1. Main Process Performance Test 

1.1 Strength 
The wet compressive strength and dry tensile strength of molding sand prepared from pulp waste liquid are affected by the following factors: 

1.1.1 Amount of pulp waste liquid added 
Under the condition that the proportion of moisture and clay is unchanged and the pile tightness is the same, the wet compressive strength and dry tensile strength will be improved with the increase of pulp waste liquid in the mixture. When the addition amount reaches 6-7%, the wet compressive strength will begin to decrease, but the dry tensile strength will not increase significantly. Through long-term measurement, the relationship between the addition amount and strength of pulp waste liquid is obtained as shown in Fig. 1. 

After adding more than a certain amount of binder, the corresponding moisture content increases, the wetting effect of sand is strengthened, and the mutual slip between sand particles is increased, thus the wet compressive strength is slightly reduced. In terms of dry tensile strength, as the adhesive film around the sand particles has reached a certain thickness, the cohesion of the adhesive is increased, thus weakening the adhesive ability of the adhesive to the sand particles, thus the increase in dry tensile strength is not significant. 

1.1.2 Quality of pulp waste liquid 
Under the condition of a certain amount of addition, the strength of the mixture depends on its specific gravity and the content of dry matter. If the ratio of pulp to waste liquid is large, the more dry matter the content is and the higher the wet compressive strength is. The surface tension of this adhesive material makes the sand mold or mud core have a relatively strong shell. Due to the joint use of this adhesive and clay, the molding sand mixture has a relatively large wet compressive strength. 

Figure 1: Relationship between Pulp Volume and Strength 

 
1.2 Drying 
After molding the waste pulp liquid sand, it is placed in the air for a period of time. Due to the evaporation of the solution, the surface will form a layer of hard shell, which will make the sand mold or mud core have higher wet compression strength before entering the kiln. When the temperature slightly exceeds 100 ℃, the sand mold can have higher strength, and the bonding effect will begin to lose when the temperature exceeds 140-150 ℃. Usually, in order to dry the sand mold or mud core thoroughly, the higher temperature will be kept at 160-180 ℃, so the pulp waste liquid sand has a lower drying temperature than the common binder. 

1.3 Sand fluidity 
As the colloid of pulp waste liquid and the moisture of sand grains wet and bond the clay between the sand grains together, the sand grains have lubricating effect and good plasticity, which is convenient to form accurate model contours and preserve geometric shapes. As the pulp waste liquid is pasty liquid, if too much water is added, the mixture will adhere to the core box, making the mold lifting operation difficult. The amount of water added to make the solid water in the mixture reach saturation, while the water content when the fluid water has not yet occurred is the most appropriate, and then the fluidity of the mixture is the best. Because the binder is used together with clay, the fine scales of clay can slip with each other when deformed, and it is itself a material with large fluidity. The plasticity of the mixture increases with the increase of clay content. 
 
1.4 Hygroscopicity 
Sand dry after storage, the stability of the surface is poo and can absorb moisture from the air.

1.5 Collapse 
The adhesive material can lose its own strength earlier after the molten metal is filled with sand mold, so it has good crushing property. 
 
2. Existing Problems and Preventive Measures 
2.1 The surface strength of sand mold or mud core decreases after high temperature drying, and the protruding edge is relatively serious.

Possible reasons: 
• After the sand mold is dried, the particles of the bonding material lose moisture, so the colloid volume of the bonding sand particles will be reduced, causing capillary cracks to disperse among the sand particles. 
• When the baking temperature of sand mold exceeds 180 ℃, the strength decreases due to the combustion of binder on the surface of mud core. 
• After parking in the air for a long time to absorb water, the strength will be reduced. 

Precautionary approach: 
• Reduce the dry baking temperature of sand mold not to exceed 180 ℃, reduce the temperature fluctuation range, and appropriately prolong the low temperature baking time. 
• After the sand is ground, it is placed for 2-3 hours to allow time for the binder to dissolve and firmly bond with clay and sand. 
• The sand piles near the mold wall shall be tightened, the connection between the inner layer and the outer layer of the sand mold skin shall be increased, and the pulp liquid reinforcing agent shall be sprayed. 
• In order to avoid moisture absorption, the storage time should be shortened. 
 
2.2 The sand mold will absorb moisture from the surrounding air to reduce its strength. 
Precautionary approach: 
• According to the strength reduction value allowed by the process card or technical conditions, the amount of binder added shall be appropriately reduced. 
• Add 0.5% heavy oil to the mixture. 
• The storage time of sand mold should be shortened when it is made.