What is Warpage? | Causes of Warpage | Warpage Variations

What is Warpage?

Warpage is an unconventional bending or twisting out of the shape of a plastic part that is easily recognizable. The cause of unnatural bent can be heating, cooling, or dampening.

The molded-in residual stress is the prime cause of warpage, which is caused by contrasting shrinkage in the molded part’s material.

One doesn’t need technical knowledge; an average consumer can notice the unconventional bending or twisting. The unintentional bending has multiple ways to be prevented, especially if the molded part was made by injection molding.

However, finding out the cause of the warpage is the most important part, often overlooked.

There are 3 main ways to determine the causes of warpage –

• How much your part warp
• Warpage direction
• Understand the mating requirements of your parts

Cause for Warpage –

Plastic Shrinkage:

Material shrinking during and after manufacturing site prime reason for warpage in injection molded plastic components. But before getting into molded components shrinkage, we should know how different plastic materials shrink.

For that, we need to start with the basics and understand how plastics melt and cool. Plastics’ melting and cooling are completely dependent on polymer and the presence of fiber reinforcements.

• Amorphous Materials
• Semi-crystalline Materials

#1 Amorphous Materials – 

The most popular amorphous materials include ABS, polystyrene, polycarbonate. Amorphous materials have a unique, intertwined molecular orientation in their natural state. When amorphous materials are melt, their molecules weaken and move away from each other.

Additionally, the friction causes single molecules to unwind and line up with the position of flow. The molecules will relax and return to the state of orientation only when the flow stops.

The molecules force squeeze them together until the temperature is dropped to their cooling point. The relaxation effect caused by the molecules, in turn, causes the shrinkage in the direction of flow.

#2 Semi-crystalline Materials –

Semi-crystalline materials are opposite to amorphous materials in terms of their structure and nature. They have thoroughly ordered and tightly packed up molecules. When the molecules are melt, their crystalline structure loosens up and lines up to maximum flow direction.

When the molecules are cooled to their cooling point rather than relaxing, they again try to re-align with the direction of flow and recrystallize tighter than their original state.

The main impact is actually felt on the perpendicular side of the flow.

Interesting Read – What is Overmolding? | Versiltality of Overmolding | Advanatges of Overmolding | Disadvantages of Overmolding

Types of Shrinkages – 

 

What is warpage? A variation in shrinkage, that’s it. In simple words, shrinkage extending in all directions uniformly becomes smaller in size but somehow retains its shape.

But any difference in the element’s directions will lead to internal stresses, and if these stresses surpass the component’s structural integrity, a warp will be formed in the ejected component.

There are three shrinkage types. Let’s dive into them.

#1 Regional:

Regional shrinkage occurs anywhere between the regions closest to the gate and regions close to the end of the fill. The variance should be easily visible between the thickest part and the thinnest part of the affected region.

One thing is for sure that one region should be shrinking more than another.

#2 Via the thickness:

When there’s shrinkage on the part’s cross-section, the shrinkage’s top part differs significantly from the bottom. The variation can force the part to bent from the side and shrink more.

#3 Directional:

Although shrinkage varies from molecule to molecule and fiber-alignment to fiber-alignment, sometimes it can also occur to the materials’ orientation. It occurs differently in both amorphous and crystalline materials.

As mentioned earlier, amorphous materials shrink more to the direction parallel to the flow; on the contrary, crystalline materials tend to have their shrinkage perpendicular to the flow direction.

Plastic Shrinkage Rates:

Why Does Warpage Variation Happen?

There are five common reasons for variations in shrinkage rates, which can cause warpage. Let’s discuss those reasons in detail.

1. Cooling Rates
2. Orientation Due to Filling
3. Mold Restraint
4. Temperature Differences via Thickness
5. Thickness Variations and Uneven Packing
6. Products Shape and form

Engaging Read – Types of Gates for Injection Molding | Applications of Gate Designs

#1 Cooling Rates – 

Semi-crystalline materials have a high cooling rate, which gives less time for the crystalline structure to form. The effect decreases the volume of shrinkage.

The same principle is applied to amorphous materials; the effect is minuscule compared to crystalline materials.

#2 Orientation Due to Filling – 

Orientation due to the filling can be attributed to polymer molecules caused by shear stress during flow. But when the polymer is at a high temperature and stress is removed, orientation will be relaxed.

The same principle, when applied to amorphous materials, there is more shrinkage parallel to the flow. And for crystalline materials, shrinkage will occur perpendicular to the flow.

#3 Mold Restraint – 

while the component is in the mold, it can’t shrink within its surface, but it can shrink directly perpendicular to its thickness, resulting in more shrinkage in the region.

After ejection, these accumulated stresses get relaxed as the component continues to cool down and creates warpage.

Mold restraint is very dependent on materials, more than it is dependent on anything else. Materials, which tend to relax more slowly have higher linear shrinkage.

#4 Temperature Differences via Thickness –

The mold temperature on the side of the cross-section will play an important role here. If the temperature is different on both sides, then shrinkage will not be uniform. In simple words, the plane on the other side will shrink more, resulting in it being tinier than the other side, making a bending movement, which will eventually turn into warpage.

#5 Thickness Variations and Uneven Packing – 

When there is a visible variation in the component’s thickness, thicker areas will naturally take longer to cool, which will result in shrinkage.

Similar things can be noticed in the areas far from the gate. If a consistent packing profile is utilized, areas closer to the gate will be denser and cool at a different speed.

 #6 Products Shape and form – 

Product design can be an unrecognizable reason for mold warpage. The component design can be a reason for multiple filling patterns resulting in shrinking plastic throughout the cavity.

Inconsistent shrink rate is also a reason for warpage to occur, especially in the areas where there is a high level of pressure loss and gain.

FAQs – 

1. What are other common injection molding defects other than Warpage?

Ans. Below are the most common injection molding defects:

1. Flow Lines
2. Sink Marks
3. Vacuum Voids
4. Surface Delamination
5. Weld Lines
6. Warping
7. Short Shots
8. Burn Marks

2. What causes part warpage?

Ans. Part warpage is caused by mold restraint or (lack of mold restraint through the thickness of the part). The difference in shrinkage between the in-plane and thickness directions can cause warpage, particularly in the corners of the part due to these sometimes being thicker than the general wall thickness.

3. What is the difference between Warpage and sink marks in injection molding?

Ans, As established easier warpage is an unconventional bending or twisting out of the shape of a plastic part that is easily recognizable. The cause of unnatural bent can be heating, cooling, or dampening. On the other hand, sink marks are areas in a molded part that where the surface is deformed into a depression. It typically occurs in the thicker read of a part and is caused by uneven cooling.

Suggested Read – 

• Top 10 Injection Molding Materials (And Why They are the Best)
• What is Hardness in Plastics? An In-Depth Guide
• What is Blow Molding? | Guide to Blow Molding Process | Blow Molding Advantages | Blow Molding Disadvantages
• What is Thermoforming | Different Stages of Thermoforming | Types of Thermoforming | Applications | Advantages & Disadvantages
• How is Plastic Made? A Simple and Detailed Explanation.
• Plastic Thermoforming Mold: Information about Male Vs. Female Molds
• Physical Properties of Plastic Materials | The Complete Guide
• 6 Best Plastic Molding Techniques | A Complete Analysis

Final Words – 

Thus, were my knowledge and insights on warpage. I have tried my best to include all the necessary information. There are other types of injection molding defects as mentioned above. However, warpage is one of the more stubborn defects which persists even after essential steps are taken to prevent it. However, if you flow the guide properly, you should easily be able not just to prevent warpage, but increase the quality of your production.

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