Chopper Gun Layup

Open Moulding by Chopper Gun Spray Up or Layup Method

Open moulding using a chopper gun or depositor is one of the most common and popular method of fibreglass manufacture especially for large parts. Recent technology in resin spray flow has enabled the reduction of VOC emissions to that approaching hand lay-up. In addition resin atomisation and controlled spray patterns reduce waste due to over-spray to a minimum. Efficient for short to medium production runs the process also allows good flexibility for laminate variation, inserts, local reinforcement and selective laminate placement.


In brief the glass fibre reinforcement, resin and catalyst are is sprayed into the mould simultaneously. The reinforcement is supplied as a string and literally 'chopped' to length (usually about 20 to 25mm during the spray-up process - hence the name chopper gun. Laminator experience and in line flow meter and or weighing systems are used to control part thickness and reinforcement distribution. In experienced hands and using thickness gauge and visual inspection the process can deliver accurate laminate and precise resin to glass ratio yielding good mechanical properties.


As stated above the laminate resulting from chopper gun layup meters the resin:glass ratio to give good strength and allows for inserts and reinforcement placement in specific areas. Additional structural laminate components such as woven rovings can be added at any time in the process and surface tissue can be specified to finish the laminate. 

As discussed elsewhere, the resin matrix is primarily used to hold the glass reinforcement in place. While the resin is relatively flexible the glass is rigid with high tensile properties. The more accurate the use of resin and the better located and tightly packed the glass matrix, the more mechanically efficient the reinforcement ie the lower the resin : glass ratio the stiffer the part. Consequently experience, quality control and accurate metering are required with chopper gun manufacture to achieve optimum results.


Depending on the the part usually one item per mould per shift is achieved. Excellent off mould finish is seen on the exterior visible surface and the interior face can be specified from raw laminate to resin rich with or without surface tissue or flow-coat if a painted or cleanable surface is required.

Inclusion of other components such as timber, metallic, plastic etc can be done during the production cycle. Skill, timing and proprietary methods are required to avoid telegraphing of the insert piece due to shrinkage movement during cure.


For the client, the primary benefits are lower cost mould and speed of production in large bulky parts.