Causes of shrinkage marks at thicker positions of plastic mold parts?

 

Introduction

Understanding Shrinkage Marks in Plastic Mold Parts

Shrinkage marks are a common and troublesome defect in the production of plastic mold parts. They present as small depressions or pits on the surface of the plastic product, which can significantly affect both the aesthetics and functionality of the final item. This issue becomes even more pronounced in thicker positions of plastic mold parts.

In industries such as automotive, electronics, and consumer goods, where the quality and appearance of plastic components are of utmost importance, shrinkage marks can lead to product rejection, increased production costs, and damage to the brand image. For example, in automotive interior parts, even a minor shrinkage mark on a dashboard component can be noticeable to the end - user, reducing the perceived quality of the vehicle.

Therefore, understanding the causes of shrinkage marks in thicker - walled plastic mold parts is crucial for manufacturers to improve product quality, enhance production efficiency, and maintain a competitive edge in the market.

Common Causes of Shrinkage Marks

1. Material - Specific Shrinkage

One of the primary factors contributing to shrinkage marks in plastic mold parts, especially at thicker positions, is the inherent shrinkage characteristics of the plastic material itself. Different plastic materials have varying shrinkage rates. For example, crystalline plastics like polyethylene (PE) and polypropylene (PP) generally have higher shrinkage rates compared to amorphous plastics such as polystyrene (PS) and polycarbonate (PC).

The following table lists the shrinkage rates of some common plastic materials:

Material	Shrinkage Rate (%)
Polypropylene (PP)	1.0 - 2.1
Polyethylene (PE) (High - Density)	1.5 - 3.0
Polycarbonate (PC)	0.4 - 0.6
Polystyrene (PS)	0.3 - 0.6
Acrylonitrile Butadiene Styrene (ABS)	0.4 - 0.7

This variation in shrinkage rates means that when choosing a material for a plastic mold part, manufacturers must consider how the material's shrinkage properties will impact the final product. If a material with a high shrinkage rate is used in a design with thick - walled sections, the likelihood of shrinkage marks occurring is significantly increased.

2. Injection Molding Process Variables

a. Temperature Fluctuations

Temperature plays a crucial role in the injection molding process and can directly influence the formation of shrinkage marks. There are two key temperature factors: the melt temperature and the mold temperature.

The melt temperature affects the viscosity of the plastic. When the melt temperature is too high, the plastic has lower viscosity and fills the mold more easily. However, it also cools and solidifies more slowly. This slower cooling can lead to greater shrinkage as the plastic has more time to contract during the cooling process. On the other hand, if the melt temperature is too low, the plastic may not fill the mold completely, or it may cool too quickly, resulting in uneven cooling and potential shrinkage marks.

The mold temperature also has a significant impact. A higher mold temperature can slow down the cooling rate of the plastic, allowing for more uniform cooling and reducing the risk of shrinkage marks. Conversely, a lower mold temperature can cause the outer layers of the plastic to cool and solidify rapidly, while the inner layers are still hot and shrinking. This differential cooling can lead to internal stresses and the formation of shrinkage marks.

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