Injection molding is demonstrating its versatility again in the production of challenging optical plastic parts such as LED lenses for automotive headlights with tolerances in the micron range. A new development in multilayer injection molding allows productivity for the production of thick-walled lenses to be increased even further
What layer sequence is most appropriate, how many layers are necessary and what savings potential can be expected? These questions can be answered with a few considerations that apply to one-sided and two-sided overmolding, but not to the bonding of two preforms (Figure 1). Since the heat removal from the connecting layers follows relatively complex laws, this alternative will not be considered. First, two assumptions are made. Firstly, known mold concepts (index plate, rotary table, sliding table) are used. Ideally, a further preform is manufactured simultaneously with the overmolding of one preform. From this the requirement, known from multicomponent injection molding, follows that the cooling times of all layers must be the same.
Second, it should be noted that only the layer manufactured first is cooled on both sides (layer 1 in Figure 1). The following layers only have contact with the mold wall on one side, while the preform borders on the other side, which, for the sake of simplicity, is regarded as an ideal insulator. To obtain the same cooling time in all stations, the following layers, which are cooled on one side, should be only half as thick as the first layer, which is cooled on both sides.
On the assumption that the first layer of a part produced from n layers is only half as thick as all the following layers, the layer thicknesses se of the first layer and sf of the following layers are as below (equations 1 and 2), where stotal describes the total thickness of the part:
The cooling time is proportional to the square of the wall thickness. The cooling time te(n) of the first layer (which is cooled on both sides), corresponding to the cooling time tf(n) of all further layers (which are cooled on one side) is described by the equation 3:
During overmolding on one side, n stations in the mold are necessary, i.e. as many stations as layers. In the sandwich variant, in which two layers lying one behind the other are produced simultaneously as in a stacked mold, only (n+1)/2 stations are necessary. In this case, the number of layers n must be odd. Overmolding on two sides, compared to on one side, does not at first offer a cooling time reduction, but provides the advantage of the stack mold, i.e. a saving of platen area and clamping force. The relative cooling time per mold station, i.e. the cooling time compared to that of a single-layer part, is described in equation 4:
The number of parts T(n) per unit time corresponds to the reciprocal of the time per part, i.e. the reciprocal of the cycle time. Instead of the cycle time, the cooling time is used here, which is permissible to a first approximation for very thick-walled parts. From the reciprocal of equation 4, the relative number of parts is thus obtained (equation 5):
However, the cooling time or number of parts are only conditionally suitable as values for an efficiency comparison. These values do not take into account the fact that multilayer processes require a larger number of cavities. However, increased efficiency can be expected from larger numbers of cavities anyway. If with single-layer processes, for example, twice the number of cavities is available, the number of parts per unit time would also be twice as high.
The productivity is therefore used for the further assessment. It is defined as the ratio between the produced parts and the production factors necessary for this, in this case the cavities. To obtain the relative productivity of multilayer molding compared to the single-layer method, equation 5 only needs to be divided by the number of cavities – i.e. by n in the case of overmolding on one side and by (n+1)/2 in the case of overmolding on both sides:
for overmolding on one side (equation 6), and
for overmolding on both sides (equation 7).