You have undoubtedly heard this term with your co-workers or when talking with machine or plastic injection molding companies for sale. However, I have read in your comments the concern to know more about this subject, so I leave you in this article a brief description of the concept and its application in our work.
Many of us have learned either in courses, books or through our mentors that we must heat the plastic. It should be in a greater percentage through friction or mechanical efforts. Moreover, the spindle generates due to the `` compression '' of the plastic in its different sections. Similarly, compression ratio (RC) is a term common to indicate the level of plastic compression that will be generated by the screw.
WHAT IS THE BENEFIT OF RC?
This level gives us an overview of the friction and mechanical stresses. The plastic will suffer, especially in the section with the highest compression of the screw. The higher RC comes by the screw; the material undergoes a higher compression. Similarly, this improves the homogeneity of the melt and increases the effects of its heating by shear.
Therefore, the plastic receives a greater amount of heat which can negatively or positively affect its properties or the process depending on the type of plastic. You must be aware to buy a plastic injection molding companies for sale.
How do Plastic injection molding companies calculate compression ratio?
It's another classic question from the Injection Molding community. To determine CR, the volume of a channel in the feeding zone and the volume of a channel in the dosing zone relate. This is often a difficult task if we do not have accurate measurements or spindle design. In addition, there is a practical standard method to determine CR.
Each type of plastic demands an ideal RC for its optimal processing in the screws. It can range from 1.2: 1 to 4.5: 1, in plastic injection molding companies for sale. This is a function of morphology, additives and material properties. In the market for spindles are those of general purpose in which the RC design comes between 2.5: 1 and 3.0: 1.
Level of Compression in Plastic injection molding companies
There are also those designed for thermoset materials. Here the CR ranges from 1.0: 1 to 1.3: 1. One way to classify the screws common for the injection molding process is according to their level of compression. Here we can find three scales: high, medium and low. Each one recommended for processing different types of plastics.
HOW TO DEFINE THE INJECTION CUSHION
The injection cushion is the amount of melt that is common in front of the screw at the end of the holding stage in plastic injection molding companies for sale. Typically, this value comes by the machine in millimeters, inches, or cubic centimeters.
This means that the cushion is a result of the process. It derives from the adjustment of the change point and the programming of the second filling stage.
WHAT RESULTING CUSHION IS RECOMMENDED?
As a general rule of thumb, for any molding process, the resulting cushion will never be "zero." If this happens, it is not guaranteed that the support stage completes in its entirety. If this happens, the percentage of weight variation between shots increases. It can result in dimensional instability in the parts and, in extreme cases, short shots or burrs.
Some plastic injection molding companies for sale recommend that the cushion not be “zero”. It is to avoid generating fatigue in the injection unit system. Moreover, it can cause premature wear of its mechanical elements, so they recommend looking for an equivalent cushion. Similarly, it result at 10% of the length of the screw diameter.
MY RECOMMENDATION, BUT NOT SOMETHING DEFINITIVE:
Personally, what has worked for me during these years of adjusting processes is to maintain a cushion equivalent to 10% of the shot size when I have a percentage of occupation of the injection unit less than 50%, and when the percentage occupancy is greater than 50% I use 5% of the shot size travel.
HOW DO I INTERPRET THE RESULTING CUSHION VALUE IN MY PROCESS?
The variation of the resultant of the cushion between cycle and cycle must be zero. In plastic injection molding companies for sale, the variation of the cushion does not exceed 0.1 mm. However, to achieve this variation requires the good adjustment of other process variables. Similarly, it comes in combination with a molding machine in good condition.
The variation in cushion size is highly dependent on the mass injected per unit length of charge. Furthermore, in this case, every tenth of a millimeter of variation in the cushion will modify the weight of the piece. The equivalent tenths of a millimeter for the piece to be out of specification in weight will be the limitation for the process.
What is the shot size in Plastic injection molding companies?
A uniform repetition of the cushion cycle by cycle, guarantees a repetitive filling of the molded parts. If there is a significant variation, consequently there will be pieces with different properties, mainly weight variation, dimensional variation and mechanical property variation.
The shot size (td) is the length in millimeters, inches or cubic centimeters of the corresponding dosing stroke. Similarly, it is to fill the mold cavities in plastic injection molding companies for sale. In other words, it is the volume that we require to be common from the injection chamber to perform the volumetric filling of the mold.
STEPS TO DETERMINE SHOT SIZE
The weight of the injection (pi) is the amount in grams (mass) of the plastic that has to be introduced into the mold to completely fill the piece or pieces. In general, this value is a specification that accompanies the mold from its design and is found in the technical data sheet of the product. If this value is not available it is required to weigh the piece or all the pieces including the branch.
For example, a cold runner mold with 4 cavities installed where each cavity weighs 10 grams and the branch 5 grams, the pi = 45xUdHPiFHQ7xbKh19G45saF1raB2ot5pag8p1Hnk4yrfXRJZskr8TMbFpVfC5tDk8eQQg63TqkW9gKhwagx6HePTaK2yXbPP). However, the weight may change when dealing with another material, especially in the case of composite or reinforced plastics.