Jackie Lau, Senior Tooling Engineer and Expert of plastic injection molding, experienced in plastic injection molding and tooling for more than 15 years. The founder and Director of Eco Molding & Sositar Mould.
1. The lifter is mainly used to form the internal undercuts of a molded plastic part, and at the same time it also offers ejection function. The mechanism features a simple structure but poor rigidity and a short travel distance. The typical structure is shown as below:
Most injection mold company usually apply the lifter structure as shown . The lifter employs the 8407 steel with a hardness of up to HRC50-52. There should be a distance of 1 – 3mm from the angle lifer to the product, with a clearance of 0.1mm on each side of the base made of the 738 steel, while the length tolerance is +1.5mm and +1.0mm on the top and bottom respectively.
At the end close to the melt flow, a 5.0mm plane surface has to be made (unnecessary when the part profile surface is plane), as well as a head that is bigger than (if not equivalent to) 1.0mm, to prevent the angle lifter from being moved by pressure during the injection molding process. A C0.2 chamfer needs to be set up on the mold core at the corner of the head.
The top of the lifter needs to be 0.03 – 0.05mm higher than the part profile surface, so as to avoid scratch during ejection.
Made of bronze, the wear block adopts the integral form and is installed under the B plate, with the main purpose to avoid lifter deformation and locate the angle lifter during the ejection process. The ejector retainer plate needs to be equipped with a limit column, which should be higher than the lifter base. When ordering a mold base for a injection mold designed with lifters, the spacer block needs to be higher, considering the travel distance of the angle lifter, so as to prevent insufficient travel distance.
The inclination of sliders should be kept between 15 and 25 degrees, and the inclination of the guide pin should be 2 degrees smaller that of the wedge. The available diameters of the guide pin are 6mm, 8mm, 10mm and 12mm, with the min and max values being 6mm and 12mm respectively. If a slider width is larger than 60 mm, deployment of 2 angle pins needs to be considered; if the width exceeds 80mm, a guide bar needs to be placed under the slider in middle.
If the injection mold sliders is too high, the starting point of the angle pin hole needs to be lowered, so as to ensure smooth travel of the slider. If slider open/close time needs to be delayed, the diameter of the guide pin hole will need to be enlarged.
When the injection mold sliders are deeper in the cavity than its own length, a wedge will not be necessary. The inclination can be designed on plate A directly. A R3 is required at the bottom. Besides, the slider doesn’t need a wear block.
When depth of the slider is mainly in the core, a wedge will be needed; if the part profile surface contacting the slider is large, or there’s kiss off or shut off on the slider, a reversed wedge should be applied and the inclination should be 10 degrees or above;
If the part profile surface contacting the slider is small, the slider can be designed as shown in Fig. 3.1.6, and the height of the wedge surface should be higher than 2/3 of the slider height.
The wear plate of the mold slider employs the 2510 steel, of which the hardness reaches up to HRC50°-52°. For all sliders with a thickness of over 50.0mm, wear blocks should be placed on the base and the back. These wear plates measure 5mm in thickness, and are 0.50mm higher than the mold base. And for all the sliders, a clearance is not needed along the travel direction of the slider (as shown in the following Fig.)
The width and height of the slider clamp – made from the 2510 steel with a hardness of up to HRC50-52 – are 20mm and 20mm respectively, while the length is dependent on slider.
For upward (including inclined ones) sliders, there are raised fine inserts and pins. When there’s an ejector pin under the slider, a spring can be applied to facilitate return.
Spring is built in the mold core and the slider (see 3.4.1A, 3.4.1B and 3.4.1C)
If the travel distance of the slider is long with an installation length of over 50mm, the spring may be mounted outside.
Air/Hydraulic Cylinder driving
If the height of the guide pin is larger than 100mm (see Fig. 3.7.1A) and the cavity slider needs to be lifted before A and B plates separate from each other, a Air/Hydraulic cylinder may be applied to drive it (see Fig. 3.7.2B).
Design Standard of Slider Inserts (Pins)
Design Standard of Slider Stops
For the undercut of a circular product with sliders on 4 sides, a stop needs to be mounted to the slider insert. It is better to design a separate stop for each individual product if there are multiple products in a single mold.
There are many types of guide pin structures in plastic injection mold application, among which the standard structural design is shown in the figure below. A guide pin has to serve as the cylindrical surface of concentric circles of different diameters. Based on structural dimensions and material requirements, a round steel bar with an appropriate size can be directly selected as the material. In addition, the technical requirements of guide pins need to be satisfied during the machining process.
Fig. 8-2 Structural Shape of a Guide Pin
Technical Requirements of Guide Pins
(1) At the joint between the guide pin and the fixed mold plate, the diameter concentricity tolerance should not exceed 1/2 of the diameter tolerance in the working portion.
(2) The cylindricity tolerance of the guide pin in the working portion should be kept within requirements.
(3) After being machined, the precision, surface quality and thermal treatment of each part of the guide pin should meet the requirements specified in the drawing. When applicable, the carburized layer on the working surface is required to be uniform usually with a thickness of 0.8 – 1.2mm.
Machining Process of Guide Pins
(1) Material Preparation & Cutting. The commonly used material of the guide pin is the 20 steel (or select materials as per the drawing). After cutting, an allowance of 3 – 5mm should be retained for facing; and an allowance of 3 – 4mm for cylindrical turning.
(2) Facing & Centering. Turn one end, retain a turning allowance of 1.5 – 2.5 mm, and drill the centered hole; turn the other end to specific dimensional requirements and drill the centered hole.
(3) Cylindrical Turning. Roughly turn the cylinder, and retain an allowance of 0.5mm on each side for grinding. When applicable, groove the guide pin to specification.
(4) Inspection. Inspect the finish sizes of the previous steps.
(5) Thermal Treatment. Follow the process, and ensure a carburization thickness of 0.8 – 1.2mm. After carburization, the quenching hardness is 58 – 62HRC.
(6) Lapping. Lap the centered hole on one end, and then lap the one on the other end.
(7) Grinding. Apply cylindrical grinder and centerless grinder to grind the cylinder. After grinding, an allowance of 0.01 – 0.05mm should be retained for lapping.
(8) Lapping. After machining, the surface of the cylinder needs to be lapped to reduce surface roughness, thus meeting the surface finish requirements.
(9) Inspection. Inspect the finish size of each step.
Texturing refers to the process that produces various patterns on the surface of a metal product through chemiosmosis, such as stripes, images, wood grain, leather pattern and stain, etc., while also including sandblasting, which means spraying glass sands directly onto the surface of a metal product.
The Purpose of Mold Texture
Improve product appearance. The texturing process is able to camouflage part of the shrinkage, welding line, parting line and steps of slider, etc.
Product surface strength can be improved via texturing and sandblasting.
Varieties of Mold Texture
Sand Pattern
Characteristics: Fast process, low cost and able to produce fine and 2D patterns.
Satin Pattern
Characteristics: Fast process and able to be applied to a plane surface. Twice as durable as the sandblasting process, and able to cover the weld lines and sagging marks on a rough surface.
Leather pattern and others
Characteristics: Durable. Product surface is abrasion resistant though cannot be fixed completely. Able to remove burnt and rust marks caused by chemical gases through surface treatment.
Other textures include stone/geometric patterns, HNDS and HN3D which are not commonly used. This time, we are going to do the satin pattern.
Procedure of Mold Texturing
Cleansing
Clean the mold cavity surface, to remove surface oil/grease.
Sealing
Apply adhesive paper or corrosion resistant coating to the cavity surface that does not need to be textured, so as to prevent corrosion. This is the most time consuming step, during which the 3 commonly used sealing materials include: Thick adhesive paper, to cover the majority part of the cavity surface; thin adhesive paper, to seal the details; and corrosion resistant coating, to cover the area that adhesive paper fails to cover, e.g. complicated curvy surfaces.
Drying
Dry the anti-corrosion coating.
Surface treatment
Carefully wipe the cavity surface to be textured using absorbent cotton, to make it free from any dirt, thus ensuring the texturing effect
Texturing
Apply a coating to the cavity surface to be textured and then soak it in the corrosive fluid. During this process, attention should be paid to the texturing status. Repeated soaking is required to get the desired textures.
Sandblasting
Sandblasting serves 2 purposes: A). To remove the residue liquid on the cavity surface after cleansing, with ammonia and pressure washer; B). To tune the gloss of the texture; different levels of gloss can be achieved by using different sands and different pressure levels.
Post treatment
Cleanse the cavity surface and apply rust protection agent before delivering the mold parts back to the mold manufacturer.
Common Post-texturing Problems
Due to the fact that the mold cavity surface is roughened after texturing, the most common problems like scratches and stickiness to the cavity may arise. In some areas, the originally small draft angle will be made smaller after texturing, or even resulting in a negative draft angle sometimes, so scratches are often caused. During the ejection process, ejector marks tend to appear due to unfavorable mold release, thus greatly affecting the part appearance.
To resolve the problem of scratches and ensure smooth mold release, the textured surface usually needs to be sandblasted to reduce the texture depth and at the same time eliminate the acute angles caused by texturing. In the practical production scenario, it is very difficult to resolve the mold release problem by adjusting injection parameters, so release agent is usually applied to the textured surface to facilitate production. From the perspective of mold, the situation may be improved by increasing the draft angle in the scratched surface area/increasing the number of ejector pins.
1. Ejector pin holes should be at least 3/32” away from other holes;
2. At least a clearance of 1/32” for pin holes on the ejector plate;
3. At least a clearance of 1/64” for pin holes on the mold plate;
4. All ejector pins should adopt the standard dimensions; the ejector base cannot be ground lower;
5. During the injection molding process of nylon, LDPE or PP, the diamter of each ejector pin must be measured, because flash might occur when the clearanc between the pin and the hole is greater than 0.02mm
6. All ejector pin holes must be vertical and glossy (Ra ~ 0.25μm);
7. For plastic materials like PP, PE and Nylon, hole diameter = pin diameter + 0.01mm; for plastic materials like HIPS, PC and ABS, hole diameter = pin diameter + 0.02mm;
8. Ejector pin should pass through the base plate, ejector plate and mold components in a straight downward way;
9. After all ejector pins are installed, the ejector plate should be able to slide downwards freely;
10. Labels in the same direction should be provided near the location of all ejector pins and screw heads, so as to prevent wrong installation;
11. All pins need to come with dowels, to prevent wrong installation (the application of square-shaped pinhead should be avoided, unless the distance between ejector pins are very small; it cannot adopt the symmetric layout, but only the one-sided way. Usually, the 1st one should be applied; apply the 3rd when there is not sufficient space);
12. Upon installation of ejector pins, everything needs to be checked before covering the back panel;
13. After installation of the supporter, use a flashlight to examine each rib and hole from the direction of the mold core, to see if there is any problem with pins or sleeve ejectors. Cover the back panel when everything’s OK;
14. When designing the location of ejector pins, in addition to guaranteeing sufficient ejection force, it has to be ensured that the product can be ejected in a straightforward way;
15. There are two types of ejector pins, i.e. though-hardened and nitrided;
15.1 Though-hardened – surface hardness is 65 – 74HRC, and steel core hardness is 50 – 55HRC;
15.2 Nitrided – the nitrided surface hardness is 65 – 74HRC.
Design of Runner Ejector Pins
Purpose:
Eject the gate/runner from cavity.
Forms of Pins
2.1 Option 1 (for general purpose)
2.2 Option 2 (for general purpose)
2.3 Applied to transparent materials like PMMA
Design of Ejector Pin Location
Pin Locations
1.1 The ejector pin should be 0.040″ – 0.100″ away from the top edge of the upper mold;
1.2 Try to place the ejector pin at the bottom of the product, such as the Pin A shown below. Try to keep a distance of at least 0.010” from mold core. Don’t place on the top (Pin B)
1.3 If an ejector pin has to be placed on a slope, Location “A” is the first option,followed by “B” and “C”, because a product tends to dislocate on a slope, and the ejector force may decline due to the existenc of the slope. If “C” is the only choice, small blocks should be added to increase ejection force.
1.4 To avoid the upper mold from being damaged by the ejector pin, the ejector pin has to be placed in location “A”.
1.4.1 The pin has to be ground lower for 0.0005”, for venting purposes;
1.4.2 A spring is needed under the return pin.
1.5 When product is too high or draft angle is too large, draw the ejection path on a layout plan, to help avoid mistake when placing the ejector pin. When the product has a large R, draw the tangent of the R on a layout plan, as a boundary for ejector pin placement.
1.6 Place ejector pins under ribs
Among the above-mentioned 5 methods, 1 is the best, and 5 is the worst.
The injection molding machine industry has experienced rapid development across the whole globe. Especially, in the era when Chinese manufacturing industry develops at a very fast pace, many Chinese and foreign injection machine brands are widely used in enterprises which produce plastic products. Among them, the more famous ones include Chen Hsong from Hong Kong, Victor Taichung from Taiwan, Haitian/Haitai/Bochuang from mainland China, Mitsubishi/Toshiba/Sumitomo from Japan, and Demag/KRAUSS MAFFEI from Germany. Comparatively speaking, Japanese injection molding machines enjoy the longest service life while German ones follow the highest safety standards.
(1) German Demag Injection Molding Machine
Demag is a world famous injection molding machine brand of the German Demag Ergotech. Its main characteristics include: the hydraulic mold clamping structure and the computer-aided optimized location control of the injection machine match quite well with preset acceleration curves, to ensure the fast and balanced mold clamping action, thus keeping mold intactness and shortening cycle time. Its hydraulic ejection mechanism is very easy to assemble and disassemble, since there is no abrupt hose while the clearance between tie bars is big enough to accommodate large sized molds. The injection molding machine employs DFE, which has achieved a closed-loop oil pressure control, thus able to meet the requirements for any high precision and high response; at the same time, it is able to minimize energy consumption, thus meeting energy-saving requirements (saves up to 30% energy when compared with conventional injection molding machines); adopts the Ergotech Control unit, which allows to switch over from injection to holding based on the required time, pressure and flow; the value can be set as per percentage or input physical values; NC4 can be used to control exterior devices, with a special page provided to set various process parameters for quality control. All process control is completed automatically by the NC4 control system.
(2) German KRAUSS MAFFEI Injection Molding Machine
KRAUSS MAFFEI is a main high-precision injection machine brand of Germany. EX, the all-electric injection molding machine KRAUSS MAFFEI has launched possesses the characteristics of fast speed, high precision, cleanness, as well as energy saving. Though designed with a very short drying cycle time, it features very high precision, thus able to guarantee the quality of molded products.
(3) Japanese Sumitomo Injection Molding Machine
Sumitomo is the no. 1 injection molding machine brand of Japan, as well as the no. 1 all-electric injection molding machine brand of the whole world. The all-electric injection molding machines of Sumitomo rank top in the world for five years in a row when market share is concerned. Sumitomo has been engaged in injection molding machine R&D and manufacturing for more than 40 years and is always well-known to the world for its high speed and high pressure. After the all-electric injection molding machines have gained widespread application, Sumitomo has become the leader of the injection molding machine industry by virtue of its strong R&D and innovative capabilities. Currently, Sumitomo has developed countless propriety technologies, such as Direct Drive, Double Center Press Platens and DST-Press etc., which enable the company to give an outstanding performance in the industry. In recent years, Sumitomo produces approx. 5,000 all-electric injection molding machines per year, ranking no. 1 among the leading Japanese all-electric injection molding machine brands for 5 consecutive years.
(4)Hong Kong Chen Hsong Injection Molding Machine
Hong Kong Chen Hsong is one of the top-selling injection molding machine manufacturers in the world, with one unit sold in every 10min on average. The company boasts an annual production capacity of nearly 15,000 units, and the whole product lineup covers the clamping force ranging from 5 to 6,500t and injection volume ranging from 1 to 100,000g. With technological innovation as the core purpose, in the early days of its foundation, Chen Hsong independently developed the first two-color blow molding machine of Hong Kong, which attracted the attention of the players in the industry. In 1960s, Chen Hsong became the first one to invent the screw injection molding machine, laying a solid foundation for the company to become the leader of the injection molding industry. Coming to the 21st century, the manufacturing technologies of Chen Hsong injection molding machines have reached the world top level. In recent years, Chen Hsong keeps launching new products and new technologies. In 2000, it launched Asia’s first PET preform injection molding system which included mold, injection machine and manipulator. In 2001, it launched the all-electric injection molding machine, and in Sept. of the same year, it launched the high-precision ultra-high speed injection molding machine.
(5)Victor Taichung Injection Molding Machine
Victor Taichung is a company in Taiwan China which has successfully developed the all-electric injection machine and realized SOP. Based on its more-than-50-year mature technologies for injection molding machine manufacturing, Victor Taichung has so far developed a complete set of injection molding machines that include five variants of 50t, 80t, 100t, 150t and 200t, as well as 11 modular combinations, which are being sold to the US, the UK, South Africa, Japan, Philippines and Malaysia, etc. In the future, it plans to launch medium-/large-size electric injection molding machines, including 250t, 300t and 350t, while extending the range of electric injection molding machines, so as to satisfy customer’s increasing demand.
(6)Ningbo Haitian Injection Molding Machine
Chinese injection molding machine manufacturers are mainly located in the southeast coast and the Pearl River Delta, among which the Ningbo region has been enjoying the strongest momentum of rapid development, so it is now the largest manufacturing base of injection molding machines in China, with an annual production capacity accounting for 1/2 of the total annual production in China and 1/3 of that across the globe. The Haitian injection molding machine series is one of the representatives. Founded more than four decades ago, Haitian Group has now become the largest production base of injection molding machines. As a high tech enterprise that’s specialized in the production of injection molding machines, Haitian is famous in the domestic injection machine industry for its high quality, high efficiency, high grade and economic returns. Its products include the plastic injection molding machines of nearly 100 types, of which the clamping force mainly ranges from 58 to 3,600t (injection volume ranging from 50 to 54,000g), boasting an annual capacity of nearly 10,000t, with both production and sales ranking no. 1 in the China market. The company has comprehensively introduced the world-class fully computer-aided and fully automatic processing centers from Japan, Germany, the US and the UK, so as to turn out the Haitian injection molding machine series with high precision and high quality, including five series of HTB, HTF, HTW, HTK and DH, and boasting more than 100 machine types, so as to meet the different needs of different customers.
As mold designers, we are all aware that, in normal circumstances, lifters are created for inner undercuts and sliders are created for outer undercuts. However, when we have undercuts on all 4 sides of a product, which makes it impossible to release the product by force, how should the mold be designed? See the following figure for the product. Shall we create lifters or sliders? Now, let me explain the mold design solutions for such products to all of you.
Design analysis
When coming across such products, we will firstly consider whether it is possible to create lifters. However, analysis shows that the undercuts in the 4 corners will not be able to be released with the help of lifters (lifters in the arrow direction).
Solution
Since lifters are not able to ensure complete release of the undercuts, we have to think about the design scheme that combines lifters and inner sliders. As shown in the figure below, green parts are inner sliders, and pink parts are lifters.
How it works
Mold core and cavity open a bit, then the slope of the wedge drives the sliders to move towards the center, so as to release the undercuts from the 4 corners of the product. At the same time, the lifters eject the product out while the undercuts are released completely. Finally, you can take the product out and close the mold for the next production run.
Injection pressure is exerted by the hydraulic mechanism of the plastic injection molding system. This pressure is transferred from the hydraulic cylinder to the molten plastic through the molding screw, under which the molten plastic will be pushed to flow into the mold sprue (also known as the primary runner of some molds), the primary runner and the sub-runner via the nozzle of the plastic injection molding machine, and then finally get into the mold cavity through the gate. This process is referred to as the injection molding process or the filling process. The purpose of the pressure is to overcome the resistance occurring when the molten plastic is flowing; or to put it another way, the resistance occurring in the flowing process needs to offset by the pressure exerted by the injection molding machine, so as to facilitate smooth filling.
During the injection molding process, the injection nozzle features the highest pressure in a bid to overcome the resistance to flow throughout the whole process. After that, the pressure shows a trend of gradual decrease from the nozzle to the melt front as the molten plastic flows further. If the mold cavity vents well, the final pressure on the melt front will be equivalent to atmospheric pressure.
The injection pressure on molten plastic is influenced by a diversity of factors. To sum up, there are 3 categories: (1). Material factors, such a material type, viscosity, etc.; (2). Structural factors, such as type, quantity and location of the runner system, shapes of mold cavity and product thickness, etc.; (3). Molding process factors.
2．Value & Time of Holding Pressure
When the plastic injection molding process is drawing to an end, the screw will stop rotating but only keep moving forward. At this point, the injection molding process enters the pressure holding phase, during which period of time the injection nozzle continuously feed materials into the cavity, to fill up the empty space caused by product shrinkage. If the pressure is not held after the cavity is filled up, the product will shrink for about 25%. In particular, shrink marks will be left near the ribs due to enormous shrinkage. Usually, the value of the holding pressure is about 85% of the top injection pressure, which, of course, is subject to actualities.
3．Back pressure
Back pressure refers to the pressure that the screw has to overcome during its return action after injecting material. The application of a high backpressure helps distribute the pigments and melt the plastic, but at the same time, it also extends the screw’s return time, decreases the length of plastic fibers and raises pressure in the injection machine. As a result, the backpressure should be kept lower, usually not exceeding 20% of the injection pressure. Some injection machines allow backpressure programming to compensate for screw travel decrease, which will reduce heat input, causing the temperature to drop. However, since it is not easy to predict the changeable result, corresponding machine adjustment will a troublesome task.
The setting of hold pressure is aimed to prevent resin backflow, while at the same time compensating for resin shrinkage caused during the cooling process, so as to achieve the optimal molding outcome. If the holding pressure is set too high, the product will be prone to flash, over filling or stress concentrating near the gate, etc.; on the other hand, if the holding pressure is too low, excessive shrinkage and dimensional instability will be likely to occur.
Holding pressure only works well along with the settings of pressure switchover point and holding time in the plastic injection molding process.
Insufficient holding pressure will lead to: 1. dents; 2. bubbles; 3. increased shrinkage rate; 4. decreased product dimensions; 5. larger dimensional fluctuation; 6. inner-layer orientation caused by melt backflow, etc.
Excessive hold pressure will lead to:
Stress in the sprue area;
Difficult mold release;
Tensile stress on the outer layer;
Gradual decrease of holding pressure during the pressure holding time may be able to (multistage holding pressure):
Reduce warpage, as well as shrinkage difference in the product molding section between the gate and the far end;
Reduce internal stress;
Reduce energy consumption.
The setting of pressure holding time is aimed to control the duration of the holding pressure effect. An insufficient holding time will result in product dimensional and weight instability. However, if the holding time is set to be too long, molding efficiency will be affected. A proper pressure holding time should last till the gate solidifies. In the meantime, appropriate coordination between the value and time of holding pressure is able to bring the effect of the procedural holding pressure into full play. The purpose of holding pressure is to seal the sprue and compensate for material shrinkage after injection is completed. As a result, the holding pressure must be greater than the internal pressure.
If the hold time is set to be shorter than the maximum effective pressure holding time, i.e. insufficient holding time, the following results may occur: 1. dents; 2. bubbles; 3. underweight; 4. smaller dimensions; 5. internal orientation caused by melt backflow; 6. greater warpage, especially for semi-crystalline materials; 7. larger dimensional fluctuations; 8. increased shrinkage, etc. The set holding time must effectively last till the sprue solidifies. Usually, a sufficient holding time is approx. 30% of the cooling time.
In general, injection pressure control is composed of first-stage pressure, second-stage (holding) pressure or more stages of injection pressure control. An appropriate pressure switchover plays an important role in the avoidance of overpressure, overflow or incomplete filling. The specific volume of a molded product is dependent on the melt pressure and temperature during the pressure holding time when the gate is closed. Every time when switching from pressure holding to product cooling, if the pressure and temperature can be kept consistent, the specific volume of the product will remain unchanged. Under consistent mold temperature conditions, the value of holding pressure is the most important parameter that determines product dimensions, while the value of holding pressure and temperature are the most important variables that influence product dimensional tolerance. For example, after injection is completed, the holding pressure decreases immediately, and when the surface layer reaches certain thickness, the holding pressure will rise again. This way, thick-walled large products can be molded with a low clamping force, so as to eliminate dents and flash etc.
Hold pressure and speed are usually 50% – 65% of the top injection pressure and speed. That is to say, the holding pressure is approx. 0.6 – 0.8MPa lower than the injection pressure that feeds plastic into the mold cavity. Since the holding pressure is lower than the injection pressure, during the relatively long holding time, the hydraulic pump will be working under a low load, so its service life will be accordingly extended. At the same time, power consumption of the pump motor will also be brought down.
While facilitating smooth and complete mold filling, holding pressure can also eliminate product defects like weld lines, dents, flash and warpage, etc. It is thus very helpful for the production of various types of parts, including thin-walled parts, multi-headed small parts, long-cycle large parts, as well as parts with an unbalanced cavity or even those with insufficient clamping force.
During the plastic injection molding process, the molten material shrinks due to cooling. However, the screw needs to keep moving forward slowly, so that the molten plastic in the barrel can continue flowing into the cavity, to compensate for the shrinkage. This process is known as pressure holding. To put it simply, its purpose is to compensate for product shrinkage, as well as ensure a stable production process. In addition, the hold pressure is also able to adjust product dimensions, and effectively eliminate weld lines, dents, flash and warpage at the same time.
As a matter of fact, hold pressure and injection mean the same thing – applying a force to push the screw forward. The only difference is that in the injection process, the screw is pushed to move at a set injection speed and the max injection pressure; during the pressure holding time, the screw is pushed to move at a set injection pressure and the top pressure holding speed.
Multistage injection molding is able to adjust the speed and pressure at which the raw material flows into the mold cavity. This way, the defect rate of some complicated structure products will be decreased, and at the same time, the small inserts in the mold will be well protected. For example, when the raw material flows into the mold, its speed and pressure can be reduced when reaching a small insert, so the insert will not be prone to damage. Multistage holding pressure is also able to reduce the occurrence of dents – another benefit that helps reduce product defects.
Changing material or color in the injection molding process certainly deserves in-depth discussion. A quick color or material change cannot only save time, but also drastically bring down the production cost.
1)Color change – a same material
In principle, when injection molding factory change the color of a same material, change from light to dark is usually easier than that from opaque to transparent.
The regular color change procedure is described as below:
Shut the feed inlet located in the lower part of the feed hopper;
Perform several empty shots, until the previous material is cleared from the barrel;
Feed the new material into the hopper;
Open the feed inlet, and pull the screw back and forth for a dozen times until changeover is completed.
When changing from an opaque material to a transparent material, the nozzle needs to be removed to clear the residue; if necessary, the screw needs to be pulled out for thorough cleaning to make sure that there is no residue hiding in the corners.
2)Color change – a different material
Concerning switch between different materials, the material change steps are performed on basis of the viscosity difference between the materials, as well as barrel temperature control.
Thermoplastics tend to adhere to metal surface at a high temperature; and the situation is the opposite when the temperature is low.
The change of materials can take advantage of this feature – make the previous material in the barrel adhere to barrel surface, and then help the high-viscosity remover material clean it with the involvement of cold screw. At this point, the screw temperature needs to be low enough, so that the previous material will not adhere to it, thus easy for purge. Therefore, the remover material needs to possess a high melt viscosity, such as high-density PE or PS.
Keep the following considerations in mind when switching materials:
Before changeover is done, the barrel temperature needs to be lower than the actual molding temperature; for example, when changing from the low molding temperature material A to the high molding temperature material B, the purging temperature of material B should be 10℃ – 20℃ lower than its molding temperature; and when changing from the high molding temperature material B to the low molding temperature material A, the purging temperature of material A should be 10℃ – 20℃ lower than its molding temperature.
Reduce screw rotation speed and screw backpressure, to prevent material temperature rise caused by frictional heat;
Try to prevent the material (molten) to be replaced from adhering to the screw;
Apply short screw travel to flush the material for several times, to achieve the best effect for material change;
If there are scars or gaps on the barrel inner surface, or the screw head, outer surface or groove, the molten material may be stuck at such locations, thus making it hard for material change.
3)Practical operation of material change
During the plastic injection molding process, changing material or color happens a lot. If sufficient basic knowledge of material change is not equipped, waste of time and money may probably be caused to the manufacturer. To respond to the challenges posed by changeover, the Germany-made CORATEX purging agent has been introduced to clear the residues left in the screw, the nozzle and the mold (especially ideal for cleaning of hot runner molds), which is very helpful for reducing the time consumed by changeover.
Material switch operation for PC
From PC to ABS
(1) Shoot all remaining PC out from the barrel;
(2) Within the molding temperature range of PC, use high-density PE to clear the residual PC from the barrel;
(3) Bring barrel temperature down below 220℃, and use ABS to clear the high-density PE, then changeover is completed.
From PC to POM
(1) Shoot all remaining PC out from the barrel;
(2) Within the molding temperature range of PC, use high-density PE to clear the residual PC from the barrel;
(3) Bring barrel temperature down below 190℃ and use POM to clear the high-density PE, then changeover is completed.
From PC to PMMA
(1) Shoot all remaining PC out from the barrel;
(2) Within the molding temperature range of PC, use high-density PE to clear the residual PC from the barrel;
(3) Bring barrel temperature down to 240℃, use non-dried (moist is not removed) PMMA to clear the high-density PE and then clear it with dried PUMA. The changeover is then completed.
From PC to PP
(1) Shoot all remaining PC out from the barrel;
(2) Within the molding temperature range of PC, use PP to clear the residual PC;
(3) Bring barrel temperature down to 200℃, and use HIPS to clear the high-density PE, then changeover is completed.
Material change operation for ABS
From ABS to PC
(1)Shoot all remaining ABS out from the barrel;
(2) Within the molding temperature range of ABS, use high-density PE to clear the residual ABS from the barrel;
(3) Raise barrel temperature to 290℃, and use PC to clear the PE, then changeover is completed.
From ABS to POM
(1) Shoot all remaining ABS out from the barrel;
(2) Within the molding temperature range of ABS, use PS to clear the residual ABS from the barrel;
(3) Bring barrel temperature down to 190℃ and use POM to clear the PS, then changeover is completed.
From ABS to PMMA
(1) Shoot all remaining ABS out from the barrel;
(2) Within the molding temperature range of ABS, use PS to clear the residual ABS from the barrel;
(3) Keep barrel temperature at 240℃, use non-dried (moist is not removed) PMMA to clear the PS and then clear it with dried PMMA. The changeover is then completed.
Material switch operation for POM
From POM to PC
(1) Shoot all remaining POM out from the barrel;
(2) Within the molding temperature range of POM, use PE to clear the residual POM from the barrel;
(3) Raise barrel temperature to 290℃ and use PC to clear the POM, then changeover is completed.
Material switch operation for PP
From PP to PC
(1) Shoot all remaining PP out from the barrel;
(2) Raise barrel temperature to 290℃ and use PC to clear the PP, then changeover is completed.
From PP to POM
(1) Shoot all remaining PP out from the barrel;
(2) Keep barrel temperature at 190℃ and use POM to clear the PP, then changeover is completed.
From PP to ABS
(1) Shoot all remaining PP out from the barrel;
(2) Keep barrel temperature at 240℃ and use ABS to clear the PP, then changeover is completed.
From PP to PMMA
(1) Shoot all remaining PP out from the barrel;
(2) Keep barrel temperature at 240℃, use non-dried PMMA to clear the residual PP from the barrel and then clear it with dried PMMA. The changeover is then completed.
From PP to HIPS
(1) Shoot all remaining PP out from the barrel;
(2) Keep barrel temperature at 240℃ and use HIPS to clear the PP, then changeover is completed.
