How much to use or do you know what your rate is?

Definition: Shrinkage 1: drawback 2: reduction in size 3: The fractional difference in corresponding dimensions between a mold cavity and the molding made in the cavity, both the molding and mold cavity being at room temperature, when measured. **Shrinkage is often found to be different in different directions, and listed in various formats, percent, inches/inch, mm/m, etc.

Shrinkage is a fact within plastic molding. The amount of shrinkage is dependent on many facts, but in most cases the processors specify the shrinkage levels based either on the data from a materials supplier's data sheet, experience and or as some say a guess. In many cases of tool building the mold maker asks what shrinkage factor should be used, and by doing so is trusting in the processor to know how much shrink they get.

In a generic sense shrinkage is the oversize that a tool must be built to achieve a final part size of the correct size. An example is that if one need a cube to be a perfect 1" x1" 1" than one would need to first know the material to be used, and second to know the rate of shrinkage. Thus if the materials was ABS and someone used a factor of 0.5% the tool would be cut to 1.005", yet this may be too little as the part thickness is thicker than what was yielding the 0.5% data point. It is also now important to know if there are other data points existing and or has the molder made similar products in their plant of which can be calculated.

Within the typical data sheet it is possible to find the shrinkage of a general sense for the material, a couple recent examples were:

Material A: Mold shrinkage @ 0.125" thick is 0.005 to 0.007

Material B: mold shrinkage @0.125 thick is listed at 0.014-0.018"

Material C: mold shrinkage @ 0.125" thick is listed as flow direction 0.003" transverse flow 0.007"

Each of the examples listed is a different material; with data taken directly from the supplier's information sheets. Material C is a filled material and shows that there is a difference in flow direction as opposed to cross flow, and seems to be fairly specific. This difference in flow to cross flow can be found in many materials not only those that are filled. Shrinkage as a volumetric change effects both thickness, length and cross flow shrinkage values differently for each and every material, even though many try to characterize it as one number.

The problem is only just beginning as all data is given only in one thickness, and that is not even close to what was described as the cube at 1" in thickness. In thinking on this one would estimate that the cube is going to shrink more than what is listed but then the question is how much more. The shrinkage of material is based on many things but wall thickness is one which is very important. Normally thicker shrinks more than thinner walls, and can really be dependent on the ejection temperature of the part. Warpage a topic unto itself is a result of variation in wall thickness and shrinkage of the various wall thicknesses.

Again is there a mold within the shop that has this wall thickness to measure and see what the actual shrinkage might be? The better question is what shrinkages are being achieved in the shop on the tools currently running.

Not many shops or processors really worry about this fact. The report from quality or first articles state that the part size is good, so they go on doing what they want, or they may hear that the part size is too big, or too small, and then try to mold out the difference by increasing or decreasing, pack, temperatures (both melt and mold) and any other little tricks to get the part into size. While this is a typical scenario the question that should be asked is what was the tool cut to? What is the steel size and how much actual shrinkage are we achieving? Yes a heck of a lot of work, but work that creates the data base for more profits in the long term. Why, because now it is defined what the actual shrinkage is per how the shops processes, and when a new tool is designed there is a better understanding as to the shrinkage to occur and what to use.

Shrinkage of the polymer is based on how it cools, how dense it has been pack out, wall thickness, gate size (related back to pack), and a few other factors.

It should also be noted that this same supplier provided great guides to shrinkage in that they showed what happens to shrinkage at various wall thickness's , mold temperature, gate opening size and injection pressures. Another material supplier provided graphs of their material and estimated shrinkage based on wall thickness.

Another great source is the SPI guidelines, "Standards & Practices of Plastic Molders" sponsored by the Molders division of the Society of the Plastics Industry, Inc.      http://www.plasticsindustry.org/ 

While these are all great resources, if one does not compare back to what is actually happening within the mold in questions than one does not know. Another example from early in my career was a tool maker whom had an association with a molding shop, whom was considered expensive for his tool quotes. The funny thing was that even though he seldom got the job on the first go around, in many cases the client came to him to fix what the other tool house did not comprehend, and that was how the various materials shrunk per the geometries used. His book which he guarded contain all the data of steel to plastic size, process conditions, gates used, and calculated shrink. On the positive side he then was awarded many of the following tools. These numbers were generated over years of doing and maintaining. The question is how is your book doing, or do you have such a book? If you are doing critical moldings of tight tolerances one would hope that your book is maintained.

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