What Causes Short Shot in Plastic Injection Molding?

short shot plastic injection molding defects

Short shot refers to incomplete filling of a mold cavity which results in the production of an incomplete part due to the insufficient plastic fluidity. The main causes are the inappropriate injection pressure and speed (including excessive pressure loss caused by resistance), which are mainly affected by the following aspects:

I. Injection Molding Machine:

(1) Insufficient injection molding capacity – This is caused by an overestimation of the capacity of the injection molding machine, which may also occur due to insufficient plasticizing capacity or insufficient injection volume. Among them, the insufficient plasticizing capability can be increased by extending the heating time, increasing the screw speed and raising the backpressure. If the injection volume is not high enough, the injection machine with a larger injection capacity can be used to solved the problem.

II. Inappropriate Mold Design

(1) Local short shot caused by the flow imbalance in each cavity of a multi-cavity mold – When the injection molding capacity of the injection molding machine is sufficient, this defect is mainly caused by the uneven flow in each gate, i.e., the mold cavities are not distributed in a balanced way.

(2) The flow range of the melt is too long, and the flow resistance is too large – The parts that hinder the flow of the melt include the nozzle, the sprue, the runner, the gate, and the thin walls of a product. The flow resistance of the nozzle can be reduced by increasing the nozzle diameter / temperature, and using a nozzle with a small flow resistance.

(3) Poor venting. When filling the cavity, air is trapped to cause counter pressure. When the melt is injected into the cavity, the cavity is often closed by the melt in the very beginning, with air trapped in the unfilled local areas. Also, because the filling speed is too fast, sometimes there is not enough time for the air to be vented through the parting surface but compressed instead, resulting in partial unfilled areas in the cavity, thus short shot in the molded part.

III. Improper Molding Process

(1) Improper injection molding process – The low barrel temperature, slow injection speed, short injection time, and insufficient backpressure lead to a shortage of plastic.

(2) Oversupply of plastic – If too much plastic enters the barrel, the injection pressure is lost due to the compression of the pellets, thus reducing the pressure required to inject the melt from the nozzle, which is necessary for injection molding, resulting in insufficient injection pressure. The solution is to adjust the amount of feed, i.e., the amount of melt, and make the amount just right for the molding.

(3) Unstable production cycle – Frequent machine shutdown, and production inconsistent with the normal cycle make some plastics stay in the barrel for too long with reduced density and viscosity, thus resulting in plastic underfill.

(4) Inappropriate mold temperature leads to lower injection speed and plastic underfill.

IV. Choice of Plastic

(1) Poor plastic fluidity – If the plastic material doesn’t feature a great fluidity, it will solidify before reaching the furthest end of the cavity or flowing to the overflow tank, which often results in a short shot.

In order to eliminate such defects, the melt / mold temperature, and the injection pressure / speed can be increased to allow the melt to reach the end of the cavity before solidifying. In such a case, a great plastic fluidity is of particular importance, so selecting the plastic with a better fluidity is also a solution. If the flow coverage is too long and the plastic cannot properly fill the injection molded part, it is recommended to change the gate location to reduce the plastic flow length.

(2) Try to evenly mix the recycled material and the raw material. Because the recycled pellets, as well as their density are relatively larger than the raw material, if the mixing is not uniform, the amount of the molten material is easily reduced, resulting in reduced injection volume accordingly.

V. Inappropriate Product Design

During the product design process, the principle of uniform wall thickness must be followed. If it is impossible to maintain a uniform wall thickness, please change the mold design in a timely manner. Increase the number of runners or ribs in the area where the feeding is difficult, so as to avoid the defect of underfill caused by too-thin wall thickness.

All in all, the reasons for short shot are multifaceted, which are also mutually restrictive and mutually influential. To reduce and correct such a defect, we need to make adjustments through an overall consideration of the relationship between these aspects, while making constant practice and accumulating experience continuously to quickly identify the causes of underfill, thereby reducing resource waste and improving product yield.

VDI 3400 Mold Texture and Drafting Angle

VDI 3400 texture for plastic injection mold

Bayer Plastics has some good Information about surfaces and injection molding available on their web- Site.
According to a (German) Document the VDI 3400 texture surfaces correspond to the following Ra / Rz Values and require the following drafting angles:

VDI3400RaRzDA-PADA-PCDA-ABS
120.40 1.50 0.5 1.0 0.5
150.56 2.40 0.5 1.0 0.5
180.80 3.30 0.5 1.0 0.5
211.12 4.70 0.5 1.0 0.5
241.60 6.50 0.5 1.5 1.0
272.24 10.50 1.0 2.0 1.5
303.15 12.50 1.5 2.0 2.0
334.50 17.50 2.0 3.0 2.5
366.30 24.00 2.5 4.0 3.0
399.00 34.00 3.0 5.0 4.0
4212.50 48.00 4.0 6.0 5.0
4518.00 69.00 5.0 7.0 6.0

DA-PA = Drafting Angle for Polyamide
DA-PC = Drafting Angle for Polycarbonate
DA-ABS = Drafting Angle for Acrilnitrile-Butadiene-Styrol

glass reinforced materials require more drafting.

VDI 3400 Table
In the following, you can determine the roughness for your application according to VDI 3400 table.

VDI3400  0-45µm µinchN3-N10ISO1302 Rt µm
00.10 4N3 
10.11 4.4  
20.12 4.8  
30.14 5.6  
40.16 6.4  
50.18 7.2N4 
60.20 8  
70.22 8.8  
80.25 10  
90.28 11.2  
100.32 12.8  
110.35 14N5 
120.40 16 1.6
130.45 18  
140.50 20  
150.56 22.4 3.2
160.63 25.2  
170.70 28  
180.80 32N65
190.90 36  
201.00 40  
211.12 44.8  
221.26 50.4  
231.40 56  
241.62 63 12
251.80 72N7 
262.00 80  
272.20 88 16
282.50 100  
292.80 112  
303.20 125N820
313.50 140  
324.00 160  
334.50 180 25
345.00 200  
355.60 224  
366.30 250N937
377.00 280  
388.00 320  
399.00 360 46
4010.00 400  
4111.20 448  
4212.60 500N1060
4314.00 560  
4416.00 640  
4518.00 760 85

What Causes Flow Lines in Plastic Injection Molding?

Definition of Flow Lines:                                               
Flow lines, also known as flow marks Linear marks on the surface of a molded product, which indicates the flow direction of the molten plastic.

flow lines marks

Injection Molding process

1.Insufficient Pressure / Holding Pressure                                               
The injection pressure and the holding pressure are not high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Increase the injection pressure and the holding pressure, to press the solidified layer against the mold surface until the product is molded, so as to prevent the occurrence of flow lines.

2.Improper Residence Time                                      
The plastic material stays in the barrel for a too short period of time, while the melt temperature is low. Even if the cavity is barely filled, the plastic cannot be compacted during pressure holding, thus leaving flow lines along the melt flow direction.

The Shot-to-Barrel Ratio should be kept between 1/1.5 and 1/4.                                      
                                       
3.Improper Cycle Time                                      
When the cycle time is too short, the plastic is not sufficiently heated in the material barrel, and the temperature of the melt is low. Even though the cavity is barely filled, the plastic cannot be compacted during pressure holding, thus leaving flow lines along the melt flow direction

The cycle time is extended until the plastic is fully melted, and the temperature of the melt is high enough to prevent flow lines along the melt flow direction.                                      
                                       
4.Barrel Temperature Too Low                      

When the barrel temperature is too low, the melt temperature will be low, and the injection pressure and holding pressure will not be high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Increase the material temperature, injection pressure and holding pressure to press the solidified layer against the mold surface until the product is molded, so as to prevent the occurrence of flow lines. The material temperature can be set by reference to material supplier’s recommendations.

The material barrel is divided into four zones: Rear, Center, Front and Nozzle. The material temperature settings should be gradually raised as it moves forward. Increase by 6°C with every zone forward.

When necessary, the temperature of the Nozzle and/or the Front are sometimes set to be the same as the Center temperature.                                       
                                       
5.Nozzle Temperature Too Low

After absorbing the heat released by the heating band, as well as the frictional heat generated by the relative movement of the plastic molecules caused by the rotation of the screw, the plastic in the barrel undergoes gradual temperature rises.

The last heating zone in the barrel is the Nozzle, where the melt should reach the desired temperature, but it must be moderately heated to maintain the optimal conditions.

If the nozzle temperature is not set high enough, due to too much heat is taken away through the contact between the nozzle and the mold, the material temperature will decrease, so that the injection pressure and the holding pressure will not be high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Raise the Nozzle temperature. The nozzle temperature is usually set to be 6°C higher than the Front temperature.                                       
                                       
Mold                                       
1.Mold Temperature Too Low                                     
If the mold temperature is too low, the material temperature will drop very fast, so the injection pressure and the holding pressure will not be high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Raise the mold temperature, maintain a high material temperature, as well as high injection pressure and holding pressure to press the solidified layer against the mold surface until the product is molded, so as to prevent the occurrence of flow lines.

The mold temperature can be set from the recommended values of the material supplier, with an increment of 6°C at each adjustment. Then perform 10 shots, and after the injection molding is stable, decide whether further adjustment is necessary according to the result.

2. Sizes of the Sprue, the Runner and/or the Gate

If the sprue, the runner, and/or the gate are too small, the flow resistance will be increased. And, if the injection pressure is not high enough, the advancement of the melt front will become slower and slower, and the plastic will become colder and colder, so that the insufficient injection pressure and holding pressure will not be able to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

It is a feasible way to simulate and analyze the filling status of the different melt transfer systems (including the sprue, the runner and the gate) on a computer with CAE (such as, C-MOLD), to find out the ideal sprue, runner, and gate sizes (including length and section related dimensions, such as diameter, etc.) 

3. Insufficient Venting

Insufficient venting will cause the melt filling to be blocked, and the melt front will not be able to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Start venting at the end of each runner section, which removes a large amount of gas before filling the cavity.

The cavity venting should not be neglected. Consider adding vents on the parting surface opposite to the gate. Correspondingly, consider adding venting ejector pins at the end of the product blind hole.

Simulate melt filling through CAE (such as, C-MOLD), which helps us quickly find out all possible last filled areas, i.e., the areas where vents must be added. The addition of a vacuum system for air extracting before and during filling is an effective venting method.

For some textured products, this may be the only way of venting.                                      
                                       
Plastic Material

1.Poor Fluidity

The mold cavity with a large flow length to thickness ratio must be filled with the plastic that features great fluidity. If the fluidity of the plastic is not good enough, the melt will be flowing slower and slower, colder and colder, so that the injection pressure and the holding pressure are not high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction. Material suppliers are able to offer professional recommendations according to specific designs:

The most flowable plastic is selected on condition that no flashing is caused.

2.Improper Application of Molding Lubricant

Usually, the lubricant content is below 1%. When the flow length to wall thickness ratio is large, the lubricant content must be moderately increased to ensure that the solidified layer is pressed against the mold surface until the product is molded, so as to prevent the occurrence of flow lines. The lubricant must be increased upon agreement with the material supplier.

Operator

1. Bad Habits

Inconsistent molding result will occur if the operator switches the door of the injection molding machine too early or too late. When the barrel heater tries to replenish heat in time due to irregular heat loss, the plastic temperature will not be uniform, thus causing the cold spot. It is not easy for the injection pressure and the holding pressure to press the solidified layer around the cold spot against the mold surface, thus leaving flow lines along the melt flow direction. Usually, the operator should be constantly educated to let everyone know the troubles caused by inconsistent molding cycles and recognize the importance of maintaining best molding practices. Appropriate work shifts are able to prevent operators from making mistakes due to exhaustless or distraction. Automated production with robots or the like is a way to maintain a consistent molding cycle.

PC/ABS injection molding defects and solutions

Seven Common Defects and Solutions for PC/ABS injection molding Products

PC/ABS injection molding defects

Flow Lines

The flow lines are caused due to the poor fluidity of the material. Unlike silver streaks, it is not caused by moisture or material decomposition, thus resulting in a different look.

Solution: The flow lines can be eliminated by raising the material temperature so as to improve the fluidity. Appropriate increase of mold temperature and decrease of injection speed can also solve the problem.

 Pitting

The shrinkage cavities are caused by short shot in the mold cavity.

Solution: Increase mold temperature and material temperature appropriately to improve material fluidity; extend the injection / pressure holding time, and raise the injection speed to improve the mold filling, or increase the size of the gate and heat the sprue to reduce and eliminate the shrinkage cavities on the products.

If the product is not appropriately designed and the material temperature is too low, not only the shrinkage cavities, but also pitting will occur. If the material temperature and the mold temperature are too high, the melt will shrink excessively during cooling, resulting in pitting.

Solution: Appropriately increase the injection speed.

 

Warpage / Deformation 

Solution: 

The warpage / deformation of PC/ABS injection molding product is caused by the unreasonable product design, the improper gate location and the inappropriate conditions of the injection molding process, so that the stress is generated inside, leading to the ununiform or excessive shrinkage. If the mold temperature is too high or uneven, it will cause the difficult product ejection, or ununiform cooling, which will also lead to warpage / deformation.

Solution:

Molding Process: Extend the injection molding cycle, lower the injection molding temperature, appropriately adjust the injection pressure and the injection speed, reduce the ejection speed, increase the ejection area, and balance the ejector force.

Product Design: Increase product wall thickness, add ribs and fillet reinforcements to reduce warpage / deformation.

Silver Streaks

Silver streaks are the most common defects with the PC/ABS injection molding material, which refers to the appearance of the silvery-white silk-like stripes on the surface of the molded product along the flow direction.

The main cause is the interference of gas, which is mainly comprised of three components:

Air: The air drawn in during the melting and injection stages;

Moisture: The moisture contained in the material itself;

Pyrolysis Gas: Gas generated by high temperature hydrolysis / thermal decomposition.

Solution: First of all, check whether the material is fully dried; after the material is confirmed to be dry, the silver streak defect can be improved by adjusting the injection molding process. At the same time, this defect is also correlated to mold venting.

Matting

Solution:

Poor dispersion: add dispersant or oil, raise temperature, and increase backpressure.

Pay attention to the mold problem. Try some other materials.

Whether the injection temperature and time are sufficient.

Adjust the mold temperature.

Product Delamination

The delamination defect of PC/ABS injection molding products is closely related to the rupture of the melt caused by high shear force. Under low shear stress or velocity conditions, small disturbances caused by various factors are suppressed by the melt, while at high shear stress or velocity, disturbances in the melt are difficult to suppress and thus develop into an unstable flow;

When a critical shear force is reached, the fracture of the fluid will be caused.

Solution:

Material: The two components of PC and ABS are partially compatible, so it is necessary to add proper compatibilizer to improve the compatibility of the two components in the process of modification. Of course, the first thing to do is to eliminate the delamination caused by the mixture.

Mold: The principle of mold design should follow the direction of minimizing shear. Generally speaking, products with densely striped surfaces are more prone to delamination (caused by the friction shear between melt and inner wall of cavity during high-speed filling). At the same time, the gate size designed too small will lead to excessive shear when the melt is flowing through the gate, and then lead to product surface delamination.

Injection Molding Process: The main purpose is to avoid excessive shear. The high-speed & high-pressure injection can be used to improve the product filling difficulty, and therefore, in the actual injection molding process, the flow resistance in the actual filling process can be reduced, so as to avoid excessive shear caused by high-speed and high-pressure injection molding process

Stretch Marks

Stretch marks usually occur due to melt rupture when the melt expands into the mold cavity during high-speed injection.

Solution:

Molding Process: This defect can be reduced by raising the material temperature and the nozzle temperature, and slowing down the injection speed, etc.

How to Control the Dimensions Tolerance of Injection Molding Products

Due to the greatly varied market demands for molds and molded products, there are many kinds of changes and thus high requirements for us in such aspects as shape, size, material, structure and more, causing us many problems and difficulties in the production process of molds and products. How to effectively control the geometric dimensions of the mold and the product is just one of them.

injection molding dimensions control

For different types of molds and products, there are different control techniques and methods for dimensions control. Personally, I usually start from the following aspects.

Mold Design Control

Firstly, we have to fully understand the technical requirements for the mold structure, material, hardness, and precision, etc., including whether the shrinkage rate of the plastic material is proper, and whether the 3D model of the product is complete, in a bid to perform reasonable analysis.

2. Thorough consideration should be given to various factors that may influence the product appearance, including shrinkage, flow marks, draft angle, weld lines and cracks of the injection molded product.

3. Try to simplify the processing method of the mold without impeding the function, shape and texture of the injection molded product.

4. Whether the selection of the parting surface is appropriate; the mold machining, product appearance and deburring of the molded part should be carefully considered.

5. Is the ejection method appropriate; use the ejector pin, the stripper plate, the ejector sleeve or other methods; whether the ejector pin and stripper plate are appropriately located.

6. What temperature control method is suitable for the plastic part; which circulation system structure is suitable for temperature control oil, temperature control water, or coolant, etc.; whether the size, quantity and location of the coolant holes are appropriate.

8. Whether the type of the gate, the size of the runner and the gate, as well as the location of the gate are appropriate.

7. Whether the injection volume, the injection pressure and the clamping force of the injection molding machine are sufficient; whether the nozzle R, and the nozzle sleeve diameter, etc. match properly.

Perform comprehensive analysis and make preparation from all of these aspects, to strictly control the product from the very beginning.

Manufacturing Process Control

Although comprehensive consideration and arrangement have been carried out in the design stage, there are still many problems and difficulties during the actual production process. We should try our best to meet the original intention of the design and find the most effective and most economical processing method in actual production.

1. Choose the economical machine tools for 2D and 3D processing.

2. We should also consider a string of production process requirements and solutions, e.g., the appropriate fixtures to facilitate preparation for production, and the rational application of cutters, so as to prevent product deformation, avoid fluctuations in product shrinkage, and eliminate product warpage caused during ejection, thus improving the precision of mold manufacturing, minimizing errors and preventing mold accuracy changes, etc.

3. Here, I’d like to mention a summary of the reasons for the dimensional error and the corresponding proportion, drafted by the British Plastics Federation (BPF):

A: The mold manufacturing error is about 1/3 for dimensions control; B: The error caused by mold tear and wear is 1/6; C: The error caused by the uneven shrinkage of the molded part is about 1/3 for dimensions control; D: The error caused by the difference between the preset shrinkage and the actual shrinkage is 1/6.

Total error = A + B + C + D, so we can see that the mold manufacturing tolerance should be kept within 1/3 of the dimensional tolerance of the molded part, otherwise it is difficult for the mold to guarantee the geometric dimensions control of the molded part.

 General Production Control

It is a common problem and phenomenon that geometric fluctuations occur after the plastic part is molded:

1. Control of material temperature and mold temperature. Different plastic materials have different requirements for temperature. Different situations will occur if the plastic features a poor fluidity or two materials are mixed. The plastic should be controlled within the optimal fluidity range, which is easy to achieve. However, the control of the mold temperature is more complicated. Different geometry, size and wall thickness of the molded product have certain requirements for the cooling system. The length of the cooling time is largely dependent on the mold temperature.

Therefore, the mold should be kept at an allowable as low as possible temperature, so as to shorten the injection cycle, and improve production efficiency. If the mold temperature changes, the shrinkage rate also changes; if the mold temperature remains stable, then the dimensional accuracy is stabilized, thereby preventing such defects of the molded part as deformation, poor glossiness, etc., and ensuring the optimal physical properties of the plastic. Of course, there is a commissioning process. In particular, the injection molding of multi-cavity molds is more complicated.

2. Pressure & Venting Control:

The proper injection pressure and clamping force should be determined when the mold is being commissioned. The air in the mold cavity and the core, and the gas generated by the plastic must be vented through the mold vents. If the gas is not smoothly vented, there will be the three molding defects of underfilling, weld marks or burns, which sometimes appear on the same part. However, as long as flashing doesn’t occur, the depth of the vents should be as deep as possible. Behind the shoulder, larger venting grooves are usually designed for the gas to vent through the shoulder quickly. If needed, a special venting groove can be designed on the ejector pin, for which the reason is the same, i.e., 1. No flashing occurs; 2. The venting is fast and effective.

3. Plastic Part Dimensional Compensation Control

Due to the difference in the shape and size of some plastic parts, deformation and warpage may occur in different situations with the change of temperature and loss of pressure after product ejection. At this time, some assistive fixtures can be used to make remedies for dimensions control in a timely manner after the molded part is released, so that when the product is naturally cooled and shaped, better adjustment and remedy results can be obtained. If strict management is ensured throughout the injection molding process, the dimensions of the injection molded part can be very well controlled.

PVC injection molding plastic resin

PVC plastic is a heat-sensitive material that features a poor injection moldability, for which the reason is that an excessively high melt temperature or an excessively long heating time easily causes PVC decomposition. Therefore, the key to molding the PVC products lies in the control of the melting temperature.
PVC injection molding plastic resin

The heat sources for melting the raw PVC material are usually from two aspects, i.e., the shear heating generated by screw motion and the resistance wire heating provided by the outer wall of the barrel, among which, the former is the main source, while the latter is mostly provided during machine operation.

Due to the unsatisfactory heat transfer property of the PVC material, although the outer portion is heated, the inner portion of the material is still in a solid state, and even when the outer portion starts to decompose due to overheating, there may still be an unmolten portion inside. Various reasons must be fully considered during product design, mold design, and the plastic injection molding process, etc. In particular, the special PVC injection molding machine from outstanding manufacturers can be selected to create the ideal product.

The following are the considerations that should be attached great importance to during the injection molding process of PVC, in the hope of helping the readers.

1.Pay attention to the following points during product design and mold design:
Try to avoid sharp corners or abrupt changes in the product, and the thickness should not change too much, so as to prevent the PVC from degrading.

2.The mold should be designed with a draft angle of more than 10°, allowing for a shrinkage rate of about 0.5%.

3.During mold runner design, pay attention to the following points

a). The mold injection gate should be slightly larger than the nozzle, and larger than the diameter of the conjunctions in the sprue, so that the PVC material is able to flow into the cavity without being blocked, while the pressure is also balanced.
b). Try to adopt the intercepting gate, so that the slag will not flow into the product and cause runner temperature reduction, thus making it easier to form.

c). The gate should be designed at the thickest part of the product, with a sufficient width guaranteed, while the length is preferably between 6-8mm, so that the PVC material is able to flow in smoothly.

d). To reduce pressure and facilitate ejection, a circular runner is preferred, with the diameter ranging from 6 to 10mm, depending on the size and weight of the product.

4. The mold temperature should be controlled by a cooling device, to keep the temperature controllable between 30°C and 60°C.

5.The mold surface should be smooth and chrome-plated to prevent corrosion.

PVC injection molding process

With regard to the injection molding process, pay attention to the following:

PVC injection molded product

  • The backpressure should be kept at about 1Mpa, because excessively high backpressure will produce excessive shear force, which will cause the PVC to decompose. At the same time, the multi-stage backpressure control should be adopted in the melting process. In the beginning when the effective length of the screw is long, the backpressure should be kept low. As the effective length of the screw decreases, the backpressure should be appropriately increased to compensate for the heat loss. Shortly before material recycling ends, the backpressure should be lowered to prevent leakage.
  • The screw speed should vary according to the diameter. Usually, when the diameter is < 60mm, the speed is 50-70rpm; when the diameter is > 70mm, the speed is 20-50rpm, so as to prevent excessive shearing and PVC degradation.
  • Usually, to inject the plastic into the cavity with a uniform speed throughout the process, the multi-stage injection speed control is adopted, of which the principle is to start with a low speed, and as the molding area increases, the injection speed must be increased to avoid cracks and shrinkage marks. The injection speed cannot be so fast, or the product surface will be denuded.
  • During the plastic injection molding process, the barrel temperature should be kept between 170-190°C. To prevent the PVC from overheating and degrading, when the temperature of the barrel reaches the set temperature, the blower should be started to cool the temperature down. On the contrary, when the temperature needs to be raised, the air blower should be shut down.

Due to the special properties of the PVC material, special PVC injection molding machines should be selected for production. Please note the following points:

  • The screw length to diameter ratio should be 20:1, and the compression ratio should be controlled between 1:1.6 and 1:1.2.
  • The screw and nozzle should be specially made and chrome-plated for the PVC material.
  • Injection pressure, speed, backpressure and temperature can be multi-stage and precision controlled. (The four-cylinder direct-lock two-plate PVC injection molding machine we produce possesses the above-mentioned performances, and we look forward to your inquiry)

Last but not the least, the following requirements should be noted in the preparation of raw materials:

  • It is necessary to prevent the raw PVC material from being mixed with other plastics, or PVC degradation might be caused. Especially, when the machine has worked with other raw materials, it is necessary to clean the barrel and the screw before molding the PVC product. (Clean the screw and barrel with the PS plastic)
  • Before shutdown, the PVC material must be completely cleaned. If it needs to be stopped for a long time, to prevent the PVC from corroding the machine, the screw and the barrel should be cleaned with PS.
  • When the PVC injection molding material is put away for more than 6 months in a hot and humid environment, it should be dried for 1-2 hours under the 60-80°C temperature conditions before use.

Mold steels P20,718 and H13 comparison

Mold steel p20,H13

P20 Mold steel

Overview:The P20 Mold Steel is a type of mold welding consumable, and the earliest is P20, followed by P20H and P20Ni. The P20 Steel is applicable to the production of molds for plastic materials, as well as those for diecasting low melting point metals. This steel material features great machinability, as well as mirror polishing properties.

Chemical composition

Element C Si Mn Mo Cr Ni
(%) 0.28~0.40 0.20~0.80 0.60~1.00 0.30~0.55 1.40~2.00 0.80~1.20

 

How to Use: The P20 Steel is pre-hardened to 285-330HB (30-36HRC), which is equivalent to 618 of Sweden and GS-2311 of Germany. It can be directly used for molding, and boasts a great dimensional stability. Pre-hardened steel is able to meet the requirements for general applications, with a mold service life of up to 500,000 cycles.

Characteristics

  • The vacuum degassing process ensures the purity of the steel, making it suitable for plastic molds that require polishing or etching treatment.
  • Supplied as a pre-hardened steel, able to be directly used for mold manufacturing without having to be heated, thus reducing the production cycle.
  • After forging and rolling, the structure is dense, and no pores or pinholes are found according to 100% ultrasonic inspection.

 

Purposes

  • Injection molds, and extrusion dies for thermoplastics
  • Blow molds for thermoplastics
  • Main parts for heavy duty molds
  • Parts for cold insulation structure
  • Commonly used in the production of TV casings, inner shell of washing machine / refrigerator, and buckets, etc.

 

718 Steel

Overview

The 718 Mold Steel, known as 3Cr2NiMo in China, is an enhanced steel on the basis of P20 (3Cr2Mo). The quality of the steel material has been so greatly improved that it is able to compensate for the weaknesses of the P20 Mold Steel, and at the same time meet the requirements that are beyond P20. Among today’s mold steel materials in China, there is also P20+Ni, in addition to the above-mentioned Chinese standard steel grades. The 718 and 718H (ASSAB) from Sweden; the PX5/PX4 (DAIDO Special Steel, Japan); the GS-711, GS-738, GS-312 and GS-318 (Thyssen, Germany); and the M238 ECOPLUS (BOHLER) from Austria are the most widely used general-purpose mold steel grades for molding plastic materials. It is often referred to as an “advanced” plastic mold steel that always serves as a benchmark for newly developed mold steels.

Purposes

The purpose of the 718 Mold Steel is the same as that of the P20 Mold Steel, but by virtue of its better hardenability and superior performance, it is able to be used for production of the forming parts of large-sized and high-end plastic molds.

  • Injection molds, and extrusion dies for thermoplastics
  • Blow molds for thermoplastics
  • Main parts for heavy duty molds
  • Parts for cold insulation structure
  • Commonly used in the production of TV casings, inner shell of washing machine / refrigerator, and buckets, etc.
  • The vacuum degassing process ensures the purity of the steel, making it suitable for plastic molds that require polishing or etching treatment.
  • Supplied as a pre-hardened steel, able to be directly used for mold manufacturing without having to be heated, thus reducing the production cycle.
  • After forging and rolling, the structure is dense, and no pores or pinholes are found according to 100% ultrasonic inspection.

 

 

Chemical composition

Element C Si Mn Mo Cr Ni
(%) 0.28~0.40 0.20~0.80 0.60~1.00 0.30~0.55 1.40~2.00 0.80~1.20

Hardening

In order to improve the mold service life to more than 800,000 molding cycles, the pre-hardened steel can be further hardened by quenching and low temperature tempering. Before quenching, pre-heat the steel under 500-600°C temperature conditions for 2-4 hours, then keep the temperature at 850-880°C for a certain period of time (at least 2 hours), oil-cool it to 50-100°C, followed by air cooling, and the hardness can reach up to 50-52HRC after quenching. To prevent cracking, immediately perform low-temperature tempering at 200 °C, after which, the hardness can be kept over 48HRC.

Nitriding

Nitriding helps obtain a high-hardness surface structure, after which, the surface hardness can reach up to 650-700HV (57-60HRC), and the service life of the mold is able to exceed 1 million cycles. The nitrided layer features a compact structure and smoothness, with improved ejection performance, as well as resistance to moisture and alkaline solutions.

 

Steel H13

Overview

mold steel H13

H13 is a hot work steel, and the grade is 4Cr5MoSiV1; an alloy tool steel is formed by adding alloying elements on the basis of the carbon steel. Among them, the alloy tool steels include: measuring and cutting tool steels, steel for impact resistant tools, cold work mold steel, hot work mold metal, non-magnetic mold steels, and plastic mold steels.

Chemical composition

Element C Si Mn Cr Mo V P S
(%) 0.32~0.45 0.80~1.20 0.20~0.50 4.75~5.50 1.10~1.75 0.80~1.20 0.030 0.03

Heating

After heat treatment, the top hardness of the H13 mold Steel reaches up to HRC52. When the minimum size of a part is greater than 300mm, the hardness after heating should be kept within HRC50.

Characteristics

As an electroslag heavy-duty steel, it boasts high hardenability and thermal crack resistance. The steel contains a high content of carbon and vanadium, allowing it to have a great wear resistance, but a relatively weaker toughness. Other features include great heat resistance, outstanding strength and hardness at high temperatures, as well as high wear resistance and toughness, excellent mechanical properties and high resistance to tempering.

Other designations that are equivalent to H13 Mold steels include:

  • AFNOR Z 40 COV 5
  • DIN 1.2344
  • UNI KU
  • JIS SKD61
  • SS 2242
  • B.S. BH 13
  • ASTM A681
  • FED QQ-T-570
  • SAE J437
  • SAE J438
  • SAE J467
  • UNS T20813
  • AMS 6408

Purposes

It is used for the production of forging dies with a high impact load, hot extrusion dies, precision forging dies; as well as die-casting molds for aluminum, copper and their alloys.

High impact polystyrene(HIPS)plastic injection molding resin material

HIPS plastic injection molding

HIPS(High impact polystyrene) is an impact-resistant polystyrene product created by adding butyl rubber particles to polystyrene. This polystyrene product is added with micron-sized rubber particles, and links the polystyrene and the rubber particles together by means of grafting.

When subject to an impact, the tip end stress of the crack propagation is released by the relatively soft rubber particles. Therefore, the crack propagation is hindered and the impact resistance is improved.

HIPS Engineering Plastic

Milky white opaque pellets, with a density of 1.05g/cm3, a melting temperature of 150-180°C, and a thermal decomposition temperature of 300°C. HIPS is one of the cheapest engineering plastics. When compared with ABS, PC/ABS and PC, the glossiness of the material is relatively poor, and so is the overall performance. HIPS is modified from PS plus butadiene. Since the impact strength of PS is very low, the product made from it is very brittle, but the toughness of butadiene is very outstanding. After joining with butadiene, the impact resistance of PS can be improved by 2-3 times. Although the impact strength of HIPS is much higher than that of PS, its comprehensive performance is not as good as that of ABS, and PC/ABS, etc. Among all engineering plastics, the impact resistance of HIPS is relatively low.

The flame-retardant high-impact HIPS material.

HIPS is a modified material of PS, which contains 5-15% of rubber component, so its toughness is about four times higher than that of PS, while the impact strength is greatly improved. It possesses the great advantages in moldability and tinting strength of PS. HIPS products are opaque. Thanks to its low water absorption rate, pre-drying is not required before production. Since the HIPS molecule contains 5-15% of rubber, the fluidity is affected to some certain extent, so the injection pressure and temperature are preferably higher. In view that it cooling rate is slower than that of PS, sufficient holding pressure, holding time and cooling time are required. Its molding cycle is slightly longer than that of PS, and the processing temperature is usually 190-240° C. There is a special “white edge” issue with the HIPS products, which can be improved by increasing mold temperature and clamping force, and reducing the holding pressure and time, etc., but welding lines is quite obvious on the product.

HIPS Properties:

HIPS are milky white and opaque pellets, with a high impact strength and toughness, and can be arbitrarily colored. In addition, it also features great moldability, chemical resistance, and electrical properties. Modified by rubber, although the impact strength and toughness of polystyrene are greatly improved, its tensile strength, bending, hardness, light resistance and thermal stability are lower when compared with the homopolymer.

Since it contains rubber content, the impact strength of high impact polystyrene is 5-10 times higher than that of GPPS, which has expanded the application range of polystyrene, allowing it to partially replace the expensive ABS materials.

HI-PS is a modified material of PS. It contains 5-15% of rubber component in the molecule. Its toughness is about four times higher than that of PS, with the impact strength greatly improved (high impact polystyrene). Now, there are flame retardant, stress crack resistant, high glossy, high impact, glass fiber reinforced and low VOC HIPS materials available. Other important properties of the standard HIPS: bending strength of 13.8-55.1MPa; tensile strength of 13.8-41.4MPa; elongation at break of 15-75%; density of 1.035-1.04g/ml; It possesses the great advantages in moldability and tinting strength of PS. HIPS products are opaque. Thanks to its low water absorption rate, pre-drying is not required before production.

Process Characteristics of HIPS:

Since the HIPS molecule contains 5-15% of rubber, the fluidity is affected to some certain extent, so the injection pressure and temperature are preferably higher. In view that its cooling rate is slower than that of PS, sufficient holding pressure, holding time and cooling time are required. Its molding cycle is slightly longer than that of PS, and the processing temperature is usually 190-240° C. The HIPS resin absorbs water very slowly, so drying is generally not required. Sometimes excessive moisture on the surface of the material will be absorbed, which will affect the surface finish of the final product. But, excessive water can be removed by drying at 160°F for 2-3 hours. There is also a special “white edge” issue with the HIPS products, which can be improved by increasing mold temperature and clamping force, and reducing the holding pressure and time, etc., but welding lines is quite obvious on the product.

Purposes:

High impact polystyrene (HI-PS) can be injection molded or extruded into various products, suitable for household appliances, electrical appliances, instrument parts & components, refrigerator lining, boards, television, tape recorder, telephone casing, stationery, toys, packaging containers, daily necessities, furniture, tableware, trays, structural foam products, disposables, instruments, household appliances and entertainment products, as well as the construction industry. The flame-retardant (UL V-0 and UL 5-V), impact-resistant polystyrene has been produced and widely applied in the production of TV cases, commercial machines and electrical products.

Steel hardness test method and conversion table

Steel hardness describes the properties of steels that resist deformation, indentation, penetration, and scratching. steel hardness is important because the inherent resistance of the surface to withstand friction or erosion by oil, steam, and water from injection molding process,there are several harness test method as below:

steel hardness test

1.Brinell Hardness (HB)
Press a hardened steel ball of a certain size (usually 10mm in diameter) against the surface of the material with a certain load (usually 3,000kg) for a period of time. After the load is removed, the ratio of the load to the area of indentation is referred to as the Brinell hardness (HB), measured in kg/mm2 (N/mm2).

2.Vickers Hardness (HV)
Use a diamond square cone presser to penetrate into the surface of the material with a load of 120kg or less, and an apex angle of 136°. Then, divide the area of indentation in the material by the load, and we get the Vickers hardness (HV) value (kgf/mm2).

3.Rockwell Hardness (HR)
When HB>450 or the sample is too small, making it impossible to apply the Brinell hardness, the Rockwell hardness measurement will take the place. A diamond cone with an apex angle of 120° or a steel ball with a diameter of 1.59-3.18mm is pressed into the surface of the material to be tested under a certain load, and the hardness of the steels is determined based on the depth of the indentation. According to the hardness of the test material, it is measured by three different scales:

The Rockwell Hardness is comprised of three different standards, i.e., HRA, HRB and HRC, referred to A scale, B scale, and C scale.

HRA: The hardness obtained by using a 60kg load and a diamond cone indenter, applicable for materials with an extremely high hardness (such as cemented carbide).

HRB: The hardness obtained by using a load of 100kg and a hardened steel ball with the diameter of 1.58mm, applicable for materials with a lower hardness (such as annealed steel, and cast iron, etc.)

HRC: The hardness obtained by using a 150kg load and a diamond cone indenter, applicable for materials with a very high hardness (such as hardened steel).

Steel hardness conversion table

There are so many different types of steel hardness tests, a conversion table, such as the one below, allows us to take one measure and estimate the relative hardness on another measurement scale.

RmN/mm2 HV HB HRC RmN/mm2 HV HB HRC
250 80 76 1030 320 304 32.2
270 85 80.7 1060 330 314 33.3
285 90 85.2 1095 340 323 34.4
305 95 90.2 1125 350 333 35.5
320 100 95 1115 360 342 36.6
335 105 99.8 1190 370 352 37.7
350 110 105 1220 380 361 38.8
370 115 109 1255 390 371 39.8
380 120 114 1290 400 380 40.8
400 125 119 1320 410 390 41.8
415 130 124 1350 420 399 42.7
430 135 128 1385 430 409 43.6
450 140 133 1420 440 418 44.5
465 145 138 1455 450 428 45.3
480 150 143 1485 460 437 46.1
490 155 147 1520 470 447 46.9
510 160 152 1555 480 456 47.7
530 165 156 1595 490 466 48.4
545 170 162 1630 500 475 49.1
560 175 166 1665 510 485 49.8
575 180 171 1700 520 494 50.5
595 185 176 1740 530 504 51.1
610 190 181 1775 540 513 51.7
625 195 185 1810 550 523 52.3
640 200 190 1845 560 532 53
660 205 195 1880 570 542 53.6
675 210 199 1920 580 551 54.1
690 215 204 1955 590 561 54.7
705 220 209 1995 600 570 55.2
720 225 214 2030 610 580 55.7
740 230 219 2070 620 589 56.3
755 235 223 2105 630 599 56.8
770 240 228 20.3 2145 640 608 57.3
785 245 233 21.3 2180 650 618 57.8
800 250 238 22.2 660 58.3
820 255 242 23.1 670 58.8
835 260 247 24 680 59.2
850 265 252 24.8 690 59.7
865 270 257 25.6 700 60.1
880 275 261 26.4 720 61
900 280 266 27.1 740 61.8
915 285 271 27.8 760 62.5
930 290 276 28.5 780 63.3
950 295 280 29.2 800 64
965 300 285 29.8 820 64.7
995 310 295 31 840 65.3
860 65.9
880 66.4
900 67
920 67.5
940 68

TPE,TPR,TPU and TPV injection molding

Thermoplastic Elastomer (TPE)

TPE-TPV-TPR-TPU injection molding

Domestically in China, it is usually known as “TPE”, basically referring to the styrene-based thermoplastic elastomer.

The styrene-based TPE (known as TPS in foreign markets) is a copolymer of butadiene or isoprene and styrene block, so its performance is similar to that of SBR.

Strengths: High elasticity of rubber and plastic injection moldability; environmentally friendly, non-toxic and safe; excellent coloring, smooth touch, weatherability, fatigue / temperature resistance, excellent processability, no need for vulcanization, and able to be recycled for cost reduction. It can be co-molded with such base materials as PP, PE, PC, PS and ABS, or molded separately.

Weaknesses: The biggest problem with SBS and SIS is their low heat resistance – the temperature should not exceed 80°C. At the same time, its strength, weatherability, oil / wear resistance, etc. are also incomparable to those of rubber.

Styrene-based TPR: Thermoplastic Rubber

TPR was originally named by the Americans. Back then, there was no uniformed nomenclature for thermoplastic elastomers. It was customary to use the English acronym TPR for thermoplastic rubbers, and TPE for thermoplastic elastomers (both are used in related articles), which was applied to the simple blends of PP and EPDM. Later, the PP/EPDM-based TPV (originally referred to as TPR, and later renamed as TPV) of AES also followed this nomenclature. After that, it became popular among the Taiwanese, as well as in mainland China. Regardless of the foreign names, the TPR and styrene-based TPE currently mean the same thing in China, indicating that it is a thermoplastic styrene-butadiene-styrene copolymer, which is commonly known as thermoplastic styrene-butadiene rubber, i.e., in China, TPE = styrene-based TPE.

However, due to historical and translation issues, the Chinese name for TPV (thermoplastic vulcanizate) is referred to as thermoplastic rubber, i.e., TPR. That is why we don’t tell our customers that our PP/EPDM-based TPV is referred to as TPR, because some customers take it for the styrene-based elastomer. If it is an SBS-based material, the price would be much lower than that of TPV.

Thermoplastic Polyurethane (AKA TPU)

The polyurethane-based TPE is a thermoplastic polyurethane rubber formed by block-bonding the hard segment of the isocyanate-reacted urethane and the soft segment of the polyester or polyether, abbreviated as TPU.

Strengths: TPU boasts an excellent mechanical strength, as well as resistance to wear, oil and flex. Especially, its wear resistance is the most outstanding; temperature resistance: -30-120°C.

Weaknesses: Poor resistance to heat, hot water and compression; prone to yellow discoloration, and mold sticking; requiring high precision processing, leading to a higher cost.

At present, wherever PVC is used, TPU can always be a substitute for it. However, in terms of the strengths of TPU, PVC is far behind. Its hardness ranges from 65A to 85D. The colors are natural, transparent, or highly translucent. TPU not only features excellent tension, tensile strength, toughness and aging resistance, but is also a mature environmentally-friendly material. At present, TPU is widely used in: shoe materials, ready-to-wear, inflatable toys, equipment for water / underwater sports, medical equipment, fitness equipment, car seat materials, umbrellas, luggage, and leather bags, etc.

TPV Thermoplastic Vulcanizate

Dynamic vulcanization refers to the chemical cross-linking of the rubber phase under the action of the cross-linking agent when the thermoplastic resin is melt-blended with the rubber, and is sheared and broken into a large number of micro-sized and crosslinked rubber particles by a strong mechanical shearing action, which are then dispersed in a continuous phase of the thermoplastic resin.

Taking PP as the hard segment and EPDM as the soft segment for melt blending, a cross-linking agent for vulcanization is added during the melt blending process. The high shearing force of such machines as an internal mixer or a screw machine, is leveraged to disperse the fully vulcanized and crosslinked EPDM rubber particles into the PP matrix in micron size (less than 1 micron).

  •  TPV possesses the heat resistance and low-compression deformation properties of the vulcanized rubber;
  • Excellent processability: It can be processed by injection molding, extrusion and other thermoplastic processing methods. It is efficient, simple and easy to operate, and requiring no extra equipment, with a high fluidity and small shrinkage.
  • Low relative density (0.90-0.97), uniform appearance, high surface quality and smooth touch.
  • Usually, TPV can be used at the temperatures from -60°C to +135°C; when not subjected to load, it is able to withstand the high temperature of up to +150°C.
  •  A wide range of hardness, 25A-70D;
  • TPV boasts excellent acid and alkali resistance, and is strongly resistant to acid-based detergents. It is able to maintain its original characteristics after long-term application in acidic environment.