The lost foam model production

The lost foam casting uses the foam plastic model to replace traditional casting production method by models. Different from other casting process, the factors which affects the casting quality are different. Because the model is the necessary consumable in the lost foam casting process, every casting production needs to consumes one model. Therefore, the model is the key point for success or failure of lost foam casting. If there is no high quality of model, there isn’t high quality of casting.

The model not only forms lost foam casting’s shape and size, but also participates in the physical and chemical reaction of the casting & modeling processes. So, the model affects both the casting size precision and casting inner quality.

In the lost foam casting, the foam plastic model is a very important point. For complex shape of castings, high casting size requirement of castings and high surface quality requirement of castings, we adopt the foaming modeling process. Whatever kinds of resin beads, the model production process is basically same.

2.3.1 The pre-foaming

     After choosing the raw material, it begins to pre-foam. The pre-foam process is short. From material feeding to the material discharging, it won’t exceed 2 minutes. However, only we master the foaming principle and process correctly, we can control pre-foaming density and surface quality precisely. The pre-foaming density is the key factor to affect the model weight. The correct master of pre-foaming process is the strong guarantee of model’s molding quality and decreasing casting’s slag inclusion defects.

2.3.1.1 The pre-foaming working principle

  When the resin beads(raw material particle) is heated to lower softening temperature, the beads can’t foam. Only the foaming agent spills over. When the temperature rises to the resin softening temperature, the beads begin to soften and has plasticity. Because the foaming agent in the bead vaporizes due to heat and produces pressure to make beads expand and form non-communicating honeycomb structure. Once the foaming hole forms, the inflation hot air makes the pressure increase gradually in foaming hole, which inflates further. In the foaming hole inflation process, the foaming agent also spills over and stops until the pressure in and out of foaming hole is equal. After cooling, the foaming bead size is fixed. The bead pre-foaming is generally made in intermittent pre-foaming machine. Different pre-foaming machine has different operation parameters. However, the operation process is basically same. The process is as follows:

Pre-foaming: The pre-foaming is the first work after starting up. According to raw material choice(EPS or STMMA), we confirm pre-foaming temperature. The pre-foaming process is to uniformly heat to the barrel’s preheating temperature. If it is steam pre-foaming machine, it needs to exclude condensate water in the barrel. The barrel temperature is uniform, which can guarantee uniform pre-foaming.

The material feeding: it has two types of manual material feeding and automatic material feeding. The manual material feeding needs to test weight every time. The automatic material feeding needs to add raw material according to weight sensor or volume. Under the precondition of fixing parameters, it pre-foams the stable density of pre-foaming material.

The heating and foaming: it is the complex control foaming process by steam heating and electric contact point pressure gauge to control barrel pressure after adding raw material to guarantee pre-foaming bead’s pre-foaming process, pressure control and guarantee barrel pressure & temperature within reasonable range. And it can effectively guarantee the  pre-foaming beads uniformity and pre-foaming effect.

The material discharging: it is the last process of heating and pre-foaming. The pre-foaming machine discharges material after fixing heating time to guarantee particle density. The material discharging process should be clean to avoid second pre-foaming which will lead to oversize particle.

The drying: The humidity of pre-foamed material(steam pre-foaming machine) is big. Under the roles of air blower and hot slices, the pre-foamed material surface is dried and scattered. By this way, the pre-foamed material won’t be blocked in the conveying process.