Features of BMC molding process:
The compression molding principle and process of BMC moulds are basically the same with other thermosetting plastics.When pressed, a certain amount of BMC moulding plastic is put into the preheated die, which is then pressed and solidified to form the required product.
1. Features of BMC molding process
1) less waste, usually only 2% to 5% of the total amount of used materials. The actual amount of material loss also depends on the shape, size and complexity of the molded products.
2) in the molding process, although BMC molded plastics contain a large amount of glass fiber, they do not produce a strong orientation of fiber. Therefore, the products have high uniformity and density, and the residual internal stress is also small.
3) during the processing, due to the fact that the filler and fiber rarely break, it can maintain high mechanical and electrical properties.
4) during pressing, due to the relatively short flow length, the abrasion of the mold cavity is not serious, and the maintenance cost of the mold is relatively low.
5) compared with injection molding, the investment cost of the molding equipment and molds used is lower, so the molding cost of the whole product is also lower.
2. Pressing molding process
When pressed into shape, a certain amount of prepared BMC moulds are put into the preheated steel moulds and then closed the moulds at a certain speed.BMC moulds flow under pressure and fill the entire cavity;Keep for a certain period of time at required temperature and pressure until the physical and chemical processes are completed and the mold is solidified, set and the best performance is achieved.
3. Preparation before pressing molding
As a wet premix, BMC molding plastics contain volatile active monomers. Do not remove the packaging materials too early before use. Otherwise, these active monomers will evaporate from BMC materials, reducing the fluidity of materials and even causing deterioration of performance.Of course, for unwrapped and unused BMC moulds, it is important to re-seal them so that they can be used next time.
1) calculation and weighing of feeding amount
Generally speaking, the first thing is to know the volume and density of the compressed products, plus burrs, flanges and other losses, and then carry out the calculation of the amount of feeding.Charge of accurate calculation, to ensure the accuracy of product geometry size and prevent lack of material or caused by excess material waste and waste of materials, has a very direct relationship, especially for BMC cannot be recycled after the forming of thermoset composites, to save materials, reduce cost, more have important practical significance.
In fact, due to the complex shape and structure of the moulded products, the volume calculation is complicated and not necessarily accurate, so the loading quantity is often estimated.For automatically operated machines, the feeding amount can be controlled within 1.5% of the total amount of soil used, and if the amount reaches 5% or exceeds this amount, the flying edge will definitely appear on the mold clamping surface.This thin layer of excess material will rapidly solidify to form a flying edge under the action of high mold temperature in the heating state.
There are a number of methods for estimating loading.Such as the so-called "shape, size simple estimation method", "density comparison method" and "injection comparison method".
After the basic loading amount is estimated by the above method and several times of pressure test, the loading and feeding amount of BMC moulded plastics can be obtained accurately.
2) preheat the mold
BMC moulding plastics is a kind of thermosetting reinforced plastics. For thermosetting plastics, the mold should be preheated to the required temperature before molding. The actual temperature is related to the type, formulation, shape and wall thickness of the pressed BMC moulding plastics, molding equipment and operating environment.It should be noted that when the mold temperature does not reach the set value and is uniform, do not feed into the mold cavity.
3) placement of inserts
In order to improve the strength of the joint of the moulded product or make it a conductive channel, it is often necessary to install the embedded part in the product.When it is necessary to set the insert, it should be installed in the mold cavity before loading, pressing.The inserts should meet the design requirements. If they are metal inserts, they should be cleaned before use.For larger metal inserts, it is necessary to warm them up before placing them, so as to prevent the defects such as rupture caused by too large shrinkage difference between materials and metals.
In the same cavity, if there are different types, different specifications of the insert, should also carefully check the placement of the insert.The dislocation of the inserts not only produces waste but also can damage the cavity.In short, inserts should be put in place, accurate and secure.
4) brush the mold release agent
For the compression molding of BMC moulding plastics, due to sufficient internal demoulding agent has been added in the component during the preparation, and the parts will be cooled and contracted after the moulding, so it is generally not necessary to brush the external demoulding agent.However, due to the good fluidity of BMC materials, it is possible for the moulding to infiltrate into the gap of the connecting surface of the formed part of the mold cavity at the time of moulding, which makes it difficult to mold. Therefore, it is also beneficial to apply some external demoulders to the mold cavity before mold closing or after mold cleaning for new or long-term use.The external demoulding agent used is generally paraffin or zinc stearate.
In the compression molding of BMC molding plastics, it is worth noting whether the molding operation is appropriate and reasonable, which will not only affect the flow of materials in the mold cavity, but also affect the quality of products, especially for products with complex shapes and structures.Therefore, it is a very important question how to put the BMC moulds into the moulds reasonably.
In most cases, the whole (mass) BMc material, which is compacted and of similar quality to the product, is put into the central position of the pressure model cavity by hand.But sometimes, also can specially drop the material to the place that may appear when pressing, such as the protruding stage, core and groove these places.The worst method is to divide the BMC molded plastics into several pieces and put them into the mold cavity. In the pressing process, the welding connection may appear when the materials flow into the junction point, so that the product will have a "weak link" of strength here.
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