Definition: Shrinkage 1: drawback 2: reduction in size
In the first section the standards were discussed and some points to remember such as the samples which predict the shrinkage is created under ideal conditions, as described by the supplier and if under ISO the molding conditions are now specified.
Given that we understand shrinkage as that function of part design and tool design which may be somewhat predictable it is important to know how much you are getting. This is an involved and sometimes hard function to do in shops. It involves the measurement or knowing the measurement of the steel and then measuring the corresponding plastic part, 24 to 48 hours later and preforming the calculation as stated in the previous article. Why? Because it can prevent the headache in processing and getting the correct size of part out of the tool. This gives data, which if documented can prevent and assist in the development of future tools to the correct size.
An example illustrates this.
Part A was molded out of ABS resin and the molder was told that the parts were too small, they are shrinking too much is what the engineer told him. You must be doing something wrong! The molder than frustrated had the tool measured, and then after allowing the parts to cool measure them. In doing the calculation he found that the shrinkage was only 0.002" per inch. Since ABS has a nominal shrinkage of 0.004 to 0.006" per inch he now knew he packed out the parts, but also realized the tool was cut incorrectly for the shrinkage. There was nothing that he could do to get the part size correct. Also on a side note he was concerned that by over packing the part he had caused undue stress to the parts and they may fail in the field.
The typical shop may say too big or too small and then leave it up to the processor to dial in the part. Without knowing the actual shrinkage, one is limiting ones process window and also the functionality of the part in the field.
The issue of process and shrinkage is a great concern, especially in this day of tool transfers. One has to understand that different process conditions well result in different part sizes for the same mold. This can be as simple as using a fast fill versus a slow fill or not using a gate freeze, or hot or cold steel temperatures. This is especially true when tools are transferred and one has to match what that other shop has done, since they were making the part to size or so they say. From that stand point one should measure one or two dimensions on the tool and the finished part to see what the tool was cut too for shrink. It can save a lot of time and headaches.
The biggest issue with predicting shrink is that we have many bosses, ribs and holes along with other part protrusions sticking through our part and or diameters holding the part from shrinking. Basically the tool can act as a shrink fixture, and in some cases molders do use the tool for just that leaving parts in the tool for extended periods of time so as to get the correct part size necessary to match some print or dimension. Is this correct no! But it is part of the molding shop mentality and design that all have tended to experience or have been through. (From a management point of view it is get me parts)
In most cases this is where experience comes into play in that you have a similar job or part and know under your conditions what it is shrinking. In some cases one can adjust with large rules of thumb such as if we have bosses and ribs the shrinkage is cut in half. This is where the history of how parts shrink in your shop from doing the homework above comes into play and you can now predict what the shrinkage well be, due to the measurements and documentation that have been done in the past.
The other issue is wall thickness and its effect on shrinkage, stated to its basic point, the thicker the wall the more shrink, the thinner the less. If one uses the standard of 0.125" (3.175 mm) as the mean for wall thickness, then above that point means more shrinkage and below that point means less shrinkage. An example of this is from polypropylene in that a chart shows that at a 0.040" wall thickness, the shrinkage is approximately 0.013"/" while at 0.300" wall thickness it is 0.025"/" while nominal at 0.125" wall thickness the shrinkage is 0.018" to 0.020"/" . The data point for the ISO specification is at 2mm wall thickness.
A word of caution here: in many cases some well use part weight to justify or specify that a part is within size, this is not true, as an adjustment of fill speed can yield two parts with identical part weight but differing part sizes.
SL Silvey
silveysplastics@hotmail.com
