OVERMOLDING UNDER LOW PRESSURE

Firstly, low-pressure overmolding has won a well-deserved place in the foundry industry. The essence of the method is that the melt under low pressure of air or gas fills the mold. 

Excessive pressure in the metal flow is necessary in order to overcome various resistances in its path. The over-molding mold according to this method is filled from the bottom up directly. 

Advantages of the low-pressure overmolding method:

• Possibility to automate and mechanize the mold filling process.
• Overmolding quality improves due to excess pressure in the liquid plastic. This makes it possible to obtain over moldings with very thin walls (3-4 mm) and a dense structure.
• The filling of the mold carries out with minimal heat loss on the path of the melt from the furnace to the mold
• The process of automatic filling of the mold cavity with plastic lasts from 1.5 to 6-8 seconds. It depends on the weight of the overmolding. This contributes to an increase in labor productivity.
• The output is overmolding with high mechanical properties, smooth surfaces, and precise geometric shapes. There is no need to machine them. 

Design and features of the overmolding

The overmolding has a retarding filter, which ensures smooth braking of the rising plastic flow, increasing the static pressure in it, and also avoiding deformation of the sand core. The filter freely passes the air displaced by the alloy from the mold cavity and does not pass the metal. The retarding filter is always made of all metal.

The braking element is a slots labyrinth formed by the outer surface of the central part and the inner surface of the detachable parts of the housing.

What is the speed of filling of overmolding?

Firstly, the mold steps, at first the melt enters slowly to the level of the collector, where it closes the electrical contact, turns on the additional pressure of the pneumatic system, and quickly fills the mold cavity. 

At the end of filling the mold cavity, the melt, closing the second contact of the electrical interlock, turns on a device that maintains the pressure in the crucible at the reached level until the overmolding is completely solidified.

How many types of molding machines?

In addition, when the overmolding completely solidifies, the so-call technological holding time switch activates, and the pressure in the crucible is automatically released. The mold is opened and the molded part is ejected.  It makes it possible to obtain thin-walled molded parts in various forms.

Currently, more than 20 varieties of thermoplastic for injection molding are known. Each type can be for certain types of products. Plastic can be used in universal or specially equipped injection molding machines, equipment, and technological methods.

Traditional injection molding

The most common type of injection molding carries out on universal injection molding machines (IMCs) and is used to manufacture parts with a wall thickness of 1.5 to 5 mm. The share of parts obtained by traditional overmolding exceeds 90% of the total number of molded thermoplastic parts.

Besides, injection molding (compression molding, press molding) involves the injection of a melt into a mold that is not completely closed. The final molding with increasing pressure in the mold occurs at the final closing of the mold. Injection molding produces simple-shaped parts of increased thickness.

Is the overmolding process at high speed?

Moreover, it has high-speed thin-walled overmolding for products with a wall thickness of 0.6 to 1.2 mm. It has its own characteristics in terms of overmolding units equipped with a hydraulic accumulator. Special equipment with a developed ventilation system, and overmolding modes that are fundamentally different from traditional overmolding.

Overmolding has thick-walled parts with a thickness of 10–30 mm. It carries out according to a technology close to traditional overmolding. It distinguishes by 2–4 ​​times longer holding time under pressure and cooling, and at the same time, the sprue diameter increases by 2–3 times.

How many producing stages of overmolding?

Furthermore, the overmolding process produces molded parts in two stages. In the first stage, the mold is partially filled with the melt during the rotation of the screw (similar to extrusion).

The final filling of the mold, holding under pressure occurs during the translational movement of the screw-piston forward without rotation. This method is used when the volume of the molded part exceeds the maximum injection volume of the injection molding machine.

How is Sandwich overmolding characterized?

Firstly, sandwich overmolding is by successive injection of two polymers through one sprue. The first polymer is creating a surface layer from a color primary polymer, and the second polymer, as if from the inside, inflates the surface layer. The inner layer can recycle or expandable polymer. The injection molding machine must equip with two injection units.

Two-component or two-color injection molding is characterized by the successive injection of different polymers using several mold cavities of different geometric shapes and volumes.

First, a plastic insert is a substrate, which moves with the mold half into another molding cavity of a larger volume. Where the insert fills with another polymer. The polymers must have good adhesion to each other. The machine equips with two injection units.

The molding machine uses high injections rates

In fact, foam molding of expandable thermoplastics carries out using high injection rates on hydraulic accumulator injection molding machines. The original thermoplastic contains a physical blowing agent (for example, isopentane) or a chemical blowing agent. There are super concentrates containing a blowing agent.

How does the molding machine work?

In addition, overmolding with gas begins with an incomplete injection of the melt. Then gas (usually nitrogen) supplies to the melt through the sprue or, directly, into the mold, bypassing the sprue. As well, gas under pressure of 50 – 120 kg/cm2 inflates the polymer shell, and presses the polymer from the inside to the inner surface of the cavity, eliminating sinks.

The structure of the overmolding has a pronounced gas cavity surrounded by a monolithic polymer. Gas injection is used for thick-walled products that do not require a monolithic polymer section.

Firstly, over the years, OEM injection molding has shown continuous development. With more than a century since its discovery, synthetic plastic is now among the most used materials in the production of goods and is present in practically all sectors.

If your company still doesn’t work with parts produced from the thermoplastic injection, then it’s time to change this scenario.

To help you make your decision, we have listed 4 reasons that will make you better understand this choice.

Why use incorporate thermoplastics in your production?

1. Versatility

Thermoplastics are materials that stand out for their great versatility. They can adapt to the most diverse uses. The OEM injection molding process allows the plastic material to take on diverse forms, ranging from plastic bags to airplane parts.

They serve as raw materials for numerous products and functionalities.

2. Reusable

The vast majority of polymers uses today have good reusability.

At a time when the excess of waste produced in the world discusses, this is a fundamental characteristic.

It is also worth remembering that in the OEM injection molding of thermoplastics it is possible to work without wasting raw material. 

3. Live in constant development

Over the years, thermoplastics have shown continuous development. Not only are new materials discovered, but also the properties of existing ones are also constantly improved.

What’s more, your products will also follow this development and thus gain in quality over time.

4. It can be adapted to your needs

Currently, plastics are even used as substitutes for wood and iron in various sectors, such as civil construction, for example. If you need a part with a higher resistance index, you can opt for so-call reinforcing loads. Through rotational OEM injection molding, it is possible to achieve this objective and still produce materials with a sophisticated design.

Therefore, it is very difficult to find a need for its production that thermoplastics are not able to supply. For these reasons and many others, your company should think about parts produced from OEM injection molding.

Enjoy the benefits of this material for your production!

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PRECISION INJECTION MOLDING METHOD

More details about this type of molding

Today, OEM injection molding is very often used in factories. This is done using advanced technology. First, molten plastic is poured into a special chamber. It can be cold or hot temperature. Subsequently, from it, through the channels, the heated substance passes into a heated mold. It makes either of steel or cast iron (this is very rare).

 The force of pressure that applies in this process depends on:

• Chemical composition of the molten substance
• The configuration and wall thickness of the future molded part

After injection molding, the part is ejected out of the mold in a special way.

Technological requirements

The molten plastic that is poured into the mold can have a different movement pattern:

• Calm
• Active turbulence
• Dispersed character

In order to obtain  molded parts of excellent quality, experts will control the following aspects:

1. Speed
2. Duration of the passage of molten metal into the mold
3. Material temperature
4. Special mold temperature
5. What are the OEM injection molding and its walls made of?
6. Area of ​​ventilation ducts and their placement

All of the above standards always depend not only on what plastic is used in molding but also on what wall thickness the future product should have.

Technological moments that are observed during injection molding

Speed ​​is an important aspect of this difficult OEM injection molding process. It must be strictly controlled. It is important to know that when it is too large, the important lubricant on the mold will erase – and the product will be impossible to get out of there.

In this case, the walls of the mold can begin to collapse from the high temperature of the molten plastic. The product will also deform. At the same time, when the material is fed very slowly, the surface of the future object will also be defective – it will become uneven.

Employees of the enterprise know that the optimal speed of OEM injection molding is from 10 to 50 meters per second. But everything also depends on the metal. For example, for steel and copper it is less, and for zinc and lead it is more.

Injection molding process

In production, a well-defined sequence of processes should be followed. First, before pouring the molten plastic, the mold is heated to the appropriate temperature for the material. The correct structure of the molding depends on whether the specialists adhere to the normalized temperature. That is, if everything is clearly executed, there will be no defects.

 

Consider the main temperature standards for plastics:

If the temperature is too high during the OEM injection molding process, the molten plastic can spread very quickly in the form and will get into the ventilation channels. As a result, there will be porosity in the molding, because it will boil due to the lack of air passing through it.

When molding under low pressure, the following conditions must be met

• Keeping the heat of the molten plastic entering the mold cavity by controlling the flow rate
• Creating a temperature gradient in the plastics flow in the direction from the mold surface to the middle zone of the plastics wall to ensure the crystallization of the molten plastics in a narrow molding layer
• Creation of excess pressure in the liquid plastic that filled the mold cavity to feed the crystallizing layers of the molding, both in thin and massive sections. 
• Maintaining a smooth, strictly according to a given regime, increase in pressure in the crucible to prevent gas breakthrough from the crucible into the mold cavity.

Always make the right choice regarding pressure and crystallization

Properly chosen excess pressure and crystallization conditions make it possible to obtain molding without air-trap and shrinkage defects. These defects can only form if the overpressure is insufficient. 

The density of the molding plastic is also favorably affected by the use of water-cooled in OEM injection molding, the workpiece then has a fine-grained structure and increased density, due to the narrowing of the solid-liquid zone in the hardening casting.

OEM injection over-molding supplier is widely common in the industry. To transform and shape this material, there are a variety of techniques common.

Thermoplastic injection and rotational molding

Both methods have their specific advantages and characteristics. In today’s text, we differentiate between the two techniques.

Thermoplastic injection x rotational molding

The thermoplastic injection consists of molding polymer granules in order to generate a plastic part. The objects created in this technique can have the most diverse shapes and dimensions. They also tend to have a constant thickness, facilitating the production of fitting pieces.

The process takes place in an OEM injection over-molding supplier with two phases, one for closing and one for injection.

• In the first phase, the solid-state material melts until it becomes a unit.
• In the second phase, the melted plastic injects into a mold in the desired shape to create the part.

It then undergoes cooling, which causes it to solidify. Thus, it gives rise to the fixed format of the final object.

Rotational molding

Rotational molding, also called rotational molding or rotational casting, is a process of molding plastic parts and transforming thermoplastics. Simple and low cost, it generates hollow or open pieces of greater or lesser dimension.

There are four stages that constitute the operation of OEM injection over-molding supplier.

The first is the dosage of the resin, which places in the mold. Then heating and rotation take place. It then passes through the cooling of the obtained part, followed by the demolding, in order to originate the final object.

Advantages and disadvantages of each process

Both thermoplastic injection and rotational molding allow a wide range of modeling. It is possible to produce parts of different shapes, colors, and dimensions with low residual stress.

On the other hand, rotational molding production time can be a disadvantage, since the molding and demolding processes are manual. Parts are exposed to high temperatures for a long time, which increases the risk of thermal degradation.

The possibility of creating parts aimed at meeting a specific need

The OEM injection over-molding supplier allows the creation of parts with great precision, while those produced by rotational molding have less exact dimensions.

As for large-scale production, injection is the most suitable for the process. After all, it is possible to produce a large number of parts in a short time. Therefore, compared to rotational molding, it is more economically advantageous.

The differences between the rotational molding and thermoplastic injection

The differences between the rotational molding and thermoplastic injection are subtle, but each of these processes is best suited to a situation. When choosing, it is important to analyze which one is most suitable for each need.

Keep following our blog for more information and tips about OEM injection over-molding suppliers.

In the plastics industry, one of the most common transformation methods is thermoplastic injection. But it is not the only one. There is also the use of vacuum forming, or vacuum thermoforming.

Below we explain what this process is and its differences from injection molding machines.

What is vacuum forming?

Vacuum forming is a polymer transformation process that has been gaining ground due to its versatility and low cost.

The process consists of the controlled heating of the polymers until they reach the appropriate temperature for the shape of the mold common.

This mold can be either positive (concave) or negative (convex)

OEM injection over-molding supplier use is for the production of parts, packaging, and wrappings and stands out for its quality and good productivity. Vacuum thermoforming does not require large industrial facilities and is easy to create molds.

Vacuum thermoforming x thermoplastic injection

Although it is a process with good advantages, thermoforming cannot be considered a substitute for injection molding machines. This is because there are some parts that can only be achieved through the use of an OEM injection over-molding supplier. Such as those that have small details or those that have a high degree of complexity.

In these cases, vacuum forming proves to be insufficient and unable to produce the same results. On the other hand, the method is excellent for prototyping and small-scale immediate production.

There is another way of deciding on which process to use is also the purpose of producing the part.

OEM injection over-molding supplier is excellent for producing parts

When looking for a supplier for your plastic parts, research your goal and the ideal method for your production. Thus, your demands will be met with assertiveness, considering your needs and choosing the process that best suits you.

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Today the injection of thermoplastics is one of the most common production processes in the manufacture of goods from different sectors. Just look around us and we can quickly realize that polymers are present in almost everything.

To make this possible, OEM injection over-molding supplier relies on different techniques that allow materials to meet different needs.

What are thermoplastic additives?

Additives for thermoplastics are chemical substances added to polymers at the time of transformation. Each of these additives will react in a different way, causing the material to gain new characteristics.

As such, they are vitally important to the enormous diversity of needs that the plastics industry is able to meet. To better understand how they work, below we separate, as an example, some widely common additives for thermoplastics.

Expanding agent

The blowing agent is common to OEM injection over-molding suppliers in expanded form, such as foam or Styrofoam. The expansion is possible thanks to the formation of gas that occurs when the incorporated additive decomposes during the heating phase.

Usually, the additive appears in powder form, and a good distribution of it is essential for good results.

Flow agent

When softened, polymers have a high viscosity that makes it difficult to fill channels and matrices. Without the application of flow agents, damage to the molding process can occur, in addition to stressing the machine to the maximum, which can cause a failure or breakage.