It is important to recognize that each plastic has its own inherent characteristics. A molded piece shrinks after being formed in the mold, which is called shrinkage.
Both designers of molded products and designers of molds should understand shrinkage’s fundamentals, even though mold designers’ concerns about shrinkage are greater.
As a result of a change in the linear dimensions of the mold and the molded piece, at room temperature, shrinkage must be calculated.
A phenolic generally shrinks between 0.001 and 0.015 inches. Per in., based on how thermosetting plastic is manufactured, the material from which molds are produced, and the conditions under which pieces are made.
It shrinks by 0.050 inches of polyethylene thermoplastics. Per in.Despite being well known and compensated for, in general, plastic shrinkage characteristics, having adequate data to predict accurate shrinkage within various sections of a molded piece can be difficult.
There are many variables and combinations of variables that require study to properly understand shrinkage, which is why technical knowledge is still insufficient.
In general, shrinkage is caused by one or a combination of the following factors, and the extent to which a piece will shrink is determined by one or a combination of them:
In the plastic, any chemical reaction;
Once the plastic is permanently molded, its temperature range and coefficient of thermal expansion determine whether it can start to cool;
The amount of compression that the material has experienced during the molding process. Molders and fabricators cannot control the first of these factors since it is an inherent property of thermosetting compounds.
Secondly, depending on the type of resin, fillers, plasticizers, etc. that a material is made up of, the coefficient of thermal expansion will vary.
As a result of the difference in thermal expansion coefficients of plastics and metals, molded articles that tightly fit mold cavities at elevated temperatures will shrink upon cooling to room temperature.
Also, the molding process can be controlled to some extent when using thermosetting resin; so, a thermoset resin molded at 300°F will have a slightly larger size compared to another thermoset resin molded in the same mold at 350°F after both have been cooled.
Molder control determines the total effect of the third factor above.
Compression reduces shrinkage in general. Molded pieces can actually be larger than the cavity in which they were molded under certain circumstances.
The following conditions may affect how tightly a mold is compressed:
An analysis of three variables was conducted in order to investigate molding temperature, molding pressure, and preheating of a transfer-molded part.
The minimum force required to fill the cavity and give a solid molding was used with both molding temperatures. Here are some immediate conclusions:
- When pressure is increased, the plastic is compressed and shrinkage is reduced.
- Shrinkage is reduced by preheating.
- In general, shrinkage in a mold increases with material flow, and shrinkage parallel to flow is greater than shrinkage in the opposite direction.
- The change in molding temperature has little impact on shrinkage. According to this explanation, the material would be more fluid at higher temperature and, thus, subject to greater compression under a given pressure, negating the effect of the higher temperature. The decrease in shrinkage over time is generally due to the piece being released from the mold at a lower temperature than when it was originally molded.
- There is an inconsistent shrinkage in thickness probably caused by slightly different closings of the mold, rather than an actual variation. Although shrinkage appears to be more pronounced in the direction of molding pressure, in general, shrinkage appears to be less. The following is an attempt to summarize the usual individual effects of the other factors listed but not discussed above, though they may often counteract each other.
|Plastic Material Name||Shrinkage Ratio (%)||Plastic Material Name||Shrinkage Ratio (%)|
|Acrylonitrile Butadiene Styrene (ABS)||0.4 〜0.9||Acrylonitrile styrene (AS)||0.2 〜0.7|
|Polystyrene (PS)||0.4 〜0.7||Ethylene vinyl acetate (EVA)||0.7 〜1.2|
|Poly Propylene (PP)||1.0 〜2.5||Poly Propylene (with 40% glass fibers)||0.2 〜0.8|
|High-Density Polyethylene (HDPE)||2.0 〜6.0||Methacrylic Acid Methyl Ester (acrylic) PMMA||0.1 〜0.4|
|Polyamide (Nylon 6)||0.5 〜1.5||Polyamide (Nylon 66)||0.8 〜1.5|
|Poly Acetal (POM)||2.0 〜2.5||Poly Butylenes Terephthalate (PBT with 30% glass fibers)||0.2 〜0.8|
|Polycarbonate (PC)||0.5 〜0.7||Poly Phenylene Sulfide (PPS with 40% glass fibers)||0.2 〜0.4|
|Liquid Crystal Polymer (LCP with 40% glass fibers)||0.2 〜0.8||Modified Polyphenylene oxide (Modified PPO)||0.1 〜0.5|
|Poly Sulfone (PSF)||0.7 〜0.8||Polyether Sulfone (PES)||0.6 〜0.8|
|Poly Ethylene Terephthalate (PET)||0.2 〜0.4||Polyether Ether Ketone (PEEK)||0.7 〜1.9|
A part having one or two deeply-curved portions that are larger in cross-section will tend to shrink non-uniformly if molding conditions are specified so that the thinner sections receive a sufficient charge or pressure.
Compression molding: positive, semi-positive, or flash; injection molding and transfer molding: location and size of gates – Both of these are influenced by mold pressure. Molding pressure depends on the size of the ram and the hydraulic pressure through the line, but it does not necessarily reach the material inside the mold. Molders are concerned with the pressure that compresses the plastic in their molds; this is often referred to as “effective pressure.”
Positive molds transmit the majority of the applied pressure to the material within the cavities; flash molds transmit a lesser amount, depending upon how quickly the flash dries. In transfer, plunger, and injection molds, undersized gates will result in excessive material constrictions; also, runner lengths and diameters, as well as the number of shifts in flow direction the plastic must travel, will cause substantially reduced pressure.
A gate’s location is also important to determining the flow of material in transfer and injection molds. The shrinkage of a transfer molded phenolic material is usually less than a compression molded phenolic material.
Material with less flow strength tends to shrink less under pressure than the materials with more flow strength, which tend to flow away under pressure and yield a less dense molding. In the same way, plasticized thermoplastic materials will shrink less when the harder materials are used than softer materials
The manner in which the materials are loaded
Utilizing preforms in compression molding will reduce shrinkage, as they facilitate a uniform distribution of the charge within mold cavities and precompress the charge.
A rate of Application of Pressure
It is up to experience to determine the proper rate of application. The mold will have a low density if the mold is closed too quickly, especially when a soft material is present.
Degree of Cure
Thermally-curable thermosetting materials will shrink more when not properly cured than adequately cured thermosetting materials. During cooling in the mold, heavy sections of thermoplastic articles must be kept free of internal voids to prevent excessive shrinkage.
Due to the different temperatures, loadings, and pressure between cavities in multiple-cavity molds, the shrinkage between pieces from each cavity can vary slightly.
It is the shrinkage that occurs during and shortly after molding that is discussed above. Some plastics will also shrink more when they age on top of this shrinkage. During aging, these materials tend to shrink more, particularly thermoset materials, though cellulose esters, such as cellulose acetate, shrink to a lesser degree depending on the type and proportion of the plasticizers used.
An example of aging shrinkage in urea-formaldehyde is shown in Table 12-2. The baking at 150°F for 24 hours was compared to one year of normal aging. Approximately 24 hours were allowed for the articles to rest at room temperature before baking began.
As a result of the restraining force of the two ears in the insert, there was a larger shrinkage when the mold was aged at the top than at the bottom. A baking test was conducted using moldings with ears 2% meters apart, and the molds cracked after baking.
As a result, the design was modified as shown in Table 12-2 in order to eliminate cracking during service. Molders need to compensate for shrinkage and they must adjust their technique in order to do so since there are limits on how much shrinkage can be controlled.
Molding shrinkage allowances must be carefully calculated for each application in order to take into consideration the tolerances considered in Standards for Tolerances on Molded Articles.
Mold shrinkage data is provided by material manufacturers for each material. Typically, shrinkage figures are determined by molding the material in a standard test mold and dividing its linear dimension by its size to calculate the unit mold shrinkage.
All shrinkage data for such molds is thus measured inch-for-inch. It may be assumed that this represents the shrinkage, inch per inch, of the molded piece for the convenience of the mold designer. So, for pieces less than 25 inches in length, this may not apply. Materials with shrinkage of 0.008 in. will be used to make these products. 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)
A test specimen’s shape may not reflect common commercially molded products, so shrinkage data cannot be generalized.
When predicting shrinkage from manufacturer data, mold designers must also consider their previous experience and those discussed above.
Adaptability is essential in mold design, so estimation of shrinkage should be considered conservative after sampling has been done.
After determining and compensating for the correct shrinkage value, the article was held within tight tolerances only after the shrinkage value was determined and compensated for in all components of the article.
Non-uniform molded pieces result in imbalanced shrinkage and warping. When the piece is removed from its mold, it results in internal stress caused by the cooling and shrinking of thin and thick sections, respectively.
A design should have a smooth transition between thin and thick elements, thanks to generous fillets. The best results are achieved when wall sections are uniformly sized throughout. Supporting ribs help prevent warping of large sections.
Adding ribs on both sides of an unsupported article can help overcome warping on the unsupported side; this is especially noticeable if the ribs are thin. A heavy section may leave surface shrinkage marks on injection molded plastic parts. The walls should be thinner and stiffened with ribs to prevent those things from happening.
Flow marks should also be obscured by making flat surfaces slightly convex with large fonts or figures. In order to prevent warping and shrinkage, cooling fixtures can be used during the cooling process.
Although the plastic shrinks towards the middle, tensions will be established within the product as it hardens and cools. If plastic is overstressed, it will crack.
By replacing plastic tubing with thin aluminum tubing instead, the problem was corrected, as plastic shrinks when the aluminum shrinks similarly, allowing a tiny amount of yielding. Since inserts are designed so that they warp and crack, the problem was corrected.
When plastic shrinks, the thin tube yields slightly, imitating the behavior of plastics. Cracking and warpage caused by inserts are closely related.