Definition: wall thickness is the thickness of a wall of plastic part from inside to outside. Shrinkage: the fractional difference in corresponding dimensions between a mold cavity and the molding made in the cavity, both measured at room temperature. It may be reported as a percent, in mils/inch, or mm/m.

One of the first rules learned in many plastic part design seminars is that one needs uniform walls, or more specifically uniform wall thickness. The reason being is that if all are uniform than flow shall be uniform, shrinkage should be uniform, and cooling well be minimized to name a few.

Shrinkage as listed in all material suppliers data sheet is typically based on an average, and a specified wall thickness. It is further based on an un-impeded part meaning that there are no ribs, holes, undercuts or other intrusions to it being allowed to shrink. Basically think of a flat bar gated on one end that than can shrink freely back to the gate. If in fact the material is unfilled it may typically shrink in all directions uniformly (isotropic) and if filled it may shrink differently in flow versus cross flow (anisotropic). It is up to part and tool designer to specify, but the molder and conditions they typically run can also play a major part in how a part shrinks.

While wall thickness that is uniform allows for a calculation of uniform shrinkage in the design state, it is dependent on flow length which may have an issue to play in what the resulting shrinkage is in actuality. Though it is hard for some to follow, as we flow material into a cavity, there is a gradual or drastic pressure loss across the cavity, (dependent on size) thus at the gate area the pressure is high and at end of fill pressure is a bit lower, which in fact equals more molecules of a plastic at the gate area and less at end of fill thus we have low shrinkage(more molecules) at the gate area and higher shrinkage (less molecules) at the end of fill. In many cases not an issue, especially with amorphous materials, but may become an issue with semi-crystalline materials due to higher shrinkage values.

The bottom line is to know the flow to thickness ratio, and use this as a reference to your shrinkage results on the actual tools. If we look at wall thickness than one can apply as some articles have stated what the flow to thickness ratio is, and given an indication as to how much clamp tonnage might be needed and as well whether we have enough injection pressure and speed to fill the part. For those needing a review Flow to thickness is: measuring the flow length from the gate to end of fill and divide by wall thickness.

The following results for flow length:

1. Less than 100:1 normal molding condition
2. Greater than 100:1 but less than 200:1 the need for upper end clamp pressure and speed/pressure to fill the part properly
3. Greater than 200:1 than higher than standards for tonnage, pressures and speeds plus steel thicknesses may need to be increased.

*** The above are just guidelines and are dependent on materials but good rules of thumb.

Example 1

This occurred with the author too many years ago in glass filled nylon, of a relatively thin wall design (1mm/ 0.040"). This particular design, was such that a thin U beam of various lengths were being molded, and in a tool which allowed for cavity insert interchange, thus runner and gate remained identical and only the flow length changed, the gating on one end to maintain flatness in part. Wall thickness were uniform and identical for all part lengths, and as can be imagined, when the part length got longer the variation in width became greater as well as length calculations as to shrinkage factors, all though it is common with glass filled material to vary flow versus cross flow, it was not at the time common (at least in that shop) to use multiple cross flow shrinkage calculations.

In the longest configuration prior to tool modification it was found that the gate end was oversized in width while the end of fill was undersized in width. The shortest and first insert was good; the second or medium length used most of the tolerances but was capable of producing acceptable parts.

In the end various shrink rates were used and the total of all tolerances were used for the part, which still allowed for a functioning part.

Example 2:

In another part of various wall thickness's it was found that for the majority of the part a rate of approximately 0.005"/inch resulted, but for a critical fit portion of the part the shrinkage was as high as 0.025"/inch. The base material was ABS which is amorphous and typically one uses anywhere between 0.003 and 0.006"/inch based on a wall thickness of 0.125". Note that nominal wall was at 0.125" thickness while the component of higher shrinkage was a bit thicker.

In reviewing the part the gate was in the thinner section, thus material was flowing from thin to thick, and cooling for the thicker section of the part was nonexistent. Thus this area experienced less pressure, and less cooling resulting in this greater shrinkage. Once this condition was realized the new components for the new detail could be cut into steel with the increased shrinkage value and the results were that it functioned properly in the finished product.

On a side note to shrinkage the issue with fill speed, (time) in that a slower fill versus a fast fill may in fact produce a difference in the shrinkage values across certain parts. For those whom mold thin walls, or find that they must both fill and pack on the filling of the part, in many cases shrinkage and the calculation of it is a trial and error the first time out and then a calculation based on history. The issue than becomes that the process should be maintained, and also that of material, or materials used for the manufacture of this product must remain constant.

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