What Is Plastic Shrinkage?

Shrinkage is a crucial property of plastics, directly impacting product molding. In plastic injection mold and injection molding industries, designers must grasp shrinkage, as it influences mold design.

For product designers, while not directly manufacturing plastic items, understanding shrinkage is vital. Otherwise, their designs might cause unnecessary issues during production, especially with thicker-walled products.

This article comprehensively explores plastic shrinkage, offering insights for both mold and product designers.

What is Plastic Shrinkage?

Plastic shrinkage refers to the percentage decrease in dimensions from the initial, uncooled size to the cooled, room temperature size. It’s not only due to thermal expansion and contraction but also related to various molding factors, hence termed molding shrinkage.

Specifically, shrinkage can be calculated using this formula:

Shrinkage = (Original size – Cooled size) / Original size × 100%

The extent of plastic shrinkage depends on factors such as material type, composition, moisture absorption, and mold temperature. For instance, crystalline plastics typically exhibit greater shrinkage than amorphous plastics.

The Impact of Shrinkage on Parts

Shrinkage affects parts in multiple ways, including product performance, appearance, and production costs.

Firstly, it diminishes part dimensional accuracy. Improperly controlled shrinkage rates can deviate part dimensions from design specifications, affecting assembly precision and fitting performance. In the automotive industry, for example, shrinkage can hinder the smooth operation of components like doors and windows, impacting overall vehicle performance and safety.

Secondly, it influences part appearance quality. As plastic parts usually have smooth surfaces, shrinkage can cause surface irregularities, diminishing product aesthetics and texture. This not only affects consumer purchasing decisions but also tarnishes a company’s brand image.

Moreover, shrinkage increases production costs. To control shrinkage rates, injection molding companies must take various measures, such as adjusting mold designs and optimizing injection molding processes. These measures require significant human and material resources, escalating production costs. Additionally, due to decreased part dimensional accuracy, companies may need secondary processing or repairs, further increasing production and time costs.

Why Product Designers Need to Know Injection Molding Shrinkage

Although injection molding factories resolve shrinkage issues during production, product designers still need to grasp shrinkage-related knowledge. Here’s why:

Optimizing designs: Understanding shrinkage allows designers to anticipate size changes during production, optimizing designs for precise, consistent results.

Material selection: Different plastics exhibit varying shrinkage levels during molding. Knowledge of shrinkage aids in selecting suitable materials based on design requirements.

Iterative design process: Anticipating and addressing shrinkage issues early shortens development cycles, expediting product launches.

Cost efficiency: Minimizing shrinkage-related problems reduces waste, rework, and delays, enhancing cost efficiency in production processes. Designers aware of shrinkage can create economically viable products.

Factors Influencing Injection Molding Shrinkage

Shrinkage rates vary among plastics due to factors like thickness, molding processes, and environmental conditions. For product designers, it’s crucial to note that:

1. Thicker walls lead to longer cooling times and greater shrinkage.
2. Features like reinforcements and engravings resist shrinkage, resulting in smaller shrinkage rates in these areas.

For mold designers, attention should be paid to how plastic shrinkage is affected, primarily in:

Molding Processes Factor

• Consistent molding temperature reduces shrinkage.
• Increased injection pressure decreases shrinkage.
• Higher melt temperature reduces shrinkage.
• Higher mold temperature increases shrinkage.
• Prolonged pressure maintains shrinkage reduction.
• Longer cooling time within the mold decreases shrinkage.
• High injection speeds slightly increase shrinkage.
• Initial shrinkage is large, stabilizing after approximately two days.

Plastic Structure Factor

• Thick-walled parts exhibit higher shrinkage rates.
• Parts with inserts have lower shrinkage rates.
• Complex shapes have smaller shrinkage rates.
• Shrinkage is typically smaller in the flow direction.
• Elongated parts show lower shrinkage along the length.
• Shrinkage along the length is smaller than thickness.

Mold Structure Factor

• Larger gate size reduces shrinkage.
• Parts farther from the gate have smaller shrinkage.
• Restricted parts of the mold exhibit less shrinkage.

Plastic Properties Factor

• Crystalline plastics exhibit greater shrinkage than amorphous ones.
• Plastics with good flowability have smaller molding shrinkage.
• Adding fillers to plastics significantly reduces shrinkage.
• Different batches of the same plastic exhibit varying shrinkage rates.

Various materials exhibit different shrinkage in injection molding

Due to the multitude of factors influencing plastic shrinkage rates, the values exhibit a considerable range of fluctuation. For instance, the shrinkage rate of ABS you might find online could be approximately 0.4% to 0.7%. To provide a more precise range, VHP Tooling has compiled several detailed tables of plastic shrinkage rates.

PA6 Plastic Shrinkage:

PA6 Plastic Shrinkage:

PP Plastic Shrinkage:

PC Plastic Shrinkage:

PC/ABS Plastic Shrinkage:

PC/PBT Plastic Shrinkage:

ABS Plastic Shrinkage:

Here’s the table based on the provided information:

How to Prevent Fluctuations in Plastic Shrinkage?

Measures to Adopt

Achieving Flow and Gate Balance

As the title mentioned, shrinkage rates vary due to resin pressure changes. In the case of single-cavity molds with multiple gates or multi-cavity molds, proper gating balancing is essential. Balancing the gates is necessary for uniform resin flow, which depends on the flow resistance within the runner. Hence, it’s preferable to achieve runner balance before gate balancing.

Mold Cavity Arrangement

To facilitate the setup of molding conditions, attention must be paid to mold cavity arrangement. Since the molten resin carries heat into the mold, under typical cavity arrangements, the mold temperature distribution forms concentric circles centered around the gate. Therefore, when selecting the arrangement of cavities in multi-cavity molds, it’s important to ensure both easy runner balance and a concentric arrangement centered around the gate.

Preventing Molding Deformation

Molding deformation occurs due to uneven shrinkage resulting in internal stress. To prevent uneven shrinkage, especially in cases like circular products with holes at the gear center, a gate must be placed at the center. However, when there’s a significant difference in shrinkage rates between the resin’s flow direction and the perpendicular direction, the disadvantage of forming an ellipse arises.

For higher roundness precision, it’s necessary to set up 3-point or 6-point gates. However, it’s crucial to ensure proper balancing of each gate. When using side gates, a 3-point gate may enlarge the inner diameter of cylindrical products. In situations where gate marks are not permissible on the surface and end faces, it’s advisable to minimize the use of internal side multi-point gates, which can yield favorable results.