation of the profile.
The paper describes the procedures used to establish a process window for the manufacture of 30mm simulated hardwood poles by vacuum pultrusion. By using a conventional (high maleic) unsaturated polyester and a combination of peroxide catalysts it was possible under factory conditions to manufacture a pole of acceptable quality and cost at a line speed of 1.2m/minute.
1. J M Methven, Microwave Assisted Pultrusion – Continuous Manufacture of Composite Profiles, Materials Technology 14(4), 183-85, 1999.
2. J M Methven, S R Ghaffariyan and A Z Abidin, Manufacture of Fibre-Reinforced Composites by Microwave Assisted Pultrusion, Polymer Composites 21(4), 15-27 August 2000.
3. M J Hewitt, UK Patents GB2202180B (8 February 1989), GB2202182B (8, February 1989), and GB2160143B (12 April 1989)