1. Economical aspects
Welcome to our course: “Cooling system: very important and yet ignored”. I’m happy to discuss with you the principles of cooling layout, coolant temperature, the use of baffle and bubbler, as well as the difference between conventional and conformal cooling. We will also talk about variotherm cooling, cycle time and warpage.
But first let us take a look to how most of the mold designers work today. We can see that they think first of parts, runner system, mold inserts and ejector pins. Less attention is payed to the most important functional unit – the cooling system. A lot of times the cooling layout is done without thinking about part specific requirements.
This course shows, which meaning the cooling has especially with regards to the economy of the mold. Simulation tools are used to make it easier to understand. Its results should be used parallel to the tool design to get the best possible cooling.
Now, why is a perfect cooling layout essential? This can be easily answered if we take a look to the requirements of the market. Like: – parts with high levels of functionality – Short cycle time – Best quality
So let‘s take a look to the injection molding process. It starts with the filling of the cavity, continues with packing and residual cooling until the mold opens and the part is ejected. Since the plastic is cooled during packing and residual cooling phase we can see that the cooling time is a total of 60-70 % of the total cycle time. This means, the shorter the needed cooling time, the more parts can be produced and the product costs will be reduced.
Moreover the cooling has a huge impact on the formation of the surface. It influences for example the crystallite formation in the surface region. Which itself counts for the ductility of the part.
In addition to that several studies show an interesting aspect if it comes to mold defects. Only 5 % of defects occur due to venting and maintenance issues. Doubled the amount, but still only 10 %, are stability issues. The runner design is responsible for up to 25 % of the defects. The main cause, which is approximately 60 %, more than all the other reasons together, depend on mold cooling.
Considering the total part costs, some simple concepts need to be understood. Let‘s take a look to the process of part development and production. It starts with the design of the part and the mold. Then it is followed by the mold making and the production of the part. At the end quality control is implied to make sure our part meets its specification.
If there is a defect somewhere along the process the “rule of 10” can be applied. It describes how much it will cost to correct a defect dependent on the process step. If we find a defect in part design it costs us 1 currency unit (say one dollar). If the same problem is identified in the next step in the process it costs 10 times more (in our example 10 dollar) to solve the defect. So from one step in the process to another the costs of solving the defect are always multiplied by 10. So it is advisable to identify possible problems as early as possible in the process. This is during part and mold design.
Another perspective to view costs is by taking a look to who proportionally contributes to the total costs of a complete project. It is kind of obvious that the portion on total costs of a project is small if we are talking about part and mold design. If it comes to mold making CAM files need to be processed and heavy machinery needs to be used. Same in injection molding: We need material, resources like energy and water as well as the injection molding machine, robots, etc. In quality control precise machines are needed to check the dimension of the part. In all three process steps (Mold making, injection molding and quality control) the portion of process costs is high.
We understand now the rule of 10 and how big the influence of each step in the process is regarding the total process cost. If we take a look to the responsibility of total project costs we can see that the designer carry the highest loads. This means while designer only create little project cost by themselves they are responsible for highest costs in the project if something is going wrong during their work (as reflected by the rule of 10).
Knowing this it becomes very clear that during design phase all measures need to be taken to identify possible problems in the process. During design phase simulation needs to be used in order to understand the impact of decisions being made. Only then project costs can be minimized.