The fact must be recognized that each plastic material has specific inherent characteristics. Shrinkage, or the contraction that takes place in a molded piece after it has been formed in the mold, comes in this category.
While the problems of shrinkage are of more significant concern to the designer of molds than to the designer of molded articles, both should understand the fundamentals of shrinkage.
Shrinkage is defined as the difference between the mold’s corresponding linear dimensions and the molded piece, both measurements being made at room temperature.
This shrinkage in phenolics generally ranges from 0.001 to 0.015 in. Per in., depending upon the type of thermosetting plastic, the material from which the mold is produced, and the conditions under which the piece is molded.
Of thermoplastic materials, polyethylene will shrink as much as 0.050 in. Per in. And nylon as much as 0.040 in. Per in.
Although the shrinkage characteristics of plastics are well recognized and are compensated for, more or less adequately, each time a mold is designed, the data available are insufficient to permit the mold designer to predict accurately the shrinkages that will occur in various sections of a molded piece.
One reason for this inadequacy of technical information is the complex nature and overlapping of the causes of shrinkage, which necessitate the study of many variables and combinations of variables.
However, the causes of shrinkage have been fairly well agreed upon, and it seems safe to say that the amount by which a piece will shrink is dependent upon one or a combination of the following factors:
- chemical reaction, if any, in the plastic;
- the temperature range over which the plastic cools after its shape has been permanently established in the mold, coupled with its coefficient of thermal expansion ;
- The degree to which the material has been compressed during molding. The first factor is an inherent property of each thermosetting compound and cannot be controlled by the molder or fabricator.
The second is controllable partially by the manufacturer of the material, in that the coefficient of thermal expansion of any plastic is influenced by the characteristics of the resins, fillers, plasticizers, etc., of which it is composed.
Since the coefficients of thermal expansion of plastics are greater than those of the metals used for molds, a molded article that exactly fills a mold cavity at an elevated temperature will be smaller than the mold cavity when both have been cooled to room temperature.
It is controllable also to some degree by the molder; thus a piece molded from the thermosetting resin at 300 °F will be slightly larger than another piece molded from the same material in the same mold at 350°F when both have been cooled to room temperature.
However, other factors may alter this relationship. These are discussed below. ,
The third factor mentioned above is controlled entirely by the molder, and its total effect is the most difficult to predict.
In general, the more the material is compressed, the less the shrinkage will be. Under certain conditions, the molded piece may actually be larger than the mold cavity in which it was formed.
Some of the conditions that may influence the extent to which a molding compound is compressed in the mold are given below:
Specifically, the variables investigated were molding temperature, pressure, and preheating of a transfer-molded piece.
The minimum pressure used with each of the two molding temperatures was that pressure that would fill the cavity and give a sound molding. The following conclusions for the piece in question are immediately evident:
- An increase in pressure, causing greater compression of the plastic, reduces shrinkage.
- Preheating reduces shrinkage.
- Shrinkage parallel to the direction of flow from the gate is greater than shrinkage across the direction of flow, or, in general, shrinkage increases with the increase of flow of material travel in the mold.
- Shrinkage is not materially affected by the change in molding temperature. This may be explained by the fact that at the higher temperature the material was more fluid and hence was compressed to a greater extent by a given pressure, with the result that the expected effect of higher temperature was neutralized. It does appear to be generally true that decreasing the temperature at which the piece is released from the mold to a temperature below that at which the plastic was molded, results in the decrease of shrinkage.
- Shrinkages in thickness is inconsistent, but probably because of slight variations in the closing of the mold, rather than actual variations in the ratio of shrinkage. However, in general, shrinkage appears to be less in the direction of the molding pressure. Although there may often be conditions that counteract each other, as discussed above, an attempt is made below to summarise the usual individual effects of the other factors listed, but not discussed, above.
Design of Piece
Uniformity of cross-section —When a piece has one or two portions considerably heavier in cross-section than the rest, it may be found that, if molding conditions are selected to mold the thinner sections properly, the thicker parts will fail to receive an adequate charge of the material, or pressure, or cure, and as a result will shrink nonuniformly.
Design of Mold
Positive, semi-positive, or flash (compression molding); location and size of gates (injection and transfer molding) — This is closely associated with molding pressure. It must always be remembered that the full calculated molding pressure, which is determined by the size of the ram and the hydraulic line pressure, does not always reach the material in the mold. The pressure in which the molder is interested is that which does the work of compressing the plastic in the mold; this may be called the “effective pressure.”
A positive mold transmits to the material in the cavity practically all of the applied pressure; a flash mold transmits an indeterminate amount, depending upon how quickly the flash over the land sets up. In transfer, plunger and injection molds, undersized gates will restrict the flow of material excessively; also length and diameter of runners, as well as the number of shifts in the direction of flow which the plastic must travel before arriving at the cavity gate, will reduce the effective pressure considerably.
In transfer and injection molds, the location of the gate also is important, since this determines the direction of the flow of material. Comparing transfer molding and compression molding of the same phenolic material, it will usually be found that the transfer molded piece will have less shrinkage.
Plasticity of Material
Of the thermosetting materials, the materials of harder flow will usually shrink somewhat less than the softer materials, which tend to flow away under pressure and to yield a less dense molding. Similarly, of the plasticized thermoplastic materials, the harder materials will usually shrink less than softer materials of the same
Manner of Loading
The use of preforms in compression molding will reduce shrinkage, by facilitating the proper distribution of the charge in the mold cavity, and by precompressing the charge.
A rate of Application of Pressure
Determination of the proper rate of application is a matter of experience. In compression molding, closing too rapidly, particularly upon a soft material, will create a “splash” effect in the mold which will produce moldings of low density.
Degree of Cure
The inadequately cured thermosetting material will usually shrink more than the same material adequately cured. Thermoplastic articles with heavy sections must be cooled in the mold long enough to avoid excessive shrinkage which may cause internal voids.
When multiple-cavity molds are used, slight variations in shrinkage between pieces from different cavities can be expected, due to small differences in temperature, loading, and pressure from the cavity to cavity.
The shrinkage discussed above is that which occurs during and immediately after molding. In addition to this shrinkage, some plastics will shrink an additional amount during aging. This shrinkage during aging is particularly noticeable in the thermosetting materials, and to varying degrees in the cellulose esters, such as cellulose acetate, depending on the type and proportion of the plasticizer used.
Table 12-2 shows the aging shrinkage that occurred in an experimental molding of urea-formaldehyde. In this case, baking for 24 hr at 150°F was considered to be equivalent to one year’s normal aging. The articles were allowed to remain at room temperature for about 24 hr before the baking was begun.
It will be noticed that the aging shrinkage at the top of the molding was greater than that at the bottom, showing the restraining influence of the two ears in the insert. When tests were made with moldings in which the ears were spaced %2 m- apart, the moldings cracked during the baking test.
Consequently, to eliminate any possibility of cracking in service, the design was changed as shown in Table 12-2. Since there are definite limitations on the extent to which the molder can control shrinkage by changes in his technique, the designers of the piece and the mold must compensate for it in their designs.
Closely related to the problems of the tolerances considered in Standards for Tolerances on Molded Articles are the allowances for molding shrinkage which must be carefully calculated for the particular plastic to be used in a given application.
Material manufacturers provide data for the mold shrinkage of each material they produce. They determine shrinkage figures by molding the material in a standard test mold and calculating the unit mold shrinkage as the difference in linear dimension between the mold at room temperature and the molded piece at room temperature, divided by the size of the mold.
Thus all such shrinkage data are given as inch per inch of mold dimension. For the convenience of the mold designer, it may be assumed that this is equivalent to the shrinkage, inch per inch of the molded piece. Consequently, for pieces up to 25 in. in length to be produced from materials with shrinkage of 0.008 in. Per in. or less, the mold dimension can be computed by the formula :
A = B (1+S)
- where A = dimension of mold
- B = dimension of molded piece
- S = unit shrinkage
For the occasional piece larger than 25 in., molded from the material of relatively high shrinkage, it is recommended that the following accurate formula be used:
A = B /(1-S)
Data on shrinkage comes from test specimens, whose shapes may not reflect common commercially molded products.
Mold designers must temper their use of shrinkage data from manufacturers with their previous experiences, as well as considering the factors discussed above.
Molds must be designed in such a way that they can be easily adapted after sampling has been done, and therefore the prediction of shrinkage should be regarded as conservative.
When the material shrinkage value was determined and compensated for in different parts of the article, and under the conditions of molding, the article was held within close tolerances only after the correct shrinkage value was determined and compensated for.
Unbalanced shrinkage and warping result from non-uniform thickness of molded pieces. It can be seen that thin sections cool faster than heavier sections, and heavy sections shrink more than thin sections, so internal stresses result as the piece is being removed from its mold.
The transition between thin and thick parts in a design should be smooth, thanks to generous fillets. When you use uniform wall sections throughout, you get the best results. The use of supporting ribs reduces warpage in large sections.
The best way to overcome warping that occurs on the unsupported side is to place ribs on both sides of the article; this is especially noticeable if the ribs are thin. Plastic injection molding produces internal voids and surface shrinkage marks with heavy sections. Those can be avoided if you design thinner walls and add ribs for stiffness.
In addition, flat surfaces with large fonts or figures should be slightly convex to obscure flow marks. The use of cooling fixtures during cooling to room temperature can prevent warping or shrinkage.
Nevertheless, as the piece hardens and cools, tensions will be set up internally in the product when the average shrinkage of the plastic is prevented. Plastic cracks when stress exceeds its strength.
The problem was corrected by replacing the plastic tubing with thin aluminum tubing in place of plastic, as aluminum has a similar expansion coefficient to plastic, allowing for a tiny amount of yielding when the plastic shrinks. Inserts tend to warp and crack because of their design.
that of plastics, and the ability of the thin tube to yield slightly when the plastic shrinks. Warpage and cracking of this sort are closely associated with the design of inserts.