Plastic Residence Time: Calculation and Process
From the specific plastic you use, your mold, temperature, material density, to the machine itself and its individual parts, there are several variables that can make or break the injection molding process.
One thing is certain: Processing for too long or too short outside of the recommended plastic residence time can cause significant quality issues. Even worse, not all resulting problems are immediately visible – such as inconsistency in melt quality, color variation, or weakened parts.
Taking the time to calculate the specific plastic residence time for the applicable manufacturing process will improve your material performance and the overall quality of the final product.
What is Plastic Residence Time?
Plastic residence time is the amount of time plastic or resin is subjected to heat during fabrication.
An injection system features a hopper, barrel, reciprocating screw, and injection nozzle. The shot size is the maximum amount of plastic injection mold that can be injected in one molding cycle. Pellets are fed through the hopper into the barrel and screw assembly. The barrel holds the reciprocating screw where the material is then melted through conduction from the electric resistance heater bands that surround the barrel. The reciprocating screw compresses, melts, and meters the material.
The size of the shot and barrel impact the length of residence time, which in turn, can affect the quality of the product. Small shot sizes that use minimal barrel capacity can result in long residence times and polymer degradation. Conversely, large shot sizes and more than 50% of barrel capacity can lead to poor melt uniformity and extended screw recovery times.
The Importance of Plastic Residence Time
Material is quickly affected by the duration of time and exposure temperature. Some polymers, according to the Injection Molding Handbook, are “hydrolytically degradable and may suffer undesirable depolymerization effects due to chemical reaction of moisture with the polymer prior to devolatilization.”
Understanding the residence time of material in the first stage of the screw can help you determine the optimal time and temperature for your manufacturing needs.
What Does Plastic Residence Time Affect?
Plastic residence time affects part quality in several ways:
- General weakness in the parts produced
- Color variation
- Degradation not visible to the eye
- An overall compromised product
It can also impact machine performance, cause inconsistency in melt quality and shot weight, as well as the melt temperature.
What You Need to Know Before Calculating Your Residence Time
Before calculating residence time, it’s important to understand all the different components involved.
The Difference Between Residence Time and Shot Size
Shot size is the amount of resin that needs to be injected into the mold. Residence time is determined by the shot size, barrel size, and the screw inventories in the channels.
The screw serves multiple functions. It moves the material through the barrel, mixes the material, compresses it to its maximum density, and forces the material into the mold. The screw’s size and design determine the space available to the plastic inside the barrel. It’s not uncommon for manufacturers to have more than one screw and barrel option.
Formulas and Tips to Find Your Residence Time Calculation
As you review different formulas and advice for determining your own residence time calculation for a new machine or a mold that hasn’t yet been built, you may want to start with a mold analysis first.
Calculations
There are a few different ways to calculate plastic residence time. Here are two options to give you a better understanding of the factors involved.
Calculation #1:
Barrel size = 3.14 x (Barrel radius)^2 x (Barrel length)
Screw size = 3.14 x (Average root radius of the screw)^2 x (Barrel length)
Shot volume = Barrel volume minus screw volume
Plastic residence time is the number of shots multiplied by your cycle time.
Calculation #2
Shot capacity divided by 1.05 x material density at room temperature = Inventory
Plastic residence time = Inventory divided by shot size x cycle time / 60
Hot Tips
- Keep melt densities in mind. You’ll want to calculate the material weight at both its solid and molten states.
- Start with a clean, uncontaminated barrel.
- The recommended barrel capacity is between 25% and 65%, plus an additional 10% for safety.
- If possible, use volume and melt densities instead of weight.
Take the Time to Make the Calculations
No matter how many shared formulas or calculations there are, plastic residence time should be calculated by each individual manufacturer to determine the ratio that works for their particular product.
Using scientific molding practices, Rosti uses recorded data to assess quality control and make any necessary adjustments to tooling, improving overall part quality and avoiding the negative effects of poorly calculated plastic residence time.
Rosti has the most advanced technology and equipment, along with the technical expertise and experience needed to successfully produce the highest quality plastic parts for your unique application. Learn how Rosti goes above and beyond to be a supplier you can trust.
