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In several of my seminars and conversations with customers, I am often asked why the dimensions of a part change over time. This change in dimensions after the part is removed from the mold is called postmold shrinkage. There are a few reasons for postmold shrinkage that we will discuss in this two-part series. In this first segment, we will discuss the reasons for postmold shrinkage related to the thermal transition of polymers known as the glass transition temperature (Tg).

There are several videos on the internet which show what happens to objects when they are quenched in liquid nitrogen. The temperature of liquid nitrogen is approximately -340°F. (Please note these experiments require extreme caution and should not be performed without supervision and proper safety gear, such as gloves and protective eyewear.) In one such video on YouTube — titled “Giant Koosh Ball in Liquid Nitrogen!” — the young scientists drop a Koosh ball into liquid nitrogen. The Koosh ball is made from an elastomeric polymer and is extremely flexible at room temperature. The ball is dropped into and rolled around in a bowl containing liquid nitrogen, and after a few seconds, the bowl is emptied onto a table, dropping the ball from a height of a couple of feet onto the solid surface. The ball shatters into pieces as if it were made of glass.

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If the plastic is below its Tg, it will be brittle; if it is above the Tg, it will be flexible.

So, what happened here? When a polymer is exposed to extremely low temperatures, the energy of its molecules is reduced and the molecules become rigid. This makes the polymer product, in our case, the Koosh ball, very brittle, and it therefore shatters into pieces. As the temperature of the broken pieces increases from the extreme cold back to room temperature, the added heat restores energy to the molecules, allowing them to become flexible again. The temperature at which the molecules transition from being rigid to flexible is called the Glass Transition Temperature (Tg). See Figure 1, which represents a scan from a differential scanning calorimeter (DSC). If the plastic is below its Tg, it will be brittle; if it is above the Tg, it will be flexible.

Fig 1 — DSC graph showing the glass transition temperature transition. Source (all images): Suhas Kulkarni

A simple definition of plastic is a polymer that can be molded. Because we are discussing molded components, we will use the word plastic instead of polymer for the remainder of this article.

Every plastic has a glass transition temperature (Tg). For example, the Tg of polystyrene (PS) is around 100°C, while the Tg of polypropylene (PP) is around -20°C. Note that these values can vary depending on the specific grade of the material.

A CD cover molded from PS will break under load at room temperature because the Tg of PS is above room temperature, making it brittle at that condition (see Figure 2). In contrast, a CD cover molded from PP will be flexible at room temperature because the Tg of PP is below room temperature. However, if the PP cover is exposed to a temperature below its Tg, such as -50°C, it will become brittle and break under load. Similarly, if the PS cover is heated to a temperature above its Tg, such as 120°C, it will become flexible.

The tragic loss of seven brilliant astronauts due to the explosion of the Challenger Space Shuttle in 1986 was traced back to failed O-rings. The night before the launch the temperature at the launch site in Florida had dropped below the Tg of the O-ring material. Under the assembly load, these O-rings cracked causing fuel leakage, which resulted in the explosion.

Tg and Shrinkage

How does Tg matter to us in molding? Consider a part being ejected out of the mold at 180°F with a room temperature of 60°F (Figure 3). The part cools to room temperature in 6 minutes. If the part is being molded with Material A, with a Tg of 100°F, then the molecules will be mobile at the ejection temperature of 180°F. The molecules will continue to be mobile until they reach the Tg of 100°F. This temperature is reached at a 3-minute mark, which suggests that the molecules will keep on shrinking for the first three minutes but will stop shrinking after that time. This is represented in Figure 4, which shows that the molded product will see a change in dimensions only for the first 3 minutes, but after 3 minutes there will be no change in the dimensions, with postmold shrinkage seen for the first 3 minutes only.

Now consider Material B with a Tg of 40°F. Because the room temperature is 60°F, the molded part will always be above the Tg, which means the molecules will always be mobile and will continue to move until they reach their equilibrium positions. In some cases, this could take a few hours or even days. The postmold shrinkage in these cases can be very high. This is represented in Figure 4 for a seal that was molded out of a TPE material that continued to shrink for about 16 hours. The final assembly with this seal was only done 24 hours after the seal was molded.

In summary, for materials which have their Tg above room temperature, postmold shrinkage will be lower than those which are molded with materials that have their Tg below the room temperature. The room temperature is essentially the molding shop’s temperature, the service temperature, the warehouse temperature or even the shipping container temperature. Going back to the original question of why parts change their dimensions over time, we can now see there are several reasons, all of which are related to the Tg and the ambient temperatures. It could also be that the dimensions of the molded parts were measured and accepted, but the parts were then stored in a warehouse in the summer with no climate control or shipped in a truck driving through hot desert-like conditions.

Controlling Quality Control

For the sake of a discussion, let us consider this very typical situation. Sample parts are collected from the molding machine and taken into the QC labs for measurement, acknowledging that QC labs are climate controlled.

• The Tg of the material being molded is 80°F.
• The molding shop temperature is 90°F.
• The temperature in the QC lab is 73°F.

The parts were molded and almost immediately taken to the quality lab. The shrinkage will now stop because the material is below its Tg. The measured dimensions are within the specification limits, so the whole batch of parts on the production floor is accepted and shipped to the customer. However, the customer complains that the parts that they received are smaller in dimensions than was reported. Why did this happen? This is because the parts that were in the boxes were exposed to the 90°F temperature that was above the Tg. The parts went through the full postmold shrinkage and were therefore smaller in dimension, whereas the parts checked in QC had not gone through the postmold shrinkage.

For parts with a long period of stabilization, a postmold shrinkage study must be done on at least the largest dimension of the part to find the stabilization time.

It is also important for us molders to educate the QC department about postmold shrinkage. I personally have come across QC inspectors who will ‘cool’ parts down in a fridge, use an air blast or even place them under water so the components are ‘ready’ to be measured. In their mind, they wait for an hour or a predetermined time to enable the parts to cool down to room temperature. In reality, we are waiting for the parts to experience the postmold shrinkage. Most of the postmold shrinkage is done within the first one or two hours following molding.

For parts with a long stabilization period, a postmold shrinkage study must be done on at least the largest dimension of the part to find the stabilization time. It is impractical to wait long times to measure the dimensions in a production environment, so there is a ‘hot spec’ which specifies what the part dimension should be 1 or 2 hours after molding. If the dimensions match at the hot spec time, then given that the process is robust, the parts it produces should duplicate the postmold shrinkage curve from shot to shot or run to run, ending up with identical dimensions after the postmold shrinkage is complete.

In the second part of this series, we will delve into the other causes of postmold shrinkage.

ABOUT THE AUTHOR: Suhas Kulkarni is the founder and president of , San Diego, an injection molding service-oriented firm focusing on scientific molding. FimmTech has developed several custom tools that help molders develop robust processes, and its seminars have trained hundreds of individuals. Kulkarni is an author of the bestselling book, “Robust Process Development and Scientific Molding,” the third edition of which was published by . Contact: 760-525–9053; suhas@fimmtech.com; .