As a mold engineer, when receiving a new product, it's essential to have certain techniques for conducting mold opening evaluations.
Therefore, it's necessary to have the capability to calculate the required machine size or determine the size of the mold to be designed. It's crucial to confirm the production equipment to accurately assess the mold opening method. This involves calculating clamping force, injection volume, molding cycle, and so on.
Before conducting the evaluation, it's essential to obtain the customer's 3D or 2D drawings. This is primarily to acquire information such as maximum external dimensions, material specifications, dimensional tolerances, and any other specific requirements such as appearance or designated manufacturing methods. If detailed information is not available, it's crucial to request the above information from the customer at least.
1. Clamping Force
When injecting plastic into the mold cavity, a significant injection pressure is required due to the near vacuum environment inside the cavity. Therefore, the injection molding machine needs a substantial clamping force to prevent the mold from being forced open. The specifications of molding machines are typically denoted in tons, representing the clamping force they possess. For instance, a machine with a 100-ton specification indicates it has a clamping force of 100 tons. If the clamping force is insufficient, it could result in product flash or even overflow. Hence, it's crucial to consider a machine with adequate tonnage for injection. Below is the calculation for clamping force:
The clamping force calculation is as follows:Clamping Force = A × P × αWhere:A: Projected area of the product
P: Average pressure inside the mold
α: Safety factor
2. Injection Volume
The maximum injection volume for each injection molding machine is determined by the size of the material barrel. If the volume of material, including runners, is too large, it may result in incomplete molding. Conversely, if it's too small, the material may reside in the barrel for an extended period during the latter stages, leading to material discoloration or degradation. Therefore, the required injection volume must be maintained within a reasonable range of proportions.
The injection volume of the material barrel :
d: Screw diameter S: Screw travel distance
Product to barrel capacity ratio :
Reasonable proportion range :
Soaking time calculation (residence time) :
In addition to ensuring a reasonable proportion for each mold injection, it's also essential to confirm how long the material will reside in the barrel. If the time is too long, the viscosity of the material in each mold injection will become unstable, making continuous production impossible. The general standard is that the material should not reside in the barrel for longer than5 minutes.
Formula :
3. Cycle Time Estimation
The injection molding cycle consists of five stages: mold opening/closing time, injection time, holding pressure time, cooling time, and ejection time (including core pin movement and automated extraction).
Typically, mold opening/closing time is 2-3 seconds, injection time is 1-2 seconds, holding pressure time is divided into hold and release segments, each about 2-3 seconds and 1 second respectively. Cooling accounts for the majority of the time, as shown in the table below.
Ejection includes the time for core pin movement, typically around 1 second for products that are not particularly large. Additionally, it includes retrieval time, often performed by a robotic arm. The experienced time varies among different manufacturers, so it's best to refer to the experience of your own company.
Adding up the times mentioned above provides an estimation of the cycle time.
4. Product and Runner Weight
The design of gates and runners for the product varies, involving many concepts in product and mold design. However, this article does not delve into the design aspect for now.
Since estimating the injection volume involves roughly estimating the weight of the entire mold, experienced engineers typically refer to similar cases with comparable volumes or structures to gauge the amount of runner material needed.
5. Material Mold Temperature
Depending on the material properties, different mold temperatures are suitable. For most materials, mold temperatures ranging from 100 to 130 degrees Celsius are sufficient. However, some high-temperature materials, such as engineering plastics like PEEK, require higher mold temperatures. Mold temperature controllers come in water and oil types, with variants including high-temperature or ultra-high-temperature models. Generally, water temperature controllers operate within the range of 30 to 120 degrees Celsius, with high-temperature variants reaching up to 180 degrees Celsius. Oil temperature controllers typically range from 30 to 200 degrees Celsius, with high-temperature models reaching up to 300 degrees Celsius. Therefore, it's essential to choose the correct mold temperature controller based on the material being used.
6. Other Equipment
In addition to basic equipment like injection molding machines and mold temperature controllers, there are several other pieces of equipment to consider throughout the process.
Extraction Equipment :
Such equipment may include robotic arms for material retrieval or three-axis clamps.
Cutting Equipment :
For products where runners are removed along with the finished parts, such as side or sprue-gated parts, it's necessary to consider placing the cutting machine either off-line or in-line and design appropriate cutting fixtures.
Preheating Equipment :
If your product involves double shots or over-molding, it may be necessary to preheat the metal parts or the first-shot product externally to a certain temperature before placing them into the mold to avoid excessive temperature differences and potential issues.
Plating Equipment :
When the finished products are ejected, it's common to place them on trays or other carriers. For larger quantities, it's necessary to have equipment that can automatically align or replace trays.
Of course, the aforementioned equipment types are not mandatory. Their necessity depends on the factory's level of automation and the production volume of the product.
The content shared today focuses on the initial evaluation required when assessing a product. It's essential to first determine the equipment and capacity needed because these factors will impact subsequent design and production processes.
