Plastic Shrinkage Explained
author: Bella
2024-11-05
The phenomenon of plastic shrinkage plays a critical role in the injection molding process
and has a substantial impact on the ultimate dimensions and shape of the final product.
In order to ensure accurate dimensions and prevent defects like warping or sink marks,
it is imperative to have a comprehensive understanding of shrinkage and the ability to compensate for it accordingly.
In this article, we aim to explore the prevalent issue of plastic shrinkage in injection molding and specifically,
to analyze the shrinkage rates of widely-used materials such as ABS, PP, and PE.
Furthermore, we will delve into the pivotal elements that impact shrinkage and explore how manufacturers can
proficiently handle these variables to guarantee the consistent manufacture of top-notch components.
What Is Plastic Shrinkage?
Plastic shrinkage occurs when the volume or dimensions of a plastic material contract during the cooling and
solidification process after injection molding from a liquid state.
This alteration in volume or dimensions arises from the reordering and cinching of molecular chains.
Plastic shrinkage doesn't just impact the precise dimensions of the end product; it can also cause injection
molding complications like warping, deformation, and unsightly sink marks.
Hence, it is imperative to comprehensively investigate and regulate plastic shrinkage during the injection
molding process to uphold the superior quality and dimensional precision of injection molded components.
How To Calculate Plastic Shrinkage?
The degree of plastic shrinkage is usually evaluated in terms of the plastic shrinkage rate. This is calculated by determining
the percentage difference between the size of the molded part while at molding temperature and its size after cooling to
room temperature following its removal from the mold.
The specific formula for calculating shrinkage is as follows：
Where (S) represents the shrinkage rate, (D) represents the mold dimensions, and (M) represents the dimensions of the molded part.
The Shrinkage Values of Various Plastics
There are notable variations in the degree of shrinkage exhibited by different plastic materials, with the primary determinants
being the type of plastic used, the degree of crystallinity, the molecular weight, and any modifications made to the material.
Thermoplastics
Thermoplastics typically exhibit a shrinkage rate ranging between 0.5% and 5%. For instance, ABS shrinkage typically falls
within the range of 0.4% to 0.7%, whereas PP shrinkage is usually between 1.0% and 2.5%.
Thermosetting plastics
Plastics such as epoxy resin and phenolic resin exhibit relatively low shrinkage rates, typically ranging between 0.2% and 0.5%.
Reinforced plastics
Glass fiber-reinforced polyamides (PA6 and PA66) exhibit lower rates of shrinkage, typically ranging from 0.1% to 0.8%.
Special plastics
Plastic materials, such as fluoroplastics, exhibit significant shrinkage rates, with some reaching up to 6%. Furthermore,
low-density polyethylene (LDPE) also shows a high level of shrinkage, reaching up to 5%.
|
Name |
Min Value (%) |
Max Value (%) |
|
ABS |
0.4 |
0.7 |
|
LDPE |
1.5 |
5.0 |
|
HDPE |
1.5 |
4.0 |
|
PP |
1.0 |
2.5 |
|
PC |
0.5 |
0.7 |
|
PA6 |
0.8 |
2.0 |
|
PA66 |
1.0 |
2.5 |
|
POM |
1.8 |
2.5 |
|
PPS |
0.1 |
0.3 |
|
PMMA |
0.2 |
1.0 |
What Are The Types Of Plastic Shrinkage？
Linear Shrinkage
This is the most prevalent type of shrinkage that occurs in plastic products during the process of injection molding.
When molten plastic is injected into a mold and allowed to cool and solidify, it's possible for the dimensions of the
resulting product to shift slightly.
This form of shrinkage is commonly attributed to the design of the mold and the cooling conditions employed.
Post-Molding Shrinkage
Upon removal from the mold, the part may undergo additional shrinkage while it stabilizes.
This phenomenon is attributed to the release of internal stresses and the realignment of
molecular chains.
Directional Shrinkage
Some types of plastics may exhibit non-uniform shrinkage in various directions caused by uneven cooling or
inadequate mold design during the molding process, leading to directional shrinkage.
Post-Treatment Shrinkage
Certain types of plastics have the potential to experience minor shifts in dimensions after undergoing
post-treatment processes, such as exposure to heat or chemicals. This is attributed to the alterations
in internal stresses that occur during these treatments.
Moisture Absorption Expansion
Some hygroscopic plastics, such as nylon, may expand upon absorbing moisture.
Although it may seem contradictory, grasping the concept of expansion can actually enhance the
dimensional stability of plastic products, as it functions in stark contrast to shrinkage.
The Main Factors Influencing Shrinkage Rates
1.Plastic Types
Various types of plastics demonstrate notable differences in their rates of plastic shrinkage.
For instance, materials such as PS, ABS, PC, and PVC that have an amorphous structure,
exhibit comparatively low rates of shrinkage. On the other hand, semi-crystalline
substances like PP, PE, PBT, POM, and PPS generally have higher rates of shrinkage.
2.Part Shape and Wall Thickness
The shape and wall thickness of a part have a significant impact on its shrinkage rate.
Uneven thickness in the wall can result in uneven shrinkage, subsequently leading to
sink marks and warping.
3.Mold Structure
The efficiency of plastic cooling is directly impacted by the cooling system design of the mold,
which in turn affects the shrinkage rate.
An expertly-crafted cooling system can efficiently hasten the cooling and solidification process,
leading to a consequent decrease in the overall shrinkage rate.
4.Process Parameters
Excessively high injection temperatures can lead to reduced viscosity in plastic melt, thus accelerating
the cooling and solidification process. Consequently, this could increase the rate of shrinkage.
Excessive injection pressure can result in increased shrinkage, which is concern that should be addressed
in the manufacturing process.
Moreover, the duration of the holding period has a direct impact on the cooling and solidification process
of the plastic, ultimately influencing the rate of shrinkage.
How Can Plastic Shrinkage Be Prevented?
In practical production, implementing the following measures can lead to improved control over the plastic shrinkage rate:
Choose suitable plastic materials and gain an understanding of their shrinkage properties.
During the mold design stage, it is crucial to prioritize a well-crafted cooling system to effectively reduce plastic shrinkage
by ensuring efficient cooling.
During the injection molding process, it is essential to adjust the molding parameters based on the plastic's properties and
mold design. This requires meticulous monitoring of temperature, pressure, and holding time.
When designing plastic products, strive for reasonable dimensions to minimize significant size discrepancies and better
manage shrinkage rates.
Conclusion
To summarize, the plastic shrinkage rate plays a vital role in the manufacturing of plastic products as it impacts their dimensional
stability and surface quality. Hence, it is a critical parameter to consider.
In order to control the shrinkage rate effectively, it is crucial to take into account various factors including the selection of materials,
mold design, and injection molding process parameters. Necessary adjustments must be made to ensure that the plastic products
conform to quality and dimensional requirements.
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Polypropylene (PP) and Nylon are both popular synthetic materials, each with distinct characteristics and benefits.
PP is valued for its light weight, whereas Nylon is prized for its exceptional resistance to abrasion.
Both nylon and polypropylene are known for their affordability, however, polypropylene is notably more
inexpensive than nylon. Accurately selecting the appropriate material for a finished product requires a
thorough understanding of their unique properties.
PP is valued for its light weight, whereas Nylon is prized for its exceptional resistance to abrasion.
Both nylon and polypropylene are known for their affordability, however, polypropylene is notably more
inexpensive than nylon. Accurately selecting the appropriate material for a finished product requires a
thorough understanding of their unique properties.
Nylon VS Polypropylene Plastic
