Injection process parameter settings
3. Injection pressure and speed
The setting of injection pressure should follow the principle of being low rather than high. As long as it can provide enough power to achieve the required injection speed and allow the melt to fill the cavity smoothly, that would be ok. Excessively high pressure can easily cause internal stress in the product. When forming products with high dimensional accuracy, we can use high pressure injection to reduce the shrinkage of the product after demoulding.
The injection speed will affect the appearance quality of the product. We should set it according to the geometry of the mold and the exhaust status. Generally, the injection speed should be increased as much as possible to reduce the filling time while ensuring a good appearance.
In the process of injection molding, when the melt flows in the mold, the mold wall will form a solidified layer which will reduce the thickness of the flowable channel. Generally, according to the different mold structure and injection speed, the mold wall will have a solidified layer of about 0.2 mm. Therefore, faster injection speed is usually used in molding.
4. Injection stroke, multi-stage injection parameters
In molding, we must first determine the injection stroke. In theory, the injection stroke can be calculated as follows:
S1 = 4 (CVp + Va) / ρDs2
In the formula: injection stroke Vp – product volume ρ – resin density C – number of cavities Va – gate volume Ds – screw diameter
In actual production, if the total weight of “product + gate” is known, the injection stroke S1 = (M / Mmax) · Smax + (5 ~ 10) mm
S1 in the formula: injection stroke, mm M-“product + gate” total weight, g Mmax-maximum injection volume of injection molding machine, g / Smax-maximum injection stroke of injection molding machine
For the different geometry of the runner system and the various parts of the mold, in order to meet the quality requirements of the product, there are different requirements for the flow state of the mold-filled melt (mainly referring to the pressure and speed during the flow) in different parts. In an injection process, when the screw advances the melt to the mold, it is required to achieve different pressures and speeds at different positions, which is called multi-stage injection molding.
In general, it is more scientific to set at least three or more injections during the molding of plastic parts. That is, the first section is the main channel. The second is from the runner to the gate, and the third is cavity filled about 90%. The remaining part is the fourth segment. Then, determine the switching position of each segment by the calculated weight method.
In actual production, we should scientifically analyze and rationally set the multi-stage injection process parameters according to product quality requirements, flow channel structure, and mold exhaust conditions. Generally, it can be set by debugging and observing. The pressure / speed required to find the switching position during injection is set to 0. Observe the direction of melt and the defect status of product, and gradually adjust until you find the reasonable position. But in the process of debugging and observation, we must pay attention to the demoulding condition of the under-injected product, so as to avoid sticking of the mold due to under-injection in some concave parts of the mold.
Other process parameters
In injection molding, in addition to the setting of several main parameters such as molding temperature, pressure, speed, time, multi-stage injection switching position, there are many other process parameters like back pressure, screw speed, screw reverse cable flow prevention and other operation parameter settings can not be ignored.
Process parameter setting example
Taking the production of nylon cable ties as an example, product quality requirements:
- The product meets the specified pull standard.
- The surface is free of all kinds of undesirable phenomena such as silver wire, bubbles, shrink marks, etc.
- After molding, the product has good tightness and no looseness.
- The material used is PA66, the mold structure is a hot runner type, and the gate type is a point gate.
Firstof all, determine the principle of process parameter setting according to product characteristics and mold structure:
- For the long flow length of the product, L / t (the ratioof flow to wall thickness ) is 511, select the high-speed injection molding.
- Gate type is point gate, use higher pressure to overcome resistance during flow;
- To ensurethe product can be filled smoothly, the melt must have good fluidity, and the molding temperature should be appropriately high;
- High-pressure and high-speed injection to the end is easy to makeflash, the molding machine must have low inertia pressure, speed switching;
- Forthe small wall thickness of the product, it’s okay without the holding pressure. Formulate the main molding process parameters.
To formulate injection process parameters, it is necessary to know about information of equipment performance, mold structure, molding materials and product quality requirements, and set each molding parameter scientifically and reasonably.
First of all, adjust the process parameters step by step according to the molding conditions of the product. The correct adjustment sequence is pressure → speed → temperature. Each time the parameter is changed, the input parameter has been confirmed by the computer before the next parameter change. Avoid changing more than two parameters at the same time.
Secondly, when the product begins stable production, keep the parameters as stable as possible and make detailed records. If the change is too large, finnd the reason in time. In addition, fix the molding process as much as possible each time the mold goes on line to facilitate the quality control of the finished product.