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.
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.
By virtue of its enormous benefits, such as light weight, outstanding toughness, easy molding and low cost, plastic is gaining more and more popularity in modern industry and the production of daily necessities, because it is an ideal substitute for glass. Especially, in the fields of optical devices and the packaging industry, it has witnessed an exceptionally rapid development. However, due to the fact that such plastic materials are required to be extremely transparent(clear), with great abrasion resistant and impact resistant features, a great deal of effort is needed in the aspects of plastic ingredients, as well as the technology, equipment and molds throughout the entire plastic injection molding process, so as to make sure that these glass substitute materials (hereinafter referred to as transparent plastics) possess an outstanding surface finish, thus meeting the application requirements.
Currently, the commonly used transparent plastic materials on the market include polymethyl methacrylate (commonly known as acrylic or acrylic glass, abbr. PMMA), polycarbonate (abbr. PC), polyethylene terephthalate (abbr. PET), transparent nylon, acrylonitrile-styrene copolymer (abbr. AS) and polysulfone (abbr. PSF), etc., among which the 3 most commonly used ones are PMMA, PC and PET. Now, we will take these 3 materials as an example, to discuss the characteristics and the injection molding process of transparent plastics.
Performance of transparent plastic materials(clear plastics)
Above all, transparent plastic materials have to be highly transparent, and then they need to be strong enough to resist abrasion, impact, heat and chemicals, with a low water absorption rate. This is the only way to guarantee that the materials can meet the transparency and durability requirements for application. The following table I shows the performance comparison among PMMA, PC and PET.
From table I we can tell, PC is an ideal choice, but the raw materials are costly and not easy to process. As a result, PMMA is the main choice (for average products). PET needs to be stretched to obtain a desired mechanical performance, so it is usually used for production of packages and containers, etc
Common problems that need to be addressed during the injection molding process of transparent plastics.
Inevitably, due to their high light transmission rate, stringent requirements are imposed on product surface finish defects like pores, black spots, discoloration and low glossiness have to be completely eliminated. Therefore, to guarantee product surface finish, throughout the entire process, close attention should be paid to raw materials, equipment, mold and even product design, with stringent or even special requirements in place. Secondly, since transparent plastic materials normally have a high melting point and poor fluidity, subtle process parameter adjustments including mold temperature, injection pressure and injection speed are usually required to make sure that the mold can be fully injected, and product deformation or cracks that are caused by internal stress can be completely eliminated.
In the following paragraphs, we will discuss the considerations for transparent plastic injection molding from the perspectives of raw material preparation, equipment/mold requirements, the injection process and raw material processing, etc.
（1）Raw material preparation and drying:
Due to the fact that even the smallest amount of impurities in the plastic may greatly affect product transparency. Therefore, during the storage, transport and feeding processes, special attention should be paid to sealing, so as to keep the raw material clean. In particular, if the raw material contains moisture, it may deteriorate after being heated. So, the raw material has to be dried during the molding process, and a dry hopper must be used for material feeding. Also note that, during the drying process, the air blown in has to be filtered and dehumidified to keep the raw material from contamination.
（2）Cleansing of barrel, screw and accessories
Before and after the molding process, in order to prevent the raw materials from contamination, and keep the recessive parts of the screw or the accessories free from used materials or impurities (in particular the resins with a poor thermal stability), each part has to be cleansed by using the screw detergent to protect it from any impurities. If there is no screw detergent, resins like PE and PS can be used to clean the screw. If the machine has to be shut down temporarily, to prevent raw material degradation caused by long time staying in the high temperature environment, the temperatures of the drier and the barrel need to be lowered. For example, barrel temperature for PC and PMMA need to be reduced to below 160℃ (for PC, the hopper temperature should be lower than to 100℃).
（3）Considerations for mold design
To prevent product defects and/or deteriorations caused by poor backflow or uneven cooling, the following points need to be paid attention to when designing a mold.
a）A consistent wall thickness, and a big-enough draft angle;
b）A smooth transition, to avoid the occurrence of pointed angles or sharp edges. Especially for PC products, no gap is allowed;
c）The gate and runner need to be wide and short, and the gate position needs to be defined based on the solidification shrinkage process; Set up a cold slug well when necessary;
d）The mold surface should be glossy, with a low roughness;
e）Adequate venting slots, to expel the air and/or gases in the molten plastic in a timely manner;
f）Except PET, the wall thickness cannot be too thin – usually, it should be thicker than 1mm;
（4）Considerations for the plastic injection process (including requirements on injection machine)
To reduce internal stress and surface defects, the following aspects of the injection molding process should be paid close attention to.
a）Select the injection machine designed with a special-purpose screw and a separate thermostatic nozzle;
b) The higher the injection temperature is, the better, but keep in mind that the temperature should be controlled below the decomposition point of the resin materials;
c）Injection pressure: Usually higher pressure is applied to cope with the high viscosity of the molten plastic. However, if the pressure is too high, internal stress will occur, leading to difficult mold release or product deformation;
d）Injection speed: Usually, the injection speed should be lower on condition that complete filling can be guaranteed. It is better to employ the slow – fast – slow multistage injection;
e）Pressure holding time & molding cycle: Keep both as short as possible on condition that complete filling can be guaranteed, and no dents or bubbles will occur, so as to minimize molten material’s staying time in the barrel;
f）Screw speed & backpressure: Keep both as low as possible on condition that plasticization quality can be guaranteed, so as to prevent the possibility of degradation;
g）Mold temperature: Product cooling effect plays an important role in determining product quality, so mold temperature has to be accurately controlled during the molding process. Whenever possible, keep the temperature as high as it can be.
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The proportional injection speed control system is extensively adopted by injection machine manufacturers. In this article, we are going to explain the benefits of applying the multi-speed injection molding process, while offering an overview on the role it plays in eliminating product defects, such as short shot, entrapped air and sink marks, etc.
By virtual of its close relation with product quality, plastic injection speed has become one of the key parameters of plastic injection molding. By defining the front, center, and rear of the feeding speed segmentation, and realize smooth transition from one set point to another, a steady molten plastic surface speed can be guaranteed to turn out the desired product.
We suggest the following speed segmentation principles:
1). The flow surface speed should be a constant;
2). Apply high-speed injection to avoid molten plastic solidification during the injection process;
3). To set the injection speed, we need to take both the fast feeding of critical areas (e.g. the runner) and the slower speed at the gate into consideration;
4). The plastic injection speed needs to ensure that the injection process immediately stops after the mold cavity is filled up, so as to avoid over flow, flash and residual stress, etc.
The several considerations for speed segmentation settings include geometric shape of the mold, other flow limitations and some uncertainties. To set the speed properly, we need to have a good understanding of the injection molding process as well as the materials, or it will be hard to control product quality. Though it is not easy to measure the speed of the molten plastic directly, we can gauge the speed indirectly via the measurement of screw moving speed or cavity pressure.
The characteristics of a material are of great importance, because polymers may decompose under a different stress, and mold temperature rise may lead to vigorous oxidation and chemical structural degradation, but at the same time, shear may lower the level of degradation, because the higher temperature has reduced material stickiness and therefore the shear force. Undoubtedly, the multistage injection speed control is very helpful for the molding process of thermally sensitive materials like PC, POM, and UPVC, as well as their ingredients.
The shape of a mold is another defining factor: the thin-walled area needs the fastest injection speed, and the thick-walled part needs the slow – fast – slow speed curve to avoid defects; to bring part quality up to standard, the setting of injection speed needs to make sure that the speed at the forefront of the molten plastic is constant. The flow speed of the molten plastic is so important that it will influence the molecular orientation and surface status of the final part. When the forefront of the molten plastic comes to an intersection, the speed should be decreased; for a complex mold with a radial expansion, we need to ensure that the throughput of the molten plastic increases in a balanced manner; as for the long runner, fast injection is needed to lessen molten plastic forefront cooling. However, the injection of high stickiness materials like PC is an exception, because if the speed is too high, the cold slug will be brought into the cavity via the gate.
The adjustment of injection speed is able to help eliminate product defects caused by the slower speed at the gate. When the molten plastic arrives at the gate via the sprue and the runner, the surface of its forefront may have solidified, or the molten plastic comes to a standstill because the runner suddenly narrows. It will not move forward until enough pressure is built up to push the molten plastic through the gate. In this instance, the pressure that passes through the gate will show a peak shape. The high pressure may do harm to the material and lead to surface defects like flow marks and burnt streaks on gate, etc. This can be resolved by slowing down the speed right before the molten plastic enters the gate, because it is able to prevent over shear at the gate location. After that, restore the injection speed to the original value. Since it is difficult to precisely slow down the injection speed at the gate location, it is a better solution to slow down the speed in the final section of the runner. We can reduce or even avoid product defects like flash, burnt marks and entrapped air by controlling the injection speed in the final section. Also, slowing down in the final section can help prevent overflow, thus avoiding flash and reducing residual stress. The entrapped air caused by poor ventilation in the final section of the mold flow path or feeding problems can also be resolved by slowing down the air venting speed, especially the air venting speed in the final section of plastic injection.
Short shot occurs because of the too low speed at the gate or the local flow blockage caused by plastic solidification. The issue can be resolved by increasing injection speed when the molten plastic is passing through the gate or when local flow is blocked.
Product defects like flow marks and burnt streaks on gate which tend to appear on thermal sensitive plastic materials are usually caused by the over shear occurring when the flow passes through the gate.
The production of smooth-surfaced parts is dependent on injection speed, yet glass fiber filled materials are even more sensitive, especially nylon. Streaks (wrinkles) are caused because the flow is not steady due to stickiness changes. A twisted flow may lead to wrinkles or uneven fog-like patterns, among which the specific defect is dependent on the level of flow unsteadiness.
When the molten plastic passes through the gate, a high injection speed will lead to high shear, causing thermal sensitive plastics to burn. The burnt plastic will then go through the cavity, reach the forefront of the flow, and finally appear on product surface.
To prevent jetting marks, injection speed setting must guarantee that the flow is able to fill the runner area quickly and then passes through the gate slowly. Essentially, the key is to find the transition point. If too early, filling time will be overextended; if too late, the excessive flow inertia will lead to jetting marks. The lower the stickiness of the molten plastic and the higher the barrel temperature, the more likely jetting marks will occur. Due to the fact that the small size gate requires high-speed and high pressure injection, it is another important factor that causes flow defects. Sink marks can spread under pressure, so the problem can be improved by reducing the pressure. Flow distance will be greatly shortened in the scenario of low mold temperature and slow screw speed, so increasing injection speed will be able to compensate for the distance. A high speed flow is able to minimize heat loss. In addition, the high shear heat will produce frictional heat, causing molten plastic temperature to rise, which helps reduce the thickening speed on the outer layer of the part. An adequate thickness must be guaranteed for the intersections inside the cavity, so as to prevent excessive pressure loss, or sink marks will appear.
In a word, most of the injection molding defects can be resolved through injection speed adjustment. Hence, the trick of adjusting the plastic injection molding process is to appropriately set the injection speed and its segmentation.
Injection molding temperature is very important for plastic injection molding,right injection molding temperature is a guarantee of product quality and production effciency.we will show what is the best injection process for different plastic material.
Front:The suck-back section is located at the forefront of the barrel, where the nozzle is situated. The temperature here should be a little lower than that of the plasticization area. Its main purpose is to prevent the molten plastic from flowing back through the nozzle under internal pressure. However, the temperature cannot be too low, or additional plasticization pressure will be required.
Center:The plasticization area is located in the middle of the barrel, where the temperature will gradually rise above the melting point. The purpose of this section is mainly to melt the plastic material. However, if the temperature is too high, the plastic material will be prone to decomposition; if the temperature is too low, it will not be helpful for the plasticization process, and screw torque will also increase.
Rear :The feeding area is close to the hopper. Usually, its temperature is set to be lower to around the melting point of the plastic material. The purpose of this section is mainly to pre-heat the material. So, if the temperature is too high, the plastic will be melted, causing screw slippage and thus affecting material delivery. However, if the temperature is too, screw torque will increase.
In the plastic injection molding industry, some newcomers often ask: why the final plastic parts have a higher gloss when the mold temperature is higher. Now in plain language, let’s explain this phenomenon, as well as how to appropriately select the right mold temperatures.
1,Influence on Product Appearance:
First of all, a too low temperature will affect the fluidity of the molten plastic, leading to incomplete filling; mold temperature influences the crystallinity of plastic materials. For ABS materials, if the temperature is too low, the final product will have a lower gloss. Compared with the filler, plastic tends to move to the surface when put under the high temperature condition. Therefore, a higher temperature will allow the plastic to contact with mold surface more closely, thus ensuring better filling, as well as higher brightness and gloss. Yet, the temperature of the plastic injection mold cannot be too high, or sticking to cavity will be caused and bright spots will also appear in some local areas of the plastic part. On the other hand, if the mold temperature is too low, the plastic part will be clamped so tight that it may be damaged during mold release, in particular the surface texture of the plastic part.
Multi-stage injection is able to solve positional problems. For example, we can employ the multi-stage injection approach to solve product gas marks caused during the filling process. In the plastic injection molding industry, the gloss level of a smooth surfaced product will be higher when the mold temperature is high, and verse versa. However, for textured PP products, the higher the temperature, the lower the gloss, and the larger the color difference – the gloss is inversely proportional to the color difference.
As a result, one of the most common problems caused by mold temperature is the rough surface of the plastic injection molded part, which is mainly because the surface temperature of the mold is too low.
2,Influence on Product Dimensions:
If mold temperature is too high, the molten plastic will probably decompose and the shrinkage rate of the plastic product will be larger when exposed to air, leading to shrunk product dimensions. When a mold is used under a low-temperature condition, if product dimensions are increased, the most likely cause is the very low surface temperature of the mold. The reason is that when the mold surface temperature is too low, the molded product will have a lower shrinkage rate when exposed in the air, so the dimensions are larger – the reason being that the low mold temperature will accelerate the molecules “freezing tendency”, creating a thicker frozen layer of the molten plastic inside the mold cavity. At the same time, the low temperature will also impede the crystallization process, therefore the shrinkage rate of the molded product is decreased. On the contrary, higher mold temperature will slow down the cooling process of molten plastic, resulting in a longer relaxation time and a lower level of tendency, while also facilitating crystallization. Thus, the actual shrinkage rate of the product will be higher.
If the startup process is very long before the dimensions stabilize, it means the temperature is not appropriately controlled, because it takes a long time for the mold to reach thermal balance.
Uneven heat radiation at some parts of the mold will greatly extend the production cycle, causing the injection molding cost to rise. A consistent mold temperature is able to minimize the fluctuation of molding shrinkage rate, thus enhancing dimensional stability. For crystalline plastics, a high temperature is helpful for the crystallization process, while a fully crystallized plastic part is subject to minimal dimensional changes during storage or application. However, the higher the crystallinity, the higher the shrinkage rate. For soft plastic materials, a low mold temperature is recommended in the molding process, which is helpful for dimensional stability. For all the materials, it is true that consistent mold temperatures and shrinkage rate are helpful for improvement of dimensional accuracy.
3. Influence on Product Deformation:
If the mold cooling system is not reasonably designed or the mold temperature is not appropriately controlled, part warpage will be caused due to insufficient cooling. To ensure proper mold temperature control, the temperature difference between the cavity side and the core side should be defined on basis of the structural characteristics of the plastic product. We can offset the shrinkage differences caused by molecular orientation and therefore avoid orientational part warpage by controlling the different cooling and shrinkage speeds at different parts of the mold while also considering the characteristic that the part tends to warp towards the side where the temperature is higher after mold release.
For fully symmetrical parts, the mold temperature should be kept consistent to guarantee cooling balance within the entire part. A consistent mold temperature and balanced cooling is able to minimize part deformation. On the contrary, if the mold temperature difference is too large, uneven part cooling will be caused, leading to inconsistent shrinkage, and the internal stress thus caused will make the plastic part warp. This is especially true for plastic parts with an uneven wall thickness and a complicated shape. The product will definitely warp towards the mold side where the temperature is higher. It is suggested that the temperatures of the cavity side and the core side should be properly selected depending on actual needs. Please refer to the Material Property Table for mold temperatures.
4,Influence on Product Mechanical Properties (Internal Stress):
A low mold temperature will lead to obvious welding lines on the plastic part, which reduces the product strength; with regard to crystalline plastics, the higher the crystallinity, the more likely stress cracks will appear on the plastic product. To reduce internal stress, the mold temperature should be kept at a moderate level (PP, PE). As for the amorphous plastic materials like PC which possess a high stickiness, the stress cracks are associated with the internal stress of the plastic part. So, it will help reduce internal stress by raising mold temperature, so as to reduce the tendency towards stress cracks. The internal stress is usually indicated by obvious stress marks.
The reason is: basically, the internal stress occurring in a molding process is caused by the different thermal shrinkage rates during cooling. After the plastic product is molded, cooling will take place and extend from the surface to the core. The surface shrinks and solidifies first, and then gradually extending to the inside. During this process, the internal stress is caused due to the difference in shrinkage speed. When the residual stress inside the part is higher than the elasticity of the resin material or the part is corroded in a chemical environment, cracks will appear on the part surface.
The research into the PC and PMMA transparent resin materials indicates that, the residual stress shows a contracted form on the surface layer but a stretched form on the inside. The compressive surface stress is dependent on the surface cooling conditions. A cold mold is able to cool down the molten resin in a very fast way, so that a high level of residual stress is produced in the molded product. Mold temperature is one of the fundamental conditions for internal stress control. A slight change in mold temperature may make a great difference to residual stress. Generally speaking, the acceptable internal stress of each product and resin material has its lowest limit. When molding a thin-walled product or the flow distance is long, the lowest mold temperature should be higher than that applied to common molding.
5,Suggestions on How to Identify the Right Mold Temperatures:
Nowadays, molds are becoming more and more complex. As a result, it is getting harder for us to create appropriate conditions for mold temperatures control. In addition to simplifying the part, the mold temperatures control system is usually a compromise solution. Therefore, the following suggestions only serve as a rough guide.
During the mold design phase, temperature control of the molded part must be taken into consideration. For example, when designing a low-volume large-size plastic injection mold, one of the most important considerations is the cooling performance. Concerning the molds for production of precision parts, or of the parts that have to meet stringent appearance or certain safety standards, a higher temperature is usually applied (ensuring lower shrinkage rate, glossier surface and consistent performance). For parts that require lower technologies and minimal production costs, a lower temperature should be applied during the molding process. Nevertheless, manufacturers should be aware of the respective weaknesses of the choices and perform careful inspection of the parts, to make sure that the produced parts are still able to satisfy customer requirements.
The purpose of wear plates is to reduce the friction caused by slider movement, ensure smooth slider movement and thus increase mold service life;
There are 2 types of wear plate: ① bottom wear plate and ② back wear plate;
5mm higher than the mold plate, the bottom wear plate is fixed on the mold plate and has direct contact with the slider;
0.5mm higher than the slider, the back wear plate is fixed on the slider and has direct contact with the wedge;
The wear plate usually measures 8mm – 10mm in height;
The contact area between the wear plate and the slider / wedge needs to be larger than 80%; when the slider is too big, wear plate can be divided into several smaller plates depending on slider size;
The wear plate applies the SK-3 material, which needs to be preheated.
The length of the angle guide pin is dependent on slider travel distance, usually a rounded number (5 times); a pocket hole needs to be set up on the mold plate since it’s longer than the slider;
The angle of an angle pin cannot be larger than 25°;
The angle pin hole on the slider is referred to as the pocket hole, with a clearance of 0.5mm of both sides and rounded off with a fillet;
A slider can only be equipped with 2 angle pins at the most, which need to be distributed evenly;
Slider travel distance = the distance between ① fillet tangent at slider pocket hole and ② the arc tangent at the bottom of the guide pin;
The angle pin usually uses the same material as the ejector pin; when the diameter is lager than 20mm, the guide pin material will be applied.
PS is an amorphous polymer with a density of around 1.04g/cm3, AKA standard plastic.
With favorable fluidity and a low water absorption rate (lower than 0.02%), it is a transparent plastic material that is easy to plastic injection molding process. PS product’s light transmissionrate are typically as high as 88 – 92%, with strong tinting strength and high hardness; It is also non-toxic and odorless.
However, PS products are highly brittle, stress cracks tend to appear (may be checked by soaking or wiping with kerosene); poor thermal resistance (60～80℃).
2. PS Applications:
Ornaments, illuminated signs, lampshades, stationery, transparent toys, daily necessities, kitchen supplies, cups, meal boxes, cassettes and mirrors, etc.
3. PS Process Characteristics:
The melting point and decomposition temperature of PS are 166℃ and 280℃ respectively. Thanks to its good fluidity and the low flow resistance, the injection pressure can be a little lower.
As the PS features a lowspecific heat, the produced parts can soon solidify after mold cooling. Its cooling speed is higher than other materials, so mold open time can be earlier. Both its plasticizing time and cooling time are a relatively shorter, leading to a shorter molding cycle.
Internally stressed plastic parts can be soaked in 65 – 80℃ water for 1 – 2 hours, then gradually cooled to the room temperature, and the internal stress can be removed.
The barrel does not need to be cleaned after the machine stops, because the PS itself can be used as the cleaning agent of other plastic materials.
ABS is an amorphous polymer with a density of around 1.05g/cm3. Its overall performance is great, with high mechanical strength, excellent impact resistance, certain surface hardness and anti-abrasion properties. Its heat resistance is as high as 90℃ (even able to be used under the 110 – 115℃ temperature conditions); good low temperature resistance (able to be used under the -40℃ temperature conditions); and allowing easy processing and easy electroplating.
But it has poor solvent resistance; easy to age when exposed to UV rays; low extension rate. ABS features a diversity of varieties and an extensive range of applications, so it is also known as “general purpose engineering plastic”.
2. ABS Applications: :
Electronic product parts, toys, casings and daily necessities, etc.
(1) ABS has poor moisture absorption and temperature resistant properties. Before the molding process, it must be dried and preheated thoroughly, to keep its moisture content below 0.03%.
(3) The best effect will be achieved when ABS is used for injection production at a medium injection speed (unless the structural complexity and thickness of the product require a higher injection speed. Gas marks tend to appear around the gate of the product.
(4) ABS should not stay for too long inside the high-temperature barrel (should be kept less than 30m), or it will easily decompose and turn yellow. When changing from another material to ABS, the plastic injection machine needs to be cleaned with PP or PE.
Acrylic (PMMA)
1. PMMA Performance:
PMMA is an amorphous polymer (commonly known as Acrylic), with a density of around 1.18g/cm3. It is extremely transparent, with a light transmission rate of as high as 92%; featuring good thermal resistance (heat deflection temperature is 98℃) and high hardness. But, if it is used as an optical product material, the surface will be easily left with scratches. Its biggest weakness is brittleness (better than that of PS).
2. PMMA Applications:
Lampshades, window glasses, signs, optical lenses, contact lenses and automotive parts, etc.
3. PMMA Process Characteristics:
PMMA has stringent processing requirements. As it is very sensitive to moisture and temperature, thorough drying is required before molding. The molten PMMA is highly sticky, so it needs to be processed under high temperature (190 – 240℃) and high pressure conditions. It is better if the mold temperature is kept between 65 and 90℃. PMMA does not feature a good thermal stability, so degradation may occur when it is put under high temperature or stays in a high temperature environment for too long. “Hollows” tend to appear inside thick PMMA products, so it is better to be processed using a big gate and under the “high material temperature, high mold temperature and slow speed” injection conditions.
Polypropylene (PP)
1. PP Performance:
PP is a crystalline polymer, with a density of as low as 0.91g/cm3. Among the commonly used plastics, PP is the lightest; and among all general-purpose plastics, PP is the most thermal resistant – its heat deflection temperature is 80 – 100℃ and can be heated in boiling water.
PP features a strong stress crack resistant property and bending fatigue resistance, and it is also commonly known as “polypropene”.
PP is lightweight, with good toughness and chemical resistance.
PP weaknesses: low dimensional accuracy, insufficient rigidity, poor weatherability. It ages and thus becomes brittle easily. During the application process, long time contact with copper should be avoided, so as to prevent the “damage by copper”.
2. PP Applications:
Various household items, transparent pot covers, chemicals delivery pipelines, chemicals containers, medical supplies, stationery, toys, wires, cups、circulating boxes, pipes and hinges, etc
PP possesses good fluidity under the melting temperature, with great molding capabilities.
High molecular orientation, leading to high shrinkage rate.
It is better that PP processing temperature is kept between 200 and 250℃, as it shows excellent thermal stability (decomposing temperature is 310℃).
To improve shrinking deformation and dents, the mold temperature should be kept between 35 and 65℃. The crystallization temperature of PP is 120 – 125℃.
The molten PP is able to flow through the narrow gaps in a mold, thus causing flash. During its melting process, PP will absorb a large amount of heat of solution (high specific heat), so the product is very hot after the mold opens.
The PP material does not need to be dried when processing. The shrinkage rate and crystallinity of PP is lower than that of PE.
Polyamide (PA)
1. PA Performance:
Commonly known as nylon, PA is also a crystalline polymer, with a density of 1.13g/cm3. Among its diversified varieties, the nylon materials that are used for injection molding typically include Nylon 6, Nylon 1010 and Nylon 610 etc.
Nylon’s advantages include high mechanical strength, high toughness, as well as fatigue resistance, smooth surface, self lubrication, low abrasion coefficient, resistance to abrasion/heat (allow long-term use below 100℃), corrosion resistance and easy processing, etc.
PA weaknesses: easy water absorption, imposing stringent requirements for injection molding, poor dimensional stability, and very hot product upon mold ejection due to the high specific heat.
Among the PP materials, PA66 has the highest mechanical strength and enjoys the most extensive scope of application. Thanks to its high crystallinity, both its rigidity and thermal resistance are very high.
2. PA Applications:
High temperature electric socket parts, electric parts, gears, bearings, rollers, pulleys, spring supports, screws, impellers, fan blades, propellers, high pressure seal gaskets, valve carriers, oil pipelines, oil containers, cables, cable ties, driving belts, grinding wheel adhesives, battery boxes, electrical insulator parts, wire cores and wires, etc.
Due to its high moisture absorption rate, PA must be dried before processing, to bring its moisture content below 0.25%. The drier the raw materials, the better, then the surface gloss of the product can be guaranteed, or it will be very rough.
PA will not soften with the rise of temperature. With the obvious melting point, it will flow the moment the temperature reaches the melting point (different from PS, PE and PP etc.); one of the rheological properties of the nylon material is that its stickiness is not sensitive to shear rate.
Due to its high fluidity, though PA fills the mold easily, flash or burrs tend to occur.
PA features both high melting point and freezing point. The molten PA can become solid at any time with the temperature falling below the freezing point, which hampers the filling and molding process, causing nozzle or sprue blockage. Therefore, high speed injection must be adopted (especially for thin-walled products or long running productions), so as to minimize pressure holding time. In addition, nylon molds need sufficient ventilating measures.
Molten PA has poor thermal stability and is prone to degradation. Normally, the barrel temperature should not exceed 300℃, and the molten material should not be heated in the barrel for more than 30m.
PA imposes high requirements for mold temperature. Its crystallinity can be controlled by mold temperature, so as to acquire the desired performance. It is better if the temperature of PA injection molding can be kept between 50 and 90℃.
Sometimes, a PA product needs “conditioning”, to improve its toughness and dimensional stability.
Polyoxymethylene (POM )
1. POM Performance:
POM is a crystalline polymer, with a density of 1.42g/cm3. It features high rigidity, and is also called “Acetal”.
It possesses many advantages, like fatigue resistance, abrasion resistance, thermal resistance and impact resistance, etc., with a low frictional coefficient and great self lubricating properties. Its heat deflection temperature is 172℃.
POM does not absorb moisture easily, so it maintains excellent dimensional stability in a moist environment, with a shrinkage rate of 2.1%, so it is not easy to control the dimensions during the injection molding process.
Able to replace most of the nonferrous metals, vehicles, machine tools, instrumental internal components, bearings, fasteners, gears, leaf springs, pipes, delivery belt components, electric boilers, pump shells, grids and tap faucets, etc.
3. POM Process Characteristics:
Before processing, the POM material does not need to be dried, but it is better be preheated (at approx. 80℃) during the process, which will benefit product dimensional stability. The processing temperature range of POM is limited between 195 and 215℃. It will decompose if staying in the barrel for too long, or the temperature exceeds 220℃, generating strongly irritant formaldehyde gas. During the injection molding process of the POM material, a high packing pressure is needed (close to the injection pressure), to minimize pressure drop. The revolving speed of the screw cannot be too high, and the residue should be kept as little as possible; POM products feature a high shrinkage rate, and are prone to shrinkage or deformation. With the high specific heat and mold temperature (80 – 100℃), POM products are very hot upon mold injection, so it is necessary to prevent your fingers from being burnt. Ideally, POM needs to be molded under the “medium pressure, medium speed, low material temperature and high mold temperature” conditions. When producing precision products, a mold temperature controller is needed to control the temperature of the mold.
Polycarbonate (PC)
1. PC Performance:
With a density of 1.2g/cm3 and excellent transparency, PC is commonly known as the bulletproof plastic. It possesses an outstanding overall performance that is characterized by “toughness and rigidity”, with high mechanical strength, great toughness, extremely high resistance to impact, outstanding thermal resistance & weatherability, accurate dimensions and high stability. Besides, it is also non-toxic and odorless.
The thermal deformation temperature of PC is between 135 and 143℃, allowing long-term application under the 120 – 130℃ temperature conditions.
PC weaknesses: Poor chemical resistance, low fluidity, sensitive to moisture, and prone to internal stress cracks, etc.
2. PC Applications:
High temperature electric products, air blower shells, transformer casings, electric tools, motor casings, tool boxes, milk bottles, beverage dispenser casings, camera parts, safety helmets, gears, food trays, medical devices, ducts, hair pins, hair dressing tools, shoe heels, structurally stronger engineering parts after being fiber reinforced, and CD discs, etc.
3. PC Process Characteristics:
The PC material is very sensitive to temperature – the stickiness of the molten PC apparently falls with the rise of temperature, leading to a faster flow speed; but it is not sensitive to pressure. To improve its fluidity, the faster way is to raise the temperature. Before processing starts, the PC material needs to be dried thoroughly (at 120℃), to keep moisture content below 0.02%; the appropriate molding conditions of PC is “high material temperature, high mold temperature, high pressure and medium speed”. The mold temperature is better be controlled between 80 and 110℃, while the ideal molding temperature is between 280 and 320℃. Gas marks tend to appear on PC product surface and the gate, and high internal stress will lead to cracks. Therefore, the PC material imposes high requirements for processing. With a low shrinkage rate (approx. 0.6%), its dimensional changes is minimal; the PC injection molded products can apply the “tempering” process to remove the internal stress.
Mold inserts are the terms relative to the integrated core – when the mold core is made of a single piece of steel, it is called an integrated core, while an insert is a part that’s inserted into the mold core made primarily of the steel material. As a matter of fact, the biggest weakness of an insert is that it weakens the rigidity of the mold as a whole. However, the application of different insert materials is able to independently enhance the rigidity of the weak parts.
Now, let’s see what the circumstances for mold inserts applications are:
Material Saving. The dimensioning of a mold core is defined by its highest point. If there is only one single point rising up regularly, this part can be inserted, so as to make the core shorter during dimensioning.
Easy Processing for mold making. During processing, EDM is the slowest equipment with the poorest precision. As a result, an insert is selected to avoid EDM processing.
Facilitate Ventilation. Inserts are usually needed for such circumstances as imperfect drainage during EDM processing and poor ventilation during molding, especially true for some deep ribs.
Guarantee Precision. The grinding machine processes the highest level of processing precision. Sometimes, to guarantee precision, the application of inserts is subject to grinding machine processing requirements.
Change Requirements. Sometimes, a customer may request to change the label of the same product, so interchangeable inserts are necessary under such circumstances.
Convenient Polishing. It is rather inconvenient to polish the mold sometimes, especially for some deep parts. As a result, an insert is needed.
Convenient Mold Modification. Some parts wear easily or have high precision requirements. Out of consideration for mold modification, an insert will be applied.
Heat Radiation (cooling)Considerations. This mainly refers to beryllium copper inserts.
With regard to processing efficiency, some large size molds are divided into several smaller inserts for separate processing to save time. Generally speaking, insects fall into 2 categories, i.e. through-hole insert and blind-end insert. Some small size inserts may adopt the Y-CUT approach to be fixed by the head, while the large size inserts, like those larger than 60*60, may adopt the form of the blind-end insert to be fixed by screws. Though the processing is relatively difficult, mold rigidity is guaranteed.
The material for mold inserts: The steel can be same as core or cavity if without shut off contact.
The material can be same as core or cavity if with shut off contact as blow picture, but 2 degrees more in hardness or different hardness.
when we design the locating feet for mold inserts, we have to consider if the inserts are easy to make, in general, the feet can be designed to 5×2.0mm or 4x3mm.
Moreover, the feet should be placed on the short side for better manufacturing
The feet need to be 0.5 -2.0mm shorter, better for making radius.
if the inserts are not a regular shape, and not suitable for feet manufacture, we can consider making 1-2 degree draft angle around, then we can press them inside tightly.
Eco molding is a very professional injection molding company since 1998, we have accumulated lots of experience in plastic injection mold design, Now we share some special mold design for the complicated product.
