What is the ejector block in plastic injection mold?

Ejector Block

1) Direct ejection can be in 2 styles: ejector and ejecting block + ejector pin.

2) The ejector is designed with gradients on its four sides: 1°on the side  that clings to the product and 3° on the other three sides.

3) When the core of plastic injection mold is integrated, the distance between the ejector and the parting surface is 0.3 – 0.5; When the core is inserted, the ejector clings to the insert. See the diagram:

ejector block design

4) The ejector is fixed with the T-base and socket head cap screws. Its matching relationship with the core is shown in the diagram:

ejector block fixation

5) When the ejecting block + ejector pin structure is applied, the positional relationship is shown in the diagram. The ejecting block is designed with gradients on its four sides: 1° on the side close to the parting surface and 3° on the other three sides.

ejector block +ejector


Weld lines defects in plastic injection molding process

Weld Lines

The flowing defects on product surface or the linear defects that are caused when melt fronts meet each other after splitting.

welding lines plastic injection molding defects

Weld lines refer to the lines that occur on a plastic product surface when the cold slug melts. In areas of mold cavity where the molten plastic meets inserts or holes, areas with inconsistent flow speed/flow interruptions, where multiple flows meet or when jetting happens during mold filling, linear weld marks tend to occur due to imperfect bonding. The existence of weld lines greatly compromises the mechanical strength of a product. The solution for overcoming weld lines is similar to that for reducing product sagging. Weld lines can be caused by many reasons, which covers the various aspects of mold, process, raw materials and product design, etc.

weld lines plastic injection molding defects

(1)Injection Machine
Poor plasticization and non-uniform melt temperature – appropriately extend the injection molding cycle, to ensure thorough plasticization, and change for a machine with a larger plasticization capacity when necessary.



①Mold surface is too cold – appropriately raise mold temperature or purposefully increase the local temperature at melt joints, to restrict cooling speed.

②Runner is too thin, narrow or shallow, and cold slug well is too small – increase runner dimensions, improve runner efficiency and at the same time enlarge the area of the cold slug well.

③Increase or reduce the sectional area of the gate, and change gate location. Gate and runner are too small – increase gate size and runner diameter; gate is too far away from the welding area – need to change gate location or add dozzle(s); when deciding gate location, try to keep the melt from flowing around inserts or holes. When jetting occurs, the gate needs to be fixed, relocated or equipped with blocking buffers. And, try to avoid the application of multiple gates.

④Insufficient mold venting or no vent at all – set up, expand or clear the vents, including venting through insert and ejection pin clearances, setting up vents in welding areas or increasing vent dimensions.

⑤Too much detergent is used – try not to use release agent.

⑥Plastic part is too thin in the welding area – need to modify mold and thicken the area.


(3)Injection Molding Process

① Effective cavity pressure is too low – need to increase injection pressure and injection time.

②Increase injection speed. A high injection speed allows the molten plastic to reach the meeting point before cooling, while a low speed allows sufficient time to let cavity air out.

③Raise melt temperature or nozzle temperature; when the temperature is high, the plastic will feature a low viscosity and good fluidity, thus resulting in slighter weld lines; when the temperature is low, decomposition of gaseous substances can be reduced.

④Try not to use release agents, esp. those containing silicone, or the flows will not be able to bond perfectly.

⑤Reduce clamping force to ensure good ventilation.

⑥Appropriately increase back pressure and adjust screw speed, so as to ensure a uniform melt temperature; increase the screw speed to lower plastic viscosity; increase back pressure to improve plastic density.


(4)Raw Materials

①Ensure the raw materials are sufficiently dried. Dry the raw materials and try not to use liquid additives.

②Improve fluidity. For plastics that feature poor fluidity or high heat sensitivity, lubricants and stabilizers should be appropriately applied; choose materials with good fluidity and high thermal resistance when necessary.


(5)Product Design

①Small thickness – increase the thickness to avoid premature solidification.

②Inappropriate insert location – make appropriate adjustments.

PVC Plastic Injection Molding Process

PVC, i.e. polyvinyl chloride, is a multi-component plastic material that is often applied in the form of pellets. In addition to the previously mentionedPVC injection Molding PVC resin powder, some other components are needed, e.g. adding plasticizer to improve its fluidity, adding stabilizer to improve its thermal stability, adding lubricant to improve its mold releasing performance, as well as fillers, colorants and coupling agents. By virtue of the different proportion of different components, PVC pellets of different mechanical properties and different hardness are thus created, which usually adopts the IP rating. PVC comes in two basic forms: rigid and flexible.

Injection molding process points of PVC are stated as below:


Due the existence of chlorine ions, the PVC material absorbs moisture slightly, so it has to be dried under 75 – 90℃ temperature conditions for about 1.5 – 2.5h before production starts.


(2)Melting Temperature
Melting temperature is one of the most important process parameters for PVC processing. If not appropriately set, material decomposition may be caused. PVC is an amorphous polymer which does not possess a clear melting point. Usually, it melts when heated to 120 – 145℃, but it is able to emit hydrogen chloride (HCl) fumes under 150℃ and emit a large amount of HCl when heated to 180℃. Since it emits poisonous gas during the injection molding process, a lot of thermal stabilizers are needed before the process starts. The adjustable scope of injection temperature is quite narrow. During plastic injection molding, the commonly used temperature is between 140 and 160℃; sometimes the temperature may reach up to 190℃, but injection time should be kept within 20min, or the material will decompose badly. Since material injection temperature is close to decomposition temperature, during injection the temperature should be kept as low as possible and the injection cycle as short as possible, so as to minimize the time for the material to stay in the barrel.


Material Temperature Parameters – phase I: 160 – 170℃, phase II: 160 – 165℃, phase III: 140 – 150℃. Due to its poor thermal stability, PVC emits hydrogen chloride (HCl) if heated in the barrel for too long, which will cause the plastic to turn yellow or black spots on product surface. Also, HCl can erode the cavity, so the dead corners of mold cavity and machine head need to be cleansed frequently.


(3)Mold Temperature
Keep the mold temperature as low as possible (usually mold temperature is kept between 30 – 45℃), shorten injection cycle, and reduce post-ejection product deformation for small-size parts, use calibrator and shrinkage controller to correct and control deformation when necessary.

(4)Runner & Gate
PVC features a poor fluidity, so mold gate and runner should be as large, short and thick as possible, with a product wall thickness of over 1.5mm, so as to minimize pressure loss and fill the cavity ASAP. All in all, it is more appropriate to apply a high pressure and a low temperature (injection pressure over 200MPa, back pressure between 0.5 and 1.5MPa, holding pressure is 20% – 30% of injection pressure).


The PVC melt features a high viscosity, with a small flow length to thickness ratio, so insufficient filling might occur for thin-walled products. Therefore, gate and runner sizes should be minimized. Shrinkage rate is 4‰; and overflow limit value 0.05mm. If small sized parts are to be produced, it is better to choose the pin-point gate or sub gate; for thicker parts, fan gate is a better option. The minimum diameter of pin-point gate or sub gate is 1mm; and the thickness of the fan gate should not be smaller than 1mm.


(5)Other Factors

Injection pressure may reach 150MPa; holding pressure may reach 100MPa. Injection speed: an appropriate injection speed must be employed to avoid material decomposition.


All copyright reserved by injection molding manufacturer Sositar Mould

Mold trial

Mold Trial Steps and Instructions 


Mold Trial Steps

(1) Before mold trial, dry the raw materials first. All materials need to be dried except PE/PP. It is a standard practice to perform the trial with new materials.

(2) Mount the mold onto the center hole and keep it aligned. It is enough to set the clamping force at 1/3 of the rated capacity (tons). During the injection process, if burs occur due to some reason rather than too high injection pressure, the clamping force can be gradually increased until burs disappear, and such a practice is able to improve mold service life.

(3) Mold opening speed, pressure and position should be appropriately adjusted, and in particular, low pressure protection should be well adjusted, because it is a new mold, of which the molded product is unknown. If a mold is equipped with slides, the opening speed cannot be too fast. For a core-pulling mold, the first thing to do is to try it manually to see whether it functions well. Or else, the mold will be damaged in case of mistake.

(4) Clean the barrel with PP or PMMA, which are used for viscosity and friction respectively. Material temperature should be kept between 220 – 240℃. Make the travel as short as possible, and apply some backpressure to cleanse with fast injection. Apply some reasonably proportioned detergent if it is not easy to clean.

(5) Condition setting/material injection volume/pressure & speed/time/quantity/position, etc. Injection pressure/injection speed/quantity are set from lower to higher according to product conditions. Cooling time/injection time should be gradually shortened as the product is taking shape, so as to prevent sticking and overfill.

(6) Packing pressure must be applied, mainly with packing material of 5 – 10mm.

(7) For molds with straight hydraulic clamping system, it is necessary to see whether the clamping force is sufficient; for the toggle type, it is necessary to check whether the crosshead is straightened, to prevent the mold from being pushed backwards during the injection process, thus avoiding mold damage caused by leakage of large burs.

(8) Since the molding conditions are unknown, release agent are needed for the first several runs. If the product is released smoothly without any ejection marks, release agent will not be necessary.

(9) Each parameter should be gradually adjusted according to the molded product. Effect of pressure/speed change will show within 2 runs; effect of material/mold temperature change will show in at least 5 minutes.

(10) The trial molded products should be stored in different bags, with defects and suggestions for improvement noted down. It is better to work in collaboration with the plastic injection mold manufacturer.

(11) Pay attention to mold temperature increase. During multiple mold runs, mold temperature should be controlled via coolant or oil temperature/water temperature/electric heating tube – select different mold temperature controls for different materials, such as hot mold (60℃ and above): nylon/PBT/POM/PPS/PET/PMMA/PC/NORYL, and cold mold (60℃ and below): PE/PP/PS/AS/ABS/PVC/PU.

(12) Air blow can be used to determine how water circulates within the mold. With regard to water pipe installation, inlet pipe should be installed at the lower end; and the return pipe, the upper end.

(13) The machine can only be stopped after raw materials in the barrel are completely injected. Then, remove the water pipe and blow air into the mold to expel water from it. Apply anti-rust agent to the inside of the mold and clean material trifles at the same time.


Mold Trial Instructions

(1) Backpressure should be adjusted according to product conditions, usually between 2 to 5kg/cm2, to achieve an even material feeding. As a precondition, backing off should not allow entry of air.

(2) When melting plastic, rotational speed can be faster for low viscosity, and should be slower for high viscosity.

(3) The setting of a cycle must ensure that the material can be fully melted for the next round of injection. If this purpose cannot be achieved, material temperature/backpressure/cooling time and change for a bigger machine will be among the important considerations.

(4) If the raw material for mold trial is PVC/POM/PBT, they need to be cleaned with PP or PE after the trial.

(5) Keep records of molding conditions and several molded products after the trial, which will be used as a reference for the next trial run, saving both time and materials for plastic injection mold company

(6) For products that have high requirements for precision, another measurement is required on the following day, for confirmation of mold modification.

(7) We have to be clear that there are rings in mm thread (thin thread), such as 10mm, 12mm, 16mm and 20mm, etc., and rings in inch (thick thread), such as 3/8〞, 1/2〞, 5/8〞 and 1〞 etc. There are also thick and thin water pipes (pipe thread), but only the application of quick joint is able to save labor hours.