1. What is thin wall injection molding?
Thin wall injection molding is one of the types of molding techniques in which thin and lightweight plastic components are created on a large level without impacting structural durability. The thin wall injection molding process uses less material and also reduces the time required for the molding cycle, improves the efficacy of the process and also reduces the cost per component. this technique enhances the overall efficiency of the process.
There are two common ways to define a thin wall:
● In the injection molding industry, we usually refer to injection molding plastic products with a wall thickness of less than 1mm as thin-walled parts, and this process is called thin-wall injection molding.
● In injection molding, when the ratio of flow length to thickness (L/T) is greater than 150, called thin wall, “L” refers to the longest distance from the melt into the mold to a fully filled cavity. “T” is the corresponding average wall thickness
2. Thin wall injection flow
In thin wall molding, the molten plastic must be thick enough to flow farther. As the following illustration shows, when the pressure pushes the plastic forward, a layer of frozen skin first forms on the outer wall of the cavity and then begins to solidify toward the center. The center of the fluid is the last to solidify. If the wall thickness is very thin, solidification will occur very quickly. This is why thin wall plastic parts must be high pressure and high speed to mold the reason.
● Ideally, molten plastic should reach all the edges of the mold before freezes.
● The injection pressure and speed are critical factors in determining how long distances the melted plastic can flow. Usually, the freezing time is proportional to the cube of the wall thickness. Therefore, for thin-walled products, the melt should flow fast enough to ensure that the melt is fully filled before freezing.
● Once filled too slowly, the partially frozen melt requires greater pressure to fill the cavity. These plastic parts may be formed under stress, and the final products are prone to stress cracking, shrinkage, or warpage.
● In many thin-wall applications, multi-valve gate applications may be the best option. By controlling the opening and closing sequence of the valve gate so that the molten plastic can advance evenly, this solution can solve the freezing issue, the shear heat and stress. Multi-valve gates are commonly used in the production of large thin-walled plastic parts.
3. Advantages of thin wall injection molding
● The largest advantage of the thin wall molding process is that it reduces material costs. As the name implies, thin walls or plastic components with thin sheets are manufactured in this molding technique which means that less plastic material is required in this molding process. Less material means less cost.
● Another largest advantage of thin-wall plastic molding is the reduced cycle time of the technique. Thin plastics manufactured in this technique are cooled in less time thus reducing the overall cycle time of the process.
● The plastic components manufactured in this technique are light in weight and are easy to carry. Therefore, the costs required for their shipment and the fuel costs are reduced.
● Due to less materials costs and short cycles, the overall productivity and output of the process are improved.
4. Application of thin wall injection molding
● Thin wall plastic molding is used to manufacture lightweight and portable packaging containers for food products and medicines as well. It is used in the packaging of bakery products, frozen foods, and meat.
● Plastic components produced by thin wall injection moldings are used in medical instruments and syringes as well.
● Thin wall plastics manufactured through the thin wall injection molding process are used in mobile phones and consumer electronics.
● Thin-walled injection molded products have applications and use in the automotive industry and also in the optical industry.
● The thin wall injection molding technique is used to manufacture lightweight and portable plastic containers, lids, storage boxes and mechanical components.
These are just a few examples. Millions of plastic products are manufactured through thin wall injection molding and these products are used in almost all industries through various means.
5. Common thin wall injection molding material
The selection of materials is an important decision in the injection molding process. In the thin wall injection molding process, a material is selected based on the shape of the component to be manufactured and the application in which we want to use that component. In the selection of materials, because the thickness of thin-wall products is less than 1mm, the flow ratio is generally more than 150, so the molding cycle of thin-wall injection molding is very short, and the molten plastic is cooled very fast in the injection process, which requires the material flowability is very high, in order to achieve rapid injection and filling.
Thermoplastics are commonly used as materials in thin-wall injection molding techniques. The following plastics are used most commonly:
1. Nylon (Polyamide)
2. High-density polyethylene (HDPE)
3. Low-density polyethylene (LDPE)
4. Polypropylene (PP)
5. PC
Thermoplastics are used in thin wall molded plastics that require bending capability. Different materials are used in thin wall injection molding according to their different geometries. Since high speeds and pressures are used in thin wall injection molding, the material should be selected such that it does not amplify the wear and tear of the mold.
6. Challenges faced during the thin wall injection molding process
The thin wall injection molding process has several benefits and is used in almost all industries throughout the world but it also faces many challenges during the process. Some of these challenges are discussed below:
● Large Capital Investment
1) Since the thin wall injection molding process needs high pressure and speeds during the process, The machines must be hard-wearing and reliable enough to bear high pressures. Only high-speed injection machines meet this requirement. thus the injecting machines required during the process are highly costly.
2) Usually for thin wall injection molding, automation servo robot auxiliary equipment is essential, and is a high-precision 5-axis servo robot, which can also ensure the consistency of product quality, for product manufacturers, it is also a lot of investment.
● Design of the mold
The design of the mold in a thin wall injection molding technique is an important parameter to consider. In thin wall molding, to reduce the timing of the cycle, the heat must be removed rapidly, and the design of the cooling system of the molds requires high technicality. Thus no compromise should be made on the quality of both the mold cooling system and design so that reliable and durable molds are designed. This is only achieved by investing huge capital in both the machine equipment and the mold.
● Steep Learning Curve
1) Learn about high-speed injection machine knowledge
The thin wall injection molding technique produces plastic parts that have thin walls. Special high-speed injection machines are used to produce these thin-walled products. The machines used for thin-wall plastic molding are equipped with the latest technology and have a variety of functions. Engineers, manufacturers, technicians and workers must embrace themselves with a complete knowledge guide and understanding of the thin wall molding machine and all of its functions.
2) Learn more molding technology knowledge
To avoid any faults in the thin-walled molded plastics and to produce high quality plastic components, the manufacturers and engineers should properly learn and comprehend the design and process constraints of the thin-wall molding technique since the process constraints of this molding technique has a very narrow range as compared to other molding processes. For example, injecting times for some thin wall components are as low as 0.1 s and any variation in this time will produce many issues in the operational process and it would be impossible to produce high-quality plastic products. Its operation window is very narrow and operation time is very small. This presents a great challenge to injection process engineers. But some plastic parts with thicker wall sections have a much bigger operating window and are easier to molding.
3) Learn automation production and mold knowledge
In automated thin wall moldings such as in the case of packaging components or water cups, complete knowledge of automation and robots is compulsory to make the molding process as smooth as possible. Moreover, the maintenance of the molds is required on daily basis in the thin wall molding process to preserve a tight tolerance level of molds. The staff and the engineers should also know how to troubleshoot efficiently to handle the small issues in the thin wall molding process and they should keep on updating themselves. To be able to identify small problems in thin wall production.
7. Difference between thin wall injection molding and traditional injection molding
There are many differences between thin-wall plastic molding and the conventional injection molding process. A few of the main differences between the two will be discussed here.
● The molding machines used in the thin wall molding process must be able to endure extremely large strains and pressures whereas in the conventional plastic molding process, there is no such condition.
● In thin wall injection molding, the plastic parts manufactured are of thin walls that cool down in less time as compared to thick wall plastic components. Thus the cycle times of thin wall plastic molding are shorter as compared to traditional plastic molding.
● The thin wall injection molding needs high strains and high speeds for the rapid filling of molds. For this purpose, highly technical and automated molding machines are required. In the conventional molding process, less pressure and speeds are required as compared to thin wall molding.
● The tolerance level for molds in thin wall injection molding is tighter as compared to traditional plastic molding because even minor defects will be more visible in thin wall plastic components.
| Standard VS. Thin Wall Molding Processing | |||
| Key Factors | Conventional | Thin Wall Molding | |
| Typical Wall (mm) | 1.5--3 | 1.3-2mm | <1mm |
| Machinery | Standard | High speed | Custom |
| Inject. Pressure, psi | 9000-14,000 | 16,000-20,000 | 20,000-35,000 |
| Hydraulic System | Standard | Standard | Accumulators on injection & clamp units. Servo valves. |
| Control System | Standard | Closed-loop on injection speed, hold pressure, decompression speed, screw rpm, backpressure, and all temperatures. | Same as at left, with resolution of 0.40 in. on speed, 14.5 psi on pressure, 0.004 in. on position, 0.01 sec on time, 1 rpm on rotation, 0.15 ton on clamp force |
| Processing | |||
| Fill Time (sec) | >2s | 1-2s | 0.1-1s |
| Cycle Time, sec | 40-60s | 20-30s | 6-10s |
| Mold | Standard | Better venting, heavier construction, more ejector pins, better polish | Extreme venting, very heavy construction, mold interlocks, precise surface preparation, extensive ejection features, mold costs 30-40% higher than standard. |
8. Common thin wall injection molding defects
Thin wall injection molding has several benefits as we discussed earlier. However, it has some common defects as well. Here, we will discuss some of these common defects and their solutions.
● Short shot
It is a flaw in the quality of the molding component. It occurs when the melting liquid in the mold solidifies before the mold cavity was filled, that is melt has condensed before the filling is completed. In the thin wall injection molding process, the thickness of the plastic is as low as 0.6-1mm and the flow length let's say is 300 mm. The length-to-thickness ratio is very large. Thin wall injection molding requires extremely high injecting speeds. Thus with this large ratio, a short shot occurs and the plastic condenses before complete filling.
● Warpage
Another common defect in thin-wall injection molding is warping. Warping is produced in the plastic component due to unintended strain or unequal cooling of the molded component. This flaw can be avoided by maintaining equal pressure and cooling time throughout the molding cycle and by balancing the cooling system of the thin wall injection molding process.
● Jetting
Jetting defects in injection molding are the result of the uneven solidification process of plastics. Jetting occurs when the initial molten resin jet enters the mold and has had enough time to begin solidifying before the cavity fills. This creates visible, curved flow lines on the workpiece surface and reduces the strength of the part. This defect is particularly prone to occur in thin-wall injection molding, for thin-wall products, the product wall is too thin, and the cooling space is too small.
● Flash/burr/spew
During injection molding, when the molten plastic escapes from the mold cavity to produce a very thin sheet of plastic, we call it a flash, also known as a burr or spew. The typical escape route is through the parting line, thimble position, or venting gate. The thin wall molding process is easier to occur this defect, because of high pressure, speed, and material with high flowability.
Other general problems linked with thin walls contain irregular cooling, flaws, highlighting texture, irregularities, fragile interwoven lines or cracks, etc.
9.Thin wall injection molding design and mold making tips or considerations
As the materials and fuel costs are increasing day by day, the thin wall molding technique is becoming more and more famous since it saves the costs required for materials and shipment as well. For example, it could be as thin as 0.6 mm for food packaging containers and as thick as 2 mm for automotive parts. The thin wall injection molding technique produces plastic parts with thin walls and small weights thus this technique requires less plastic material and since plastic walls are very thin, less time is needed for their cooling. But the thin wall injection molding process is highly technical and faces a few challenges as well.
● Molding plastic raw material selection
In the thin-wall injection molding method, the choice of material is more important than in conventional molding methods since thin-walled plastic components are difficult to manufacture and all kinds of molding materials used in conventional molding methods cannot be used here. Moreover, the selection of molding material is also dependent upon the shape and the application of the molded plastic component.
Thin wall injection molding requirements for raw materials: high flowability, high impact strength, high heat distortion temperature, high thermal stability, low directionality and nice dimensional stability, and also consider low-temperature impact stiffness, mechanical assembly and appearance quality of plastic raw materials. As wall thickness decreases, plastics with better physical properties are needed to maintain product strength.
● Avoid sharp corners (using radius corner)
In the wall injection molding process, the mold opening is very thin. Moreover, the filling resistance of the molten plastic is very high. Thus the thin walling plastic parts should be designed in a manner that they must escape sharp corners and perforation is decreased. It also can reduce filling resistance and stress.
● Mold material selection
In thin wall injection molding, molten plastic is injected into the mold cavities at high speeds and high pressures for rapid filling. so the impact of the molten material on the surface of the mold will be particularly violent. so it is important to select the right mold material as well when making the mold. it is usually necessary to use steel with hardness above HRC50-55, and the mold steel material must be heat treated, which can effectively prevent the wear of the mold. In traditional injection molding techniques, P20 steel is used commonly. But for thin wall injection molding, molds steel must be more strong, tough, hard and more reliable. For this purpose, H13, Nak80, S136, etc higher-grade steels are used to make molds more secure and durable.
● Add ribs
In injection molding procedures, to provide durability, and robustness and improve the strength of the plastic components, ribs are added. The ribs are half or less in thickness than the thickness of the plastic components they provide support and strength.
However, Incorrectly designed ribs can result in warpage, shrinkage, voids, and cracking, causing more trouble than they’re worth. To avoid these problems, in the case of thin wall injection molding, especially when the walls of components are less than 1 mm, design ribs to be 50% of wall thickness or less, keep small radii, space ribs apart by three times the wall thickness, and add 1°of draft angle or greater. Otherwise, the paper-thin rib is very easy to break when injection molding.
● Polish
The polishing of the mold surface is also very critical, as much as possible to polish the location, and reduce the molten plastic flow resistance, and the friction of the plastic filling. Especially with thin ribs location, the poor polish will heavily affect the parts stripping.
● Mold cost
The Mold made with better wear-resistant hard steel and optimized designs can end up costing 25-35% more than traditional regular molds. This is mainly due to the high requirements of the mold processing cost is more expensive, it needs more sophisticated equipment machining, and requires more processing procedures and time. However, these additional costs can easily be offset by the increased performance and lifetime of the mold.
● Big draft angel
Because thin wall plastic parts shrinkage is very small, the ribs and bosses edge of the draft Angle is larger than the regular mold (usually 2 times) so that the plastic components manufactured through thin wall injection molding can easy to stripping. If not using a bigger draft, it will heavily affect the parts stripping.
● Uniform wall thickness
The uniform thickness of the walls of the components is very crucial to achieve in thin wall injection molding. Uneven wall thickness will produce flaws in the molded parts and will also cause many problems such as warps, deformation, stress marks, and short shots. For the best results, keep your wall thickness a minimum of 0.9 mm or greater. But for some very small parts, the thin wall can be thinner than regular plastic parts.
● Add support columns, use a thick moldboard for the mold, and mold interlocks structure
To prevent deflection in the cavity region, use support columns as they can help fit the mold. In thin wall molding tooling, pre-loading support columns between 0.1mm-0.15mm are common. Thicker cavity support plates and additional support columns will help offset mold deformation. Mold interlocks will likewise help minimize tool shifting.
● Ample Cooling system
A sufficient cooling system is required to cool the mold cavities in thin wall injection molding. The cooling system of thin wall injection molding is a bit tough and technical as compared to conventional injection molding. Cold flow cooling can better cool the mold and improve the production cycle. A big challenge for thin wall molding tooling is to provide enough mold cooling when cycle times are so short.
Common Helpful cooling aids include:
1) Non-looping cooling lines are usually located directly in the cores, cavities blocks and inserts to help keep the mold surface temperature as consistent as possible.
2) Using the larger diameter cooling lines to increase flow through the tool (rather than relying on lower coolant temperatures)
3) where necessary, inserts made of special high-conductivity metals for faster heat transfer. Installing a beryllium copper insert on top of the core is a common nice solution to increase heat dissipation. This ensures better cooling of the mold, reducing cycle time
● Ample Venting system
For thin wall molding, high pressure, and high speed is necessary, and the injection time is very short, then a nice venting system is particularly important, otherwise, the cavity's compressed gas will burn raw materials, and occur a burning phenomenon.
1) Use venting sizes recommended for the material and increase the number of vents around the part. The common venting system of thin wall molding tooling requires venting the core pins and ejector pins, in addition to venting along the parting line. The pins used to provide ventilation in thin wall molding are 0.500-1mm in width and 0.02 to 0.04mm in depth.
2) The venting sheet arrangement is as far as possible perpendicular to the molten plastic final convergence location, which can highest effectively increase the exhaust gas efficiency.
3) We can also install a negative pressure valve in the mold to pre-vacuum the mold cavity and then fill it, which can reduce the resistance of air compression.
● Ejector system
The ejector system used for thin wall injection molding is a bit different from the ejector systems of traditional injecting molding techniques. The ejection of plastic thin-wall parts is prone to some obstacles. First, thin walls and ribs location are easier to damage than thick ones. Secondly, the smallest wall thickness has a smaller shrinkage, which can easily interfere with the release of plastic parts
1) The pins used in the ejector systems of thin wall injection molding are bigger than the size of the pins used in traditional injection molding. Moreover, the number of pins is twice the number of pins used in conventional injection molding.
2) Many plastic thin-wall parts use blade ejectors, and sleeve ejectors on ribs and bosses to avoid damage to the parts.
3) For some plastic thin wall parts, stripping can also be aided by polishing all internal features(special is thin and deep ribs), employing mold surface treatments (such as paint nickel-PTFE), or using sufficient draft angles.
● Using a hot runner system
To avoid any loss in the high temperature provided when thin wall injection molding, hot runners are used. a hot runner system is almost essential, especially for thin wall molding. A hot runner system can control the molten plastic temperature at any time according to the production needs. And using a hot runner system does not need trimmed gate. Save cost, improved efficiency and increase product surface appearance.
● Well-design Gate
In the thin wall injection molding process, gates should be designed with extra care. As high pressures and speeds are required in this molding method, therefore the size of the gate should be greater than the size of the walls of the plastic component to avoid any wear and tear and material shear. When gates are designed in such a manner, they avoid any chances of freeze-off as well.
1) The product mold design of thin wall molding should pay special attention to the arrangement of the gate so that can be balanced as much as possible and the whole product can be filled in a balanced way.
2) Since thin wall molding must be filled with high speed and pressure, the high shear rate in the gate area may lead to increased resin temperature and shear stress. The gate size should be large enough to minimize pressure drop but still, freeze fast enough not to interfere with the faster production cycles obtained from thin wall molding. Using mold flow analysis can help find the optimal balance between dwell time and pressure drop.
3) When gate directly onto a thin wall with, pinpoint, a sprue, or hot-drop, use gate wells to reduce material shear and stress at the gate, improving filling, and reduce part damage when stripping.
10. Thin wall injection molding manufacturing considerations or tips
● Using a high-speed Injection Machine
1) The thin wall injection molding technique needs high pressure and speed for the accurate placement of molten plastics in thin wall molds. If the speed and pressure of the process are not enough, the molten plastic will be cooled before the complete filling of the cavities. High-speed machines designed for molding thin-walled parts typically provide injection pressures in excess of 30,000 psi.
2) The clamping mechanism of the injection molding machine with thin wall plastic products requires high precision and stability of movement. In the field of packaging is usually used in-mold decoration process, before injection, quickly and accurately put the label into the cavity, and then take out the molding products, usually using an efficient servo robot-assisted production, which requires the injection molding machine to open and close the mold action accurate, rapid response, smooth action.
3) High-speed injection machines provide high consistency of process. Ensuring stable process parameters is another challenge for high-speed injection molding machines, and this is the key to successful thin-walled products, because the range of adjustment of molding parameters is narrow, as even small deviations can lead to differences between qualified parts and defective products.
4) Compared with traditional injection molding, molding thin wall products require higher clamping forces to compensate for the increased injection pressure. Typically, these range from 5 to 8 tons/square inch, compared to 3 to 5 tons/square inch in traditional molding. These high-speed injection molding machines typically also feature accumulator-assisted clamps to accommodate fast cycling time.
● Importance of molding process parameters
The process parameters in thin wall injection molding are very important since the window of the process parameters for thin wall injection molding is very narrow. Thus to manufacture high-quality thin wall plastic parts, the process parameters must be maintained stability. 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, flash, and another molding defect, and need to control the molding temperature and screw sheer speed, hold pressure, speed and other parameters according to the heat-sensitive and shear-sensitive characteristics of plastic. Also, need to combine with mold venting to reduce the filling resistance.
● Mold temperature
Mold temperature is the key to the thin-wall injection molding process. The higher the mold temperature, the better the plastic fluidity, which provides indelible help for thin-wall molding with the difficult flow. The right mold temperature allows the injected molten plastic to flow more easily, making better surface quality.
● Automatic servo robot production
Automatic servo robots can improve production efficiency, save cost, and maintain quality consistency. Servo robots are used to place and remove the parts from the molds. High-speed production is not possible to achieve without automated servo robots.
● Regular maintenance of the mold
The thin wall injection molding process needs a very tight tolerance and high-quality surface mold. This requires proper maintenance of the molds on regular basis otherwise it will affect the quality of the plastic molded parts. Regular maintenance includes Treatment of foreign bodies on the surface of the mold core cavity, oil contamination of mold inserts, lubrication of guide column guide sleeve, etc.
● Injection speed is the critical point in thin wall molding
In addition to high pressure, a fast filling speed that avoids freezing until the cavity is fully filled is critical to the success of thin-wall molding. Rapid filling and high pressure can inject molten plastic material into the cavity at high speed to prevent the gate freeze. Thin-walled plastic parts usually require an injection filling speed of more than 1000mm/s, and may even require a filling speed of more than 1200mm/ s. At the same time, the ultra-fast acceleration provided by the high-speed machine is also the key to ensuring that the filling is completed in a very short time.
11. How to make a thin wall injection molding plastic parts successfully?
Injection molding is a combination of material science and precision manufacturing, and it all starts with tooling. The precision of the mold is the first step, then selecting the right raw material, and keeping in mind its relationship to the tooling. In summary, creating the mold with the highest standards and applying molding parameters that best interact with the viscosity of the material is essential for filling the thin wall parts.
The thin wall injection molding process can be implemented successfully if we reflect the specific design and production consideration tips carefully. However, here we will discuss the most important factors that are crucial for the success of the thin wall injection molding process. These are described below:
1. Careful selection of the molding material and the design of the tooling is very important in thin-wall plastic molding.
2. High-precision machining and maintaining the mold with tight tolerance are necessary.
3. To achieve high speed and pressure for thin wall molding, the usage of a high-speed injecting machine is compulsory.
4. The process parameters for thin wall injection molding must be fulfilled.
5. Manual operation of thin wall molding efficiency is too low, automation is the essential requirement to successfully implement the thin wall injection molding process.
12. Conclusion
The demand for thin wall plastic components is increasing throughout the world because of their durability, shorter cycle times, and less source material requirement. Thin wall plastics can fulfill the maximum of the needs of consumers. Thin wall plastic molding is one of the many kinds of traditional injection molding methods that manufactures plastics that are light in weight, strong and reliable, and economical. Thin wall injection molding is increasing in each industry with time. It has its pros and cons. The above guide will help you decide if it is the right choice for you or not.
