Monday, July 29, 2013

Recycled plastic contraction and expansion caused by temperature changes

As temperatures rise, recycled plastic expands, and as it gets colder so it will contract. Any construction will have to allow the material to expand or shrink; especially in the case of large constructions, such as fencing along dozens of metres. Due to the long length expansion and shrinkage will become too large to be intercepted by the flexibility of the construction. In the case of smaller constructions it depends a little on how much the construction can transform and by doing so can intercept expansion/shrinkage by itself. Nevertheless we advise to always take this material behaviour into account.

Mixed recycled plastic profiles will expand to a maximum of 1.5 mm per metre plank at 10°C. In the case of outdoor applications we expect that mounting normally takes place at a temperature between 10 and 20°C. We assume that -20°C is the minimum temperature. In summer, when the sun is shining on the planks, temperature can rise to approximately 50°C, particularly in the case of black. The largest possible temperature difference will then be + or -40°C. Therefore it has to be taken into account that the length of the plank can become 6 mm longer or 6 mm shorter per span metre.

Example expansion/shrinkage
If in figure 1 the centre to centre distance of poles is 1.25 metre and the plank is attached to one of the poles, you should take into account an expansion of 1.25 x 6 = 7.5 mm (ÄL in figure 4.2). Shrinkage could then also be 7.5 mm. Therefore the slit hole has to have a width of 15 mm (centre to centre). When mounting the screw has to be placed in the middle of the slit hole (see top image of figure 4.2) and, although it has to be tightened somewhat, it must not be tightened completely. The plank underneath still has to be able to be shoved. Further there has to be some free expansion space next to the plank. So planks must not be placed directly next to one another. This is size S in figure 1, which is 15 mm in this example; 7.5 mm for expansion of the plank on the left side and 7.5 mm for the plank on the right side.