Everything you need to know about thin wall injection molding process

Table of Contents

Thin-wall injection molding is a process used to make thin plastic parts. This process is typically used for small, complex parts that require a high degree of precision. In this blog post, we will discuss the basics of thin-wall injection molding and how it can be used to produce high-quality plastic parts.

A simple way of looking at it is that when the wall thickness is less than 1mm it is called a thin wall. More comprehensively, the definition of a thin wall is related to the process/wall thickness ratio, the viscosity of the plastic, and the heat transfer coefficient.

Thin-wall injection molding has different requirements in terms of product design, machine selection, mold manufacturing, and molding process because of its unique shape characteristics, and each step will affect the final product quality.

What is the thin wall?

The injection molding industry usually refers to injection molded products with a wall thickness of less than 1mm as thin-walled parts, and this process is called thin-wall injection molding.

Compared with traditional injection molding, the wall thickness of the product is much thinner, but the core or professional difference is the difference in the flow length ratio (usually the process at the end of the mainstream molding process is called L, the thickness of the product is t, L/t is called the flow length ratio) when L/t>150, it can be called thin wall injection molding.

Why use thin wall injection molding?

The cost of plastic is usually a large percentage of the cost of the finished product, say 50-80%. A thin wall helps to reduce this ratio.

As consumer electronic devices such as cell phones,digital cameras, and PDAs become smaller and lighter, the plastic parts are designed to be thinner and thinner.

The nature of thin-wall filling

The mold wall is cold, and when melt fills the mold cavity, the mold wall will set up a cured layer, thus reducing the thickness of the flowable channel. This situation is more serious when the wall thickness is thinner. 1mm wall thickness has a 0.2mm thick cured layer, leaving a flow path of 0.6mm thick.

A 0.5mm wall thickness has a 0.2mm thick cured layer, leaving a 0.1mm thick flow path. When the filling is not completed and the flow channel disappears due to the thick curing layer, the finished product will not be filled.

Other features of thin wall injection molding

High-speed filling

Thin-wall injection molding, therefore, requires a high injection speed to fill the injection mold cavity when the cured layer is not too thick.

High injection pressure is not necessary. He just makes up for the lack of injection speed by injecting melt hard into the unfilled cavity. This not only increases the required clamping force but also creates high internal stresses in the finished product, which deforms after demolding.

The injection speed of a general-purpose injection molding machine is around 100mm/s, which cannot cope with thin walls injection. The injection speed can be increased by 25% by increasing the oil pump. Double pump injection is 70% higher.

Some manufacturers use regenerative injection to exchange injection pressure for injection speed. It is used when the initial injection does not require high injection pressure. The injection speed can be increased by more than 100%.

The nitrogen cylinder can store the energy of the oil pump in the form of pressure and release it during injection, which is a formal method to increase the injection speed significantly. Injection speed is divided into four categories as follows.

Low speed 200-300mm/s; medium speed 300-600mm/s; high speed 600-1000mm/s; super high speed 1000-2000mm/s. Domestic injection molding machines can reach the medium speed grade.

Low inertia injection

The only high-speed injection cannot meet all the requirements of thin wall injection. What should also be considered are high acceleration and high deceleration. At the beginning of injection, the screw is stationary. From standstill to full speed, e.g. 400mm/s, the screw has to be accelerated.

If the whole injection time is only 0.5s, it is desired to reach full speed in 0.05s and the acceleration rate is more than 8 G. On the contrary, if the acceleration time takes 0.3s, it is not suitable. The reason is that the average speed is pulled down by the low acceleration.

Ignoring the melt viscosity resistance, a=F/m. a is the acceleration rate, F is the thrust, and m is the mass. Therefore, thin wall injection also requires large thrust force and small mass.

High rigidity oil circuit

The pressure oil is elastic and is to be considered when talking about 0.05s acceleration. Large cylinder piston area, short stroke, and short tubing can reduce the effect of elasticity. The rigidity of the oil circuit will also be improved when the hose can be replaced by a rigid hose.

Servo valve

The response of the servo valve is faster than the general proportional valve. It can be most effective when filling the mold cavity and turning to pressure-holding. If the response is not timely, the plastic material will be overflowed and the finished product will have a burr.

Fully closed-loop control

The use of servo valves, generally with full closed-loop control, can achieve the injection speed, holding pressure, and back pressure control.

The fully closed-loop control monitors the relevant variable (speed or pressure) and notifies the servo valve to correct when there is a deviation from the set amount.

Simply put, fully closed-loop control improves the stability (repeatability) of injection molding and reduces the scrap rate.

Controller

The controller, commonly known as a computer, has to issue the command to finish injection and switch to pressure-holding the moment the mold cavity is filled.

When the injection speed is 400mm/s and the allowable electronic scale deviation is 0.1mm, the controller can only have a deviation of 0.25ms. The controller is required to scan the injection electronic scale every 0.1ms.

If the controller adopts “real-time” control, it does not use scanning, and when the electronic ruler measures that the pressure-holding point has been reached, it generates an interruption, and the controller handles it “instantly”, which can also achieve the requirement of high stability.

Short injection cycle

The cycle time can be saved by opening the mold and ejecting it at the same time, which is about one second.

Mechanical structure

To achieve a cycle time of four seconds, the opening and closing of the mold must be fast and stable (no vibration).

A proportional valve is used to open and close the mold with a braking function. The high rigidity of the frame also helps. The variation of the mold plate directly affects the thickness of the mold cavity.

When the uniform wall thickness is 0.5mm, the deformation of the mold plate should be controlled below 0.05mm. Therefore, the rigidity of the injection mold plate should be high (suitable reinforcement, proper thickness of the mold plate), and the space of the four columns should not be too large.

Plasticizing ability

To do a good job of plasticizing in a four-second cycle, the plasticizing capacity of the screw should be increased or a pneumatic sealing nozzle should be used to extend the plasticizing time.

Double thread design can improve the plasticizing capacity. A longer screw with an L/D ratio of 24-25 can increase the heat absorption area and also increase the plasticization effectively.

The extra high screw speed increases the screw surface speed to more than 1m/s, which has no negative effect on the commonly used PP materials.

The pneumatic nozzle allows the plasticization to continue when the mold is opened and closed, but the injection molding machine must have two power sources, such as two oil pumps, to achieve this.

Mold

Like the template of the injection molding machine, the template of the mold should be thick to reduce the variation.

High-speed injection molding should be well exhausted. Sufficient exhaust slots, the use of breathable mold steel, and vacuum are the methods. The machining precision of the mold is required to achieve a uniform thickness of the circumference of four thinner walls.

The requirements of multi-cavity molds are even higher. The molds are equipped with an ejector and blowing device to make the finished products fall faster after demolding and close the mold immediately.

Plastic

The melt index (MI) of PP plastic is as high as 60(g/10min), such as Basell’s Moplen RP1086. Many finished products are made of PS/ABS because of the toughness of PC and the fluidity of ABS, which are also suitable for thin-wall injection molding.

High fluidity material selection for thin-wall injection molding

In terms of material selection, since the thickness of packaging thin-walled products is less than 0.6mm and the flow length ratio is generally greater than 200, the molding cycle of thin-walled injection molding is very short and the products are cooled very quickly during the injection molding process, which requires very high fluidity of the material to achieve rapid injection and filling.

At present, thin-wall injection molding is mainly used in polyolefin materials, such as PP, and PE, in addition to PET, PS, PE, ABS, PC, and other applications.

Thin-wall injection molding product design considering the arrangement of the inlet port

The product design of thin wall injection molding should pay special attention to the arrangement of the inlet port so that the inlet glue can be balanced as much as possible and the whole product can be filled in a balanced way.

In addition, special consideration should be given to the exhaust design. For some products with wide and thin wall thickness, the exhaust sheet can be used for exhaust. Note that the exhaust sheet arrangement is as far as possible perpendicular to the final convergence line, which can most effectively increase the exhaust efficiency.

For example, in the cell phone battery case, because the thickness of the cell phone is getting thinner and thinner, the electromagnetic case is also hoped to make the thickness as thin as possible. So the back of the battery case will usually do an exhausting piece, in the filling of trapped gas can be effective gas out of the mold.

Thin-wall injection molding mold material, exhaust slot, and gate design

There are many factors to be considered for thin wall injection molding, such as mold material, mold exhaust, gate, etc.

First of all, the material of the mold is very important, because thin-wall injection molding is usually used for high pressure and rapid filling, so the impact of the material on the surface of the mold will be particularly violent, so it is usually necessary to use steel with hardness above HRC55 degrees, and the material needs to be heat-treated, which can effectively prevent the wear of the mold.

The polishing of the mold surface is also very important, as much as possible to polish the location, reduce the flow resistance, and prevent the friction of the plastic filling.

Secondly, the exhaust of the mold is also very important, as mentioned earlier can be designed in the structure of the product exhaust piece, can also be used in the form of a negative pressure valve in the mold, the mold pre-vacuum, and then filled, which can reduce the resistance to air compression.

The end of the exhaust slot needs to be designed according to the different materials, not only not to run the flying edge can also be an effective exhaust, but at the same time need to pay attention to all the exhaust channels need to lead to the outside of the mold.

In the gate design, it is necessary to consider using a big gate or multi-gate to feed the rubber, or even consider using needle valve type timing control to feed the rubber, so that the material can be filled effectively, and at the same time, it should be noted that the cooling time of thin wall parts is very fast, and the runner and gate need to increase the thickness to prevent cooling closure.

Thin-wall injection molding with a high-speed machine

A thin wall injection molding machine needs to choose a high-speed machine, you can choose a machine with speed increasing mechanism.

For example, the screw plunger type is a classic thin-wall injection molding machine, using screw storage, and plunger injection rubber, molding injection speed is fast, and can well meet the thin-wall injection molding needs.

In addition, some nitrogen boosters, cylinder boosters, and other equipment are also available as thin-wall injection molding machine options. To meet the requirements of fast filling, a thin-wall injection molding machine is required to have the following conditions.

Injection pressure should be high, 200 MPa or more (general injection molding machine pressure is below 200 MPa); clamping force is large, the clamping force provided by the clamping mechanism on the projection area of each square inch of the product is 5-8 tons (conventional is 3-5 tons); response speed is fast, it is understood that the injection speed is up to 2200 mm / s, the response time is 11 milliseconds, etc.

In addition, the clamping mechanism of the thin-walled injection molding machine also has higher requirements for precision and smoothness of movement.

The packaging field usually adopts an in-mold labeling process, before the injection, quickly and accurately place the label in the cavity and remove the molded products, which requires high-efficiency robotic action, requiring accurate, fast response and smooth movement of opening and closing the mold.

Thin wall injection molding process should control the pressure and speed

The filling time of thin-wall injection molding is very short, many filling time is less than 0.5s, it is impossible to follow the speed curve or truncate the pressure in such a short time, so it is necessary to use a high-resolution microprocessor to control the injection molding machine.

In the whole injection molding process of thin-walled products, pressure and speed should be controlled independently of each other at the same time.

The method of filling the stage with speed control and then switching to pressure control in the pressure-holding stage of conventional injection molding machines is no longer applicable.

Thin-wall injection molding usually uses high pressure to fill quickly, but we should pay attention to the injection process may be burnt, trapped gas, and other problems, and need to control the molding temperature and screw sheer speed and other parameters according to the heat-sensitive and shear-sensitive characteristics of plastic. Also, need to combine with mold exhaust to reduce the filling resistance.

Conclusion

Thin-wall molding is an important branch of modern injection molding, which can meet the needs of many products in terms of structure, lightweight, molding cycle time, and cost.

Thin-wall molding has been widely used in the field of injection molding, we should start from the design source and combine all the factors of machine, mold, and process to optimize the successful thin wall molding process and optimize the quality of products.

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